| 1. |
- Orozco-Duarte, Rogelio, et al.
(författare)
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Synthetic photometry of OB star clusters with stochastically sampled IMFs : analysis of models and HST observations
- 2022
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 509:1, s. 522-549
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Tidskriftsartikel (refereegranskat)abstract
- We present a pilot library of synthetic NUV, U, B, V, and I photometry of star clusters with stochastically sampled IMFs and ionized gas for initial masses, Mi = 103, 104, and 105 M⊙; t = 1, 3, 4, and 8 Myr; Z = 0.014 and Z = 0.002; and log(US) = −2 and −3. We compare the library with predictions from deterministic models and observations of isolated low-mass (<104 M⊙) star clusters with co-spatial compact H II regions. The clusters are located in NGC 7793, one of the nearest galaxies observed as part of the HST LEGUS and Hα-LEGUS surveys. (1) For model magnitudes that only account for the stars: (a) the residual |deterministic mag - median stochastic mag| can be ≥0.5 mag, even for Mi = 105 M⊙; and (b) the largest spread in stochastic magnitudes occurs when Wolf–Rayet stars are present. (2) For Mi = 105 M⊙: (a) the median stochastic mag with gas can be >1.0 mag more luminous than the median stochastic magnitude without gas; and (b) nebular emission lines can contribute with >50 per cent and >30 per cent to the total emission in the V and I bands, respectively. (3) Age-dating OB-star clusters via deterministic tracks in the U-B versus V-I plane is highly uncertain at Z = 0.014 for Mi ∼ 103 M⊙ and Z = 0.002 for Mi ∼ 103–105 M⊙. (4) For low-mass clusters, the V-band extinction derived with stochastic models significantly depends on the value of log(US). (5) The youngest clusters tend to have higher extinction. (6) The majority of clusters have multi-peaked age PDFs. (7) Finally, we discuss the importance of characterizing the true variance in the number of stars per mass bin in nature.
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| 2. |
- Arellano-Cordova, Karla Z., et al.
(författare)
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CLASSY V : The Impact of Aperture Effects on the Inferred Nebular Properties of Local Star-forming Galaxies
- 2022
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 935:2
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Tidskriftsartikel (refereegranskat)abstract
- Strong nebular emission lines are an important diagnostic tool for tracing the evolution of star-forming galaxies across cosmic time. However, different observational setups can affect these lines, and the derivation of the physical nebular properties. We analyze 12 local star-forming galaxies from the COS Legacy Spectroscopy SurveY (CLASSY) to assess the impact of using different aperture combinations on the determination of the physical conditions and gas-phase metallicity. We compare optical spectra observed with the Sloan Digital Sky Survey Data Release aperture, which has a 3'' diameter similar to COS, IFU, and long-slit spectra, including new LBT/MODS observations of five CLASSY galaxies. We calculate the reddening, electron densities and temperatures, metallicities, star formation rates, and equivalent widths (EWs). We find that measurements of the electron densities and temperatures, and metallicity remained roughly constant with aperture size, indicating that the gas conditions are relatively uniform for this sample. However, using IFU observations of three galaxies, we find that the E(B − V) values derived from the Balmer ratios decrease (by up to 53%) with increasing aperture size. The values change most significantly in the center of the galaxies, and level out near the COS aperture diameter of 2 5. We examine the relative contributions from the gas and stars using the Hα and [O iii] λ5007 EWs as a function of aperture light fraction, but find little to no variations within a given galaxy. These results imply that the optical spectra provide nebular properties appropriate for the far-UV CLASSY spectra, even when narrow 1 0 long-slit observations are used.
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| 3. |
- Berg, Danielle A., et al.
(författare)
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The COS Legacy Archive Spectroscopy Survey (CLASSY) Treasury Atlas
- 2022
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 261:2
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Tidskriftsartikel (refereegranskat)abstract
- Far-ultraviolet (FUV; ∼1200–2000 Å) spectra are fundamental to our understanding of star-forming galaxies, providing a unique window on massive stellar populations, chemical evolution, feedback processes, and reionization. The launch of the James Webb Space Telescope will soon usher in a new era, pushing the UV spectroscopic frontier to higher redshifts than ever before; however, its success hinges on a comprehensive understanding of the massive star populations and gas conditions that power the observed UV spectral features. This requires a level of detail that is only possible with a combination of ample wavelength coverage, signal-to-noise, spectral-resolution, and sample diversity that has not yet been achieved by any FUV spectral database. We present the Cosmic Origins Spectrograph Legacy Spectroscopic Survey (CLASSY) treasury and its first high-level science product, the CLASSY atlas. CLASSY builds on the Hubble Space Telescope (HST) archive to construct the first high-quality (S/N1500 Å ≳ 5/resel), high-resolution (R ∼ 15,000) FUV spectral database of 45 nearby (0.002 < z < 0.182) star-forming galaxies. The CLASSY atlas, available to the public via the CLASSY website, is the result of optimally extracting and coadding 170 archival+new spectra from 312 orbits of HST observations. The CLASSY sample covers a broad range of properties including stellar mass (6.2 < log M⋆(M⊙) < 10.1), star formation rate (−2.0 < log SFR (M⊙ yr−1) < +1.6), direct gas-phase metallicity (7.0 < 12+log(O/H) < 8.8), ionization (0.5 < O32 < 38.0), reddening (0.02 < E(B − V) < 0.67), and nebular density (10 < ne (cm−3) < 1120). CLASSY is biased to UV-bright star-forming galaxies, resulting in a sample that is consistent with the z ∼ 0 mass–metallicity relationship, but is offset to higher star formation rates by roughly 2 dex, similar to z ≳ 2 galaxies. This unique set of properties makes the CLASSY atlas the benchmark training set for star-forming galaxies across cosmic time.
