Extended Ly\alpha Emission in Nearby Galaxies

The Lyman alpha Reference Sample: Extended Lyman alpha Halos Produced at Low Dust Content

Matthew Hayes22affiliation: Université de Toulouse; UPS-OMP; IRAP; Toulouse, France 33affiliation: CNRS; IRAP; 14, avenue Edouard Belin, F-31400 Toulouse, France , Göran Östlin44affiliation: Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden , Daniel Schaerer55affiliation: Geneva Observatory, University of Geneva, 51 Chemin des Maillettes, CH-1290 Versoix, Switzerland 33affiliation: CNRS; IRAP; 14, avenue Edouard Belin, F-31400 Toulouse, France , Anne Verhamme55affiliation: Geneva Observatory, University of Geneva, 51 Chemin des Maillettes, CH-1290 Versoix, Switzerland , J. Miguel Mas-Hesse66affiliation: Centro de Astrobiología (CSIC–INTA), Departamento de Astrofísica, POB 78, 28691 Villanueva de la Cañada, Spain. , Angela Adamo77affiliation: Max Planck Institute for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany. , Hakim Atek88affiliation: Laboratoire d’Astrophysique, École Polytechnique Fédérale de Lausanne (EPFL), Observatoire, CH-1290 Sauverny, Switzerland. , John M. Cannon99affiliation: Department of Physics and Astronomy, Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, USA. , Florent Duval44affiliation: Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden , Lucia Guaita44affiliation: Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden , E. Christian Herenz1010affiliation: Leibniz-Institut für Astrophysik (AIP), An der Sternwarte 16, D-14482 Potsdam, Germany. , Daniel Kunth1111affiliation: Institut d’Astrophysique de Paris, UMR 7095 CNRS & UPMC, 98 bis Bd Arago, 75014 Paris, France. , Peter Laursen1212affiliation: Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark. , Jens Melinder44affiliation: Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden , Ivana Orlitová55affiliation: Geneva Observatory, University of Geneva, 51 Chemin des Maillettes, CH-1290 Versoix, Switzerland 1313affiliation: Astronomical Institute of the Academy of Sciences, Bočnı´II 1401/1a, CZ-141 31 Praha 4, Czech Republic. , Héctor Otí-Floranes1414affiliation: Instituto de Astronomía, Universidad Nacional Autónoma de México, Apdo. Postal 106, Ensenada B. C. 22800 Mexico 1515affiliation: Dpto. de Física Moderna, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain 66affiliation: Centro de Astrobiología (CSIC–INTA), Departamento de Astrofísica, POB 78, 28691 Villanueva de la Cañada, Spain. , and Andreas Sandberg44affiliation: Department of Astronomy, Oskar Klein Centre, Stockholm University, AlbaNova University Centre, SE-106 91 Stockholm, Sweden matthew.hayes@irap.omp.eu

We report on new imaging observations of the Lyman alpha emission line (Ly), performed with the Hubble Space Telescope, that comprise the backbone of the Lyman alpha Reference Sample (LARS). We present images of 14 starburst galaxies at redshifts in continuum-subtracted Ly, H, and the far ultraviolet continuum. We show that Ly is emitted on scales that systematically exceed those of the massive stellar population and recombination nebulae: as measured by the Petrosian 20 percent radius, , Ly radii are larger than those of H by factors ranging from 1 to 3.6, with an average of 2.4. The average ratio of Ly-to-FUV radii is 2.9. This suggests that much of the Ly light is pushed to large radii by resonance scattering. Defining the Relative Petrosian Extension of Ly compared to H, = /, we find  to be uncorrelated with total Ly luminosity. However  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 photons throughout the interstellar medium and drive a large extended Ly halo.

Subject headings:
Physical data and processes: Radiative transfer — Galaxies: evolution — Galaxies: formation — Galaxies: starburst — Cosmology: observations
slugcomment: Accepted by ApJL11affiliationtext: Based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program #12310.

