| 1. |
- Melinder, Jens, 1977-, et al.
(författare)
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The Lyα Reference Sample. XIV. Lyα Imaging of 45 Low-redshift Star-forming Galaxies and Inferences on Global Emission
- 2023
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Ingår i: Astrophysical Journal Supplement Series. - 0067-0049 .- 1538-4365. ; 266:1
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Tidskriftsartikel (refereegranskat)abstract
- We present Lyα imaging of 45 low-redshift star-forming galaxies observed with the Hubble Space Telescope. The galaxies have been selected to have moderate to high star formation rates (SFRs) using far-ultraviolet (FUV) luminosity and Hα equivalent width criteria, but no constraints on Lyα luminosity. We employ a pixel stellar continuum fitting code to obtain accurate continuum-subtracted Lyα, Hα, and Hβ maps. We find that Lyα is less concentrated than FUV and optical line emission in almost all galaxies with significant Lyα emission. We present global measurements of Lyα and other quantities measured in apertures designed to capture all of the Lyα emission. We then show how the escape fraction of Lyα relates to a number of other measured quantities (mass, metallicity, star formation, ionization parameter, and extinction). We find that the escape fraction is strongly anticorrelated with nebular and stellar extinction, weakly anticorrelated with stellar mass, but no conclusive evidence for correlations with other quantities. We show that Lyα escape fractions are inconsistent with common dust extinction laws, and discuss how a combination of radiative transfer effects and clumpy dust models can help resolve the discrepancies. We present an SFR calibration based on Lyα luminosity, where the equivalent width of Lyα is used to correct for nonunity escape fraction, and show that this relation provides a reasonably accurate SFR estimate. We also show stacked growth curves of Lyα for the galaxies that can be used to find aperture loss fractions at a given physical radius.
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| 2. |
- Atek, Hakim, et al.
(författare)
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Influence of physical galaxy properties on Ly alpha escape in star-forming galaxies
- 2014
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 561, s. A89-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Among the different observational techniques used to select high-redshift galaxies, the hydrogen recombination line Lyman-alpha (Ly alpha) is of particular interest because it gives access to the measurement of cosmological quantities such as the star formation rate (SFR) of distant galaxy populations. However, interpreting this line and calibrating such observables are still subject to serious uncertainties. Aims. In this context, it important to understand the mechanisms responsible for the attenuation of Ly alpha emission, and under what conditions the Ly alpha emission line can be used as a reliable star formation diagnostic tool. Methods. We used a sample of 24 Ly alpha emitters at z similar to 0.3 with an optical spectroscopic follow-up to calculate the Ly alpha escape fraction and its dependence upon different physical properties. We also examined the reliability of Ly alpha as a SFR indicator. We combined these observations with a compilation of Ly alpha emitters selected at z = 0-0.3 from the literature to assemble a larger sample. Results. We confirm that the Ly alpha escape fraction clearly depends on the dust extinction following the relation f(esc)(Ly alpha) = C-Ly alpha x 10(-0.4 E(B-V) kLy alpha) where k(Ly alpha) similar to 6.67 and C-Ly alpha = 0.22. However, the correlation does not follow the expected curve for a simple dust attenuation. A higher attenuation can be attributed to a scattering process, while f(esc)(Ly alpha) values that are clearly above the continuum extinction curve can be the result of various mechanisms that can lead to an enhancement of the Ly alpha output. We also observe that the strength of Ly alpha and the escape fraction appear unrelated to the galaxy metallicity. Regarding the reliability of Ly alpha as a SFR indicator, we show that the deviation of SFR(Ly alpha) from the true SFR (as traced by the UV continuum) is a function of the observed SFR(UV), which can be seen as the decrease in f(esc)(Ly alpha) with increasing UV luminosity. Moreover, we observe redshift dependence of this relationship, revealing the underlying evolution of f(esc)(Ly alpha) with redshift.