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| 4. |
- Calzetti, Daniela, et al.
(författare)
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Revisiting Attenuation Curves : The Case of NGC 3351
- 2021
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 913:1
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Tidskriftsartikel (refereegranskat)abstract
- Multiwavelength images from the far-UV (similar to 0.15 mu m) to the submillimeter of the central region of the galaxy NGC 3351 are analyzed to constrain its stellar populations and dust attenuation. Despite hosting a similar to 1 kpc circumnuclear starburst ring, NGC 3351 deviates from the IRX-beta relation, the relation between the infrared-to-UV luminosity ratio and the UV continuum slope beta that other starburst galaxies follow. To understand the reason for the deviation, we leverage the high angular resolution of archival near-UV-to-near-IR Hubble Space Telescope images to divide the ring into similar to 60-180 pc size regions and model each individually. We find that the UV slope of the combined intrinsic (dust-free) stellar populations in the central region is redder than what is expected for a young model population. This is due to the region's complex star formation history, which boosts the near-UV emission relative to the far-UV. The resulting net attenuation curve has a UV slope that lies between those of the starburst attenuation curve (Calzetti et al. 2000) and the Small Magellanic Cloud extinction curve; the total-to-selective attenuation value, R'(V) = 4.93, is larger than both. As found for other star-forming galaxies, the stellar continuum of NGC 3351 is less attenuated than the ionized gas, with E(B - V)(star) = 0.40 E(B - V)(gas). The combination of the red intrinsic stellar population and the new attenuation curve fully accounts for the location of the central region of NGC 3351 on the IRX-beta diagram. Thus, the observed characteristics result from the complex mixture of stellar populations and dust column densities in the circumnuclear region. Despite being a sample of one, these findings highlight the difficulty of defining attenuation curves of general applicability outside the regime of centrally concentrated starbursts.
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| 5. |
- Della Bruna, Lorenza, et al.
(författare)
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Studying the ISM at ∼10 pc scale in NGC 7793 with MUSE : I. Data description and properties of the ionised gas
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 635
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Tidskriftsartikel (refereegranskat)abstract
- Context. Studies of nearby galaxies reveal that around 50% of the total H alpha luminosity in late-type spirals originates from diffuse ionised gas (DIG), which is a warm, diffuse component of the interstellar medium that can be associated with various mechanisms, the most important ones being leaking HII regions, evolved field stars, and shocks.Aims. Using MUSE Wide Field Mode adaptive optics-assisted data, we study the condition of the ionised medium in the nearby (D=3.4 Mpc) flocculent spiral galaxy NGC 7793 at a spatial resolution of similar to 10 pc. We construct a sample of HII regions and investigate the properties and origin of the DIG component.Methods. We obtained stellar and gas kinematics by modelling the stellar continuum and fitting the H alpha emission line. We identified the boundaries of resolved HII regions based on their H alpha surface brightness. As a way of comparison, we also selected regions according to the H alpha/[SII] line ratio; this results in more conservative boundaries. Using characteristic line ratios and the gas velocity dispersion, we excluded potential contaminants, such as supernova remnants (SNRs) and planetary nebulae (PNe). The continuum subtracted HeII map was used to spectroscopically identify Wolf Rayet stars (WR) in our field of view. Finally, we computed electron densities and temperatures using the line ratio [SII]6716/6731 and [SIII]6312/9069, respectively. We studied the properties of the ionised gas through BPT emission line diagrams combined with velocity dispersion of the gas.Results. We spectroscopically confirm two previously detected WR and SNR candidates and report the discovery of the other seven WR candidates, one SNR, and two PNe within our field of view. The resulting DIG fraction is between similar to 27 and 42% depending on the method used to define the boundaries of the HII regions (flux brightness cut in H alpha = 6.7x10(-18) erg s(-1) cm(-2) or H alpha/[SII] = 2.1, respectively). In agreement with previous studies, we find that the DIG exhibits enhanced [SII]/H alpha and [NII]/H alpha ratios and a median temperature that is similar to 3000 K higher than in HII regions. We also observe an apparent inverse correlation between temperature and H alpha surface brightness. In the majority of our field of view, the observed [SII]6716/6731 ratio is consistent within 1 sigma with n(e)< 30 cm(-3), with an almost identical distribution for the DIG and HII regions. The velocity dispersion of the ionised gas indicates that the DIG has a higher degree of turbulence than the HII regions. Comparison with photoionisation and shock models reveals that, overall, the diffuse component can only partially be explained via shocks and that it is most likely consistent with photons leaking from density bounded HII regions or with radiation from evolved field stars. Further investigation will be conducted in a follow-up paper.
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| 6. |
- Della Bruna, Lorenza, et al.