1. Introduction

The Lyman alpha emission line (Ly), emitted by the spontaneous de-excitation over the electronic transition in neutral hydrogen (Hi), is now an established observational probe of evolving galaxies in the high- Universe (Cowie & Hu, 1998; Rhoads et al., 2000). Exploitation of Ly has resulted in significant galaxy surveys (Ouchi et al., 2008; Nilsson et al., 2009; Guaita et al., 2010; Adams et al., 2011), the next generations of which will recover vast numbers of galaxies. However the Hi abundance in most galaxies, combined with the large Ly absorption cross section of ground-state hydrogen, suggests that most Ly will be absorbed and re-scattered by the same transition that created it. Thus most Ly photons are thought to be subject to multiple scattering events as they encounter neutral gas, resulting in a complicated radiative transport (Neufeld, 1990; Verhamme et al., 2006; Laursen et al., 2009).

Figure 1.— False-color images of the LARS galaxies 01 to 08. Red encodes continuum-subtracted H, green the FUV continuum, and blue shows continuum-subtracted Ly. Images have been adaptively filtered to show detail. Scales in kpc are given on the side. Intensity scales are logarithmic, with intensity cut levels set to show detail.

Because Hi is often found at distances that exceed the size of stellar disks and star-forming regions (Yun et al., 1994; Meurer et al., 1996; Cannon et al., 2004), characteristic Ly scale lengths may be expected to be substantially larger than those of, for example, the FUV continuum or H. Indeed this has been well observed at high (e.g. Fynbo et al. 2001; Rauch et al. 2008; Steidel et al. 2011, although see also Feldmeier et al. 2013) and low (Mas-Hesse et al., 2003; Östlin et al., 2009), and studied extensively by simulation (Laursen et al., 2009; Barnes & Haehnelt, 2010; Zheng et al., 2011; Verhamme et al., 2012).

In this Letter we present images from the Lyman alpha Reference Sample (LARS). The LARS program (Östlin et al., in prep; Hayes et al., in prep) is targeting 14 UV-selected star-forming galaxies in the nearby Universe, all of which have been imaged in Ly, H, H, and five UV/optical continuum bands. Many other observations, both in hand and ongoing, are providing gas covering fractions and kinematics, and measuring the Hi mass and extent directly. HST imaging allows us to probe spatial scales down to 28 pc in individual galaxies, quantify the extent of Ly, and compare it with other wavelengths and derived properties. This Letter discusses the extension of Ly radiation. In Section 2 we briefly summarize the data and show the new images. In Section 3 we quantify the sizes of the galaxies in Ly, FUV, and H, and discuss them with reference to high- measurements in Section 4. In Section 5 we show how a low dust content seems to be a necessary prerequisite in order to produce this extended emission. We assume a cosmology of .

2. LARS Images

LARS consists of 14 star-forming galaxies selected by FUV luminosity from the GALEX all-sky surveys, and imaged with Hubble Space Telescope cameras ACS/SBC, ACS/WFC, and WFC3/UVIS. The sample selection, observations, and data processing are described in detail in Östlin et al. (in prep). FUV luminosities range between and 10.7, overlapping much of the luminosity range of Lyman-break Galaxy (LBG) surveys, and are listed in Table 1.

We use the Lyman alpha eXtraction software (LaXs, Hayes et al. 2009) to produce continuum-subtracted Ly and H images, corrected for underlying stellar absorption and contamination from [Nii]. In 1 arcsec square boxes away from the targets we measure r.m.s. background noise of  erg s cm in Ly,  erg s cm Å in the FUV, and  erg s cm in H. Total Ly luminosities range from 0 (non-detection) and  erg s with a median of  erg s; roughly seven of the objects would be recovered by the deepest Ly surveys (Hayes et al. in prep).

We present our first imaging results in this paper as a series of RGB composite images in Figures 1 and 2. In green we encode the far UV continuum, which traces the unobscured massive stars, and roughly incorporates the sites that produce the ionizing photons. In the red we show continuum-subtracted H, which traces the nebulae where the aforementioned ionizing photons are reprocessed into the recombination line spectrum. The continuum subtracted Ly observation is encoded in blue. The images have been adaptively smoothed using a variable Gaussian kernel (FILTER/ADAPTIVE in ESO/MIDAS), in order to enhance positive regions of low surface brightness emission. The intensity scaling of all the images is logarithmic, and the levels are set to show the maximum of structure and the level at which the faintest features fade into the background.