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| 3. |
- Bridge, Joanna S., et al.
(författare)
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The Ly alpha Reference Sample. VIII. Characterizing Ly alpha Scattering in Nearby Galaxies
- 2018
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 852:1
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Tidskriftsartikel (refereegranskat)abstract
- We examine the dust geometry and Ly alpha scattering in the galaxies of the Lyman Alpha Reference Sample (LARS), a set of 14 nearby (0.02 < z < 0.2) Ly alpha-emitting and starbursting systems with Hubble Space Telescope Ly alpha, H alpha, and H beta imaging. We find that the global dust properties determined by line ratios are consistent with other studies, with some of the LARS galaxies exhibiting clumpy dust media, while others of them show significantly lower Ly alpha emission compared to their Balmer decrement. With the LARS imaging, we present Ly alpha/H alpha and H alpha/H beta maps with spatial resolutions as low as similar to 40 pc, and use these data to show that in most galaxies, the dust geometry is best modeled by three distinct regions: a central core where dust acts as a screen, an annulus where dust is distributed in clumps, and an outer envelope where Ly alpha photons only scatter. We show that the dust that affects the escape of Ly alpha is more restricted to the galaxies' central regions, while the larger Ly alpha halos are generated by scattering at large radii. We present an empirical modeling technique to quantify how much Ly alpha scatters in the halo, and find that this characteristic scattering distance correlates with the measured size of the Ly alpha halo. We note that there exists a slight anti-correlation between the scattering distance of Ly alpha and global dust properties.
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| 4. |
- Duval, Florent, et al.
(författare)
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The Lyman alpha reference sample VI. Lyman alpha escape from the edge-on disk galaxy Mrk 1486
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 587
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Tidskriftsartikel (refereegranskat)abstract
- Context. Recent numerical simulations suggest that the strength of the Lyman alpha (Ly alpha) line of star-forming disk galaxies strongly depends on the inclination at which they are observed: from edge-on to face-on, we expect to see a change from a strongly attenuated Ly alpha line to a strong Ly alpha emission line.Aims. We aim to understand how a strong Ly alpha emission line is able to escape from the low-redshift highly inclined (edge-on) disk galaxy Mrk 1486 (z similar to 0.0338). To our knowledge, this work is the first observational study of Ly alpha transport inside an edge-on disk galaxy.Methods. Using a large set of HST imaging and spectroscopic data, we investigated the interstellar medium (ISM) structure and the dominant source of Ly alpha radiation inside Mrk 1486. Moreover, using a 3D Monte Carlo Ly alpha radiation transfer code, we studied the radiative transfer of Ly alpha and UV continuum photons inside a 3D geometry of neutral hydrogen (HI) and dust that models the ISM structure at the galaxy center. Our numerical simulations predicted the Ly alpha line profile that we then compared to the one observed in the HST/COS spectrum of Mrk 1486.Results. While a pronounced Ly alpha absorption line emerges from the disk of Mrk 1486, very extended Ly alpha structures are observed at large radii from the galaxy center: a large Ly alpha-halo and two very bright Ly alpha regions located slightly above and below the disk plane. The analysis of IFU H alpha spectroscopic data of Mrk 1486 indicates the presence of two bipolar outflowing halos of HI gas at the same location as these two bright Ly alpha regions. Comparing different diagnostic diagrams (such as [OIII](5007)/H beta versus [OI](6300)/H alpha) to photo-and shock-ionization models, we find that the Ly alpha production of Mrk 1486 is dominated by photoionization inside the galaxy disk. From this perspective, our numerical simulations succeed in reproducing the strength and shape of the observed Ly alpha emission line of Mrk 1486 by assuming a scenario in which the Ly alpha photons are produced inside the galaxy disk, travel along the outflowing halos, and scatter on cool HI materials toward the observer.Conclusions. Extended bipolar galactic winds are frequently observed from star-forming disk galaxies. Given the advantage Ly alpha photons take of such outflowing HI materials to easily escape from Mrk 1486, this mechanism may explain the origin of strong Ly alpha emission lines frequently observed from highly inclined galaxies at high-redshift. This therefore challenges the robustness of the expected viewing-angle effect on the Ly alpha properties of star-forming disk galaxies.
