Laura S. Douglas

Lyman-break galaxies at z ~ 5 – I. First significant stellar mass assembly in galaxies that are not simply z ~ 3 LBGs at higher redshift

We determine the ensemble properties of z ∼ 5 Lyman-break galaxies (LBGs) selected as V-band dropouts to i AB < 26.3 in the Chandra Deep Field-South using their rest-frame UV-to-visible spectral energy distributions. By matching the selection and performing the same analysis that has been used for z ∼ 3 samples, we show clear differences in the ensemble properties of two samples of LBGs which are separated by 1 Gyr in look-back time. We find that z ∼ 5 LBGs are typically much younger (<100Myr) and have lower stellar masses (∼10 9 M ⊙ ) than their z ∼ 3 counterparts (which are typically ∼ few x 10 10 M ⊙ and ∼320 Myr old). The difference in mass is significant even when considering the presence of an older, underlying population in both samples. Such young and moderately massive systems dominate the luminous z ∼ 5 LBG population (≥70 per cent), whereas they comprise ≤30 per cent of LBG samples at z ∼ 3. This result, which we demonstrate is robust under all reasonable modelling assumptions, shows a clear change in the properties of the luminous LBGs between z ∼ 5 and z ∼ 3. These young and moderately massive z ∼ 5 LBGs appear to be experiencing their first (few) generations of large-scale star formation and are accumulating their first significant stellar mass. Their dominance in luminous LBG samples suggests that z ∼ 5 witnesses a period of wide-spread, recent galaxy formation. As such, z ∼ 5 LBGs are the likely progenitors of the spheroidal components of present-day massive galaxies. This is supported by their high stellar mass surface densities, and is consistent with their core phase-space densities, as well as the ages of stars in the bulge of our Galaxy and other massive systems. With implied formation redshifts of z ∼ 6-7, these luminous z ∼ 5 LBGs could have only contributed to the UV photon budget at the end of reionization. However, their high star formation rates per unit area suggest these systems host outflows or winds that enrich the intragalactic and intergalactic media with metals, as has been established for z ∼ 3 LBGs. Their estimated young ages are consistent with inefficient metal-mixing on galaxy-wide scales. Therefore these galaxies may contain a significant fraction of Population III stars as proposed for z ∼ 3 LBGs by Jiminez & Haimann.

Spectroscopy of z 5 Lyman Break Galaxies in the ESO Remote Galaxy Survey

We present the global results of a large spectroscopic survey carried out in order to identify z∼ 5 Lyman break galaxies (LBGs) across 10 widely separated ∼45 arcmin2 fields to a depth of IAB= 26.3. The redshifts of seventy 4.6 3, the typical z∼ 5 LBGs have metallicities a factor of 3 lower than those of LBGs at z∼ 3. Hubble Space Telescope imaging of a subset of the LBGs indicates that a large majority of the spectroscopically confirmed LBGs in our sample are members of multiple systems (on ∼ arcsec scales) and/or show disturbed morphology. Using local LBG analogues as a model, this multiplicity could be explained either by super-starburst regions within a larger unseen structure or by a high incidence of merging events at this epoch. The current data cannot distinguish between these two possibilities. The surface density of z∼ 5 LBGs in two of the 10 fields is considerably higher than in the rest. Both show clear spikes in their redshift distributions indicating strong three-dimensional clustering in these fields. Against an expectation of about one source per 0.1 in redshift between 4.8 < z < 5.6, one field has seven identified objects between 5.11 < z < 5.21 and the other has 17 between 4.95 < z < 5.15. Neither structure can be bound given their depth in redshift and probably extend beyond the observed fields. The three-dimensional distances between LBGs in the structures are too large for them to have triggered their starbursts through mutual gravitational interaction, and so it is likely that the short-lived LBGs represent only a small fraction of the baryons in the structures.

A limit on the number density of bright z≈ 7 galaxies

We present a survey of bright optical dropout sources in two deep, multiwavelength surveys comprising 11 widely separated fields, aimed at constraining the galaxy luminosity function at z≈ 7 for sources at 5–10 L* (z= 6). Our combined survey area is 225 arcmin2 to a depth of JAB= 24.2 (3σ) and 135 arcmin2 to J= 25.3 (4σ). We find that infrared data longwards of 2 μm are essential for classifying optical dropout sources, and in particular for identifying cool Galactic star contaminants. Our limits on the number density of high-redshift sources are consistent with current estimates of the Lyman break galaxy luminosity function at z= 6.

Photometric selection of z∼ 5 Lyman break galaxies in the ESO Remote Galaxy Survey

