Eric Gawiser

DAMPED LYα SYSTEMS

Observations of damped Lyman alpha systems offer a unique window on the neutral-gas reservoirs that gave rise to galaxies at high redshifts. This review focuses on critical properties such as the H I and metal content of the gas and on independent evidence for star formation. Together, these provide an emerging picture of gravitationally bound objects in which accretion of gas from the IGM replenishes gas consumed by star formation. Other properties such as dust content, molecular content, ionized-gas content, gas kinematics, and galaxy identifications are also reviewed. These properties point to a multiphase ISM in which radiative and hydrodynamic feedback processes are present. Numerical simulations and other types of models used to describe damped Lyman alpha systems within the context of galaxy formation are also discussed.

Lyα Emission-Line Galaxies at z = 3.1 in the Extended Chandra Deep Field-South

We describe the results of an extremely deep, 0.28 deg^2 survey for z = 3.1 Ly-alpha emission-line galaxies in the Extended Chandra Deep Field South. By using a narrow-band 5000 Anstrom filter and complementary broadband photometry from the MUSYC survey, we identify a statistically complete sample of 162 galaxies with monochromatic fluxes brighter than 1.5 x 10^-17 ergs cm^-2 s^-1 and observers frame equivalent widths greater than 80 Angstroms. We show that the equivalent width distribution of these objects follows an exponential with a rest-frame scale length of w_0 = 76 +/- 10 Angstroms. In addition, we show that in the emission line, the luminosity function of Ly-alpha galaxies has a faint-end power-law slope of alpha = -1.49 +/- 0.4, a bright-end cutoff of log L^* = 42.64 +/- 0.2, and a space density above our detection thresholds of 1.46 +/- 0.12 x 10^-3 h70^3 galaxies Mpc^-3. Finally, by comparing the emission-line and continuum properties of the LAEs, we show that the star-formation rates derived from Ly-alpha are ~3 times lower than those inferred from the rest-frame UV continuum. We use this offset to deduce the existence of a small amount of internal extinction within the host galaxies. This extinction, coupled with the lack of extremely-high equivalent width emitters, argues that these galaxies are not primordial Pop III objects, though they are young and relatively chemically unevolved.

Lyα-Emitting Galaxies at z = 3.1: L* Progenitors Experiencing Rapid Star Formation

We studied the clustering properties and multiwavelength spectral energy distributions of a complete sample of 162 Lyα-emitting (LAE) galaxies at z 3.1 discovered in deep narrowband MUSYC imaging of the Extended Chandra Deep Field-South. LAEs were selected to have observed frame equivalent widths >80 A and emission line fluxes >1.5 × 10-17 ergs cm-2 s-1. Only 1% of our LAE sample appears to host AGNs. The LAEs exhibit a moderate spatial correlation length of r0 = 3.6 Mpc, corresponding to a bias factor b = 1.7, which implies median dark matter halo masses of log10 Mmed = 10.9 M☉. Comparing the number density of LAEs, 1.5 ± 0.3 × 10-3 Mpc-3, with the number density of these halos finds a mean halo occupation ~1%-10%. The evolution of galaxy bias with redshift implies that most z = 3.1 LAEs evolve into present-day galaxies with L 3 galaxy populations typically evolve into more massive galaxies. Halo merger trees show that z = 0 descendants occupy halos with a wide range of masses, with a median descendant mass close to that of L*. Only 30% of LAEs have sufficient stellar mass (>~3 × 109 M☉) to yield detections in deep Spitzer IRAC imaging. A two-population SED fit to the stacked UBVRIzJK+[3.6, 4.5, 5.6, 8.0] μm fluxes of the IRAC-undetected objects finds that the typical LAE has low stellar mass (1.0 × 109 M☉), moderate star formation rate (2 ± 1 M☉ yr-1), a young component age of 20 Myr, and little dust (AV < 0.2). The best-fit model has 20% of the mass in the young stellar component, but models without evolved stars are also allowed.

