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Register a new userA peculiar galaxy appears at redshift 11: properties of a moderate redshift interloper
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Matthew Hayes
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Laporte et al. (2011) reported a very high redshift galaxy candidate: a lensed J-band dropout (A2667-J1). J1 has a photometric redshift of z=9.6-12, the probability density function for which permits no low or intermediate z solution. We here report new spectroscopic observations of this galaxy with VLT/XShooter, which show clear [OIII]5007AA, Ly-alpha, H-alpha, and H-beta emission and place the galaxy firmly at z=2.082. The oxygen lines contribute only ~25% to the H-band flux, and do not significantly affect the dropout selection of J1. After correcting the broadband fluxes for line emission, we identify two roughly equally plausible natures for A2667-J1: either it is young heavily reddened starburst, or a maximally old system with a very pronounced 4000AA break, upon which a minor secondary burst of star formation is superimposed. Fits show that to make a 3 sigma detection of this object in the B-band (V-band), imaging of depth AB=30.2 (29.5) would be required - despite the relatively bright NIR magnitude, we would need optical data of equivalent depth to the Hubble Ultra Deep Field to rule out the mid-z solution on purely photometric grounds. Assuming that this stellar population can be scaled to the NIR magnitudes of recent HST/WFC3 IR-selected galaxies, we conclude that infeasibly deep optical data AB~32 would be required for the same level of security. There is a population of galaxies at z~2 with continuum colours alone that mimic those of our z=7-12 candidates.
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In the optical sky, minutes-duration transients from cosmological distances are rare. Known objects that give rise to such transients include gamma-ray bursts (GRBs), the most luminous explosions in the universe that have been detected at redshift as high as z ~ 9.4. These high-redshift GRBs and their associated emission can be used to probe the star formation and reionization history in the era of cosmic dawn. Here we report a near-infrared transient with an observed duration shorter than 245 s coincident with the luminous star-forming galaxy GN-z11 at z ~ 11. The telluric absorption shown in the near-infrared spectrum indicates its origin from above the atmosphere. We can rule out the possibility of known man-made objects or moving objects in the Solar system based on the observational information and our current understanding of the properties of these objects. Since some long-duration GRBs are associated with a bright ultraviolet (UV) or optical flash, we investigate the possibility that the detected signal arose from a rest-frame UV flash associated with a long GRB from GN-z11. Despite the very low probability of being a GRB, we find that the spectrum, brightness, and duration of the transient are consistent with such an interpretation. Our result may suggest that long GRBs can be produced as early as 420 million years after the Big Bang.
From a search of a ~ 2400 square degree region covered by both the SDSS and UKIDSS databases, we have attempted to identify galaxies at z ~ 0.5 that are consistent with their being essentially unmodified examples of the luminous passive compact galaxies found at z ~ 2.5. After isolating good candidates via deeper imaging, we further refine the sample with Keck moderate-resolution spectroscopy and laser-guide-star adaptive-optics imaging. For 4 of the 5 galaxies that so far remain after passing through this sieve, we analyze plausible star-formation histories based on our spectra in order to identify galaxies that may have survived with little modification from the population formed at high redshift. We find 2 galaxies that are consistent with having formed > 95% of their mass at z > 5. We attempt to estimate masses both from our stellar population determinations and from velocity dispersions. Given the high frequency of small axial ratios, both in our small sample and among samples found at high redshifts, we tentatively suggest that some of the more extreme examples of passive compact galaxies may have prolate morphologies.
The construction of catalogues of a particular type of galaxy can be complicated by interlopers contaminating the sample. In spectroscopic galaxy surveys this can be due to the misclassification of an emission line; for example in the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX) low redshift [OII] emitters may make up a few percent of the observed Ly${alpha}$ emitter (LAE) sample. The presence of contaminants affects the measured correlation functions and power spectra. Previous attempts to deal with this using the cross-correlation function have assumed sources at a fixed redshift, or not modelled evolution within the adopted redshift bins. However, in spectroscopic surveys like HETDEX, where the contamination fraction is likely to be redshift dependent, the observed clustering of misclassified sources will appear to evolve strongly due to projection effects, even if their true clustering does not. We present a practical method for accounting for the presence of contaminants with redshift-dependent contamination fractions and projected clustering. We show using mock catalogues that our method, unlike existing approaches, yields unbiased clustering measurements from the upcoming HETDEX survey in scenarios with redshift-dependent contamination fractions within the redshift bins used. We show our method returns auto-correlation functions with systematic biases much smaller than the statistical noise for samples with at least as high as 7 per cent contamination. We also present and test a method for fitting for the redshift-dependent interloper fraction using the LAE-[OII] galaxy cross-correlation function, which gives less biased results than assuming a single interloper fraction for the whole sample.
Galaxies had their most significant impact on the Universe when they assembled their first generations of stars. Energetic photons emitted by young, massive stars in primeval galaxies ionized the intergalactic medium surrounding their host galaxies, cleared sight-lines along which the light of the young galaxies could escape, and fundamentally altered the physical state of the intergalactic gas in the Universe continuously until the present day. Observations of the Cosmic Microwave Background, and of galaxies and quasars at the highest redshifts, suggest that the Universe was reionised through a complex process that was completed about a billion years after the Big Bang, by redshift z~6. Detecting ionizing Ly-alpha photons from increasingly distant galaxies places important constraints on the timing, location and nature of the sources responsible for reionisation. Here we report the detection of Ly-a photons emitted less than 600 million years after the Big Bang. UDFy-38135539 is at a redshift z=8.5549+-0.0002, which is greater than those of the previously known most distant objects, at z=8.2 and z=6.97. We find that this single source is unlikely to provide enough photons to ionize the volume necessary for the emission line to escape, requiring a significant contribution from other, probably fainter galaxies nearby.
We analyse the Tully-Fisher relation at moderate redshift from the point of view of the underlying stellar populations, by comparing optical and NIR photometry with a phenomenological model that combines population synthesis with a simple prescription for chemical enrichment. The sample comprises 108 late-type galaxies extracted from the FORS Deep Field (FDF) and William Herschel Deep Field (WHDF) surveys at z<1 (median redshift z=0.45). A correlation is found between stellar mass and the parameters that describe the star formation history, with massive galaxies forming their populations early (zFOR~3), with star formation timescales, tau1~4Gyr; although with very efficient chemical enrichment timescales (tau2~1Gyr). In contrast, the stellar-to-dynamical mass ratio - which, in principle, would track the efficiency of feedback in the baryonic processes driving galaxy formation - does not appear to correlate with the model parameters. On the Tully-Fisher plane, no significant age segregation is found at fixed circular speed, whereas at fixed stellar-to-dynamical mass fraction, age splits the sample, with older galaxies having faster circular speeds at fixed Ms/Mdyn. Although our model does not introduce any prior constraint on dust reddening, we obtain a strong correlation between colour excess and stellar mass.
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