Orbitally-driven Peierls state in spinels

We consider the superstructures, which can be formed in spinels containing on B-sites the transition-metal ions with partially filled t2g levels. We show that, when such systems are close to itinerant state (e.g. have an insulator-metal transition), there may appear in them an orbitally-driven Peierls state. We explain by this mechanism the very unusual superstructures observed in CuIr2S4 (octamers) and MgTi2O4 (chiral superstructures) and suggest that similar phenomenon should be observed in NaTiO2 and possibly in some other systems.Comment: 4 pages, 3 figure

The Dearth of z~10 Galaxies in all HST Legacy Fields -- The Rapid Evolution of the Galaxy Population in the First 500 Myr

We present an analysis of all prime HST legacy fields spanning >800 arcmin^2 for the search of z~10 galaxy candidates and the study of their UV luminosity function (LF). In particular, we present new z~10 candidates selected from the full Hubble Frontier Field (HFF) dataset. Despite the addition of these new fields, we find a low abundance of z~10 candidates with only 9 reliable sources identified in all prime HST datasets that include the HUDF09/12, the HUDF/XDF, all the CANDELS fields, and now the HFF survey. Based on this comprehensive search, we find that the UV luminosity function decreases by one order of magnitude from z~8 to z~10 at all luminosities over a four magnitude range. This also implies a decrease of the cosmic star-formation rate density by an order of magnitude within 170 Myr from z~8 to z~10. We show that this accelerated evolution compared to lower redshift can entirely be explained by the fast build-up of the dark matter halo mass function at z>8. Consequently, the predicted UV LFs from several models of galaxy formation are in good agreement with this observed trend, even though the measured UV LF lies at the low end of model predictions. In particular, the number of only 9 observed candidate galaxies is lower, by ~50%, than predicted by galaxy evolution models. The difference is generally still consistent within the Poisson and cosmic variance uncertainties. However, essentially all models predict larger numbers than observed. We discuss the implications of these results in light of the upcoming James Webb Space Telescope mission, which is poised to find much larger samples of z~10 galaxies as well as their progenitors at less than 400 Myr after the Big Bang.Comment: 13 pages, 6 figures, minor updates to match accepted versio

The size-star formation relation of massive galaxies at 1.5<z<2.5

We study the relation between size and star formation activity in a complete sample of 225 massive (M > 5 x 10^10 Msun) galaxies at 1.5<z<2.5, selected from the FIREWORKS UV-IR catalog of the CDFS. Based on stellar population synthesis model fits to the observed restframe UV-NIR SEDs, and independent MIPS 24 micron observations, 65% of galaxies are actively forming stars, while 35% are quiescent. Using sizes derived from 2D surface brightness profile fits to high resolution (FWHM_{PSF}~0.45 arcsec) groundbased ISAAC data, we confirm and improve the significance of the relation between star formation activity and compactness found in previous studies, using a large, complete mass-limited sample. At z~2, massive quiescent galaxies are significantly smaller than massive star forming galaxies, and a median factor of 0.34+/-0.02 smaller than galaxies of similar mass in the local universe. 13% of the quiescent galaxies are unresolved in the ISAAC data, corresponding to sizes <1 kpc, more than 5 times smaller than galaxies of similar mass locally. The quiescent galaxies span a Kormendy relation which, compared to the relation for local early types, is shifted to smaller sizes and brighter surface brightnesses and is incompatible with passive evolution. The progenitors of the quiescent galaxies, were likely dominated by highly concentrated, intense nuclear star bursts at z~3-4, in contrast to star forming galaxies at z~2 which are extended and dominated by distributed star formation.Comment: 6 pages, 4 figures, accepted for publication in Ap

Newly Discovered Bright z~9-10 Galaxies and Improved Constraints on Their Prevalence Using the Full CANDELS Area