Spitzer Mid- to Far-Infrared Flux Densities of Distant Galaxies

We study the infrared (IR) properties of high-redshift galaxies using deep Spitzer 24, 70, and 160 μm data. Our primary interest is to improve the constraints on the total IR luminosities, L_(IR), of these galaxies. We combine the Spitzer data in the southern Extended Chandra Deep Field with a K_s-band-selected galaxy sample and photometric redshifts from the Multiwavelength Survey by Yale-Chile. We used a stacking analysis to measure the average 70 and 160 μm flux densities of 1.5 250 μJy and 1.5 250 μJy have S_(70)/S_(24) flux ratios comparable to sources with X-ray detections or red rest-frame IR colors, suggesting that warm dust possibly heated by AGNs may contribute to the high 24 μm emission. Based on the average 24-160 μm flux densities, nearly all 24 μm-selected galaxies at 1.5 6 × 10^(12) L_⊙ are quite rare, with a surface density ~30 ± 10 deg^(-2), corresponding to ~2 ± 1 × 10^(-6) Mpc^(-3) over 1.5 < z < 2.5.

A CRITICAL ASSESSMENT OF PHOTOMETRIC REDSHIFT METHODS: A CANDELS INVESTIGATION

We present results from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) photometric redshift methods investigation. In this investigation, the results from 11 participants, each using a different combination of photometric redshift code, template spectral energy distributions (SEDs), and priors, are used to examine the properties of photometric redshifts applied to deep fields with broadband multi-wavelength coverage. The photometry used includes U-band through mid-infrared filters and was derived using the TFIT method. Comparing the results, we find that there is no particular code or set of template SEDs that results in significantly better photometric redshifts compared to others. However, we find that codes producing the lowest scatter and outlier fraction utilize a training sample to optimize photometric redshifts by adding zero-point offsets, template adjusting, or adding extra smoothing errors. These results therefore stress the importance of the training procedure. We find a strong dependence of the photometric redshift accuracy on the signal-to-noise ratio of the photometry. On the other hand, we find a weak dependence of the photometric redshift scatter with redshift and galaxy color. We find that most photometric redshift codes quote redshift errors (e.g., 68% confidence intervals) that are too small compared to that expected from the spectroscopic control sample. We find that all codes show a statistically significant bias in the photometric redshifts. However, the bias is in all cases smaller than the scatter; the latter therefore dominates the errors. Finally, we find that combining results from multiple codes significantly decreases the photometric redshift scatter and outlier fraction. We discuss different ways of combining data to produce accurate photometric redshifts and error estimates.

The LABOCA survey of the Extended Chandra Deep Field-South: a photometric redshift survey of submillimetre galaxies

We derive photometric redshifts from 17-band optical to mid-infrared photometry of 78 robust radio, 24-mu m and Spitzer IRAC counterparts to 72 of the 126 submillimetre galaxies (SMGs) selected at 870 mu m by LABOCA observations in the Extended Chandra Deep Field-South (ECDF-S). We test the photometric redshifts of the SMGs against the extensive archival spectroscopy in the ECDF-S. The median photometric redshift of identified SMGs is z = 2.2 +/- 0.1, the standard deviation is sigma(z) = 0.9 and we identify 11 (similar to 15 per cent) high-redshift (z >= 3) SMGs. A statistical analysis of sources in the error circles of unidentified SMGs identifies a population of possible counterparts with a redshift distribution peaking at z = 2.5 +/- 0.2, which likely comprises similar to 60 per cent of the unidentified SMGs. This confirms that the bulk of the undetected SMGs are coeval with those detected in the radio/mid-infrared. We conclude that at most similar to 15 per cent of all the SMGs are below the flux limits of our IRAC observations and thus may lie at z greater than or similar to 3 and hence at most similar to 30 per cent of all SMGs have z greater than or similar to 3. We estimate that the full S(870 mu m) > 4mJy SMG population has a median redshift of 2.5 +/- 0.5. In contrast to previous suggestions, we find no significant correlation between submillimetre flux and redshift. The median stellar mass of the SMGs derived from spectral energy distribution fitting is (9.1 +/- 0.5) x 10(10)M(circle dot) although we caution that the uncertainty in the star formation histories results in a factor of similar to 5 uncertainty in these stellarmasses. Using a single temperature modified blackbody fit with beta = 1.5, the median characteristic dust temperature of SMGs is 37.4 +/- 1.4K. The infrared luminosity function shows that SMGs at z = 2-3 typically have higher far-infrared luminosities and luminosity density than those at z = 1-2. This is mirrored in the evolution of the star formation rate density (SFRD) for SMGs which peaks at z similar to 2. The maximum contribution of bright SMGs to the global SFRD (similar to 5 per cent for SMGs with S(870 mu m) greater than or similar to 4mJy or similar to 50 per cent extrapolated to SMGs with S(870 mu m) > 1mJy) also occurs at z similar to 2.

