| 1. |
- 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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| 2. |
- 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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| 3. |
- 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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| 4. |
- 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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| 5. |
- 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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| 6. |
- 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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