| 1. |
- Adamo, Angela, et al.
(author)
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Legacy ExtraGalactic UV Survey with The Hubble Space Telescope : Stellar Cluster Catalogs and First Insights Into Cluster Formation and Evolution in NGC 628
- 2017
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In: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 841:2
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Journal article (peer-reviewed)abstract
- We report the large effort that is producing comprehensive high-level young star cluster (YSC) catalogs for a significant fraction of galaxies observed with the Legacy ExtraGalactic UV Survey (LEGUS) Hubble treasury program. We present the methodology developed to extract cluster positions, verify their genuine nature, produce multiband photometry (from NUV to NIR), and derive their physical properties via spectral energy distribution fitting analyses. We use the nearby spiral galaxy NGC 628 as a test case for demonstrating the impact that LEGUS will have on our understanding of the formation and evolution of YSCs and compact stellar associations within their host galaxy. Our analysis of the cluster luminosity function from the UV to the NIR finds a steepening at the bright end and at all wavelengths suggesting a dearth of luminous clusters. The cluster mass function of NGC 628 is consistent with a power-law distribution of slopes similar to-2 and a truncation of a few times 10(5) M-circle dot. After their formation, YSCs and compact associations follow different evolutionary paths. YSCs survive for a longer time frame, confirming their being potentially bound systems. Associations disappear on timescales comparable to hierarchically organized star-forming regions, suggesting that they are expanding systems. We find massindependent cluster disruption in the inner region of NGC 628, while in the outer part of the galaxy there is little or no disruption. We observe faster disruption rates for low mass (<= 10(4) M-circle dot) clusters, suggesting that a massdependent component is necessary to fully describe the YSC disruption process in NGC 628.
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| 2. |
- Grasha, K., et al.
(author)
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The spatial relation between young star clusters and molecular clouds in M51 with LEGUS
- 2019
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 483:4, s. 4707-4723
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Journal article (peer-reviewed)abstract
- We present a study correlating the spatial locations of young star clusters with those of molecular clouds in NGC 5194, in order to investigate the time-scale over which clusters separate from their birth clouds. The star cluster catalogues are from the Legacy ExtraGalactic UV Survey (LEGUS) and the molecular clouds from the Plateau de Bure Interefrometer Arcsecond Whirpool Survey (PAWS). We find that younger star clusters are spatially closer to molecular clouds than older star clusters. The median age for clusters associated with clouds is 4 Myr, whereas it is 50 Myr for clusters that are sufficiently separated from a molecular cloud to be considered unassociated. After similar to 6 Myr, the majority of the star clusters lose association with their molecular gas. Younger star clusters are also preferentially located in stellar spiral arms where they are hierarchically distributed in kpc-size regions for 50-100 Myr before dispersing. The youngest star clusters are more strongly clustered, yielding a two-point correlation function with alpha = -0.28 +/- 0.04, than the giant molecular cloud (GMCs) (alpha = -0.09 +/- 0.03) within the same PAWS field. However, the clustering strength of the most massive GMCs, supposedly the progenitors of the young clusters for a star formation efficiency of a few percent, is comparable (alpha = -0.35 +/- 0.05) to that of the clusters. We find a galactocentric dependence for the coherence of star formation, in which clusters located in the inner region of the galaxy reside in smaller star-forming complexes and display more homogeneous distributions than clusters further from the centre. This result suggests a correlation between the survival of a cluster complex and its environment.
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| 3. |
- Messa, Matteo, et al.
(author)
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The young star cluster population of M51 with LEGUS - I. A comprehensive study of cluster formation and evolution
- 2018
-
In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 473:1, s. 996-1018
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Journal article (peer-reviewed)abstract
- Recently acquired WFC3 UV (F275W and F336W) imaging mosaics under the Legacy Extragalactic UV Survey (LEGUS), combined with archival ACS data of M51, are used to study the young star cluster (YSC) population of this interacting system. Our newly extracted source catalogue contains 2834 cluster candidates, morphologically classified to be compact and uniform in colour, for which ages, masses and extinction are derived. In this first work we study the main properties of the YSC population of the whole galaxy, considering a mass-limited sample. Both luminosity and mass functions follow a power-law shape with slope -2, but at high luminosities and masses a dearth of sources is observed. The analysis of the mass function suggests that it is best fitted by a Schechter function with slope -2 and a truncation mass at 1.00 +/- 0.12 x 10(5) M-circle dot . Through Monte Carlo simulations, we confirm this result and link the shape of the luminosity function to the presence of a truncation in the mass function. A mass limited age function analysis, between 10 and 200 Myr, suggests that the cluster population is undergoing only moderate disruption. We observe little variation in the shape of the mass function at masses above 1 x 10(4) M-circle dot over this age range. The fraction of star formation happening in the form of bound clusters in M51 is similar to 20 per cent in the age range 10-100 Myr and little variation is observed over the whole range from 1 to 200 Myr.
