High-redshift galaxies and low-mass stars

The sensitivity available to near-infrared surveys has recently allowed us to probe the galaxy population at z ≈ 7 and beyond. The existing Hubble Wide Field Camera 3 (WFC3) and Visible and Infrared Survey Telescope for Astronomy (VISTA) Infrared Camera (VIRCam) instruments allow deep surveys to be undertaken well beyond 1 μm – a capability that will be further extended with the launch and commissioning of the James Webb Space Telescope (JWST). As new regions of parameter space in both colour and depth are probed, new challenges for distant galaxy surveys are identified. In this paper, we present an analysis of the colours of L- and T-dwarf stars in widely used photometric systems. We also consider the implications of the newly identified Y-dwarf population – stars that are still cooler and less massive than T-dwarfs for both the photometric selection and spectroscopic follow-up of faint and distant galaxies. We highlight the dangers of working in the low-signal-to-noise regime, and the potential contamination of existing and future samples. We find that Hubble/WFC3 and VISTA/VIRCam Y-drop selections targeting galaxies at z ∼ 7.5 are vulnerable to contamination from T- and Y-class stars. Future observations using JWST, targeting the z ∼ 7 galaxy population, are also likely to prove difficult without deep medium-band observations. We demonstrate that single emission line detections in typical low-signal-to-noise spectroscopic observations may also be suspect, due to the unusual spectral characteristics of the cool dwarf star population

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 SEDs. By matching the selection and performing the same analysis that has been used for z~3 samples, we show clear differences in the properties of two samples of LBGs which are separated by ~1Gyr in lookback time. We find that z~5 LBGs are typically much younger (<100Myr) and have lower stellar masses (10^9Msol) than their z~3 counterparts. 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%), whereas they comprise <30% of LBG samples at z~3. This result is robust under all reasonable modelling assumptions. These intense starbursts 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, their core phase-space densities, as well as the ages of stars in the bulge of our Galaxy and other massive systems. Their high star formation rates per unit area suggest that these systems host outflows or winds that enrich the intra- and inter-galactic media with metals. Their estimated young ages are consistent with inefficient metal-mixing on galaxy-wide scales. Therefore these galaxies may contain a significant fraction of metal-free stars as has been proposed for z~3 LBGs (Jimenez & Haiman 2006). [Abridged]Comment: Accepted for publication in MNRAS. 21 pages, 9 postscript figures. For a PDF file with high resolution figures, see http://www-astro.physics.ox.ac.uk/~averma

Early Quenching of Massive Protocluster Galaxies Around <i>z</i>=2.2 Radio Galaxies

Radio galaxies are among the most massive galaxies in the high-redshift Universe and are known to often lie in protocluster environments. We have studied the fields of seven z = 2.2 radio galaxies with High Acuity Wide field K-band Imager (HAWK-I) narrow-band and broad-band imaging in order to map out their environment using Hα emitters (HAEs). The results are compared to the blank field HAE survey HiZELS. All of the radio galaxy fields are overdense in HAEs relative to a typical HiZELS field of the same area and four of the seven are richer than all except one of 65 essentially random HiZELS subfields of the same size. The star formation rates of the massive HAEs are lower than those necessary to have formed their stellar population in the preceding Gyr – indicating that these galaxies are likely to have formed the bulk of their stars at higher redshifts, and are starting to quench

Luminosity functions of cluster galaxies. The near-ultraviolet luminosity function at < z > ∼ 0.05

We derive NUV luminosity functions for 6471 NUV detected galaxies in 28 0.02 < z < 0.08 clusters and consider their dependence on cluster properties. We consider optically red and blue galaxies and explore how their NUV LFs vary in several cluster subsamples, selected to best show the influence of environment. Our composite LF is well fit by the Schechter form with M*(NUV) = -18.98 +/- 0.07 and alpha = -1.87 +/- 0.03 in good agreement with values for the Coma centre and the Shapley supercluster, but with a steeper slope and brighter L* than in Virgo. The steep slope is due to the contribution of massive quiescent galaxies that are faint in the NUV. There are significant differences in the NUV LFs for clusters having low and high X-ray luminosities and for sparse and dense clusters, though none are particularly well fitted by the Schechter form, making a physical interpretation of the parameters difficult. When splitting clusters into two subsamples by X-ray luminosity, the ratio of low to high NUV luminosity galaxies is higher in the high X-ray luminosity subsample (i.e., the luminosity function is steeper across the sampled luminosity range). In subsamples split by surface density, when characterised by Schechter functions the dense clusters have an M* about a magnitude fainter than that of the sparse clusters and alpha is steeper (-1.9 vs. -1.6, respectively). The differences in the data appear to be driven by changes in the LF of blue (star-forming) galaxies. This appears to be related to interactions with the cluster gas. For the blue galaxies alone, the luminosity distributions indicate that for high L-X and high velocity dispersion cluster subsamples (i.e., the higher mass clusters), there are relatively fewer high UV luminosity galaxies (or correspondingly a relative excess of low UV luminosity galaxies) in comparison the lower mass cluster subsamples

