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Register a new userExtremely Small Sizes for Faint z~2-8 Galaxies in the Hubble Frontier Fields: A Key Input For Establishing their Volume Density and UV Emissivity
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We provide the first observational constraints on the sizes of the faintest galaxies lensed by the Hubble Frontier Fields (HFF) clusters. Ionizing radiation from faint galaxies likely drives cosmic reionization, and the HFF initiative provides a key opportunity to find such galaxies. Yet, we cannot really assess their ionizing emissivity without a robust measurement of their sizes, since this is key to quantifying both their prevalence and the faint-end slope to the UV luminosity function. Here we provide the first such size constraints with 2 new techniques. The first utilizes the fact that the detectability of highly-magnified galaxies as a function of shear is very dependent on a galaxys size. Only the most compact galaxies will remain detectable in regions of high shear (vs. a larger detectable size range for low shear), a phenomenon we carefully quantify using simulations. Remarkably, however, no correlation is found between the surface density of faint galaxies and the predicted shear, using 87 faint high-magnification mu>10 z~2-8 galaxies seen behind the first 4 HFF clusters. This can only be the case if such faint (~-15 mag) galaxies have significantly smaller sizes than luminous galaxies. We constrain their half-light radii to be <~30 mas (<160-240 pc). As a 2nd size probe, we rotate and stack 26 faint high-magnification sources along the major shear axis. Less elongation is found than even for objects with an intrinsic half-light radius of 10 mas. Together these results indicate that extremely faint z~2-8 galaxies have near point-source profiles in the HFF dataset (half-light radii conservatively <30 mas and likely 5-10 mas). These results suggest smaller completeness corrections and hence much lower volume densities for faint z~2-8 galaxies and shallower faint-end slopes than have been derived in many recent studies (by factors of ~2-3 and by dalpha>~0.1-0.3).
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We present the results of a new search for galaxies at redshift z ~ 9 in the first two Hubble Frontier Fields with completed HST WFC3/IR and ACS imaging. To ensure robust photometric redshift solutions, and to minimize incompleteness, we confine our search to objects with H_{160} < 28.6 (AB mag), consider only image regions with an rms noise sigma_{160} > 30 mag (within a 0.5-arcsec diameter aperture), and insist on detections in both H_{160} and J_{140}. The result is a survey covering an effective area (after accounting for magnification) of 10.9 sq. arcmin, which yields 12 galaxies at 8.4 < z < 9.5. Within the Abell-2744 cluster and parallel fields we confirm the three brightest objects reported by Ishigaki et al. (2014), but recover only one of the four z > 8.4 sources reported by Zheng et al. (2014). In the MACSJ0416.1-240 cluster field we report five objects, and explain why each of these eluded detection or classification as z ~ 9 galaxies in the published searches of the shallower CLASH data. Finally, we uncover four z ~ 9 galaxies from the previously unsearched MACSJ0416.1-240 parallel field. Based on the published magnification maps we find that only one of these 12 galaxies is likely boosted by more than a factor of two by gravitational lensing. Consequently we are able to perform a fairly straightforward reanalysis of the normalization of the z ~ 9 UV galaxy luminosity function as explored previously in the HUDF12 programme. We conclude that the new data strengthen the evidence for a continued smooth decline in UV luminosity density (and hence star-formation rate density) from z ~ 8 to z ~ 9, contrary to recent reports of a marked drop-off at these redshifts. This provides further support for the scenario in which early galaxy evolution is sufficiently extended to explain cosmic reionization.