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| 4. |
- Ashworth, G., et al.
(author)
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Exploring the IMF of star clusters : a joint SLUG and LEGUS effort
- 2017
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 469:2, s. 2464-2480
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Journal article (peer-reviewed)abstract
- We present the implementation of a Bayesian formalism within the Stochastically Lighting Up Galaxies (SLUG) stellar population synthesis code, which is designed to investigate variations in the initial mass function (IMF) of star clusters. By comparing observed cluster photometry to large libraries of clusters simulated with a continuously varying IMF, our formalism yields the posterior probability distribution function (PDF) of the cluster mass, age and extinction, jointly with the parameters describing the IMF. We apply this formalism to a sample of star clusters from the nearby galaxy NGC 628, for which broad-band photometry in five filters is available as part of the Legacy ExtraGalactic UV Survey (LEGUS). After allowing the upper-end slope of the IMF (a3) to vary, we recover PDFs for the mass, age and extinction that are broadly consistent with what is found when assuming an invariant Kroupa IMF. However, the posterior PDF for a3 is very broad due to a strong degeneracy with the cluster mass, and it is found to be sensitive to the choice of priors, particularly on the cluster mass. We find only a modest improvement in the constraining power of a3 when adding Ha photometry from the companion Ha-LEGUS survey. Conversely, Ha photometry significantly improves the age determination, reducing the frequency of multi-modal PDFs. With the aid of mock clusters, we quantify the degeneracy between physical parameters, showing how constraints on the cluster mass that are independent of photometry can be used to pin down the IMF properties of star clusters.
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| 5. |
- Calzetti, D., et al.
(author)
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THE BRIGHTEST YOUNG STAR CLUSTERS IN NGC 5253
- 2015
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 811:2
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Journal article (peer-reviewed)abstract
- The nearby dwarf starburst galaxy NGC 5253 hosts a number of young, massive star clusters, the two youngest of which are centrally concentrated and surrounded by thermal radio emission (the radio nebula). To investigate the role of these clusters in the starburst energetics, we combine new and archival Hubble Space Telescope images of NGC 5253 with wavelength coverage from 1500 angstrom 1.9 mu m in 13 filters. These include H alpha, P beta, and P alpha, and the imaging from the Hubble Treasury Program LEGUS (Legacy Extragalactic UV Survey). The extraordinarily well-sampled spectral energy distributions enable modeling with unprecedented accuracy the ages, masses, and extinctions of the nine optically brightest clusters (M-V < -8.8) and the two young radio nebula clusters. The clusters have ages similar to 1-15 Myr and masses similar to 1 x 10(4)-2.5 x 10(5) M-circle dot. The clusters' spatial location and ages indicate that star formation has become more concentrated toward the radio nebula over the last similar to 15 Myr. The most massive cluster is in the radio nebula; with a mass similar to 2.5 x 10(5) M-circle dot and an age similar to 1 Myr, it is 2-4 times less massive and younger than previously estimated. It is within a dust cloud with AV similar to 50 mag, and shows a clear near-IR excess, likely from hot dust. The second radio nebula cluster is also similar to 1 Myr old, confirming the extreme youth of the starburst region. These two clusters account for about half of the ionizing photon rate in the radio nebula, and will eventually supply about 2/3 of the mechanical energy in present-day shocks. Additional sources are required to supply the remaining ionizing radiation, and may include very massive stars.
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| 6. |
- Cignoni, M., et al.
(author)
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Star Formation Histories of the LEGUS Dwarf Galaxies. I. Recent History of NGC 1705, NGC 4449, and Holmberg II
- 2018
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 856:1
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Journal article (peer-reviewed)abstract
- We use Hubble Space Telescope observations from the Legacy Extragalactic UV Survey to reconstruct the recent star formation histories (SFHs) of three actively star-forming dwarf galaxies, NGC 4449, Holmberg II, and NGC 1705, from their UV color-magnitude diagrams (CMDs). We apply a CMD fitting technique using two independent sets of stellar isochrones, PARSEC-COLIBRI and MIST, to assess the uncertainties related to stellar evolution modeling. Irrespective of the adopted stellar models, all three dwarfs are found to have had almost constant star formation rates (SFRs) in the last 100-200 Myr, with modest enhancements (a factor of similar to 2) above the 100 Myr averaged SFR. Significant differences among the three dwarfs are found in terms of the overall SFR, the timing of the most recent peak, and the SFR/area. The initial mass function of NGC. 1705 and Holmberg II is consistent with a Salpeter slope down to approximate to 5 M-circle dot, whereas it is slightly flatter, s = -2.0, in NGC 4449. The SFHs derived with the two different sets of stellar models are consistent with each other, except for some quantitative details, attributable to their input assumptions. They also share the drawback that all synthetic diagrams predict a clear separation in color between the upper main-sequence and helium-burning stars, which is not apparent in the data. Since neither differential reddening, which is significant in NGC 4449, nor unresolved binaries appear to be sufficient to fill the gap, we suggest this calls for a revision of both sets of stellar evolutionary tracks.