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 AGN. Out of a sample of more than fifty 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 ionisation-potential emission-line indicating the presence of a hard ionising continuum from an AGN. Like most objects in our sample the rest-frame-UV spectrum shows the UV continuum breaking across a Ly_alpha line. Uniquely within this sample of LBGs, emission from NV is also detected, a clear signature of AGN photo-ionisation. The object is spatially resolved in HST imaging. This, and the comparatively high Ly_alpha/NV flux ratio indicates that the majority of the Ly_alpha (and the UV continuum longward of it) originates from stellar photo-ionisation, a product of the ongoing starburst in the Lyman break galaxy. 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 (I(AB)=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 are found within luminous starbursting galaxies. We discuss the cosmological implications of this discovery.Comment: 7 pages, 3 figures, MNRAS in pres

Morphological evolution of cluster red sequence galaxies in the past 9 Gyr

Galaxies arrive on the red sequences of clusters at high redshift (z > 1) once their star formation is quenched and evolve passively thereafter. However, we have previously found that cluster red sequence galaxies (CRSGs) undergo significant morphological evolution subsequent to the cessation of star formation, at some point in the past 9-10 Gyr. Through a detailed study of a large sample of cluster red sequence galaxies spanning 0.2 < z < 1.4 we elucidate the details of this evolution. Below z similar to 0.5-0.6 (in the last 5-6 Gyr) there is little or no morphological evolution in the population as a whole, unlike in the previous 4-5 Gyr. Over this earlier time (i) disc-like systems with Sersic n < 2 progressively disappear, as (ii) the range of their axial ratios similarly decreases, removing the most elongated systems (those consistent with thin discs seen at an appreciable inclination angle) and (iii) radial colour gradients (bluer outwards) decrease in an absolute sense from significant age-related gradients to a residual level consistent with the metallicity-induced gradients seen in low-redshift cluster members. The distribution of their effective radii shows some evidence of evolution, consistent with growth of at most a factor < 1.5 between z similar to 1.4 and similar to 0.5, significantly less than for comparable field galaxies, while the distribution of their central (<1 kpc) bulge surface densities shows no evolution at least at z < 1. A simple model involving the fading and thickening of a disc component after comparatively recent quenching (after z similar to 1.5) around an otherwise passively evolving older spheroid component is consistent with all of these findings

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 z=5.12. Our Australia Telescope Compact Array observations were centered between two spectroscopically-confirmed z=5.12 galaxies. We place upper limits on the molecular gas masses in these two galaxies of M(H_2) <1.7 x 10^10 M_sun and <2.9 x 10^9 M_sun (2 sigma), 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 z=5.1245 +/- 0.0001. This source, detected in the CO(2-1) transition but undetected in CO(1-0), has an integrated line flux of 0.106 +/- 0.012 Jy km/s, yielding an inferred gas mass M(H_2)=(1.9 +/- 0.2) x 10^10 M_sun. Molecular line emitters without detectable counterparts at optical and infrared wavelengths may be crucial tracers of structure and mass at high redshift.Comment: 4 pages, accepted for publication in ApJ Letter

Environmental effects on the UV upturn in local clusters of galaxies

We explore the dependence of ultraviolet (UV) upturn colours in early-type cluster galaxies on the properties of their parent clusters (such as velocity dispersion and X-ray luminosity) and on the positions and kinematics of galaxies within them. We use a sample of 24 nearby clusters with highly complete spectroscopy and optical/infrared data to select a suitable sample of red-sequence galaxies, whose far-ultraviolet and NUV magnitudes we measure from archival GALEX data. Our results show that the UV upturn colour has no dependence on cluster properties and has the same range in all clusters. There is also no dependence on the projected position within clusters or on line-of-sight velocity. Therefore, our conclusion is that the UV upturn phenomenon is an intrinsic feature of cluster early-type galaxies, irrespective of their cluster environment

Evolution of the Ultraviolet Upturn at 0.3 < z < 1: Exploring Helium-rich Stellar Populations

We measure the near-UV (rest-frame similar to 2400 angstrom) to optical color for early-type galaxies in 12 clusters at 0.3 = 3, resembling globular clusters with multiple stellar populations in our Galaxy. This suggests that elliptical galaxies and globular clusters share similar chemical evolution and star formation histories. The vast majority of the stellar mass in these galaxies also must have been in place at z > 3

The XXL Survey XI: ATCA 2.1 GHz continuum observations

We present 2.1 GHz imaging with the Australia Telescope Compact Array (ATCA) of a 6.5 deg^2 region within the XXM-Newton XXL South field using a band of 1.1-3.1 GHz. We achieve an angular resolution of 4.7" x 4.2" in the final radio continuum map with a median rms noise level of 50 uJy/beam. We identify 1389 radio sources in the field with peak S/N >=5 and present the catalogue of observed parameters. We find that 305 sources are resolved, of which 77 consist of multiple radio components. These number counts are in agreement with those found for the COSMOS-VLA 1.4 GHz survey. We derive spectral indices by a comparison with the Sydney University Molongolo Sky Survey (SUMSS) 843MHz data. We find an average spectral index of -0.78 and a scatter of 0.28, in line with expectations. This pilot survey was conducted in preparation for a larger ATCA program to observe the full 25 deg^2 southern XXL field. When complete, the survey will provide a unique resource of sensitive, wide-field radio continuum imaging with complementary X-ray data in the field. This will facilitate studies of the physical mechanisms of radio-loud and radio-quiet AGNs and galaxy clusters, and the role they play in galaxy evolution. The source catalogue is publicly available online via the XXL Master Catalogue browser and the Centre de Donn\'ees astronomiques de Strasbourg (CDS).Comment: 7 pages, 6 figures. Accepted by A&A 13th October 201