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Register a new userGOLDRUSH. IV. Luminosity Functions and Clustering Revealed with ~4,000,000 Galaxies at z~2-7: Galaxy-AGN Transition, Star Formation Efficiency, and Implication for Evolution at z>10
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We present new measurements of rest-UV luminosity functions and angular correlation functions from 4,100,221 galaxies at z~2-7 identified in the Subaru/Hyper Suprime-Cam survey and CFHT Large-Area U-band Survey. The obtained luminosity functions at z~4-7 cover a very wide UV luminosity range of ~0.002-2000L*uv combined with previous studies, revealing that the dropout luminosity function is a superposition of the AGN luminosity function dominant at Muv<-24 mag and the galaxy luminosity function dominant at Muv>-22 mag, consistent with galaxy fractions based on 1037 spectroscopically-identified sources. Galaxy luminosity functions estimated from the spectroscopic galaxy fractions show the bright end excess beyond the Schechter function at >2sigma levels, which is possibly made by inefficient mass quenching, low dust obscuration, and/or hidden AGN activity. By analyzing the correlation functions at z~2-6 with halo occupation distribution models, we find a weak redshift evolution (within 0.3 dex) of the ratio of the star formation rate (SFR) to the dark matter accretion rate, SFR/(dMh/dt), indicating the almost constant star formation efficiency at z~2-6, as suggested by our earlier work at z~4-7. Meanwhile, the ratio gradually increases with decreasing redshift at z<5 within 0.3 dex, which quantitatively reproduces the redshift evolution of the cosmic SFR density, suggesting that the evolution is primarily driven by the increase of the halo number density due to the structure formation, and the decrease of the accretion rate due to the cosmic expansion. Extrapolating this calculation to higher redshifts assuming the constant efficiency suggests a rapid decrease of the SFR density at z>10 with $rho_mathrm{SFR}propto10^{-0.5(1+z)}$, which will be directly tested with JWST.
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We present a joint analysis of the rest-frame ultraviolet (UV) luminosity functions of continuum-selected star-forming galaxies and galaxies dominated by active galactic nuclei (AGNs) at $z sim$ 4. These 3,740 $z sim$ 4 galaxies are selected from broad-band imaging in nine photometric bands over 18 deg$^2$ in the textit{Spitzer}/HETDEX Exploratory Large Area Survey (SHELA) field. The large area and moderate depth of our survey provide a unique view of the intersection between the bright end of the galaxy UV luminosity function (M$_{AB}<-$22) and the faint end of the AGN UV luminosity function. We do not separate AGN-dominated galaxies from star-formation-dominated galaxies, but rather fit both luminosity functions simultaneously. These functions are best fit with a double power-law (DPL) for both the galaxy and AGN components, where the galaxy bright-end slope has a power-law index of $-3.80pm0.10$, and the corresponding AGN faint-end slope is $alpha_{AGN} = -1.49^{+0.30}_{-0.21}$. We cannot rule out a Schechter-like exponential decline for the galaxy UV luminosity function, and in this scenario the AGN luminosity function has a steeper faint-end slope of $-2.08^{+0.18}_{-0.11}$. Comparison of our galaxy luminosity function results with a representative cosmological model of galaxy formation suggests that the molecular gas depletion time must be shorter, implying that star formation is more efficient in bright galaxies at $z=4$ than at the present day. If the galaxy luminosity function does indeed have a power-law shape at the bright end, the implied ionizing emissivity from AGNs is not inconsistent with previous observations. However, if the underlying galaxy distribution is Schechter, it implies a significantly higher ionizing emissivity from AGNs at this epoch.
We investigate the dependence of galaxy clustering at $z sim 4 - 7$ on UV-luminosity and stellar mass. Our sample consists of $sim$ 10,000 Lyman-break galaxies (LBGs) in the XDF and CANDELS fields. As part of our analysis, the $M_star - M_{rm UV}$ relation is estimated for the sample, which is found to have a nearly linear slope of $dlog_{10} M_star / d M_{rm UV} sim 0.44$. We subsequently measure the angular correlation function and bias in different stellar mass and luminosity bins. We focus on comparing the clustering dependence on these two properties. While UV-luminosity is only related to recent starbursts of a galaxy, stellar mass reflects the integrated build-up of the whole star formation history, which should make it more tightly correlated with halo mass. Hence, the clustering segregation with stellar mass is expected to be larger than with luminosity. However, our measurements suggest that the segregation with luminosity is larger with $simeq 90%$ confidence (neglecting contributions from systematic errors). We compare this unexpected result with predictions from the textsc{Meraxes} semi-analytic galaxy formation model. Interestingly, the model reproduces the observed angular correlation functions, and also suggests stronger clustering segregation with luminosity. The comparison between our observations and the model provides evidence of multiple halo occupation in the small scale clustering.