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| 7. |
- Cook, D. O., et al.
(author)
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Star cluster catalogues for the LEGUS dwarf galaxies
- 2019
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 484:4, s. 4897-4919
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Journal article (peer-reviewed)abstract
- We present the star cluster catalogues for 17 dwarf and irregular galaxies in the HST Treasury Program 'Legacy ExtraGalactic UV Survey' (LEGUS). Cluster identification and photometry in this sub-sample are similar to that of the entire LEGUS sample, but special methods were developed to provide robust catalogues with accurate fluxes due to low cluster statistics. The colours and ages are largely consistent for two widely used aperture corrections, but a significant fraction of the clusters are more compact than the average training cluster. However, the ensemble luminosity, mass, and age distributions are consistent suggesting that the systematics between the two methods are less than the random errors. When compared with the clusters from previous dwarf galaxy samples, we find that the LEGUS catalogues are more complete and provide more accurate total fluxes. Combining all clusters into a composite dwarf galaxy, we find that the luminosity and mass functions can be described by a power law with the canonical index of -2 independent of age and global SFR binning. The age distribution declines as a power law, with an index of approximate to -0.80 +/- 0.15, independent of cluster mass and global SFR binning. This decline of clusters is dominated by cluster disruption since the combined star formation histories and integrated-light SFRs are both approximately constant over the last few hundred Myr. Finally, we find little evidence for an upper-mass cut-off (< 2 sigma) in the composite cluster mass function, and can rule out a truncation mass below approximate to 10(4.5)M(circle dot) but cannot rule out the existence of a truncation at higher masses.
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| 8. |
- Dobbs, C. L., et al.
(author)
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The properties, origin and evolution of stellar clusters in galaxy simulations and observations
- 2017
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 464:3, s. 3580-3596
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Journal article (peer-reviewed)abstract
- We investigate the properties and evolution of star particles in two simulations of isolated spiral galaxies, and two galaxies from cosmological simulations. Unlike previous numerical work, where typically each star particle represents one 'cluster', for the isolated galaxies we are able to model features we term 'clusters' with groups of particles. We compute the spatial distribution of stars with different ages, and cluster mass distributions, comparing our findings with observations including the recent LEGUS survey. We find that spiral structure tends to be present in older (100s Myr) stars and clusters in the simulations compared to the observations. This likely reflects differences in the numbers of stars or clusters, the strength of spiral arms, and whether the clusters are allowed to evolve. Where we model clusters with multiple particles, we are able to study their evolution. The evolution of simulated clusters tends to follow that of their natal gas clouds. Massive, dense, long-lived clouds host massive clusters, whilst short-lived clouds host smaller clusters which readily disperse. Most clusters appear to disperse fairly quickly, in basic agreement with observational findings. We note that embedded clusters may be less inclined to disperse in simulations in a galactic environment with continuous accretion of gas on to the clouds than isolated clouds and correspondingly, massive young clusters which are no longer associated with gas tend not to occur in the simulations. Caveats of our models include that the cluster densities are lower than realistic clusters, and the simplistic implementation of stellar feedback.
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| 9. |
- Grasha, K., et al.