(författare)
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Studying the ISM at ∼10 pc scale in NGC 7793 with MUSEII : Constraints on the oxygen abundance and ionising radiation escape (Corrigendum)
- 2022
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 663
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Tidskriftsartikel (refereegranskat)abstract
- Table 3 in the original paper reports inaccurate values of Q0exp;YSC and Q0obs(and hence of the related quantities Qexp;tot, Qexptot=Qobs, and fesc). The error in Q0exp;YSC results from a mistake in the sampling of the cluster probability distribution functions (PDFs), whereas Q0obswas mistakenly computed from non-dereddened values of L(Hα). In Table 1 we provide a revised version of the original Table 3, and in Figs. 1 and 2 we show revised versions of the original Figs. 10 and 11. The revised values do not change any of the conclusions stated in the original paper. In the following we provide revised text for the affected sections. (Figure Presented).
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| 7. |
- Della Bruna, Lorenza, et al.
(författare)
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Studying the ISM at similar to 10 pc scale in NGC 7793 with MUSE : II. Constraints on the oxygen abundance and ionising radiation escape
- 2021
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 650
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Tidskriftsartikel (refereegranskat)abstract
- Context. Feedback from massive stars a ffects the interstellar medium (ISM) from the immediate surroundings of the stars (parsec scales) to galactic (kiloparsec) scales. High-spatial resolution studies of H ii regions are critical to investigate how this mechanism operates.Aims. We study the ionised ISM in NGC7793 with the MUSE instrument at ESO Very Large Telescope (VLT), over a field of view (FoV) of similar to 2 kpc2 and at a spatial resolution of similar to 10 pc. The aim is to link the physical conditions of the ionised gas (reddening, ionisation status, abundance measurements) within the spatially resolved H ii regions to the properties of the stellar populations producing Lyman continuum photons.Methods. The analysis of the MUSE dataset, which provides a map of the ionised gas and a census of Wolf Rayet stars, is complemented with a sample of young star clusters (YSCs) and O star candidates observed with the Hubble Space Telescope (HST) and of giant molecular clouds traced in CO(2-1) emission with the Atacama Large Millimeter /submillimeter Array (ALMA). We estimated the oxygen abundance using a temperature-independent strong-line method. We determined the observed total amount of ionising photons ( Q(H0)) from the extinction corrected H ff luminosity. This estimate was then compared to the expected Q(H0) obtained by summing the contributions of YSCs and massive stars. The ratio of the two values gives an estimate for the escape fraction ( fesc) of photons in the region of interest. We used the [S ii] /[O iii] ratio as a proxy for the optical depth of the gas and classified H ii regions into ionisation bounded, or as featuring channels of optically thin gas. We compared the resulting ionisation structure with the computed fesc. We also investigated the dependence of fesc on the age spanned by the stellar population in each region.Results. We find a median oxygen abundance of 12 + log (O =H) similar to 8 :37, with a scatter of 0.25 dex, which is in agreement with previous estimates for our target. We furthermore observe that the abundance map of H ii regions is rich in substructures, surrounding clusters and massive stars, although clear degeneracies with photoionisation are also observed. From the population synthesis analysis, we find that YSCs located in H ii regions have a higher probability of being younger and less massive as well as of emitting a higher number of ionising photons than clusters in the rest of the field. Overall, we find fesc;H ii = 0:67+0:08 0:12 for the population of H ii regions. We also conclude that the sources of ionisation observed within the FoV are more than su fficient to explain the amount of di ffuse ionised gas (DIG) observed in this region of the galaxy. We do not observe a systematic trend between the visual appearance of H ii regions and fesc, pointing to the e ffect of 3D geometry in the small sample probed.
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| 8. |
- Finn, Molly K., et al.
(författare)
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ALMA-LEGUS. I. The Influence of Galaxy Morphology on Molecular Cloud Properties
- 2024
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 964:1
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Tidskriftsartikel (refereegranskat)abstract
- We present a comparative study of the molecular gas in two galaxies from the Legacy ExtraGalactic UV Survey (LEGUS) sample: barred spiral NGC 1313 and flocculent spiral NGC 7793. These two galaxies have similar masses, metallicities, and star formation rates, but NGC 1313 is forming significantly more massive star clusters than NGC 7793, especially young massive clusters (<10 Myr, >104M⊙). Using Atacama Large Millimeter/submillimeter Array (ALMA) CO(2–1) observations of the two galaxies with the same sensitivity and resolution (13 pc), we directly compare the molecular gas in these two similar galaxies to determine the physical conditions responsible for their large disparity in cluster formation. By fitting size–line width relations for the clouds in each galaxy, we find that NGC 1313 has a higher intercept than NGC 7793, implying that its clouds have higher kinetic energies at a given size scale. NGC 1313 also has more clouds near virial equilibrium than NGC 7793, which may be connected to its higher rate of massive cluster formation. However, these virially bound clouds do not show a stronger correlation with young clusters than with the general cloud population. We find surprisingly small differences between the distributions of molecular cloud populations in the two galaxies, though the largest of those differences is that NGC 1313 has higher surface densities and lower freefall times.
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| 9. |
- Finn, Molly K., et al.
