Lucia Guaita

THE EVOLUTION OF Lyα-EMITTING GALAXIES BETWEEN z = 2.1 AND z = 3.1

We describe the results of a new, wide-field survey for z=3.1 Ly-alpha emission-line galaxies (LAEs) in the Extended Chandra Deep Field South (ECDF-S). By using a nearly top-hat 5010 Angstrom filter and complementary broadband photometry from the MUSYC survey, we identify a complete sample of 141 objects with monochromatic fluxes brighter than 2.4E-17 ergs/cm^2/s and observers-frame equivalent widths greater than ~ 80 Angstroms (i.e., 20 Angstroms in the rest-frame of Ly-alpha). The bright-end of this dataset is dominated by x-ray sources and foreground objects with GALEX detections, but when these interlopers are removed, we are still left with a sample of 130 LAE candidates, 39 of which have spectroscopic confirmations. This sample overlaps the set of objects found in an earlier ECDF-S survey, but due to our filters redder bandpass, it also includes 68 previously uncataloged sources. We confirm earlier measurements of the z=3.1 LAE emission-line luminosity function, and show that an apparent anti-correlation between equivalent width and continuum brightness is likely due to the effect of correlated errors in our heteroskedastic dataset. Finally, we compare the properties of z=3.1 LAEs to LAEs found at z=2.1. We show that in the ~1 Gyr after z~3, the LAE luminosity function evolved significantly, with L* fading by ~0.4 mag, the number density of sources with L > 1.5E42 ergs/s declining by ~50%, and the equivalent width scale-length contracting from 70^{+7}_{-5} Angstroms to 50^{+9}_{-6} Angstroms. When combined with literature results, our observations demonstrate that over the redshift range z~0 to z~4, LAEs contain less than ~10% of the star-formation rate density of the universe.

Ly alpha-EMITTING GALAXIES AT z=2.1 : STELLAR MASSES, DUST, AND STAR FORMATION HISTORIES FROM SPECTRAL ENERGY DISTRIBUTION FITTING

We study the physical properties of 216 z similar or equal to 2.1 Ly alpha-emitting galaxies (LAEs) discovered in an ultra-deep narrow-MUSYC image of the ECDF-S. We fit their stacked spectral energ ...

The Lyman Alpha Reference Sample. Ii. Hubble Space Telescope Imaging Results, Integrated Properties, And Trends

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.

THE LYMAN ALPHA REFERENCE SAMPLE: EXTENDED LYMAN ALPHA HALOS PRODUCED AT LOW DUST CONTENT*

We report on new imaging observations of the Lyman alpha emission line (Lyα), 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 Lyα, Hα, and the far ultraviolet continuum. We show that Lyα is emitted on scales that systematically exceed those of the massive stellar population and recombination nebulae: as measured by the Petrosian 20% radius, R P20, Lyα radii are larger than those of Hα by factors ranging from 1 to 3.6, with an average of 2.4. The average ratio of Lyα-to-FUV radii is 2.9. This suggests that much of the Lyα light is pushed to large radii by resonance scattering. Defining the Relative Petrosian Extension of Lyα compared to Hα, ξLyα = R Lyα P20/R Hα P20, we find ξLyα to be uncorrelated with total Lyα luminosity. However, ξLyα 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 Lyα photons throughout the interstellar medium and drive a large extended Lyα halo.

The Lyα reference sample. I. Survey outline and first results for Markarian 259

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.

THE LYMAN ALPHA REFERENCE SAMPLE. V. THE IMPACT OF NEUTRAL ISM KINEMATICS ANDGEOMETRY ON Lyα ESCAPE

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 d ...

Magellan/MMIRS near-infrared multi-object spectroscopy of nebular emission from star-forming galaxies at 2 < z < 3

Aims. To investigate the ingredients, which allow star-forming galaxies to present Ly alpha line in emission, we studied the kinematics and gas phase metallicity of the interstellar medium. Methods. We used multi-object near-infrared spectroscopy with Magellan/MMIRS to study nebular emission from z similar or equal to 2-3 star-forming galaxies discovered in three MUSYC fields. Results. We detected emission lines from four active galactic nuclei and 13 high-redshift star-forming galaxies, including H alpha lines down to a flux of (4 +/- 1)E-17 erg s(-1) cm(-2). This yielded seven new redshifts. The most common emission line detected is [OIII]5007, which is sensitive to metallicity. We were able to measure metallicity (Z) for two galaxies and to set upper (lower) limits for another two (two). The metallicity values are consistent with 0.3 < Z/Z(circle dot) < 1.2, 12 + log (O/H) similar to 8.2-8.8. Comparing the Ly alpha central wavelength with the systemic redshift, we find Delta(vLy alpha-[OIII]5007) = 70 270 km s(-1). Conclusions. High-redshift star-forming galaxies, Ly alpha emitting (LAE) galaxies, and H alpha emitters appear to be located in the low mass, high star-formation rate (SFR) region of the SFR versus stellar mass diagram, confirming that they are experiencing burst episodes of star formation, which are building up their stellar mass. Their metallicities are consistent with the relation found for z <= 2.2 galaxies in the Z versus stellar mass plane. The measured Delta(vLy alpha[OIII]5007) values imply that outflows of material, driven by star formation, could be present in the z similar to 2 3 LAEs of our sample. Comparing with the literature, we note that galaxies with lower metallicity than ours are also characterized by similar Delta(vLy alpha[OIII]5007) velocity off sets. Strong F([OIII]5007) is detected in many Ly alpha emitters. Therefore, we propose the F(Ly alpha)/F([OIII]5007) flux ratio as a tool for the study of high-redshift galaxies; while influenced by metallicity, ionization, and Ly alpha radiative transfer in the ISM, it may be possible to calibrate this ratio to primarily trace one of these effects.