(author)
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Connecting young star clusters to CO molecular gas in NGC 7793 with ALMA-LEGUS
- 2018
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 481:1, s. 1016-1027
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Journal article (peer-reviewed)abstract
- We present an investigation of the relationship between giant molecular cloud (GMC) properties and the associated stellar clusters in the nearby flocculent galaxy NGC 7793. We combine the star cluster catalogue from the HST LEGUS (Legacy ExtraGalactic UV Survey) programme with the 15 pc resolution ALMA CO(2-1) observations. We find a strong spatial correlation between young star clusters and GMCs such that all clusters still associated with a GMC are younger than 11 Myr and display a median age of 2 Myr. The age distribution increases gradually as the cluster-GMC distance increases, with star clusters that are spatially unassociated with molecular gas exhibiting a median age of 7 Myr. Thus, star clusters are able to emerge from their natal clouds long before the time-scale required for clouds to disperse. To investigate if the hierarchy observed in the stellar components is inherited from the GMCs, we quantify the amount of clustering in the spatial distributions of the components and find that the star clusters have a fractal dimension slope of -0.35 +/- 0.03, significantly more clustered than the molecular cloud hierarchy with slope of -0.18 +/- 0.04 over the range 40-800 pc. We find, however, that the spatial clustering becomes comparable in strength for GMCs and star clusters with slopes of -0.44 +/- 0.03 and -0.45 +/- 0.06, respectively, when we compare massive (> 10(5) M-circle dot) GMCs to massive and young star clusters. This shows that massive star clusters trace the same hierarchy as their parent GMCs, under the assumption that the star formation efficiency is a few per cent.
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| 10. |
- Grasha, K., et al.
(author)
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The Hierarchical Distribution of the Young Stellar Clusters in Six Local Star-forming Galaxies
- 2017
-
In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 840:2
-
Journal article (peer-reviewed)abstract
- We present a study of the hierarchical clustering of the young stellar clusters in six local (3–15 Mpc) star-forminggalaxies using Hubble Space Telescope broadband WFC3/UVIS UV and optical images from the Treasury Program LEGUS (Legacy Extra Galactic UV Survey). We identified 3685 likely clusters and associations, each visually classified by their morphology, and we use the angular two-point correlation function to study the clustering of the sestellar systems. We find that the spatial distribution of the young clusters and associations are clustered with respect to each other, forming large, unbound hierarchical star-forming complexes that are in general very young. The strength of the clustering decreases with increasing age of the star clusters and stellar associations, becoming more homogeneously distributed after ∼ 40–60 Myr and on scales larger than a few hundred parsecs. In all galaxies, the associations exhibit a global behavior that is distinct and more strongly correlated from compact clusters. Thus, populations of clusters are more evolved than associations in terms of their spatial distribution, traveling significantly from their birth site within a few tens of Myr, whereas associations show evidence of disruption occurring very quickly after their formation. The clustering of the stellar systems resembles that of a turbulent interstellar medium that drives the star formation process, correlating the components in unbound star-forming complexes in a hierarchical manner, dispersing shortly after formation, suggestive of a single, continuous mode of star formation across all galaxies.
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| 11. |
- Grasha, K., et al.
(author)
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THE SPATIAL DISTRIBUTION OF THE YOUNG STELLAR CLUSTERS IN THE STAR-FORMING GALAXY NGC 628
- 2015
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 815:2
-
Journal article (peer-reviewed)abstract
- We present a study of the spatial distribution of the stellar cluster populations in the star-forming galaxy NGC 628. Using Hubble Space Telescope broadband WFC3/UVIS UV and optical images from the Treasury Program LEGUS (Legacy ExtraGalactic UV Survey), we have identified 1392 potential young (less than or similar to 100 Myr) stellar clusters within the galaxy using a combination of visual inspection and automatic selection. We investigate the clustering of these young stellar clusters and quantify the strength and change of clustering strength with scale using the two-point correlation function. We also investigate how image boundary conditions and dust lanes affect the observed clustering. The distribution of the clusters is well fit by a broken power law with negative exponent a. We recover a weighted mean index of alpha similar to -0.8 for all spatial scales below the break at 3.13 (158 pc at a distance of 9.9 Mpc) and an index of alpha similar to - 0.18 above 158 pc for the accumulation of all cluster types. The strength of the clustering increases with decreasing age and clusters older than 40 Myr lose their clustered structure very rapidly and tend to be randomly distributed in this galaxy, whereas the mass of the star cluster has little effect on the clustering strength. This is consistent with results from other studies that the morphological hierarchy in stellar clustering resembles the same hierarchy as the turbulent interstellar medium.
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| 12. |
- Hannon, Stephen, et al.