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| 5. |
- Hayes, Matthew, et al.
(författare)
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Continuum subtracting Lyman-alpha images : Low redshift studies using the Solar Blind Channel of HST/ACS
- 2009
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 0004-6256 .- 1538-3881. ; 138:3, s. 911-922
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Tidskriftsartikel (refereegranskat)abstract
- We are undertaking an imaging study of local star-forming galaxies in the Lyman-alpha (Lyα) emission line using the Solar Blind Channel (SBC) of the Advanced Camera for Surveys onboard the Hubble Space Telescope. Observations have been obtained in Lyα and H-alpha (Hα) and six line-free continuum filters between ~1500 Å and the I band. In a previous article, we demonstrated that the production of Lyα line-only images (i.e., continuum subtraction) in the SBC-only data set is nontrivial and that supporting data is a requirement. We here develop various methods of continuum subtraction and assess their relative performance using a variety of spectral energy distributions (SEDs) as input. We conclude that simple assumptions about the behavior of the ultraviolet continuum consistently lead to results that are wildly erroneous, and determine that a spectral fitting approach is essential. Moreover, fitting of a single component stellar or stellar+nebular spectrum is not always sufficient for realistic template SEDs and, in order to successfully recover the input observables, care must be taken to control the contribution of nebular gas and any underlying stellar population. Independent measurements of the metallicity must first be obtained, while details of the initial mass function play only a small role. We identify the need to bin together pixels in our data to obtain signal-to-noise ratios (S/Ns) of around 10 in each band before processing. At S/N = 10, we are able to recover Lyα fluxes accurate to within around 30% for Lyα lines with intrinsic equivalent width (W Lyα) of 10 Å. This accuracy improves to 10% for W Lyα = 100 Å. We describe the method of image processing applied to the observations presented in Östlin et al. and the associated data release. We also present simulations for an observing strategy for an alternative low-redshift Lyα imaging campaign using ACS/SBC using adjacent combinations of long-pass filters to target slightly higher redshift.
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| 6. |
- Hayes, Matthew, et al.
(författare)
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Escape of about five per cent of Lyman-alpha photons from high-redshift star-forming galaxies
- 2010
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Ingår i: Nature. - : Springer Science and Business Media LLC. - 0028-0836 .- 1476-4687. ; 464:7288, s. 562-565
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Tidskriftsartikel (refereegranskat)abstract
- The Lyman-alpha (Ly alpha) emission line is the primary observational signature of star-forming galaxies at the highest redshifts(1), and has enabled the compilation of large samples of galaxies with which to study cosmic evolution(2-5). The resonant nature of the line, however, means that Ly alpha photons scatter in the neutral interstellar medium of their host galaxies, and their sensitivity to absorption by interstellar dust may therefore be greatly enhanced. This implies that the Ly alpha luminosity may be significantly reduced, or even completely suppressed. Hitherto, no unbiased empirical test of the escaping fraction (f(esc)) of Ly alpha photons has been performed at high redshifts. Here we report that the average f(esc) from star-forming galaxies at redshift z=2.2 is just 5 per cent by performing a blind narrowband survey in Ly alpha and H alpha. This implies that numerous conclusions based on Ly alpha-selected samples will require upwards revision by an order of magnitude and we provide a benchmark for this revision. We demonstrate that almost 90 per cent of star-forming galaxies emit insufficient Ly alpha to be detected by standard selection criteria(2-5). Both samples show an anti-correlation of f(esc) with dust content, and we show that Ly alpha- and H alpha-selection recovers populations that differ substantially in dust content and f(esc).