Figure 2.— Same as Figure 1 except for LARS galaxies 09 to 14. The black square in LARS 09 masks a UV-bright field star.

Immediately it can be seen that Ly morphologies bear limited resemblance to those of the FUV and H. In some cases Ly appears to be almost completely absent: LARS 04 and 06 in particular show only small hints of Ly emission that contribute negligibly towards filling in the global absorption, and the composites are dominated by UV and H light. Ly is strongly absorbed, particularly in the central regions of these objects. Others show copious Ly emission and reveal morphological structures that are not seen at other wavelengths. Most obviously, LARS 01, 02, 05, 07, 12, and 14 show large-scale halos of Ly emission that completely encompass the star-forming regions, although the same phenomenon is visible to some extent in all the objects, even the absorbers.

We have discussed this extended Ly emission in depth in the past (Hayes et al., 2005, 2007; Atek et al., 2008; Östlin et al., 2009). However now, with an observational setup that is more sensitive to faint levels of Ly and a larger and UV-selected sample (Östlin et al, in prep), we are able to robustly quantify and contrast these sizes and the relative extension of Ly.

3. Apertures, Sizes and Global Quantities

In order to quantify the sizes of the galaxies at various wavelengths, we adopt the Petrosian radius (Petrosian, 1976) with index of : i.e. the radius, , at which the local surface brightness is 20 percent the average surface brightness inside of . In Hayes et al. (in prep) we will show the Ly extent of some objects to be so large that ACS/SBC cannot capture the full flux, and hence measurements like 50 percent light radius are not robust. Indeed Petrosian radii were developed to be depth-independent measures of size. We note from experimentation, however, that very similar conclusions are reached using other definitions. The choice of gives a size for every Ly-emitting galaxy in the sample except LARS 09, for which even at the full extent of the SBC we do not come close to crossing the threshold. We reach the edge of the detector at ( kpc) and can expect the true extent of Ly to be much larger. For the 11 galaxies in which  is well measured, its determination is robust, and would not change were the observations deeper or the field-of-view larger. , is computed for Ly, H, and the FUV continuum, and listed in Table 1. Based upon aperture-matched H and H imaging and standard Case B assumptions, we recover up to 60 % of the intrinsic Ly flux, although the median value is just  % (Hayes et al. in prep).

LARS Common name R.A. Dec. -slope H/H
ID :: :: kpc kpc kpc kpc
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13)
01 Mrk 259 13:28:44.0 +43:55:49.9 0.028 9.92 1.18 1.29    4.36 7.87   3.37 –1.83 3.08
02 09:07:04.9 +53:26:56.5 0.030 9.48 1.12 1.17    2.67 8.41   2.27 –2.02 3.08
03 Arp 238 13:15:35.1 +62:07:27.2 0.031 9.52 0.84 0.97    0.75 8.68   0.77 –0.57 5.18
04 13:07:28.2 +54:26:50.7 0.033 9.93 3.79 1.57 9.22 –1.76 3.48
05 Mrk 1486 13:59:51.0 +57:26:23.0 0.034 10.0 0.93 1.24    3.24 9.49   2.61 –2.09 3.06
06 KISSR 2019 15:45:44.5 +44:15:49.9 0.034 9.20 3.65 0.66 9.48 –1.85 2.96
07 IRAS 1313+2938 13:16:03.9 +29:22:54.2 0.038 9.75 0.85 0.89    3.01 10.5   3.37 –1.94 3.37
08 12:50:13.7 +07:34:44.2 0.038 10.2 5.01 3.89    4.35 10.5   1.12 –0.90 4.09
09 IRAS 0820+2816 08:23:54.9 +28:06:22.8 0.047 10.5 5.00 4.21 12.0 12.9 2.85 –1.52 3.48
10 Mrk 0061 13:01:41.5 +29:22:53.2 0.057 9.74 2.34 2.63    5.49 15.5    2.08 –1.36 3.93
11 14:03:47.1 +06:28:15.0 0.084 10.7 8.00 6.81    15.5 22.1    2.27 –1.50 4.60
12 SBS 0934+547 09:38:13.5 +54:28:25.3 0.102 10.5 1.78 2.03    7.06 26.3    3.48 –1.92 3.21
13 IRAS 0147+1254 01:50:28.4 +13:08:59.2 0.147 10.6 3.83 4.68    8.12 36.0    1.74 –1.53 4.07
14 09:26:00.3 +44:27:36.0 0.181 10.7 0.79 1.62    5.86 42.7    3.62 –2.22 3.13