(författare)
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ALMA-LEGUS. II. The Influence of Subgalactic Environments on Molecular Cloud Properties
- 2024
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 964:1
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Tidskriftsartikel (refereegranskat)abstract
- We compare the molecular cloud properties in subgalactic regions of two galaxies, barred spiral NGC 1313, which is forming many massive clusters, and flocculent spiral NGC 7793, which is forming significantly fewer massive clusters despite having a similar star formation rate to NGC 1313. We find that there are larger variations in cloud properties between different regions within each galaxy than there are between the galaxies on a global scale, especially for NGC 1313. There are higher masses, line widths, pressures, and virial parameters in the arms of NGC 1313 and the center of NGC 7793 than in the interarm and outer regions of the galaxies. The massive cluster formation of NGC 1313 may be driven by its greater variation in environment, allowing more clouds with the necessary conditions to emerge, although no one parameter seems primarily responsible for the difference in star formation. Meanwhile NGC 7793 has clouds that are as massive and have as much kinetic energy as the clouds in the arms of NGC 1313, but have densities and pressures more similar to those in the interarm regions and so are less inclined to collapse and form stars. The cloud properties in NGC 1313 and NGC 7793 suggest that spiral arms, bars, interarm regions, and flocculent spirals each represent distinct environments with regard to molecular cloud populations. We see surprisingly little difference in surface density between the regions, suggesting that the differences in surface densities frequently seen between arm and interarm regions in lower-resolution studies are indicative of the sparsity of molecular clouds, rather than differences in their true surface density.
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| 10. |
- Gazagnes, Simon, et al.
(författare)
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Interpreting the Si ii and C ii Line Spectra from the COS Legacy Archive Spectroscopic SurveY Using a Virtual Galaxy from a High-resolution Radiation-hydrodynamic Simulation
- 2023
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 952:2
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Tidskriftsartikel (refereegranskat)abstract
- Observations of low-ionization state metal lines provide crucial insights into the interstellar medium (ISM) of galaxies, yet, disentangling the physical processes responsible for the emerging line profiles is difficult. This work investigates how mock spectra generated using a single galaxy in a radiation-hydrodynamical simulation can help us interpret observations of a real galaxy. We create 22,500 C ii and Si ii spectra from the virtual galaxy at different times and through multiple lines of sight and compare them with the 45 observations of low-redshift star-forming galaxies from the COS Legacy Spectroscopic SurveY (classy). We find that the mock profiles provide accurate replicates of the observations of 38 galaxies with a broad range of stellar masses (106–109M⊙) and metallicities (0.02–0.55 Z⊙). Additionally, we highlight that aperture losses explain the weakness of the fluorescent emission in several classy spectra and must be accounted for when comparing simulations to observations. Overall, we show that the evolution of a single simulated galaxy can produce a large diversity of spectra whose properties are representative of galaxies of comparable or smaller masses. Building upon these results, we explore the origin of the continuum, residual flux, and fluorescent emission in the simulation. We find that these different spectral features all emerge from distinct regions in the galaxy's ISM, and their characteristics can vary as a function of the viewing angle. While these outcomes challenge simplified interpretations of down-the-barrel spectra, our results indicate that high-resolution simulations provide an optimal framework to interpret these observations.
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| 11. |
- Gouliermis, Dimitrios A., et al.
(författare)
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Hierarchical star formation across the grand-design spiral NGC 1566
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 468:1, s. 509-530
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Tidskriftsartikel (refereegranskat)abstract
- We investigate how star formation is spatially organized in the grand-design spiral NGC 1566 from deep Hubble Space Telescope photometry with the Legacy ExtraGalactic UV Survey. Our contour-based clustering analysis reveals 890 distinct stellar conglomerations at various levels of significance. These star- forming complexes are organized in a hierarchical fashion with the larger congregations consisting of smaller structures, which themselves fragment into even smaller and more compact stellar groupings. Their size distribution, covering a wide range in length-scales, shows a power law as expected from scale-free processes. We explain this shape with a simple 'fragmentation and enrichment' model. The hierarchical morphology of the complexes is confirmed by their mass-size relation that can be represented by a power law with a fractional exponent, analogous to that determined for fractal molecular clouds. The surface stellar density distribution of the complexes shows a lognormal shape similar to that for supersonic non-gravitating turbulent gas. Between 50 and 65 per cent of the recently formed stars, as well as about 90 per cent of the young star clusters, are found inside the stellar complexes, located along the spiral arms. We find an age difference between young stars inside the complexes and those in their direct vicinity in the arms of at least 10 Myr. This time-scale may relate to the minimum time for stellar evaporation, although we cannot exclude the in situ formation of stars. As expected, star formation preferentially occurs in spiral arms. Our findings reveal turbulent-driven hierarchical star formation along the arms of a grand-design galaxy.
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| 12. |
- Hu, Weida, et al.
(författare)
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CLASSY VII Lyα Profiles : The Structure and Kinematics of Neutral Gas and Implications for LyC Escape in Reionization-era Analogs
- 2023
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 956:1
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Tidskriftsartikel (refereegranskat)abstract
- Lyα line profiles are a powerful probe of interstellar medium (ISM) structure, outflow speed, and Lyman-continuum escape fraction. In this paper, we present the Lyα line profiles of the Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY, a sample rich in spectroscopic analogs of reionization-era galaxies. A large fraction of the spectra show a complex profile, consisting of a double-peaked Lyα emission profile in the bottom of a damped, Lyα absorption trough. Such profiles reveal an inhomogeneous ISM. We successfully fit the damped Lyα absorption and the Lyα emission profiles separately, but with complementary covering factors, a surprising result because this approach requires no Lyα exchange between high-NH i and low-NH i paths. The combined distribution of column densities is qualitatively similar to the bimodal distributions observed in numerical simulations. We find an inverse relation between Lyα peak separation and the [O iii]/[O ii] flux ratio, confirming that the covering fraction of Lyman-continuum-thin sightlines increases as the Lyα peak separation decreases. We combine measurements of Lyα peak separation and Lyα red peak asymmetry in a diagnostic diagram, which identifies six Lyman-continuum leakers in the COS Legacy Archive Spectrocopy SurveY (CLASSY) sample. We find a strong correlation between the Lyα trough velocity and the outflow velocity measured from interstellar absorption lines. We argue that greater vignetting of the blueshifted Lyα peak, relative to the redshifted peak, is the source of the well-known discrepancy between shell-model parameters and directly measured outflow properties. The CLASSY sample illustrates how scattering of Lyα photons outside the spectroscopic aperture reshapes Lyα profiles because the distances to these compact starbursts span a large range.