(author)
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H α morphologies of star clusters : a LEGUS study of H II region evolution time-scales and stochasticity in low-mass clusters
- 2019
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 490:4, s. 4648-4665
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Journal article (peer-reviewed)abstract
- The morphology of H II regions around young star clusters provides insight into the time-scales and physical processes that clear a cluster's natal gas. We study similar to 700 young clusters (<= 10 Myr) in three nearby spiral galaxies (NGC 7793, NGC 4395, and NGC 1313) using Hubble Space Telescope (HST) imaging from LEGUS (Legacy ExtraGalactic Ultraviolet Survey). Clusters are classified by their H alpha morphology (concentrated, partially exposed, no-emission) and whether they have neighbouring clusters (which could affect the clearing time-scales). Through visual inspection of the HST images, and analysis of ages, reddenings, and stellar masses from spectral energy distributions fitting, together with the (U - B), (V - I) colours, we find (1) the median ages indicate a progression from concentrated (similar to 3Myr), to partially exposed (similar to 4Myr), to no H alpha emission (>5Myr), consistent with the expected temporal evolution of H II regions and previous results. However, (2) similarities in the age distributions for clusters with concentrated and partially exposed H alpha morphologies imply a short time-scale for gas clearing (less than or similar to 1 Myr). Also, (3) our cluster sample's median mass is similar to 1000 M-circle dot, and a significant fraction (similar to 20 per cent) contain one or more bright red sources (presumably supergiants), which can mimic reddening effects. Finally, (4) the median E(B - V) values for clusters with concentrated H alpha and those without H alpha emission appear to be more similar than expected (similar to 0.18 versus similar to 0.14, respectively), but when accounting for stochastic effects, clusters without H alpha emission are less reddened. To mitigate stochastic effects, we experiment with synthesizing more massive clusters by stacking fluxes of clusters within each H alpha morphological class. Composite isolated clusters also reveal a colour and age progression for H alpha morphological classes, consistent with analysis of the individual clusters.
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| 13. |
- Kahre, L., et al.
(author)
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Extinction Maps and Dust-to-gas Ratios in Nearby Galaxies with LEGUS
- 2018
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 855:2
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Journal article (peer-reviewed)abstract
- We present a study of the dust-to-gas ratios in five nearby galaxies: NGC 628 (M74), NGC 6503, NGC 7793, UGC 5139 (Holmberg I), and UGC 4305 (Holmberg II). Using Hubble Space Telescope broadband WFC3/UVIS UV and optical images from the Treasury program Legacy ExtraGalactic UV Survey (LEGUS) combined with archival HST/Advanced Camera for Surveys data, we correct thousands of individual stars for extinction across these five galaxies using an isochrone-matching (reddening-free Q) method. We generate extinction maps for each galaxy from the individual stellar extinctions using both adaptive and fixed resolution techniques and correlate these maps with neutral H I and CO gas maps from the literature, including the H I Nearby Galaxy Survey and the HERA CO-Line Extragalactic Survey. We calculate dust-to-gas ratios and investigate variations in the dust-to-gas ratio with galaxy metallicity. We find a power-law relationship between dust-to-gas ratio and metallicity, consistent with other studies of dust-to-gas ratio compared to metallicity. We find a change in the relation when H-2 is not included. This implies that underestimation of N-H2 in low-metallicity dwarfs from a too-low CO-to-H-2 conversion factor X-CO could have produced too low a slope in the derived relationship between dust-to-gas ratio and metallicity. We also compare our extinctions to those derived from fitting the spectral energy distribution (SED) using the Bayesian Extinction and Stellar Tool for NGC 7793 and find systematically lower extinctions from SED fitting as compared to isochrone matching.
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| 14. |
- Messa, Matteo, et al.
(author)
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The young star cluster population of M51 with LEGUS - II. Testing environmental dependences
- 2018
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 477:2, s. 1670-1694
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Journal article (peer-reviewed)abstract
- It has recently been established that the properties of young star clusters (YSCs) can vary as a function of the galactic environment in which they are found. We use the cluster catalogue produced by the Legacy ExtragalacticUVSurvey (LEGUS) collaboration to investigate cluster properties in the spiral galaxy M51. We analyse the cluster population as a function of galactocentric distance and in arm and inter-arm regions. The cluster mass function exhibits a similar shape at all radial bins, described by a power law with a slope close to -2 and an exponential truncation around 10(5) M-circle dot. While the mass functions of the YSCs in the spiral arm and inter-arm regions have similar truncation masses, the inter-arm region mass function has a significantly steeper slope than the one in the arm region, a trend that is also observed in the giant molecular cloud mass function and predicted by simulations. The age distribution of clusters is dependent on the region considered, and is consistent with rapid disruption only in dense regions, while little disruption is observed at large galactocentric distances and in the inter-arm region. The fraction of stars forming in clusters does not show radial variations, despite the drop in the H-2 surface density measured as a function of galactocentric distance. We suggest that the higher disruption rate observed in the inner part of the galaxy is likely at the origin of the observed flat cluster formation efficiency radial profile.
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| 15. |
- Rousseau-Nepton, L., et al.
