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Register a new userUV & U-band luminosity functions from CLAUDS and HSC-SSP -- I. Using four million galaxies to simultaneously constrain the very faint and bright regimes to $z sim 3$
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We constrain the rest-frame FUV (1546AA), NUV (2345AA) and U-band (3690AA) luminosity functions (LFs) and luminosity densities (LDs) with unprecedented precision from $zsim0.2$ to $zsim3$ (FUV, NUV) and $zsim2$ (U-band). Our sample of over 4.3 million galaxies, selected from the CFHT Large Area $U$-band Deep Survey (CLAUDS) and HyperSuprime-Cam Subaru Strategic Program (HSC-SSP) data lets us probe the very faint regime (down to $M_mathrm{FUV},M_mathrm{NUV},M_mathrm{U} simeq -15$ at low redshift) while simultaneously detecting very rare galaxies at the bright end down to comoving densities $<10^{-5}$ Mpc$^{-3}$. Our FUV and NUV LFs are well fitted by single Schechter functions, with faint-end slopes that are very stable up to $zsim2$. We confirm, but self-consistently and with much better precision than previous studies, that the LDs at all three wavelengths increase rapidly with lookback time to $zsim1$, and then much more slowly at $1<z<2$--$3$. Evolution of the FUV and NUV LFs and LDs at $z<1$ is driven almost entirely by the fading of the characteristic magnitude, $M^star_{UV}$, while at $z>1$ it is due to the evolution of both $M^star_{UV}$ and the characteristic number density $phi^star_{UV}$. In contrast, the U-band LF has an excess of faint galaxies and is fitted with a double-Schechter form; $M^star_mathrm{U}$, both $phi^star_mathrm{U}$ components, and the bright-end slope evolve throughout $0.2<z<2$, while the faint-end slope is constant over at least the measurable $0.05<z<0.6$. We present tables of our Schechter parameters and LD measurements that can be used for testing theoretical galaxy evolution models and forecasting future observations.
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We study the UV luminosity functions (LFs) at $zsim 4$, $5$, $6,$ and $7$ based on the deep large-area optical images taken by the Hyper Suprime-Cam (HSC) Subaru strategic program (SSP). On the 100 deg$^2$ sky of the HSC SSP data available to date, we make enormous samples consisting of a total of 579,565 dropout candidates at $zsim 4-7$ by the standard color selection technique, 358 out of which are spectroscopically confirmed by our follow-up spectroscopy and other studies. We obtain UV LFs at $z sim 4-7$ that span a very wide UV luminosity range of $sim 0.002 - 100 , L_{rm UV}^ast$ ($-26 < M_{rm UV} < -14$ mag) by combining LFs from our program and the ultra-deep Hubble Space Telescope legacy surveys. We derive three parameters of the best-fit Schechter function, $phi^ast$, $M_{rm UV}^ast$, and $alpha$, of the UV LFs in the magnitude range where the AGN contribution is negligible, and find that $alpha$ and $phi^ast$ decrease from $zsim 4$ to $7$ with no significant evolution of $M_{rm UV}^ast$. Because our HSC SSP data bridge the LFs of galaxies and AGNs with great statistical accuracy, we carefully investigate the bright end of the galaxy UV LFs that are estimated by the subtraction of the AGN contribution either aided with spectroscopy or the best-fit AGN UV LFs. We find that the bright end of the galaxy UV LFs cannot be explained by the Schechter function fits at $> 2 sigma$ significance, and require either double power-law functions or modified Schechter functions that consider a magnification bias due to gravitational lensing.
We present Ly$alpha$ and ultraviolet-continuum (UV) luminosity functions (LFs) of galaxies and active galactic nuclei (AGN) at $z=2.0-3.5$ determined by the un-targetted optical spectroscopic survey of the Hobby-Eberly Telescope Dark Energy Experiment (HETDEX). We combine deep Subaru imaging with HETDEX spectra resulting in $11.4$ deg$^2$ of fiber-spectra sky coverage, obtaining $18320$ galaxies spectroscopically identified with Ly$alpha$ emission, $2126$ of which host type 1 AGN showing broad (FWHM$~>1000$ km s$^{-1}$) Ly$alpha$ emission lines. We derive the Ly$alpha$ (UV) LF over 2 orders of magnitude covering bright galaxies and AGN in $log L_mathrm{Lyalpha}/mathrm{[erg~s^{-1}]}=43.3-45.5$ ($-27<M_mathrm{UV}<-20$) by the $1/V_mathrm{max}$ estimator. Our results reveal the bright-end hump of the Ly$alpha$ LF is composed of type 1 AGN. In conjunction with previous spectroscopic results at the faint end, we measure a slope of the best-fit Schechter function to be $alpha_mathrm{Sch}=-1.70^{+0.13}_{-0.14}$, which indicates $alpha_mathrm{Sch}$ steepens from $z=2-3$ towards high redshift. Our UV LF agrees well with previous AGN UV LFs, and extends to faint-AGN and bright-galaxy regimes. The number fraction of Ly$alpha$-emitting objects ($X_mathrm{LAE}$) increases from $M_mathrm{UV}^*sim-21$ to bright magnitude due to the contribution of type 1 AGN, while previous studies claim that $X_mathrm{Lyalpha}$ decreases from faint magnitude to $M_mathrm{UV}^*$, suggesting a valley in the $X_mathrm{Lyalpha}-$magnitude relation at $M_mathrm{UV}^*$. Comparing our UV LF of type 1 AGN at $z=2-3$ with those at $z=0$, we find that the number density of faint ($M_mathrm{UV}>-21$) type 1 AGN increases from $zsim2$ to $zsim0$ as opposed to the evolution of bright ($M_mathrm{UV}<-21$) type 1 AGN, suggesting the AGN downsizing in the rest-frame UV luminosity.