We report the results of an expanded search for z~9-10 candidates over the ~883 arcmin^2 CANDELS+ERS fields. This study adds 147 arcmin^2 to the search area we consider over the CANDELS COSMOS, UDS, and EGS fields, while expanding our selection to include sources with bluer J_{125}-H_{160} colors than our previous J_{125}-H_{160}>0.5 mag selection. In searching for new z~9-10 candidates, we make full use of all available HST, Spitzer/IRAC, and ground-based imaging data. As a result of our expanded search and use of broader color criteria, 3 new candidate z~9-10 galaxies are identified. We also find again the z=8.683 source previously confirmed by Zitrin+2015. This brings our sample of probable z~9-11 galaxy candidates over the CANDELS+ERS fields to 19 sources in total, equivalent to 1 candidate per 47 arcmin^2 (1 per 10 WFC3/IR fields). To be comprehensive, we also discuss 28 mostly lower likelihood z~9-10 candidates, including some sources that seem to be reliably at z>8 using the HST+IRAC data alone, but which the ground-based data show are much more likely at z<4. One case example is a bright z~9.4 candidate COS910-8 which seems instead to be at z~2. Based on this expanded sample, we obtain a more robust LF at z~9 and improved constraints on the volume density of bright z~9 and z~10 galaxies. Our improved z~9-10 results again reinforce previous findings for strong evolution in the UV LF at z>8, with a factor of ~10 evolution seen in the luminosity density from z~10 to z~8.Comment: 22 pages, 12 figures, 6 tables, accepted for publication in the Astrophysical Journa

First Frontier Field Constraints on the Cosmic Star-Formation Rate Density at z~10 - The Impact of Lensing Shear on Completeness of High-Redshift Galaxy Samples

We search the complete Hubble Frontier Field dataset of Abell 2744 and its parallel field for z~10 sources to further refine the evolution of the cosmic star-formation rate density (SFRD) at z>8. We independently confirm two images of the recently discovered triply-imaged z~9.8 source by Zitrin et al. (2014) and set an upper limit for similar z~10 galaxies with red colors of J_125-H_160>1.2 in the parallel field of Abell 2744. We utilize extensive simulations to derive the effective selection volume of Lyman-break galaxies at z~10, both in the lensed cluster field and in the adjacent parallel field. Particular care is taken to include position-dependent lensing shear to accurately account for the expected sizes and morphologies of highly-magnified sources. We show that both source blending and shear reduce the completeness at a given observed magnitude in the cluster, particularly near the critical curves. These effects have a significant, but largely overlooked, impact on the detectability of high-redshift sources behind clusters, and substantially reduce the expected number of highly-magnified sources. The detections and limits from both pointings result in a SFRD which is higher by 0.4+-0.4 dex than previous estimates at z~10 from blank fields. Nevertheless, the combination of these new results with all other estimates remain consistent with a rapidly declining SFRD in the 170 Myr from z~8 to z~10 as predicted by cosmological simulations and dark-matter halo evolution in LambdaCDM. Once biases introduced by magnification-dependent completeness are accounted for, the full six cluster and parallel Frontier Field program will be an extremely powerful new dataset to probe the evolution of the galaxy population at z>8 before the advent of the JWST.Comment: 10 pages, 7 figures, changed to match accepted version to appear in Ap

Rest-Frame Optical Emission Lines in z~3.5 Lyman Break selected Galaxies: The Ubiquity of Unusually High [OIII]/Hbeta Ratios at 2 Gyr

We present K-band spectra of rest-frame optical emission lines for 24 star-forming galaxies at z~3.2-3.7 using MOSFIRE on the Keck 1 telescope. Strong rest-frame optical [O III] and Hbeta emission lines were detected in 18 LBGs. The median flux ratio of [O III]5007 to Hbeta is 5.1+/-0.5, a factor of 5-10x higher than in local galaxies with similar stellar masses. The observed Hbeta luminosities are in good agreement with expectations from the estimated star-formation rates, and none of our sources are detected in deep X-ray stacks, ruling out significant contamination by active galactic nuclei. Combining our sample with a variety of LBGs from the literature, including 49 galaxies selected in a very similar manner, we find a high median ratio of [OIII]/Hbeta = 4.8+0.8-1.7. This high ratio seems to be an ubiquitous feature of z~3-4 LBGs, very different from typical local star-forming galaxies at similar stellar masses. The only comparable systems at z~0 are those with similarly high specific star-formation rates, though ~5x lower stellar masses. High specific star-formation rates either result in a much higher ionization parameter or other unusual conditions for the interstellar medium, which result in a much higher [OIII]/Hbeta line ratio. This implies a strong relation between a global property of a galaxy, the specific star-formation rate, and the local conditions of ISM in star-forming regions.Comment: 14 pages, 8 figures, 5 color, published in ApJ, updated to reflect published versio