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| 13. |
- James, Bethan L., et al.
(författare)
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CLASSY. II. A Technical Overview of the COS Legacy Archive Spectroscopic Survey
- 2022
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Ingår i: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 262:2
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Tidskriftsartikel (refereegranskat)abstract
- The COS Legacy Archive Spectroscopic SurveY (CLASSY) is designed to provide the community with a spectral atlas of 45 nearby star-forming galaxies that were chosen to cover similar properties to those seen at high z (z > 6). The prime high-level science product of CLASSY is accurately coadded UV spectra, ranging from ∼1000 to 2000 Å, derived from a combination of archival and new data obtained with HST's Cosmic Origins Spectrograph (COS). This paper details the multistage technical processes of creating this prime data product and the methodologies involved in extracting, reducing, aligning, and coadding far-ultraviolet and near-ultraviolet (NUV) spectra. We provide guidelines on how to successfully utilize COS observations of extended sources, despite COS being optimized for point sources, and best-practice recommendations for the coaddition of UV spectra in general. Moreover, we discuss the effects of our reduction and coaddition techniques in the scientific application of the CLASSY data. In particular, we find that accurately accounting for flux calibration offsets can affect the derived properties of the stellar populations, while customized extractions of NUV spectra for extended sources are essential for correctly diagnosing the metallicity of galaxies via C iii] nebular emission. Despite changes in spectral resolution of up to ∼25% between individual data sets (due to changes in the COS line-spread function), no adverse affects were observed on the difference in velocity width and outflow velocities of isolated absorption lines when measured in the final combined data products, owing in part to our signal-to-noise regime of S/N < 20.
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| 14. |
- Jung, Dooseok Escher, et al.
(författare)
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Universal Upper End of the Stellar Initial Mass Function in the Young and Compact LEGUS Clusters
- 2023
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 954:2
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Tidskriftsartikel (refereegranskat)abstract
- We investigate the variation in the upper end of the stellar initial mass function (uIMF) in 375 young and compact star clusters in five nearby galaxies within ∼5 Mpc. All the young stellar clusters (YSCs) in the sample have ages ≲ 4 Myr and masses above 500 M⊙, according to standard stellar models. The YSC catalogs were produced from Hubble Space Telescope images obtained as part of the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. They are used here to test whether the uIMF is universal or changes as a function of the cluster's stellar mass. We perform this test by measuring the Hα luminosity of the star clusters as a proxy for their ionizing photon rate, and charting its trend as a function of cluster mass. Large cluster numbers allow us to mitigate the stochastic sampling of the uIMF. The advantage of our approach relative to previous similar attempts is the use of cluster catalogs that have been selected independently of the presence of Hα emission, thus removing a potential sample bias. We find that the uIMF, as traced by the Hα emission, shows no dependence on cluster mass, suggesting that the maximum stellar mass that can be produced in star clusters is universal, in agreement with previous findings.
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| 15. |
- Krumholz, Mark R., et al.
(författare)
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STAR CLUSTER PROPERTIES IN TWO LEGUS GALAXIES COMPUTED WITH STOCHASTIC STELLAR POPULATION SYNTHESIS MODELS
- 2015
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 812:2
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Tidskriftsartikel (refereegranskat)abstract
- We investigate a novel Bayesian analysis method, based on the Stochastically Lighting Up Galaxies (slug) code, to derive the masses, ages, and extinctions of star clusters from integrated light photometry. Unlike many analysis methods, slug correctly accounts for incomplete initial mass function (IMF) sampling, and returns full posterior probability distributions rather than simply probability maxima. We apply our technique to 621 visually confirmed clusters in two nearby galaxies, NGC 628 and NGC 7793, that are part of the Legacy Extragalactic UV Survey (LEGUS). LEGUS provides Hubble Space Telescope photometry in the NUV, U, B, V, and I bands. We analyze the sensitivity of the derived cluster properties to choices of prior probability distribution, evolutionary tracks, IMF, metallicity, treatment of nebular emission, and extinction curve. We find that slug's results for individual clusters are insensitive to most of these choices, but that the posterior probability distributions we derive are often quite broad, and sometimes multi-peaked and quite sensitive to the choice of priors. In contrast, the properties of the cluster population as a whole are relatively robust against all of these choices. We also compare our results from slug to those derived with a conventional non-stochastic fitting code, Yggdrasil. We show that slug's stochastic models are generally a better fit to the observations than the deterministic ones used by Yggdrasil. However, the overall properties of the cluster populations recovered by both codes are qualitatively similar.
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| 16. |
- Mingozzi, Matilde, et al.