A large sample of spectroscopically confirmed galaxies at 1.4<z<3.7, with complementary imaging in the near- and mid-IR from the ground and from Hubble and Spitzer, is used to infer the average star formation histories (SFHs) of typical galaxies from z~7 to 2. For a subset of 302 galaxies at 1.5<z<2.6, we perform a comparison of star formation rates (SFRs) determined from SED modeling (SFRs[SED]) and those calculated from deep Keck UV and Spitzer/MIPS 24 micron imaging (SFRs[IR+UV]). Exponentially declining SFHs yield SFRs[SED] that are 5-10x lower on average than SFRs[IR+UV], indicating that declining SFHs may not be accurate for typical galaxies at z>2. The SFRs of z~2-3 galaxies are directly proportional to their stellar masses M*, with unity slope---a result that is confirmed with Spitzer/IRAC stacks of 1179 UV-faint (R>25.5) galaxies---for M*>5e8 Msun and SFRs >2 Msun/yr. We interpret this result in the context of several systematic biases that can affect determinations of the SFR-M* relation. The average specific SFRs at z~2-3 are similar within a factor of two to those measured at z>4, implying an average SFH where SFRs increase with time. A consequence of these rising SFHs is that (a) a substantial fraction of UV-bright z~2-3 galaxies had faint sub-L* progenitors at z>4; and (b) gas masses must increase with time from z=7 to 2, over which time the net cold gas accretion rate---as inferred from the specific SFR and the Kennicutt-Schmidt relation---is ~2-3x larger than the SFR . However, if we evolve to higher redshift the SFHs and masses of the halos that are expected to host L* galaxies at z~2, we find that <10% of the baryons accreted onto typical halos at z>4 actually contribute to star formation at those epochs. These results highlight the relative inefficiency of star formation even at early cosmic times when galaxies were first assembling. [Abridged]
Using the VLA and ALMA, we have obtained CO(2-1), [C II], [N II] line emission and multiple dust continuum measurements in a sample of normal galaxies at $z=5-6$. We report the highest redshift detection of low-$J$ CO emission from a Lyman Break Galaxy, at $zsim5.7$. The CO line luminosity implies a massive molecular gas reservoir of $(1.3pm0.3)(alpha_{rm CO}/4.5,M_odot$ (K km s$^{-1}$ pc$^2)^{-1})times10^{11},M_odot$, suggesting low star formation efficiency, with a gas depletion timescale of order $sim$1 Gyr. This efficiency is much lower than traditionally observed in $zgtrsim5$ starbursts, indicating that star forming conditions in Main Sequence galaxies at $zsim6$ may be comparable to those of normal galaxies probed up to $zsim3$ to-date, but with rising gas fractions across the entire redshift range. We also obtain a deep CO upper limit for a Main Sequence galaxy at $zsim5.3$ with $sim3$ times lower SFR, perhaps implying a high $alpha_{rm CO}$ conversion factor, as typically found in low metallicity galaxies. For a sample including both CO targets, we also find faint [N II] 205$,mu$m emission relative to [C II] in all but the most IR-luminous normal galaxies at $z=5-6$, implying more intense or harder radiation fields in the ionized gas relative to lower redshift. These radiation properties suggest that low metallicity may be common in typical $sim$10$^{10},M_odot$ galaxies at $z=5-6$. While a fraction of Main Sequence star formation in the first billion years may take place in conditions not dissimilar to lower redshift, lower metallicity may affect the remainder of the population.
We study the role of feedback from supernovae and black holes in the evolution of the star formation rate function (SFRF) of z~4-7 galaxies. We use a new set of cosmological hydrodynamic simulations, ANGUS (AustraliaN GADGET-3 early Universe Simulations), run with a modified and improved version of the parallel TreePM-smoothed particle hydrodynamics code GADGET-3 called P-GADGET3(XXL), that includes a self-consistent implementation of stellar evolution and metal enrichment. In our simulations both Supernova (SN) driven galactic winds and Active Galactic Nuclei (AGN) act simultaneously in a complex interplay. The SFRF is insensitive to feedback prescription at z>5, meaning that it cannot be used to discriminate between feedback models during reionisation. However, the SFRF is sensitive to the details of feedback prescription at lower redshift. By exploring different SN driven wind velocities and regimes for the AGN feedback, we find that the key factor for reproducing the observed SFRFs is a combination of strong SN winds and early AGN feedback in low mass galaxies. Conversely, we show that the choice of initial mass function and inclusion of metal cooling have less impact on the evolution of the SFRF. When variable winds are considered, we find that a non-aggressive wind scaling is needed to reproduce the SFRFs at z>4. Otherwise, the amount of objects with low SFRs is greatly suppressed and at the same time winds are not effective enough in the most massive systems.
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