Probing the Dawn of Galaxies at z~9-12: New Constraints from HUDF12/XDF and CANDELS Data

We present a comprehensive analysis of z>8 galaxies based on ultra-deep WFC3/IR data. We constrain the evolution of the UV luminosity function (LF) and luminosity densities from z~11 to z~8 by exploiting all the WFC3/IR data over the Hubble Ultra-Deep Field from the HUDF09 and the new HUDF12 program, in addition to the HUDF09 parallel field data, as well as wider area WFC3/IR imaging over GOODS-South. Galaxies are selected based on the Lyman Break Technique in three samples centered around z~9, z~10 and z~11, with seven z~9 galaxy candidates, and one each at z~10 and z~11. We confirm a new z~10 candidate (with z=9.8+-0.6) that was not convincingly identified in our first z~10 sample. The deeper data over the HUDF confirms all our previous z>~7.5 candidates as genuine high-redshift candidates, and extends our samples to higher redshift and fainter limits (H_160~29.8 mag). We perform one of the first estimates of the z~9 UV LF and improve our previous constraints at z~10. Extrapolating the lower redshift UV LF evolution should have revealed 17 z~9 and 9 z~10 sources, i.e., a factor ~3x and 9x larger than observed. The inferred star-formation rate density (SFRD) in galaxies above 0.7 M_sun/yr decreases by 0.6+-0.2 dex from z~8 to z~9, in good agreement with previous estimates. The low number of sources found at z>8 is consistent with a very rapid build-up of galaxies across z~10 to z~8. From a combination of all current measurements, we find a best estimate of a factor 10x decrease in the SFRD from z~8 to z~10, following (1+z)^(-11.4+-3.1). Our measurements thus confirm our previous finding of an accelerated evolution beyond z~8, and signify a rapid build-up of galaxies with M_UV<-17.7 within only ~200 Myr from z~10 to z~8, in the heart of cosmic reionization.Comment: 21 pages, 13 figures, 6 tables; submitted to Ap

A Spectroscopic Redshift Measurement for a Luminous Lyman Break Galaxy at z=7.730 using Keck/MOSFIRE

We present a spectroscopic redshift measurement of a very bright Lyman break galaxy at z=7.7302+-0.0006 using Keck/MOSFIRE. The source was pre-selected photometrically in the EGS field as a robust z~8 candidate with H=25.0 mag based on optical non-detections and a very red Spitzer/IRAC [3.6]-[4.5] broad-band color driven by high equivalent width [OIII]+Hbeta line emission. The Lyalpha line is reliably detected at 6.1 sigma and shows an asymmetric profile as expected for a galaxy embedded in a relatively neutral inter-galactic medium near the Planck peak of cosmic reionization. The line has a rest-frame equivalent width of EW0=21+-4 A and is extended with V_FWHM=360+90-70 km/s. The source is perhaps the brightest and most massive z~8 Lyman break galaxy in the full CANDELS and BoRG/HIPPIES surveys, having assembled already 10^(9.9+-0.2) M_sol of stars at only 650 Myr after the Big Bang. The spectroscopic redshift measurement sets a new redshift record for galaxies. This enables reliable constraints on the stellar mass, star-formation rate, formation epoch, as well as combined [OIII]+Hbeta line equivalent widths. The redshift confirms that the IRAC [4.5] photometry is very likely dominated by line emission with EW0(OIII+Hbeta)= 720-150+180 A. This detection thus adds to the evidence that extreme rest-frame optical emission lines are a ubiquitous feature of early galaxies promising very efficient spectroscopic follow-up in the future with infrared spectroscopy using JWST and, later, ELTs.Comment: 6 pages, 4 figures, small updates to match ApJL accepted versio