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| 7. |
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| 8. |
- Hayes, Matthew, et al.
(författare)
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ON THE REDSHIFT EVOLUTION OF THE Ly alpha ESCAPE FRACTION AND THE DUST CONTENT OF GALAXIES
- 2011
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 730:1, s. 8-
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Tidskriftsartikel (refereegranskat)abstract
- The Ly alpha emission line has been proven to be a powerful tool for studying evolving galaxies at the highest redshift. However, in order to use Ly alpha as a physical probe of galaxies, it becomes vital to know the Ly alpha escape fraction (f(esc)(Ly alpha)). Unfortunately, due to the resonant nature of Ly alpha, f(esc)(Ly alpha) may vary unpredictably and requires empirical measurement. Here, we compile Ly alpha luminosity functions (LFs) between redshifts z = 0 and 8 and, combined with Ha and ultraviolet data, assess how f(esc)(Ly alpha) evolves with redshift. We find a strong upward evolution in f(esc)(Ly alpha) over the range z = 0.3-6, which is well fit by the power law f(esc)(Ly alpha) proportional to (1 + z)(xi) with xi = (2.57(-0.12)(+0.19)). This predicts that f(esc)(Ly alpha) should reach unity at z = 11.1. By comparing f(esc)(Ly alpha) and E(B-V) in individual galaxies we derive an empirical relationship between f(esc)(Ly alpha) and E(B-V), which includes resonance scattering and can explain the redshift evolution of f(esc)(Ly alpha) between z = 0 and 6 purely as a function of the evolution in the dust content of galaxies. Beyond z approximate to 6.5, f(esc)(Ly alpha) drops more substantially, an effect attributed to either ionizing photon leakage, or an increase in the neutral gas fraction of the intergalactic medium. While distinguishing between these two scenarios may be extremely challenging, by framing the problem this way we remove the uncertainty of the halo mass from Ly alpha-based tests of reionization. We finally derive a new method by which to estimate the dust content of galaxies, based purely upon the observed Ly alpha and UV LFs. These data are characterized by an exponential with an e-folding scale of z(EBV) approximate to 3.4.
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| 9. |
- Hayes, Matthew, et al.
(författare)
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The escape of Lyman photons from a young starburst : The case of Haro 11
- 2007
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Wiley-Blackwell. - 0035-8711 .- 1365-2966. ; 382:4, s. 1465-1480
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Tidskriftsartikel (refereegranskat)abstract
- Lyman α (Lyα) is one of the dominant tools used to probe the star-forming galaxy population at high redshift (z). However, astrophysical interpretations of data drawn from Lyα alone hinge on the Lyα escape fraction which, due to the complex radiative transport, may vary greatly. Here, we map the Lyα emission from the local luminous blue compact galaxy Haro 11, a known emitter of Lyα and the only known candidate for low-z Lyman continuum emission. To aid in the interpretation, we perform a detailed ultraviolet and optical multiwavelength analysis and model the stellar population, dust distribution, ionizing photon budget, and star-cluster population. We use archival X-ray observations to further constrain properties of the starburst and estimate the neutral hydrogen column density. The Lyα morphology is found to be largely symmetric around a single young star-forming knot and is strongly decoupled from other wavelengths. From general surface photometry, only very slight correlation is found between Lyα and Hα, E(B−V), and the age of the stellar population. Only around the central Lyα bright cluster do we find the Lyα/Hα ratio at values predicted by the recombination theory. The total Lyα escape fraction is found to be just 3 per cent. We compute that ∼90 per cent of the Lyα photons that escape do so after undergoing multiple resonance scattering events, masking their point of origin. This leads to a largely symmetric distribution and, by increasing the distance that photons must travel to escape, decreases the escape probability significantly. While dust must ultimately be responsible for the destruction of Lyα, it plays a little role in governing the observed morphology, which is regulated more by interstellar medium kinematics and geometry. We find tentative evidence for local Lyα equivalent width in the immediate vicinity of star clusters being a function of cluster age, consistent with hydrodynamic studies. We estimate the intrinsic production of ionizing photons and put further constraints of ∼9 per cent on the escaping fraction of photons at 900 Å.