C II* Absorption in Damped Lyα Systems. I. Star Formation Rates in a Two-Phase Medium

We describe a technique that for the first time measures star formation rates (SFRs) in damped Lyα systems (DLAs) directly. We assume that massive stars form in DLAs and that the far-ultraviolet (FUV) radiation they emit heats the gas by the grain photoelectric mechanism. We infer the heating rate by equating it to the cooling rate measured by the strength of C II* λ1335.7 absorption. Since the heating rate is proportional to the product of the dust-to-gas ratio, the grain photoelectric heating efficiency, and the SFR per unit area, , we can deduce for DLAs in which the cooling rate and dust-to-gas ratio have been measured. We consider models in which the dust consists of carbonaceous grains and silicate grains. We present two-phase models in which the cold neutral medium (CNM) and warm neutral medium (WNM) are in pressure equilibrium. In the CNM model the line of sight passes through CNM and WNM gas, while in the WNM model the line of sight passes only through WNM gas. Since the grain photoelectric heating efficiency is at least an order of magnitude higher in the CNM than in the WNM, most of the C II* absorption arises in the CNM in the CNM model. We use the measured C II* absorption lines to derive for a sample of ≈30 DLAs in which κ has been inferred from element depletion patterns. We show that the inferred corresponds to an average over the star-forming volume of the DLA rather than to local star formation along the line of sight. We obtain the average and show that = 10-2.2 M☉ yr-1 kpc-2 for the CNM solution and = 10-1.3 M☉ yr-1 kpc-2 for the WNM solution. Interestingly, the SFR per unit area in the CNM solution is similar to that measured in the Milky Way interstellar medium.

Spectroscopic Identification of Massive Galaxies at z ~ 2.3 with Strongly Suppressed Star Formation*

We present first results of a spectroscopic survey targeting K-selected galaxies at z = 2.0-2.7 using the Gemini near-infrared spectrograph (GNIRS). We obtained near-infrared spectra with a wavelength coverage of 1.0-2.5 μm for 26 K-bright galaxies (K < 19.7) selected from the Multi-wavelength Survey by Yale-Chile (MUSYC) using photometric redshifts. We successfully derived spectroscopic redshifts for all 26 galaxies using rest-frame optical emission lines or the redshifted Balmer/4000 A break. Twenty galaxies have spectroscopic redshifts in the range 2.0 < z < 2.7, for which bright emission lines like Hα and [O III] fall in atmospheric windows. Surprisingly, we detected no emission lines for nine of these 20 galaxies. The median 2 σ upper limit on the rest-frame equivalent width of Hα for these nine galaxies is ~10 A. The stellar continuum emission of these same nine galaxies is best fitted by evolved stellar population models. The best-fit star formation rate (SFR) is zero for five out of nine galaxies and is consistent with zero within 1 σ for the remaining four. Thus, both the Hα measurements and the independent stellar continuum modeling imply that 45% of our K-selected galaxies are not forming stars intensely. This high fraction of galaxies without detected line emission and low SFRs may imply that the suppression of star formation in massive galaxies occurs at higher redshift than is predicted by current cold dark matter (CDM) galaxy formation models. However, obscured star formation may have been missed, and deep mid-infrared imaging is needed to clarify this situation.