We present the comprehensive analyses of faint dropout galaxies up to $zsim10$ with the first full-depth data set of Abell 2744 lensing cluster and parallel fields observed by the Hubble Frontier Fields (HFF) program. We identify $54$ dropouts at $zsim5-10$ in the HFF fields, and enlarge the size of $zsim9$ galaxy sample obtained to date. Although the number of highly magnified ($musim10$) galaxies is small due to the tiny survey volume of strong lensing, our study reaches the galaxies intrinsic luminosities comparable to the deepest-field HUDF studies. We derive UV luminosity functions with these faint dropouts, carefully evaluating the combination of observational incompleteness and lensing effects in the image plane by intensive simulations including magnification, distortion, and multiplication of images, with the evaluations of mass model dependences. Our results confirm that the faint-end slope, $alpha$, is as steep as $-2$ at $zsim6-8$, and strengthen the evidence of the rapid decrease of UV luminosity densities, $rho_mathrm{UV}$, at $z>8$ from the large $zsim9$ sample. We examine whether the rapid $rho_mathrm{UV}$ decrease trend can reconcile with the large Thomson scattering optical depth, $tau_mathrm{e}$, measured by CMB experiments allowing a large space of free parameters such as average ionizing photon escape fraction and stellar-population dependent conversion factor. No parameter set can reproduce both the rapid $rho_mathrm{UV}$ decrease and the large $tau_mathrm{e}$. It is possible that the $rho_mathrm{UV}$ decrease moderates at $zgtrsim11$, that the free parameters significantly evolve towards high-$z$, or that there exist additional sources of reionization such as X-ray binaries and faint AGNs.
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We investigate the intra-cluster light (ICL) in the 6 Hubble Frontier Field clusters at $0.3<z<0.6$. We employ a new method, which is free from any functional form of the ICL profile, and exploit the unprecedented depth of this Hubble Space Telescope imaging to map the ICLs diffuse light out to clustrocentric radii $Rsim300$kpc ($mu_{rm ICL}sim27$mag arcsec$^{-2}$). From these maps, we construct radial color and stellar mass profiles via SED fitting and find clear negative color gradients in all systems with increasing distance from the Brightest Cluster Galaxy (BCG). While this implies older/more metal rich stellar components in the inner part of the ICL, we find the ICL mostly consists of a $<2$Gyr population, and plausibly originated with $log M_*/M_odot<10$ cluster galaxies. Further, we find 10-15% of the ICLs mass at large radii ($>150$kpc) lies in a younger/bluer stellar population ($sim1$Gyr), a phenomenon not seen in local samples. We attribute this light to the higher fraction of starforming/(post-)starburst galaxies in clusters at $zsim0.5$. Ultimately, we find the ICLs total mass to be $log M_{rm *}^{rm ICL}/M_odotsim11$-12, constituting 5%-20% of the clusters total stellar mass, or about a half of the value at $zsim0$. The above implies distinct formation histories for the ICL and BCGs/other massive cluster galaxies; i.e. the ICL at this epoch is still being constructed rapidly ($sim40M_odot$yr$^{-1}$), while the BCGs have mostly completed their evolution. To be consistent with the ICL measurements of local massive clusters, such as the Virgo, our data suggest mass acquisition mainly from quiescent cluster galaxies is the principal source of ICL material in the subsequent $sim$5 Gyr of cosmic time.
The discovery of Ultra-Faint Dwarf (UFD) galaxies in the halo of the Milky Way extends the faint end of the galaxy luminosity function to a few hundred solar luminosities. This extremely low luminosity regime poses a significant challenge for the photometric characterization of these systems. We present a suite of simulations aimed at understanding how different observational choices related to the properties of a low luminosity system impact our ability to determine its true structural parameters such as half-light radius and central surface brightness. We focus on estimating half-light radii (on which mass estimates depend linearly) and find that these numbers can have up to 100% uncertainties when relatively shallow photometric surveys, such as SDSS, are used. Our simulations suggest that to recover structural parameters within 10% or better of their true values: (a) the ratio of the field-of-view to the half-light radius of the satellite must be greater than three, (b) the total number of stars, including background objects should be larger than 1000, and (c) the central to background stellar density ratio must be higher than 20. If one or more of these criteria are not met, the accuracy of the resulting structural parameters can be significantly compromised. In the context of future surveys such as LSST, the latter condition will be closely tied to our ability to remove unresolved background galaxies. Assessing the reliability of measured structural parameters will become increasingly critical as the next generation of deep wide-field surveys detects UFDs beyond the reach of current spectroscopic limits.
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