TO STACK OR NOT TO STACK: SPECTRAL ENERGY DISTRIBUTION PROPERTIES OF Lyα-EMITTING GALAXIES AT z = 2.1

We use the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey (CANDELS) GOODS-S multi-wavelength catalog to identify counterparts for 20 Ly alpha emitting (LAE) galaxies at z = 2.1. We build several types of stacked spectral energy distributions (SEDs) of these objects. We combine photometry to form average and median flux-stacked SEDs, and postage-stamp images to form average and median image-stacked SEDs. We also introduce scaled flux stacks that eliminate the influence of variation in overall brightness. We use the SED fitting code SpeedyMC to constrain the physical properties of individual objects and stacks. Our LAEs at z = 2.1 have stellar masses ranging from 2 x 10(7) M-circle dot to 8 x 10(9) M-circle dot (median = 3 x 10(8) M-circle dot), ages ranging from 4 Myr to 500 Myr (median = 100 Myr), and E(B - V) between 0.02 and 0.24 (median = 0.12). Although still low, this represents significantly more dust reddening than has been reported for LAEs at higher redshifts. We do not observe strong correlations between Ly alpha equivalent width (EW) and age or E(B - V). The Ly alpha radiative transfer (q) factors of our sample are predominantly close to one and do not correlate strongly with EW or E(B - V). The absence of strong correlations with EW or q implies that Ly alpha radiative transfer is highly anisotropic and/or prevents Ly alpha photons from scattering in dusty regions. The SED parameters of the flux stacks match the average and median values of the individual objects, with the flux-scaled median SED performing best with uncertainties reduced by a factor of two. Median image-stacked SEDs provide a poor representation of the median individual object, and none of the stacking methods capture the large dispersion of LAE properties.

THE CURIOUS CASE OF Lyα EMITTERS: GROWING YOUNGER FROM z ∼ 3 to z ∼ 2?

Lyα emitting (LAE) galaxies are thought to be progenitors of present-day L* galaxies. Clustering analyses have suggested that LAEs at z ~ 3 might evolve into LAEs at z ~ 2, but it is unclear whether the physical nature of these galaxies is compatible with this hypothesis. Several groups have investigated the properties of LAEs using spectral energy distribution (SED) fitting, but direct comparison of their results is complicated by inconsistencies in the treatment of the data and in the assumptions made in modeling the stellar populations, which are degenerate with the effects of galaxy evolution. By using the same data analysis pipeline and SED fitting software on two stacked samples of LAEs at z = 3.1 and z = 2.1, and by eliminating several systematic uncertainties that might cause a discrepancy, we determine that the physical properties of these two samples of galaxies are dramatically different. LAEs at z = 3.1 are found to be old (age ~1 Gyr) and metal-poor (Z Z ☉). The difference in the observed stellar ages makes it very unlikely that z = 3.1 LAEs evolve directly into z = 2.1 LAEs. Larger samples of galaxies, studies of individual objects, and spectroscopic measurements of metallicity at these redshifts are needed to confirm this picture, which is difficult to reconcile with the effects of 1 Gyr of cosmological evolution.

STACKED REST-FRAME ULTRAVIOLET SPECTRA OF Lyα-EMITTING AND CONTINUUM-SELECTED GALAXIES AT 2 < z < 3.5

We present properties of individual and composite rest-UV spectra of continuum- and narrowband-selected star-forming galaxies (SFGs) at a redshift of 2 20 {\AA}, the canonical limit to be classified as a LAE. We divide our dataset into subsamples based on properties we are able to measure for each individual galaxy: Ly{\alpha} equivalent width, rest-frame UV colors, and redshift. Among our subsample of galaxies with R 20 {\AA} have bluer UV continua, weaker low-ionization interstellar absorption lines, weaker C IV absorption, and stronger Si II* nebular emission than those with W_{Ly{\alpha}} 20 {\AA} exhibit weaker Ly{\alpha} emission at lower redshifts, although we caution that this could be caused by spectroscopic confirmation of low Ly{\alpha} equivalent width galaxies being harder at z~3 than z~2.