The LABOCA Survey of the Extended Chandra Deep Field South : Clustering of submillimetre galaxies

We present a measurement of the spatial clustering of submillimetre galaxies (SMGs) at z= 1-3. Using data from the 870μm Large APEX Bolometer Camera (LABOCA) submillimetre survey of the Extended Chandra Deep Field-South, we employ a novel technique to measure the cross-correlation between SMGs and galaxies, accounting for the full probability distributions for photometric redshifts of the galaxies. From the observed projected two-point cross-correlation function we derive the linear bias and characteristic dark matter halo masses for the SMGs. We detect clustering in the cross-correlation between SMGs and galaxies at the >4σ level. Accounting for the clustering of galaxies from their autocorrelation function, we estimate an autocorrelation length for SMGs of r o = 7.7 -2.3 +1.8 h -1 Mpc assuming a power-law slope γ= 1.8, and derive a corresponding dark matter halo mass of log(M halo[h -1M ⊙]) = 12.8 -0.5 +0.3. Based on the evolution of dark matter haloes derived from simulations, we show that that the z= 0 descendants of SMGs are typically massive (~2-3L *) elliptical galaxies residing in moderate- to high-mass groups (log(M halo[h -1M ⊙]) = 13.3 -0.5 +0.3). From the observed clustering we estimate an SMG lifetime of ~100Myr, consistent with lifetimes derived from gas consumption times and star formation time-scales, although with considerable uncertainties. The clustering of SMGs at z~ 2 is consistent with measurements for optically selected quasi-stellar objects (QSOs), supporting evolutionary scenarios in which powerful starbursts and QSOs occur in the same systems. Given that SMGs reside in haloes of characteristic mass ~6 × 10 12h -1M ⊙, we demonstrate that the redshift distribution of SMGs can be described remarkably well by the combination of two effects: the cosmological growth of structure and the evolution of the molecular gas fraction in galaxies. We conclude that the powerful starbursts in SMGs likely represent a short-lived but universal phase in massive galaxy evolution, associated with the transition between cold gas-rich, star-forming galaxies and passively evolving systems.

The HETDEX pilot survey - II. The evolution of the Lyα escape fraction from the ultraviolet slope and luminosity function of 1.9 < z < 3.8 LAEs

We study the escape of Lyα photons from Lyα emitting galaxies (LAEs) and the overall galaxy population using a sample of 99 LAEs at 1.9 (3-6) × 1042 erg s–1 (0.25-0.5 L*), have a mean E(B – V) = 0.13 ± 0.01, implying an attenuation of ~70% in the UV. They show a median UV uncorrected SFR = 11 M ☉ yr–1, dust-corrected SFR = 34 M ☉ yr–1, and Lyα equivalent widths (EWs) which are consistent with normal stellar populations. We measure a median Lyα escape fraction of 29%, with a large scatter and values ranging from a few percent to 100%. The Lyα escape fraction in LAEs correlates with E(B – V) in a way that is expected if Lyα photons suffer from similar amounts of dust extinction as UV continuum photons. This result implies that a strong enhancement of the Lyα EW with dust, due to a clumpy multi-phase interstellar medium (ISM), is not a common process in LAEs at these redshifts. It also suggests that while in other galaxies Lyα can be preferentially quenched by dust due to its scattering nature, this is not the case in LAEs. We find no evolution in the average dust content and Lyα escape fraction of LAEs from z ~ 4 to 2. We see hints of a drop in the number density of LAEs from z ~ 4 to 2 in the redshift distribution and the Lyα luminosity function, although larger samples are required to confirm this. The mean Lyα escape fraction of the overall galaxy population decreases significantly from z ~ 6 to z ~ 2, in agreement with recent results. Our results point toward a scenario in which star-forming galaxies build up significant amounts of dust in their ISM between z ~ 6 and 2, reducing their Lyα escape fraction, with LAE selection preferentially detecting galaxies which have the highest escape fractions given their dust content. The fact that a large escape of Lyα photons is reached by z ~ 6 implies that better constraints on this quantity at higher redshifts might detect re-ionization in a way that is uncoupled from the effects of dust.