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| 10. |
- Hayes, Matthew, et al.
(författare)
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THE LYMAN ALPHA REFERENCE SAMPLE : EXTENDED LYMAN ALPHA HALOS PRODUCED AT LOW DUST CONTENT
- 2013
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Ingår i: Astrophysical Journal Letters. - 2041-8205 .- 2041-8213. ; 765:2, s. L27-
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Tidskriftsartikel (refereegranskat)abstract
- We report on new imaging observations of the Lyman alpha emission line (Ly alpha), performed with the Hubble Space Telescope, that comprise the backbone of the Lyman alpha Reference Sample. We present images of 14 starburst galaxies at redshifts 0.028 < z < 0.18 in continuum-subtracted Ly alpha, H alpha, and the far ultraviolet continuum. We show that Ly alpha is emitted on scales that systematically exceed those of the massive stellar population and recombination nebulae: as measured by the Petrosian 20% radius, RP20, Ly alpha radii are larger than those of H alpha by factors ranging from 1 to 3.6, with an average of 2.4. The average ratio of Ly alpha-to-FUV radii is 2.9. This suggests that much of the Ly alpha light is pushed to large radii by resonance scattering. Defining the Relative Petrosian Extension of Ly alpha compared to H alpha, xi(Ly alpha) = R-P20(Ly alpha)/R-P20(H alpha), we find xi(Ly alpha) to be uncorrelated with total Ly alpha luminosity. However, xi(Ly alpha) is strongly correlated with quantities that scale with dust content, in the sense that a low dust abundance is a necessary requirement (although not the only one) in order to spread Ly alpha photons throughout the interstellar medium and drive a large extended Ly alpha halo.
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| 11. |
- Hayes, Matthew, et al.
(författare)
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THE LYMAN ALPHA REFERENCE SAMPLE. II. HUBBLE SPACE TELESCOPE IMAGING RESULTS, INTEGRATED PROPERTIES, AND TRENDS
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 782:1
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Tidskriftsartikel (refereegranskat)abstract
- We report new results regarding the Ly alpha output of galaxies, derived from the Lyman Alpha Reference Sample, and focused on Hubble Space Telescope imaging. For 14 galaxies we present intensity images in Ly alpha, H alpha, and UV, and maps of H alpha/H beta, Ly alpha equivalent width (EW), and Ly alpha/H alpha. We present Ly alpha and UV radial light profiles and show they are well-fitted by Sersic profiles, but Ly alpha profiles show indices systematically lower than those of the UV (n approximate to 1-2 instead of greater than or similar to 4). This reveals a general lack of the central concentration in Ly alpha that is ubiquitous in the UV. Photometric growth curves increase more slowly for Ly alpha than the far ultraviolet, showing that small apertures may underestimate the EW. For most galaxies, however, flux and EW curves flatten by radii approximate to 10 kpc, suggesting that if placed at high-z only a few of our galaxies would suffer from large flux losses. We compute global properties of the sample in large apertures, and show total Ly alpha luminosities to be independent of all other quantities. Normalized Ly alpha throughput, however, shows significant correlations: escape is found to be higher in galaxies of lower star formation rate, dust content, mass, and nebular quantities that suggest harder ionizing continuum and lower metallicity. Six galaxies would be selected as high-z Ly alpha emitters, based upon their luminosity and EW. We discuss the results in the context of high-z Ly alpha and UV samples. A few galaxies have EWs above 50 angstrom, and one shows f(esc)(Ly alpha) of 80%; such objects have not previously been reported at low-z.
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| 12. |
- Herenz, Edmund Christian, et al.