(author)
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SIGNALS : I. Survey description
- 2019
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 489:4, s. 5530-5546
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Journal article (peer-reviewed)abstract
- SIGNALS, the Star formation, Ionized Gas, and Nebular Abundances Legacy Survey, is a large observing programme designed to investigate massive star formation and HII regions in a sample of local extended galaxies. The programme will use the imaging Fourier transform spectrograph SITELLE at the Canada-France-Hawaii Telescope. Over 355 h (54.7 nights) have been allocated beginning in fall 2018 for eight consecutive semesters. Once completed, SIGNALS will provide a statistically reliable laboratory to investigate massive star formation, including over 50 000 resolved HII regions: the largest, most complete, and homogeneous data base of spectroscopically and spatially resolved extragalactic HII regions ever assembled. For each field observed, three datacubes covering the spectral bands of the filters SN1 (363386 nm), SN2 (482-513 nm), and SN3 (647-685 nm) are gathered. The spectral resolution selected for each spectral band is 1000, 1000, and 5000, respectively. As defined, the project sample will facilitate the study of small-scale nebular physics and many other phenomena linked to star formation at a mean spatial resolution of similar to 20 pc. This survey also has considerable legacy value for additional topics, including planetary nebulae, diffuse ionized gas, and supernova remnants. The purpose of this paper is to present a general outlook of the survey, notably the observing strategy, galaxy sample, and science requirements.
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| 16. |
- Sabbi, E., et al.
(author)
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The Resolved Stellar Populations in the LEGUS Galaxies
- 2018
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In: Astrophysical Journal Supplement Series. - : American Astronomical Society. - 0067-0049 .- 1538-4365. ; 235:1
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Journal article (peer-reviewed)abstract
- The Legacy ExtraGalactic UV Survey (LEGUS) is a multiwavelength Cycle 21 Treasury program on the Hubble Space Telescope. It studied 50 nearby star-forming galaxies in 5 bands from the near-UV to the I-band, combining new Wide Field Camera 3 observations with archival Advanced Camera for Surveys data. LEGUS was designed to investigate how star formation occurs and develops on both small and large scales, and how it relates to the galactic environments. In this paper we present the photometric catalogs for all the apparently single stars identified in the 50 LEGUS galaxies. Photometric catalogs and mosaicked images for all filters are available for download. We present optical and near-UV color-magnitude diagrams for all the galaxies. For each galaxy we derived the distance from the tip of the red giant branch. We then used the NUV color-magnitude diagrams to identify stars more massive than 14 Me, and compared their number with the number of massive stars expected from the GALEX FUV luminosity. Our analysis shows that the fraction of massive stars forming in star clusters and stellar associations is about constant with the star formation rate. This lack of a relation suggests that the timescale for evaporation of unbound structures is comparable or longer than 10 Myr. At low star formation rates this translates to an excess of mass in clustered environments as compared to model predictions of cluster evolution, suggesting that a significant fraction of stars form in unbound systems.
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| 17. |
- Turner, J. A., et al.
(author)
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An ALMA/HST Study of Millimeter Dust Emission and Star Clusters
- 2019
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 884:2
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Journal article (peer-reviewed)abstract
- We present results from a joint ALMA/HST study of the nearby spiral galaxy NGC.628. We combine the Hubble Space Telescope (HST) Legacy ExtraGalactic UV Survey (LEGUS) database of over 1000 stellar clusters in NGC.628 with ALMA Cycle 4 mm/submillimeter observations of the cold dust continuum that span similar to 15.kpc(2) including the nuclear region and western portions of the galaxy's disk. The resolution-1 ''.1 or approximately 50 pc at the distance of NGC.628-allows us to constrain the spatial variations in the slope of the millimeter dust continuum as a function of the ages and masses of the nearby stellar clusters. Our results indicate an excess of dust emission in the millimeter, assuming a typical cold dust model for a normal star-forming galaxy, but little correlation of the dust continuum slope with stellar cluster age or mass. For the depth and spatial coverage of these observations, we cannot substantiate the millimeter/submillimeter excess arising from the processing of dust grains by the local interstellar radiation field. We detect a bright unknown source in NGC.628 in ALMA bands 4 and 7 with no counterparts at other wavelengths from ancillary data. We speculate this is possibly a dust-obscured supernova.
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| 18. |
- Calzetti, D., et al.
