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Register a new userSpitzer Constraints on the Stellar Populations of Lyman-Alpha Emitting Galaxies at z = 3.1
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We investigate the stellar populations of a sample of 162 Lyman-alpha emitting galaxies (LAEs) at z = 3.1 in the Extended Chandra Deep Field South, using deep Spitzer IRAC data available from the GOODS and SIMPLE surveys to derive reliable stellar population estimates. We divide the LAEs according to their rest-frame near-IR luminosities into IRAC-detected and IRAC-undetected samples. About 70% of the LAEs are undetected in 3.6 micron down to [3.6] = 25.2 AB. Stacking analysis reveals that the average stellar population of the IRAC-undetected sample has an age of ~ 200 Myr and a mass of ~ 3x10^8 solar masses, consistent with the expectation that LAEs are mostly young and low-mass galaxies. On the other hand, the IRAC-detected LAEs are on average significantly older and more massive, with an average age > 1 Gyr and mass ~ 10^10 solar masses. Comparing the IRAC colors and magnitudes of the LAEs to z ~ 3 Lyman break galaxies (LBGs) shows that the IRAC-detected LAEs lie at the faint blue end of the LBG color-magnitude distribution, suggesting that IRAC-detected LAEs may be the low mass extension of the LBG population. We also present tentative evidence for a small fraction (~ 5%) of obscured AGN within the LAE sample. Our results suggest that LAEs posses a wide range of ages and masses. Additionally, the presence of evolved stellar populations inside LAEs suggests that the Lyman-alpha luminous phase of galaxies may either be a long-lasting or recurring phenomenon.
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We present a sample of 33 spectroscopically confirmed z ~ 3.1 Ly$alpha$-emitting galaxies (LAEs) in the Cosmological Evolution Survey (COSMOS) field. This paper details the narrow-band survey we conducted to detect the LAE sample, the optical spectroscopy we performed to confirm the nature of these LAEs, and a new near-infrared spectroscopic detection of the [O III] 5007 AA line in one of these LAEs. This detection is in addition to two [O III] detections in two z ~ 3.1 LAEs we have reported on previously (McLinden et al 2011). The bulk of the paper then presents detailed constraints on the physical characteristics of the entire LAE sample from spectral energy distribution (SED) fitting. These characteristics include mass, age, star-formation history, dust content, and metallicity. We also detail an approach to account for nebular emission lines in the SED fitting process - wherein our models predict the strength of the [O III] line in an LAE spectrum. We are able to study the success of this prediction because we can compare the model predictions to our actual near-infrared observations both in galaxies that have [O III] detections and those that yielded non-detections. We find a median stellar mass of 6.9 $times$ 10$^8$ M$_{odot}$ and a median star formation rate weighted stellar population age of 4.5 $times$ 10$^6$ yr. In addition to SED fitting, we quantify the velocity offset between the [O III] and Ly$alpha$ lines in the galaxy with the new [O III] detection, finding that the Ly$alpha$ line is shifted 52 km s$^{-1}$ redwards of the [O III] line, which defines the systemic velocity of the galaxy.
We selected 40 candidate Lyman Alpha Emitting galaxies (LAEs) at z ~=3.1 with observed frame equivalent widths >150A and inferred emission line fluxes >2.5x10^-17 ergs/cm^2/s from deep narrow-band and broad-band MUSYC images of the Extended Chandra Deep Field South. Covering 992 sq. arcmin, this is the largest ``blank field surveyed for LAEs at z ~3, allowing an improved estimate of the space density of this population of 3+-1x10^-4 h_70^3/Mpc^3. Spectroscopic follow-up of 23 candidates yielded 18 redshifts, all at z ~=3.1. Over 80% of the LAEs are dimmer in continuum magnitude than the typical Lyman break galaxy spectroscopic limit of R= 25.5 (AB), with a median continuum magnitude R ~=27 and very blue continuum colors, (V-z) ~=0. Over 80% of the LAEs have the right UVR colors to be selected as Lyman break galaxies, but only 10% also have R<=25.5. Stacking the UBVRIzJK fluxes reveals that LAEs have stellar masses ~=5x10^8 h_70^-2 M_sun and minimal dust extinction, A_V < ~ 0.1. Inferred star formation rates are ~=6 h_70^-2 M_sun/yr, yielding a cosmic star formation rate density of 2x10^-3 h_70 M_sun/yr/Mpc^3. None of our LAE candidates show evidence for rest-frame emission line equivalent widths EW_rest>240A which might imply a non-standard IMF. One candidate is detected by Chandra, implying an AGN fraction of 2+-2% for LAE candidate samples. In summary, LAEs at z ~ 3 have rapid star formation, low stellar mass, little dust obscuration and no evidence for a substantial AGN component.