(författare)
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The Lyman alpha reference sample VII. Spatially resolved H alpha kinematics
- 2016
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 587
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Tidskriftsartikel (refereegranskat)abstract
- We present integral field spectroscopic observations with the Potsdam Multi-Aperture Spectrophotometer of all 14 galaxies in the z similar to 0.1 Lyman Alpha Reference Sample (LARS). We produce 2D line-of-sight velocity maps and velocity dispersion maps from the Balmer alpha (H alpha) emission in our data cubes. These maps trace the spectral and spatial properties of the LARS galaxies' intrinsic Ly alpha radiation field. We show our kinematic maps that are spatially registered onto the Hubble Space Telescope H alpha and Lyman alpha (Ly alpha) images. We can conjecture a causal connection between spatially resolved H alpha kinematics and Ly alpha photometry for individual galaxies, however, no general trend can be established for the whole sample. Furthermore, we compute the intrinsic velocity dispersion sigma(0), the shearing velocity v(shear), and the v(shear)/sigma(0) ratio from our kinematic maps. In general LARS galaxies are characterised by high intrinsic velocity dispersions (54 km s(-1) median) and low shearing velocities (65 km s(-1) median). The v(shear/sigma 0) values range from 0.5 to 3.2 with an average of 1.5. It is noteworthy that five galaxies of the sample are dispersion-dominated systems with v(shear)/sigma(0) < 1, and are thus kinematically similar to turbulent star-forming galaxies seen at high redshift. When linking our kinematical statistics to the global LARS Ly alpha properties, we find that dispersion-dominated systems show higher Ly alpha equivalent widths and higher Ly alpha escape fractions than systems with v(shear)/sigma(0) > 1. Our result indicates that turbulence in actively star-forming systems is causally connected to interstellar medium conditions that favour an escape of Ly alpha radiation.
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| 13. |
- Melinder, Jens, et al.
(författare)
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Lyman alpha escape from 45 star forming galaxies– the Lyα Reference Sample XIV
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We present Lyα imaging of 45 low redshift star-forming galaxies observed with the Hubble space telescope. The galaxies have been selected to have moderate to high star formation rates using FUVluminosity and Hα equivalent width criteria, but no constraints on Lyα luminosity have been applied. We employ a pixel SED fitting code to obtain accurate continuum subtracted Lyα, Hα and Hβ maps. We find that Lyα is less concentrated than FUV and optical line emission in almost all galaxies with significant Lyα emission. We present global measurements of Lyα and other quantities measured in apertures designed to capture all of the Lyα emission. We then show how the escape fraction ofLyα relates to a number of other measured quantities (mass, metallicity, star formation, ionization parameter, and extinction). We find that the escape fraction is strongly anti-correlated with both nebular and stellar extinction, weakly anti-correlated with stellar mass, but no conclusive evidence for correlations to other quantities. We show that Lyα escape fractions are inconsistent with common dust extinction laws and discuss how a combination of radiative transfer effects and clumpy dust models can help resolve the discrepancies. We present a star formation rate calibration based on Lyαluminosity, where the equivalent width of Lyα is used to correct for non-unity escape fraction, and show that this relation provides a reasonably accurate calibration but with a large scatter. We also show stacked growth curves of Lyα for the galaxies that can be used to find aperture loss fractions at a given physical radius
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| 14. |
- Rivera-Thorsen, Thöger E., et al.