We describe the selection of a sample of photometrically defined Lyman break galaxies (LBGs) at z∼ 5 using the multiwavelength imaging data of the ESO (European Southern Observatory) Remote Galaxy Survey. The data are drawn from 10 widely separated fields covering a total sky area of 275 arcmin2. Starting with a simple colour (R−I > 1.3) and magnitude (I < 26.3) cut to isolate the Lyman break and then refining the sample by applying further optical and near-infrared photometric criteria we identify a sample of 253 LBG candidates. We carefully model the completeness of this sample and the factors that affect its reliability. There is considerable overlap between this sample and a spectroscopically confirmed sample drawn from the same survey and this allows us to determine the reliability of the optical photometric selection (∼60 per cent) and to show that the reliability can be significantly improved (to ∼80 per cent) by applying near-infrared waveband criteria to exclude very red contaminants. Even this high level of reliability may compromise some statistical studies of LBG properties. We show that over 30 per cent of the highest reliability candidates have multiple ultraviolet (UV) luminous components and/or disturbed morphology in Hubble Space Telescope imaging, though it is unclear whether this represents multiple interacting/merging sources or individual large sources with multiple UV bright regions. Using this sample we confirm that the normalization of the bright end of the z= 5 UV luminosity function (down to M*) is lower than the same at z= 4 by a factor of 3. Using a Schechter fit we determine M*UV=−20.9 ± 0.2. We discuss whether it is reasonable to expect the UV luminosity function to follow a Schechter function, given the UV emission is short lived and stochastic, and does not necessarily trace the underlying mass of the galaxy.

Discovery of a single faint AGN in a large sample of z > 5 Lyman break galaxies

As part of a large spectroscopic survey of z > 5 Lyman break galaxies (LBGs), we have identified a single source which is clearly hosting an active galactic nucleus (AGN). Out of a sample of more than 50 spectroscopically confirmed R-band dropout galaxies at z∼ 5 and above, only J104048.6−115550.2 at z= 5.44 shows evidence for a high ionization potential emission line indicating the presence of a hard ionizing continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Lyα line. Uniquely within this sample of LBGs, emission from N V is also detected, a clear signature of AGN photoionization. The object is spatially resolved in Hubble Space Telescope (HST) imaging. This, and the comparatively high Lyα/N V flux ratio indicates that the majority of the Lyα (and the UV continuum longward of it) originates from stellar photoionization, a product of the ongoing starburst in the LBG. Even without the AGN emission, this object would have been photometrically selected and spectroscopically confirmed as a Lyman break in our survey. The measured optical flux (IAB= 26.1) is therefore an upper limit to that from the AGN and is of order 100 times fainter than the majority of known quasars at these redshifts. The detection of a single object in our survey volume is consistent with the best current models of high redshift AGN luminosity function, providing a substantial fraction of such AGN is found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.

Gemini H-band imaging of the field of a z = 10 candidate

We present a deep H-band image of the field of a candidate z = 10 galaxy magnified by the foreground (z = 0.25) cluster A1835. The image was obtained with NIRI on Gemini-North to better constrain the photometry and investigate the morphology of the source. The image is approximately 1 mag deeper and has better spatial resolution (seeing was 04-05) than the existing H-band image obtained with ISAAC on the Very Large Telescope by Pello et al. The object is not detected in our new data. Given the published photometry (HAB = 25.0), we would have expected it to have been detected at more than ~7 σ in a 14 diameter aperture. We obtain a limit of HAB > 26.0 (3 σ) for the object. A major part of the evidence that this object is at z = 10 was the presence of a strong continuum break between the J and H bands, attributed to absorption of all continuum shortward of 1216 A in the rest frame of the object. Our H-band nondetection substantially reduces the magnitude of any break and therefore weakens the case that this object is at z = 10. Without a clear continuum break, the identification of an emission line at 1.33745 μm as Lyα at z ≈ 10 is less likely. We show that the width and flux of this line are consistent with an alternative emission line such as [O III] λ5007 from an intermediate-redshift H II/dwarf galaxy.

Large-Scale Structure Traced by Molecular Gas at High Redshift

We present observations of redshifted CO(1-0) and CO(2-1) in a field containing an overdensity of Lyman break galaxies (LBGs) at . Our Australia Telescope Compact Array observations were centered between two spectroscopically confirmed galaxies. We place upper limits on the molecular gas masses in these two galaxies of M(H2) < 1.7 × 1010 and <2.9 × 109 M (2 σ), comparable to their stellar masses. We detect an optically faint line emitter situated between the two LBGs which we identify as warm molecular gas at . This source, detected in the CO(2-1) transition but undetected in CO(1-0), has an integrated line flux of Jy km s−1 , yielding an inferred gas mass M(H2) = M . Molecular line emitters without detectable counterparts at optical and infrared wavelengths may be crucial tracers of structure and mass at high redshift.

Intense starbursts at z~5: first significant stellar mass assembly in the progenitors of present-day spheroids

High redshift galaxies play a key role in our developing understanding of galaxy formation and evolution. Since such galaxies are being studied within a Gyr of the big bang, they provide a unique probe of the physics of one of the first generations of large-scale star-formation. We have performed a complete statistical study of the physical properties of a robust sample of z~5 UV luminous galaxies selected using the Lyman-break technique. The characteristic properties of this sample differ from LBGs at z~3 of comparable luminosity in that they are a factor of ten less massive (~few×10 9 M ⊙ ) and the majority (~70%) are considerably younger ( ⊙ yr −1 kpc −2 ), suggesting they drive outflows and winds that enrich the intra- and inter-galactic media with metals. These properties imply that the majority of z~5 LBGs are in formation meaning that most of their star-formation has likely occurred during the last few crossing times. They are experiencing their first (few) generations of large-scale star formation and are accumulating their first significant stellar mass. As such, z~5 LBGs are the likely progenitors of the spheroidal components of present-day massive galaxies (supported by their high stellar mass surface densities and their core phase-space densities).