Note. – Coordinates (3 and 4) are J2000. Redshifts (5) are derived from SDSS. (6) are in solar luminosities. (7–9) are the Petrosian radii with , . (10) shows the physical scale corresponding to an angular size of 14 arcsec at the redshift of each galaxy – this corresponds to half the diametric size of the ACS/SBC (28 arcsec / 2) and describes the maximum usable scale to which we can probe Ly. (11) gives the relative extension of Ly relative toH. (12) gives the UV slope, , derived from HST imaging. (13) gives the H/H ratio, derived from SDSS spectroscopy.

Table 1The LARS sample: properties and sizes.

We compare the light radii graphically in Figure 3. The plots show  vs. , a comparison that could be made at high-, and  vs. , a comparison that more directly conveys the difference between the observed and intrinsic Ly sizes. Clearly, though, there is little difference in the result: Ly radii are, on average, substantially larger than corresponding FUV or H radii. In Table 1 we also report the Relative Petrosian Extension of Ly compared to H, , which is simply defined as /. 12 galaxies show net emission of Ly, where all except for one (LARS 03) has . The galaxy with the largest extension is LARS 14, for which we measure =3.6. It is not clear whether globally absorbing galaxies LARS 04 and 06 become emitters on larger scales, but if so their  must be larger than the radius of the SBC chip, implying that  must exceed 5.3 and 13.4, respectively. That would make them the most extended objects in the sample. Excluding these two galaxies, and also LARS 09 for which we can only provide a lower limit, the sample mean (median) is computed as 2.43 (2.28).

Figure 3.— Comparison of the Petrosian radii (), , in continuum-subtracted Ly, H, and the UV continuum. In cases where a galaxy is a net Ly absorber its size is undefined, and  has been set to a small negative value – it could in principle also be very large. The Left panel shows how Ly sizes compare with the FUV, which can be similarly derived at high-. The Right panel makes the same comparison against , which directly contrasts the intrinsic and emitted Ly sizes. When net Ly emission is found it is systematically extended, taking mean values of /=2.9 (left) and /=2.4 (right).

4. Relevance for high-redshift studies

It is important to note that FUV radii imply that all the galaxies would be effectively unresolved by ground-based observations if they were at . The largest is 8 kpc, which corresponds to the 1 arcsec resolution that could be expected from the seeing. However one of the objects has a Ly radius of 15.5 kpc: recovering this total flux at would require an aperture of at least 2 arcsec. Some objects are also highly elongated and were they pushed to the high- Universe, much of their Ly could also be unmeasured if circular apertures are used.

Ly emission more extended than the FUV has been reported in numerous high- samples. Fynbo et al. (2003) remarked upon a few such objects at the brighter end of the luminosity distribution of the 27 narrowband selected galaxies, and the extremely deep spectroscopic observations of Rauch et al. (2008) uncovered 28 Ly galaxies, ten of which were classified as extended. Samples of Ly blobs (e.g. Matsuda et al., 2012; Prescott et al., 2012) may be many times the size of their counterpart galaxies, if indeed counterparts are identified at all. Here we report that every galaxy in the sample that emits Ly does so by producing a halo; on average the halo is over twice the linear size of H and the FUV.