(författare)
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CLASSY IV. Exploring UV Diagnostics of the Interstellar Medium in Local High-z Analogs at the Dawn of the JWST Era
- 2022
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 939:2
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Tidskriftsartikel (refereegranskat)abstract
- The Cosmic Origins Spectrograph (COS) Legacy Archive Spectroscopic SurveY (CLASSY) provides the first high-resolution spectral catalog of 45 local high-z analogs in the ultraviolet (UV; 1200–2000 Å) to investigate their stellar and gas properties. Here we present a toolkit of UV interstellar medium (ISM) diagnostics, analyzing the main emission lines of CLASSY spectra (N iv] λλ1483,87, C iv λλ1548,51, He iiλ1640, O iii]λλ1661,6, Si iii] λλ1883,92, C iii] λ1907,9). Specifically, our aim is to provide accurate diagnostics for the reddening E(B − V), electron density ne, electron temperature Te, metallicity 12+log(O/H), and ionization parameter log(U), taking the different ISM ionization zones into account. We calibrate our UV toolkit using well-known optical diagnostics, analyzing archival optical spectra for all CLASSY targets. We find that UV density diagnostics estimate ne values that are ∼1–2 dex higher (e.g., ne(C iii]λλ1907,9) ∼ 104 cm−3) than those inferred from their optical counterparts (e.g., ne([S ii]λλ6717,31) ∼ 102 cm−3; ne([Ar iv]λλ4714,41) ∼ 103 cm−3). Te derived from the hybrid ratio [O iii] λ1666/λ5007 proves to be reliable, implying differences in determining 12+log(O/H) compared to the optical counterpart O iii] λ4363/[O iii] λ5007 within ∼ ±0.3 dex. We also investigate the relation between the stellar and gas E(B − V), finding consistent values at high specific star formation rates (sSFRs; log(sSFR) ≳ -8 yr−1), while at low sSFRs we confirmed an excess of dust attenuation in the gas. Finally, we investigate UV line ratios and equivalent widths to provide correlations with 12+log(O/H) and log(U), but note that there are degeneracies between the two. With this suite of UV-based diagnostics, we illustrate the pivotal role CLASSY plays in understanding the chemical and physical properties of high-z systems that JWST can observe in the rest-frame UV.
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| 17. |
- Mingozzi, Matilde, et al.
(författare)
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CLASSY. VIII. Exploring the Source of Ionization with UV Interstellar Medium Diagnostics in Local High-z Analogs
- 2024
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 962:1
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Tidskriftsartikel (refereegranskat)abstract
- In the current JWST era, rest-frame UV spectra play a crucial role in enhancing our understanding of the interstellar medium (ISM) and stellar properties of the first galaxies in the epoch of reionization (z > 6). Here, we compare well-known and reliable optical diagrams sensitive to the main ionization source (i.e., star formation, SF; active galactic nuclei, AGN; and shocks) to UV counterparts proposed in the literature—the so-called "UV–BPT diagrams"—using the HST COS Legacy Archive Spectroscopic SurveY (CLASSY), which is the largest high-quality, high-resolution, and broad-wavelength range atlas of far-UV spectra for 45 local star-forming galaxies. In particular, we explore where CLASSY UV line ratios are located in the different UV diagnostic plots, taking into account state-of-the-art photoionization and shock models, and, for the first time, the measured ISM and stellar properties (e.g., gas-phase metallicity, ionization parameter, carbon abundance, and stellar age). We find that the combination of C iii] λλ1907,9 He iiλ1640 and O iii] λ1666 can be a powerful tool to separate between SF, shocks, and AGN at subsolar metallicities. We also confirm that alternative diagrams without O iii] λ1666 still allow us to define an SF-locus, with some caveats. Diagrams including C ivλλ1548,51 should be taken with caution given the complexity of this doublet profile. Finally, we present a discussion detailing the ISM conditions required to detect UV emission lines, visible only in low gas-phase metallicity (12 + log(O/H) ≲ 8.3) and high ionization parameter (log(U) ≳ −2.5) environments. Overall, CLASSY and our UV toolkit will be crucial in interpreting the spectra of the earliest galaxies that JWST is currently revealing.
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| 18. |
- Sirressi, Mattia, et al.
(författare)
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CLusters in the UV as EngineS (CLUES). I. Survey Presentation and FUV Spectral Analysis of the Stellar Light
- 2022
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 0004-6256 .- 1538-3881. ; 164:5
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Tidskriftsartikel (refereegranskat)abstract
- The CLusters in the Uv as EngineS (CLUES) survey is a Cosmic Origins Spectrograph (COS) campaign aimed at acquiring the 1130–1770 Å rest-frame spectroscopy of very young (<20 Myr) and massive (>104 M⊙) star clusters in galaxies that are part of the Hubble treasury program Legacy ExtraGalactic UV Survey. In this first paper of a series, we describe the CLUES sample consisting of 20 young star clusters and report their physical properties as derived by both multiwavelength photometry and far-UV (FUV) spectroscopy with Hubble Space Telescope. Thanks to the synergy of the two different data sets, we build a coherent picture of the diverse stellar populations found in each region (with sizes of 40–160 pc). We associate the FUV-brightest stellar population to the central targeted star cluster and the other modeled population to the diffuse stars that are included in the COS aperture. We observe better agreement between photometric and spectroscopic ages for star clusters younger than 5 Myr. For clusters older than 5 Myr, photometry and spectroscopy measurements deviate, with the latter producing older ages, due to the degeneracy of photometric models. FUV spectroscopy enables us to better constrain the stellar metallicities, a parameter that optical colors are insensitive to. Finally, the derived E(B − V) are quite similar, with a tendency for FUV spectroscopy to favor solutions with higher extinctions. The recovered masses are in agreement within a factor of 2 for all of the clusters.