(author)
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Spatially Resolved Dust, Gas, and Star Formation in the Dwarf Magellanic Irregular NGC 4449
- 2018
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 852:2
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Journal article (peer-reviewed)abstract
- We investigate the relation between gas and star formation in subgalactic regions, similar to 360. pc to similar to 1.5. kpc in size, within the nearby starburst dwarf NGC 4449, in order to separate the underlying relation from the effects of sampling at varying spatial scales. Dust and gas mass surface densities are derived by combining new observations at 1.1. mm, obtained with the AzTEC instrument on the Large Millimeter Telescope, with archival infrared images in the range 8-500 mu m from the Spitzer Space Telescope and the Herschel Space Observatory. We extend the dynamic range of our millimeter (and dust) maps at the faint end, using a correlation between the far-infrared/millimeter colors F(70)/F(1100) (and F(160)/F(1100)) and the mid-infrared color F(8)/F(24) that we establish for the first time for this and other galaxies. Supplementing our data with maps of the extinction-corrected star formation rate (SFR) surface density, we measure both the SFR-molecular gas and the SFR-total. gas relations in NGC 4449. We find that the SFR-molecular. gas relation is described by a power law with an exponent that decreases from similar to 1.5 to similar to 1.2 for increasing region size, while the exponent of the SFR-total. gas relation remains constant with a value of similar to 1.5 independent of region size. We attribute the molecular law behavior to the increasingly better sampling of the molecular cloud mass function at larger region sizes; conversely, the total gas law behavior likely results from the balance between the atomic and molecular gas phases achieved in regions of active star formation. Our results indicate a nonlinear relation between SFR and gas surface density in NGC 4449, similar to what is observed for galaxy samples.
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| 19. |
- Cook, D. O., et al.
(author)
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Fraction of stars in clusters for the LEGUS dwarf galaxies
- 2023
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In: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 519:3, s. 3749-3775
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Journal article (peer-reviewed)abstract
- We study the young star cluster populations in 23 dwarf and irregular galaxies observed by the Hubble Space Telescope (HST) Legacy ExtraGalactic Ultraviolet Survey (LEGUS), and examine relationships between the ensemble properties of the cluster populations and those of their host galaxies: star formation rate (SFR) density (ΣSFR). A strength of this analysis is the availability of SFRs measured from temporally resolved star formation histories that provide the means to match cluster and host galaxy properties on several time-scales (1–10, 1–100, and 10–100 Myr). Nevertheless, studies of this kind are challenging for dwarf galaxies due to the small numbers of clusters in each system. We mitigate these issues by combining the clusters across different galaxies with similar ΣSFR properties. We find good agreement with a well-established relationship (MVbrightest–SFR), but find no significant correlations between ΣSFR and the slopes of the cluster luminosity function, mass function, nor the age distribution. We also find no significant trend between the fraction of stars in bound clusters at different age ranges (Γ1–10, Γ10–100, and Γ1–100) and ΣSFR of the host galaxy. Our data show a decrease in Γ over time (from 1–10 to 10–100 Myr) suggesting early cluster dissolution, though the presence of unbound clusters in the youngest time bin makes it difficult to quantify the degree of dissolution. While our data do not exhibit strong correlations between ΣSFR and ensemble cluster properties, we cannot rule out that a weak trend might exist given the relatively large uncertainties due to low number statistics and the limited ΣSFR range probed.
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| 20. |
- Finn, Molly K., et al.
(author)
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ALMA-LEGUS. I. The Influence of Galaxy Morphology on Molecular Cloud Properties
- 2024
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 964:1
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Journal article (peer-reviewed)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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| 21. |
- Finn, Molly K., et al.
(author)
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ALMA-LEGUS. II. The Influence of Subgalactic Environments on Molecular Cloud Properties
- 2024
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In: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 964:1
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Journal article (peer-reviewed)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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| 22. |
- Grasha, K., et al.
(author)
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Hierarchical Star Formation in Turbulent Media : Evidence from Young Star Clusters
- 2017
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 842:1
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Journal article (peer-reviewed)abstract
- We present an analysis of the positions and ages of young star clusters in eight local galaxies to investigate the connection between the age difference and separation of cluster pairs. We find that star clusters do not form uniformly but instead are distributed so that the age difference increases with the cluster pair separation to the 0.25-0.6 power, and that the maximum size over which star formation is physically correlated ranges from similar to 200. pc to similar to 1 kpc. The observed trends between age difference and separation suggest that cluster formation is hierarchical both in space and time: clusters that are close to each other are more similar in age than clusters born further apart. The temporal correlations between stellar aggregates have slopes that are consistent with predictions of turbulence acting as the primary driver of star formation. The velocity associated with the maximum size is proportional to the galaxy's shear, suggesting that the galactic environment influences the maximum size of the star-forming structures.
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| 23. |
- Lin, Zesen, et al.