We present the results of a high-spatial-resolution study of the line emission in a sample of z=3.1 Lyman-Alpha-Emitting Galaxies (LAEs) in the Extended Chandra Deep Field-South. Of the eight objects with coverage in our HST/WFPC2 narrow-band imaging, two have clear detections and an additional two are barely detected (~2-sigma). The clear detections are within ~0.5 kpc of the centroid of the corresponding rest-UV continuum source, suggesting that the line-emitting gas and young stars in LAEs are spatially coincident. The brightest object exhibits extended emission with a half-light radius of ~1.5 kpc, but a stack of the remaining LAE surface brightness profiles is consistent with the WFPC2 point spread function. This suggests that the Lyman Alpha emission in these objects originates from a compact (<~2 kpc) region and cannot be significantly more extended than the far-UV continuum emission (<~1 kpc). Comparing our WFPC2 photometry to previous ground-based measurements of their monochromatic fluxes, we find at 95% (99.7%) confidence that we cannot be missing more than 22% (32%) of the Lyman Alpha emission.
We present a rest-frame ultraviolet morphological analysis of 108 z=2.1 Lyman Alpha Emitters (LAEs) in the Extended Chandra Deep Field South (ECDF-S) and compare it to a similar sample of 171 LAEs at z=3.1. Using Hubble Space Telescope (HST) images from the Galaxy Evolution from Morphology and SEDs survey, Great Observatories Origins Deep Survey, and Hubble Ultradeep Field, we measure size and photometric component distributions, where photometric components are defined as distinct clumps of UV-continuum emission. At both redshifts, the majority of LAEs have observed half-light radii <~ 2 kpc, but the median half-light radius rises from 1.0 kpc at z=3.1 to 1.4 kpc at z=2.1. A similar evolution is seen in the sizes of individual rest-UV components, but there is no evidence for evolution in the number of multi-component systems. In the z=2.1 sample, we see clear correlations between the size of an LAE and other physical properties derived from its SED. LAEs are found to be larger for galaxies with higher stellar mass, star formation rate, and dust obscuration, but there is no evidence for a trend between equivalent width and half-light radius at either redshift. The presence of these correlations suggests that a wide range of objects are being selected by LAE surveys at z~2, including a significant fraction of objects for which a massive and moderately extended population of old stars underlies the young starburst giving rise to the Lyman alpha emission.
We present the first results of a project, LSD, aimed at obtaining spatially-resolved, near-infrared spectroscopy of a complete sample of Lyman-Break Galaxies at z~3. Deep observations with adaptive optics resulted in the detection of the main optical lines, such as [OII], Hbeta and [OIII], which are used to study sizes, SFRs, morphologies, gas-phase metallicities, gas fractions and effective yields. Optical, near-IR and Spitzer/IRAC photometry is used to measure stellar mass. We obtain that morphologies are usually complex, with the presence of several peaks of emissions and companions that are not detected in broad-band images. Typical metallicities are 10-50% solar, with a strong evolution of the mass-metallicity relation from lower redshifts. Stellar masses, gas fraction, and evolutionary stages vary significantly among the galaxies, with less massive galaxies showing larger fractions of gas. In contrast with observations in the local universe, effective yields decrease with stellar mass and reach solar values at the low-mass end of the sample. This effect can be reproduced by gas infall with rates of the order of the SFRs. Outflows are present but are not needed to explain the mass-metallicity relation. We conclude that a large fraction of these galaxies are actively creating stars after major episodes of gas infall or merging.
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