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| 19. |
- Sirressi, Mattia, 1995-, et al.
(författare)
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CLusters in the Uv as EngineS (CLUES). II. Subkiloparsec-scale Outflows Driven by Stellar Feedback
- 2024
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Ingår i: Astronomical Journal. - 0004-6256 .- 1538-3881. ; 167:4
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the far-ultraviolet (1130−1770 Å rest frame) spectroscopy of 20 young (<50 Myr) and massive (>104M⊙) star clusters (YSCs) in 11 nearby star-forming galaxies. We probe the interstellar gas intervening along the line of sight, detecting several metal absorption lines of a wide range of ionization potentials, from 6.0 to 77.5 eV. Multiple-component Voigt fits to the absorption lines are used to study the kinematics of the gas. We find that nearly all targets in the sample feature gas outflowing from 30 up to 190 km s−1, often in both the neutral and ionized phases. The outflow velocities correlate with the underlying stellar population properties directly linked to the feedback: the mass of the YSCs, the photon production rate, and the instantaneous mechanical luminosity produced by stellar winds and supernovae. We detect a neutral inflow in four targets, which we interpret as likely not associated with the star cluster but tracing larger-scale gas kinematics. A comparison between the outflows' energy and that produced by the associated young stellar populations suggests an average coupling efficiency of 10% with a broad scatter. Our results extend the relation found in previous works between galactic outflows and the host galaxy star formation rate to smaller scales, pointing toward the key role that clustered star formation and feedback play in regulating galaxy growth.
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| 20. |
- Stark, Daniel P., et al.
(författare)
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Ultraviolet emission lines in young low-mass galaxies at z similar or equal to 2 : physical properties and implications for studies at z > 7
- 2014
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 445:3, s. 3200-3220
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Tidskriftsartikel (refereegranskat)abstract
- We present deep spectroscopy of 17 very low mass (M* similar or equal to 2.0 x 10(6)-1.4 x 10(9) M-circle dot) and low luminosity (M-UV similar or equal to -13.7 to -19.9) gravitationally lensed galaxies in the redshift range z similar or equal to 1.5-3.0. Deep rest-frame ultraviolet spectra reveal large equivalent width emission from numerous emission lines (N IV], O III], C IV, Si III], C III]) which are rarely seen in individual spectra of more massive star-forming galaxies. C III] is detected in 16 of 17 low-mass star-forming systems with rest-frame equivalent widths as large as 13.5 angstrom. Nebular C (IV) emission is present in the most extreme C III] emitters, requiring an ionizing source capable of producing a substantial component of photons with energies in excess of 47.9 eV. Photoionization models support a picture whereby the large equivalent widths are driven by the increased electron temperature and enhanced ionizing output arising from metal-poor gas and stars (0.04-0.13 Z(circle dot)), young stellar populations (6-50 Myr), and large ionization parameters (log U = -2.16 to -1.84). The young ages implied by the emission lines and continuum spectral energy distributions (SEDs) indicate that the extreme line emitters in our sample are in the midst of a significant upturn in their star formation activity. The low stellar masses, blue UV colours, and large specific star formation rates of our sample are similar to those of typical z greater than or similar to 6 galaxies. Given the strong attenuation of Ly alpha in z greater than or similar to 6 galaxies, we suggest that C III] is likely to provide our best probe of early star-forming galaxies with ground-based spectrographs and one off the most efficient means of confirming z greater than or similar to 10 galaxies with the James Webb Space Telescope.
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| 21. |
- Whitmore, Bradley C., et al.
(författare)
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LEGUS and H-alpha-LEGUS Observations of Star Clusters in NGC 4449 : Improved Ages and the Fraction of Light in Clusters as a Function of Age
- 2020
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 889:2
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Tidskriftsartikel (refereegranskat)abstract
- We present a new catalog and results for the cluster system of the starburst galaxy NGC 4449, based on multiband imaging observations taken as part of the LEGUS and H-alpha-LEGUS surveys. We improve the spectral energy fitting method used to estimate cluster ages, and find that the results, particularly for older clusters, are in better agreement with those from spectroscopy. The inclusion of H-alpha measurements, the role of stochasticity for low-mass clusters, the assumptions about reddening, and the choices of SSP model and metallicity all have important impacts on the age dating of clusters. A comparison with ages derived from stellar color-magnitude diagrams for partially resolved clusters shows reasonable agreement, but large scatter in some cases. The fraction of light found in clusters relative to the total light (i.e., T-L) in the U, B, and V filters in 25 different approximate to kiloparsec-size regions throughout NGC 4449 correlates with both the specific region luminosity, R-L, and the dominant age of the underlying stellar population in each region. The observed cluster age distribution is found to decline over time as dN/d tau proportional to tau(gamma), with gamma = -0.85 +/- 0.15, independent of cluster mass, and is consistent with strong, early cluster disruption. The mass functions of the clusters can be described by a power law with dN/dM proportional to M-beta and beta = -1.86 +/- 2, independent of cluster age. The mass and age distributions are quite resilient to differences in age-dating methods. There is tentative evidence for a factor of 2-3 enhancement in both the star and cluster formation rate approximate to 100-300 Myr ago, indicating that cluster formation tracks star formation generally. The enhancement is probably associated with an earlier interaction event.
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| 22. |
- Wofford, Aida, et al.