By stacking narrowband images of LBGs at , Steidel et al. (2011) reported Ly halos that extend many tens of kpc, probably probing the neutral circumgalactic medium (CGM) out to the virial radius. Subdividing the full sample by Ly properties, the halos at radii larger than 20–30 physical kpc show very similar scale lengths in all subsamples (although different central surface brightnesses), even when central Ly absorption is found. At small radii the subsamples exhibit profiles that differ markedly, dropping rapidly to for the Ly-absorbing sample but steepening by varying degrees in all others. Even the steepest central profiles, however, still run much flatter than those of the stellar continuum, and this change likely marks the onset of higher density gaseous disks or similar.

From the various Ly profiles of Steidel et al. (2011) we calculate  using the same method as for our sample, and dividing by  from the continuum profile we obtain  (now relative to the UV). These raw values range between =3.8 for the non-LAEs, and 5.9 for the LAE-only sample, and are notably bigger than our largest . However, under the assumption that the inner and outer profiles mark physically different regimes that may not be the same in low- galaxies, we also subtract the exponential halo fits of Steidel et al. (2011) and repeat the exercise; this yields a range of =0.84 to 2.0. This is now smaller than many of our values, although close to the average and the dispersion of the high- sample is obviously lost in the stacking process. On the other hand, the UV continuum profile of Steidel et al. (2011) is dominated by atmospheric seeing. If we instead use the continuum effective radius of BM/BX galaxies and LBGs from HST imaging (Mosleh et al., 2011) we compute  kpc, which would increase all the  quoted above by a factor of 2.5.  from the raw data would then become much larger than we measure in the local universe (up to 15), and  in halo-subtracted profiles that are roughly consistent (2.1 to 4.8).

Figure 4.— Correlations between the Ly vs. H extension, , and measures of the galaxy reddening. The Left panel shows the UV slope, measured from HST imaging, while the Right panel shows the nebular attenuation measured from SDSS spectroscopy at H and H. Net absorbing galaxies are set to zero and ringed, but could in principle also be very extended. Rank correlation coefficients of the Spearman test and the associated probability of the no-correlation hypothesis (accounting for ties and the small sample) are given in the top-right corners. Symbols are the same as Figure 3.

LARS observations probe scales far below the tens of kpc sampled at high- on a case-by-case basis. The galaxies likely include the range between, or roughly bracketing, the averaged subsamples of Steidel et al. (2011). Our imaging also suggests this extension to be a very common property of Ly-emitting galaxies, and its onset begins almost immediately in the inner few kpc: we find seven galaxies with FUV Petrosian radii below 2 kpc, five of which have corresponding Ly radii three times larger.

It is also noteworthy that Steidel et al. (2011) find different median dust attenuations for the Ly-emitting and non-emitting subsamples, almost precisely as we did in Hayes et al. (2010). LAEs, which show extended central peaks, were determined to have stellar =0.09 magnitudes (c.f. 0.085 in Hayes et al. 2010) while absorbers show =0.19 (c.f. 0.23 for our H-selected sample). Adopting the prescription of Meurer et al. (1999) the stellar  measurements for the Steidel et al. samples correspond to slopes111UV continuum flux density, parameterized by a power-law of the form . of (LAEs) and (Ly absorbers). Bluntly accounting for a factor of 2.27 that connects stellar  to its nebular equivalent in local starbursts (Calzetti et al., 2000), the same stellar  would equate to H/H ratios of 3.5 (LAEs) and 4.4 (absorbers). In the next Section we will show case-by-case that Ly halos systematically become more extended with decreasing dust contents.

5. Lyman alpha extension and dust contents

In Hayes et al. (in prep) we compute many global properties for the sample, in order to study the processes complicit in Ly transport. Indeed that paper will include a complete analysis of correlations between Ly transmission, halo sizes, and many other properties; in this Letter we restrict ourselves to observables that scale with the dust content. It is noteworthy for the moment, however, that we find no correlation between  and the total Ly luminosity. In Figure 4 we show how  compares with both the UV continuum slope and the H/H ratio. We note that the SDSS fibers are on average smaller than the Ly radii, but do capture the bulk of the nebular emission, and fluxes can easily be measured without contamination of [Nii] and stellar absorption. Since Meurer et al. (1999) has been used almost ubiquitously as a proxy of stellar attenuation in high- galaxies; here we measure from aperture-matched HST imaging using the FUV (SBC/F140LP or F150LP) and the band (UVIS/F336W or F390W) filters. With colors between and our objects have similar UV slopes to the vast majority of those found in Ly-emitting galaxies (Blanc et al., 2011). Similarly the H/H ratio is the canonical probe of nebular reddening (i.e. that which is to zeroth order expected for Ly) used in studies of low- and Galactic nebulae. and H/H are listed in Table 1.