(författare)
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THE LYMAN ALPHA REFERENCE SAMPLE. V. THE IMPACT OF NEUTRAL ISM KINEMATICS ANDGEOMETRY ON Lyα ESCAPE
- 2015
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 805:14
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Tidskriftsartikel (refereegranskat)abstract
- We present high-resolution far-UV spectroscopy of the 14 galaxies of the Lyα Reference Sample; a sample of strongly star-forming galaxies at low redshifts (0.028 < z < 0.18). We compare the derived properties to global properties derived from multi-band imaging and 21 cm H i interferometry and single-dish observations, as well as archival optical SDSS spectra. Besides the Lyα line, the spectra contain a number of metal absorption features allowing us to probe the kinematics of the neutral ISM and evaluate the optical depth and and covering fraction of the neutral medium as a function of line of sight velocity. Furthermore, we show how this, in combination with the precise determination of systemic velocity and good Lyα spectra, can be used to distinguish a model in which separate clumps together fully cover the background source, from the "picket fence" model named by Heckman et al. We find that no one single effect dominates in governing Lyα radiative transfer and escape. Lyα escape in our sample coincides with a maximum velocity-binned covering fraction of 0.9 and bulk outflow velocities of 50 km s−1, although a number of galaxies show these characteristics and yet little or no Lyα escape. We find that Lyα peak velocities, where available, are not consistent with a strong backscattered component, but rather with a simpler model of an intrinsic emission line overlaid by a blueshifted absorption profile from the outflowing wind. Finally, we find a strong anticorrelation between Hα equivalent width and maximum velocity-binned covering factor, and propose a heuristic explanatory model.
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| 15. |
- Östlin, Göran, et al.
(författare)
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THE Ly alpha REFERENCE SAMPLE. I. SURVEY OUTLINE AND FIRST RESULTS FOR MARKARIAN 259
- 2014
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 797:1
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Tidskriftsartikel (refereegranskat)abstract
- The Ly alpha Reference Sample (LARS) is a substantial program with the Hubble Space Telescope (HST) that provides a sample of local universe laboratory galaxies in which to study the detailed astrophysics of the visibility and strength of the Ly alpha line of neutral hydrogen. Ly alpha is the dominant spectral line in use for characterizing high-redshift (z) galaxies. This paper presents an overview of the survey, its selection function, and HST imaging observations. The sample was selected from the combined GALEX+Sloan Digital Sky Survey catalog at z = 0.028-0.19, in order to allow Ly alpha to be captured with combinations of long-pass filters in the Solar Blind Channel (SBC) of the Advanced Camera for Surveys (ACS) onboard HST. In addition, LARS utilizes H alpha and H beta narrowband and u, b, i broadband imaging with ACS and the Wide Field Camera 3 (WFC3). In order to study galaxies in which large numbers of Ly alpha photons are produced (whether or not they escape), we demanded an H alpha equivalent width W(H alpha) >= 100 angstrom. The final sample of 14 galaxies covers far-UV (FUV, lambda similar to 1500 angstrom) luminosities that overlap with those of high-z Ly alpha emitters (LAEs) and Lyman break galaxies (LBGs), making LARS a valid comparison sample. We present the reduction steps used to obtain the Ly alpha images, including our LARS eXtraction software (LaXs), which utilizes pixel-by-pixel spectral synthesis fitting of the energy distribution to determine and subtract the continuum at Ly alpha. We demonstrate that the use of SBC long-pass-filter combinations increase the signal-to-noise ratio by an order of magnitude compared to the nominal Ly alpha filter available in SBC. To exemplify the science potential of LARS, we also present some first results for a single galaxy, Mrk 259 (LARS #1). This irregular galaxy shows bright and extended (indicative of resonance scattering) but strongly asymmetric Ly alpha emission. Spectroscopy from the Cosmic Origins Spectrograph on board HST centered on the brightest UV knot shows a moderate outflow in the neutral interstellar medium (probed by low ionization stage absorption features) and Ly alpha emission with an asymmetric profile. Radiative transfer modeling is able to reproduce the essential features of the Ly alpha line profile and confirms the presence of an outflow. From the integrated photometry we measure an Ly alpha luminosity of L-Ly alpha= 1.3x10(42) erg s(-1) an equivalent width W(Ly alpha) = 45 angstrom and an FUV absolute magnitude M-FUV = -19.2 (AB). Mrk 259 would hence be detectable in high-z Ly alpha and LBG surveys. The total Ly alpha escape fraction is 12%. This number is higher than the low-z average, but similar to that at z > 4, demonstrating that LARS provides a valid comparison sample for high-z galaxy studies.