(författare)
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Candidate LBV stars in galaxy NGC 7793 found via HST photometry plus MUSE spectroscopy
- 2020
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 493:2, s. 2410-2428
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Tidskriftsartikel (refereegranskat)abstract
- Only about 19 Galactic and 25 extragalactic bonafide luminous blue variables (LBVs) are known to date. This incomplete census prevents our understanding of this crucial phase of massive star evolution which leads to the formation of heavy binary black holes via the classical channel. With large samples of LBVs one could better determine the duration and maximum stellar luminosity which characterize this phase. We search for candidate LBVs (cLBVs) in a new galaxy, NGC 7793. For this purpose, we combine high spatial resolution images from two Hubble Space Telescope (HST) programs with optical spectroscopy from the Multi Unit Spectroscopic Explorer (MUSE). By combining PSF-fitting photometry measured on F547M, F657N, and F814W images, with restrictions on point-like appearance (at HST resolution) and Ha luminosity, we find 100 potential cLBVs, 36 of which fall in the MUSE fields. Five of the latter 36 sources are promising cLBVs which have M-V <= -7 and a combination of: H alpha with a P-Cygni profile; no [O I]lambda 6300 emission; weak or no [O III]lambda 5007 emission; large [N II]/H alpha relative to HII regions; and [S II]lambda 6716/[S II]lambda 6731 similar to 1. It is not clear if these five cLBVs are isolated from O-type stars, which would favour the binary formation scenario of LBVs. Our study, which approximately covers one fourth of the optical disc of NGC 7793, demonstrates how by combining the above HST surveys with multi-object spectroscopy from 8-m class telescopes, one can efficiently find large samples of cLBVs in nearby galaxies.
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| 23. |
- Wofford, Aida, et al.
(författare)
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Extreme broad He ii emission at high and low redshifts : the dominant role of VMS in NGC 3125-A1 and CDFS131717
- 2023
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Ingår i: Monthly notices of the Royal Astronomical Society. - 0035-8711 .- 1365-2966. ; 523:3, s. 3949-3966
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Tidskriftsartikel (refereegranskat)abstract
- Super star cluster (SSC) A1 (3.1 x10(5) M-circle dot) in NGC 3125 has one of the strongest ( EW = 4.6 +/- 0.5 A) broad ( F W H M = 1131 +/- 40 km s(-1)) He II lambda 1640 emission lines in the nearby universe and constitutes an important template for interpreting observations of extreme He II emitters out to redshifts of z similar to 2 - 3. We use Cosmic Origins Spectrograph (COS) observations of A1 to show that there is no significant contamination of the He II line with nebular emission and that the line is redshifted by 121 +/- 17 km s(-1) relative to ISM lines. We compare the COS G130M + G160M observations of A1 to recent binary BPASS and single-star Charlot & Bruzual (C&B) simple stellar population (SSP) models with very massive stars (VMS) of up to 300 M-circle dot. On the other hand, we suggest why BPASS models fail to reproduce A1's He II emission. On the other hand, a C&B model with Z = 0.008, age = 2.2 Myr, and VMS approaching the Eddington limit provides an excellent fit to the He II emission and fits reasonably well C III lambda 1175, N V lambda lambda 1238, 1241, and C IV lambda lambda 1548, 1551. We present O V lambda 1371 line-profile predictions showing that this line constitutes an important tracer of youth and VMS in galaxies. Finally, we discuss the presence of VMS in CDFS131717, a highly star-forming low-metallicity galaxy located at z = 3.071, which has a tentative detection of O V absorption and strong broad He II emission. These features are rare and hint to the presence of short-lived VMS in the galaxy. Our results show the effect of the latest developments of stellar wind theory and the importance of accounting for VMS in models.
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| 24. |
- Wofford, Aida, et al.
(författare)
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Stars and gas in the most metal-poor galaxies - I. COS and MUSE observations of SBS 0335-052E
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 500:3, s. 2908-2927
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Tidskriftsartikel (refereegranskat)abstract
- Among the nearest most metal-poor starburst-dwarf galaxies known, SBS 0335-052E is the most luminous in integrated nebular He II.4686 emission. This makes it a unique target to test spectral synthesis models and spectral interpretation tools of the kind that will be used to interpret future rest-frame UV observations of primeval galaxies. Previous attempts to reproduce its He II.4686 luminosity found that X-ray sources, shocks, and single Wolf-Rayet stars are not main contributors to the He IIionizing budget; and that only metal-free single rotating stars or binary stars with a top-heavy IMF and an unphysically low metallicity can reproduce it. We present new UV (COS) and optical (MUSE) spectra that integrate the light of four super star clusters in SBS 0335-052E. Nebular He II, [CIII], CIII], C IV, and OIII] UV emission lines with equivalent widths between 1.7 and 5 A and a CIV lambda lambda 1548, 1551 P-Cygni like profile are detected. Recent extremely metal-poor shock + precursor models and binary models fail to reproduce the observed optical emission-line ratios. We use different sets of UV and optical observables to test models of constant star formation with single non-rotating stars that account for very massive stars as blueshifted OV lambda 1371 absorption is present. Simultaneously fitting the fluxes of all high-ionization UV lines requires an unphysically low metallicity. Fitting the P-Cygni like + nebular components of CIV lambda lambda 1548, 1551 does not constrain the stellar metallicity and time since the beginning of star formation. We obtain 12+log(O/H)= 7.45 +/- 0.04 and log(C/O)= -0.45(-0.04)(+0.03) for the galaxy. Model testing would benefit from higher spatial resolution UV and optical spectroscopy of the galaxy.
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