Both measures of dust content strongly anti-correlate with  although the sample is small ( defined sizes in Ly). To assess its significance we compute the Spearman rank correlation coefficient, , which yields and for the anti-correlation of  with and H/H, respectively. This corresponds to likelihoods of the null hypothesis – that this correlation arises purely by chance – amounting to 0.7 percent (UV slope), and 3.6 percent (H/H).

The halo–dust phenomenon appears not to be a direct effect of radiative transfer. We have performed new test simulations with the McLya code (Verhamme et al., 2006), by tuning the gas-to-dust ratio in the synthetic galaxy of Verhamme et al. (2012). Indeed the surface brightness does scale with dust abundance but the light profile (therefore ) does not, and the –dust trend must be a secondary correlation. A scenario is needed in which galaxies decrease the relative size of their Hi envelopes as the absolute dust content increases. A sequence in which neutral gas settles into the galaxy (reducing ) and subsequently forms stars (creating more dust) would explain the trend, but without yet having obtained spatially resolved Hi data this is conjecture.

Scattering also has the potential to spread Ly over such an area that its surface brightness decreases greatly. In such a case, scattered radiation measured at large radii may not be sufficient to recover flux from a broad central absorption, making  observationally undefined when it is actually very large. The trend of  increasing in bluer galaxies, then, is also able to explain the undefined sizes of LARS 04 and 06, at their measured dust abundance. Similar considerations would also explain the non-detection of Ly in local gas-rich but metal- and dust-poor dwarf starbursts such as i Zw 18 and SBS 0335–052 (Kunth et al., 1994; Mas-Hesse et al., 2003; Östlin et al., 2009), as discussed in Atek et al. (2009b).

We have empirically shown before (Atek et al., 2009a; Hayes et al., 2010) that the global escape fraction of Ly photons anti-correlates strongly with attenuation (also Kornei et al. 2010 in LBGs). We now demonstrate that at lower , the more strongly emitting galaxies are likely to also spread their Ly over larger surfaces. Thus while they do transmit more of their Ly, it may be that more of the transferred Ly is observationally lost outside photometric apertures. This may also explain the lack of correlation between Ly/H and  observed by Giavalisco et al. (1996), compared to trends seen in other samples: the aperture of the IUE probed just 3 kpc at and if more Ly is lost in bluer galaxies the Ly/Balmer ratios would be artificially lowered in in such systems. This could in part mask an underlying correlation. By a similar token, galaxies that can very efficiently scatter Ly photons may not be recovered at all, despite frequently showing very blue UV colors. Determining precisely how Ly profiles are modified for a given set of host properties will provide a cornerstone for interpreting future large high- surveys.

M.H. received support from Agence Nationale de la recherche bearing the reference ANR-09-BLAN-0234-01. G.Ö. is a Swedish Royal Academy of Sciences research fellow supported by a grant from Knut and Alice Wallenberg foundation, and also acknowledges support from the Swedish research council (VR) and the Swedish National Space Board (SNSB). A.V. benefits from the fellowship ‘Boursière d’excellence de l’Université de Genève’. H.A. and D.K. are supported by the Centre National d’Études Spatiales (CNES) and the Programme National de Cosmologie et Galaxies (PNCG). I.O. acknowledges the Sciex fellowship. H.O.F. acknowledges financial support from CONACYT grant 129204, and Spanish FPI grant BES-2006-13489. H.O.F. and J.M.M.H. are partially funded by Spanish MICINN grants CSD2006-00070 (CONSOLIDER GTC), AYA2010-21887-C04- 02 (ESTALLIDOS) and AYA2011-24780/ESP. We thank C. Steidel for making the high- Ly profiles available for our comparisons in Section 4. Facilities: HST (ACS,WFC3).


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