The deep, wide-area (~800-900 arcmin**2) near-infrared/WFC3/IR + Spitzer/IRAC observations over the CANDELS fields have been a remarkable resource for constraining the bright end of high redshift UV luminosity functions (LFs). However, the lack of HST 1.05-micron observations over the CANDELS fields has made it difficult to identify z~9-10 sources robustly, since such data are needed to confirm the presence of an abrupt Lyman break at 1.2 microns. We report here on the successful identification of many such z~9-10 sources from a new HST program (z9-CANDELS) that targets the highest-probability z~9-10 galaxy candidates with observations at 1.05 microns, to search for a robust Lyman-break at 1.2 microns. The potential z~9-10 candidates are preselected from the full HST, Spitzer/IRAC S-CANDELS observations, and the deepest-available ground-based optical+near-infrared observations. We identified 15 credible z~9-10 galaxies over the CANDELS fields. Nine of these galaxies lie at z~9 and 5 are new identifications. Our targeted follow-up strategy has proven to be very efficient in making use of scarce HST time to secure a reliable sample of z~9-10 galaxies. Through extensive simulations, we replicate the selection process for our sample (both the preselection and follow-up) and use it to improve current estimates for the volume density of bright z~9 and z~10 galaxies. The volume densities we find are 5(-2)(+3)x and 8(-3)(+9)x lower, respectively, than found at z~8. When compared with the best-fit evolution (i.e., dlog_{10} rho(UV)/dz=-0.29+/-0.02) in the UV luminosities densities from z~8 to z~4 integrated to 0.3L*(z=3) (-20 mag), these luminosity densities are 2.6(-0.9)(+1.5)x and 2.2(-1.1)(+2.0)x lower, respectively, than the extrapolated trends. Our new results are broadly consistent with the accelerated evolution scenario at z>8, as seen in many theoretical models.
We estimate the evolution of the galaxy-galaxy merger fraction for $M_star>10^{10.5}M_odot$ galaxies over $0.25<z<1$ in the $sim$18.6 deg$^2$ deep CLAUDS+HSC-SSP surveys. We do this by training a Random Forest Classifier to identify merger candidates from a host of parametric morphological features, and then visually follow-up likely merger candidates to reach a high-purity, high-completeness merger sample. Correcting for redshift-dependent detection bias, we find that the merger fraction at $z=0$ is 1.0$pm$0.2%, that the merger fraction evolves as $(1+z)^{2.3 pm 0.4}$, and that a typical massive galaxy has undergone $sim$0.3 major mergers since $z=1$. This pilot study illustrates the power of very deep ground-based imaging surveys combined with machine learning to detect and study mergers through the presence of faint, low surface brightness merger features out to at least $zsim1$.
The HUDF09 data are the deepest near-IR observations ever, reaching to 29.5 mag. Luminosity functions (LF) from these new HUDF09 data for 132 zsim7 and zsim8 galaxies are combined with new LFs for zsim5-6 galaxies and the earlier zsim4 LF to reach to very faint limits (<0.05 L*(z=3)). The faint-end slopes alpha are steep: -1.79+/-0.12 (zsim5), -1.73+/-0.20 (zsim6), -2.01+/-0.21 (zsim7), and -1.91+/-0.32 (zsim8). Slopes alphalesssim-2 lead to formally divergent UV fluxes, though galaxies are not expected to form below sim-10 AB mag. These results have important implications for reionization. The weighted mean slope at zsim6-8 is -1.87+/-0.13. For such steep slopes, and a faint-end limit of -10 AB mag, galaxies provide a very large UV ionizing photon flux. While current results show that galaxies can reionize the universe by zsim6, matching the Thomson optical depths is more challenging. Extrapolating the current LF evolution to z>8, taking alpha to be -1.87+/-0.13 (the mean value at zsim6-8), and adopting typical parameters, we derive Thomson optical depths of 0.061_{-0.006}^{+0.009}. However, this result will change if the faint-end slope alpha is not constant with redshift. We test this hypothesis and find a weak, though uncertain, trend to steeper slopes at earlier times (dalpha/dzsim-0.05+/-0.04), that would increase the Thomson optical depths to 0.079_{-0.017}^{+0.063}, consistent with recent WMAP estimates (tau=0.088+/-0.015). It may thus not be necessary to resort to extreme assumptions about the escape fraction or clumping factor. Nevertheless, the uncertainties remain large. Deeper WFC3/IR+ACS observations can further constrain the ionizing flux from galaxies.
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