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| 16. |
- Östlin, Göran, et al.
(författare)
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The Lyman alpha morphology of local starburst galaxies : release of calibrated images
- 2009
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Ingår i: Astronomical Journal. - : American Astronomical Society. - 0004-6256 .- 1538-3881. ; 138, s. 923-940
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Tidskriftsartikel (refereegranskat)abstract
- We present reduced and calibrated high resolution Lyman-alpha (Lyα) images for a sample of six local star-forming galaxies. Targets were selected to represent a range in luminosity and metallicity and to include both known Lyα emitters and nonemitters. Far ultraviolet imaging was carried out with the Solar Blind Channel of the Advanced Camera for Surveys on the Hubble Space Telescope (HST) in the F122M (Lyα online) and F140LP (continuum) filters. The resulting Lyα images are the product of careful modeling of both the stellar and nebular continua, facilitated by supporting HST imaging at λ 2200, 3300, 4400, 5500, Hα, and 8000 Å, combined with Starburst 99 evolutionary synthesis models, and prescriptions for dust extinction on the continuum. In all, the resulting morphologies in Lyα, Hα, and UV continuum are qualitatively very different and we show that the bulk of Lyα emerges in a diffuse component resulting from resonant scattering events. Lyα escape fractions, computed from integrated Hα luminosities and recombination theory, are found never to exceed 14%. Internal dust extinction is estimated in each pixel and used to correct Lyα fluxes. However, the extinction corrections are far too small (by factors from 2.6 to infinity) to reconcile the emerging global Lyα luminosities with standard recombination predictions. Surprisingly, when comparing the global equivalent widths of Lyα and Hα, the two quantities appear to be anticorrelated, which may be due to the evolution of mechanical feedback from the starburst. This calls for caution in the interpretation of Lyα observations in terms of star formation rates. The images presented have a physical resolution 3 orders of magnitude better than attainable at high redshifts from the ground with current instrumentation and our images may therefore serve as useful templates for comparing with observations and modeling of primeval galaxy formation. We therefore provide the reduced Lyα, Hα, and continuum images to the community.
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| 17. |
- Östlin, Göran, et al.
(författare)
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The Source of Leaking Ionizing Photons from Haro11 : Clues from HST/COS Spectroscopy of Knots A, B, and C
- 2021
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 912:2
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Tidskriftsartikel (refereegranskat)abstract
- Understanding the escape of ionizing (Lyman continuum) photons from galaxies is vital for determining how galaxies contributed to reionization in the early universe. While directly detecting the Lyman continuum from high-redshift galaxies is impossible due to the intergalactic medium, low-redshift galaxies in principle offer this possibility but require observations from space. The first local galaxy for which Lyman continuum escape was found is Haro 11, a luminous blue compact galaxy at z = 0.02, where observations with the FUSE satellite revealed an escape fraction of 3.3%. However, the FUSE aperture covers the entire galaxy, and it is not clear from where the Lyman continuum is leaking out. Here we utilize Hubble Space Telescope/Cosmic Origins Spectrograph spectroscopy in the wavelength range 1100-1700 angstrom of the three knots (A, B, and C) of Haro 11 to study the presence of Ly alpha emission and the properties of intervening gas. We find that all knots have bright Ly alpha emission. UV absorption lines, originating in the neutral interstellar medium, as well as lines probing the ionized medium, are seen extending to blueshifted velocities of 500 km s(-1) in all three knots, demonstrating the presence of an outflowing multiphase medium. We find that knots A and B have large covering fractions of neutral gas, making LyC escape along these sightlines improbable, while knot C has a much lower covering fraction (less than or similar to 50%). Knot C also has the the highest Ly alpha escape fraction, and we conclude that it is the most likely source of the escaping Lyman continuum detected in Haro 11.
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