(author)
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The Age Dependence of Mid-infrared Emission around Young Star Clusters
- 2020
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 896:1
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Journal article (peer-reviewed)abstract
- Using the star cluster catalogs from the Hubble Space Telescope program Legacy Extragalactic UV survey (LEGUS) and 8 mu m images from the IRAC camera on the Spitzer Space Telescope for five galaxies within 5 Mpc, we investigate how the 8 mu m dust luminosity correlates with the stellar age on the 30-50 pc scale of star-forming regions. We construct a sample of 97 regions centered at local peaks of 8 mu m emission, each containing one or more young star cluster candidates from the LEGUS catalogs. We find a tight anticorrelation with a Pearson correlation coefficient ofr = -0.84 0.05 between the mass-normalized dust-only 8 mu m luminosity and the age of stellar clusters younger than 1 Gyr; the 8 mu m luminosity decreases with increasing age of the stellar population. Simple assumptions on a combination of stellar and dust emission models reproduce the observed trend. We also explore how the scatter of the observed trend depends on assumptions of stellar metallicity, polycyclic aromatic hydrocarbon (PAH) abundance, fraction of stellar light absorbed by dust, and instantaneous versus continuous star formation models. We find that variations in stellar metallicity have little effect on the scatter, while PAH abundance and the fraction of dust-absorbed light bracket the full range of the data. We also find that the trend is better explained by continuous star formation, rather than instantaneous burst models. We ascribe this result to the presence of multiple star clusters with different ages in many of the regions. Upper limits of the dust-only 8 mu m emission as a function of age are provided.
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| 24. |
- Linden, S. T., et al.
(author)
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Star Cluster Formation and Evolution in M101 : An Investigation with the Legacy Extragalactic UV Survey
- 2022
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 935:2
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Journal article (peer-reviewed)abstract
- We present Hubble Space Telescope WFC3/UVIS (F275W, F336W) and ACS/WFC optical (F435W, F555W, and F814W) observations of the nearby grand-design spiral galaxy M101 as part of the Legacy Extragalactic UV Survey (LEGUS). Compact sources detected in at least four bands were classified by both human experts and the convolutional neural network StarcNet. Human experts classified the 2351 brightest sources, retrieving N-c = 965 star clusters. StarcNet, trained on LEGUS data not including M101, classified all 4725 sources detected in four bands, retrieving N-c = 2270 star clusters. The combined catalog represents the most complete census to date of compact star clusters in M101. We find that for the 2351 sources with both a visual- and machine-learning classification StarcNet is able to reproduce the human classifications at high levels of accuracy (similar to 80%-90%), which is equivalent to the level of agreement between human classifiers in LEGUS. The derived cluster age distribution implies a disruption rate of dN/d tau proportional to tau(-0.45 +/- 0.14) over 10(7) < tau < 10(8.5) yr for cluster masses >= 10(3.55) M-circle dot for the central region of M101 and dN/d tau proportional to tau(-0.02 +/- 0.15) for cluster masses >= 10(3.38) M-circle dot in the northwest region of the galaxy. The trends we recover are weaker than those of other nearby spirals (e.g., M51) and starbursts, consistent with the M101 environment having a lower-density interstellar medium, and providing evidence in favor of environmentally dependent cluster disruption in the central, southeast, and northwest regions of M101.
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| 25. |
- Ryon, J. E., et al.
(author)
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Effective Radii of Young, Massive Star Clusters in Two LEGUS Galaxies
- 2017
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In: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 841:2
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Journal article (peer-reviewed)abstract
- We present a study of the effective (half-light) radii and other structural properties of a systematically selected sample of young, massive star clusters (>= 5. x. 10(3) M-circle dot and <= 200 Myr) in two nearby spiral galaxies, NGC. 628 and NGC. 1313. We use Hubble Space Telescope (HST) WFC3/UVIS and archival ACS/WFC data obtained by the Legacy Extragalactic UV Survey (LEGUS), an HST Treasury Program. We measure effective radii with GALFIT, a two-dimensional image-fitting package, and with a new technique to estimate effective radii from the concentration index of observed clusters. The distribution of effective radii from both techniques spans similar to 0.5-10. pc and peaks at 2-3. pc for both galaxies. We find slight positive correlations between effective radius and cluster age in both galaxies, but no significant relationship between effective radius and galactocentric distance. Clusters in NGC. 1313 display a mild increase in effective radius with cluster mass, but the trend disappears when the sample is divided into age bins. We show that the vast majority of the clusters in both galaxies are much older than their dynamical times, suggesting they are gravitationally bound objects. We find that about half of the clusters in NGC. 628 are underfilling their Roche lobes, based on their Jacobi radii. Our results suggest that the young, massive clusters in NGC. 628 and NGC. 1313 are expanding, due to stellar mass loss or two-body relaxation, and are not significantly influenced by the tidal fields of their host galaxies.
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