The Reionization of Carbon

Observations suggest that CII was more abundant than CIV in the intergalactic medium towards the end of the hydrogen reionization epoch. This transition provides a unique opportunity to study the enrichment history of intergalactic gas and the growth of the ionizing background (UVB) at early times. We study how carbon absorption evolves from z=10-5 using a cosmological hydrodynamic simulation that includes a self-consistent multifrequency UVB as well as a well-constrained model for galactic outflows to disperse metals. Our predicted UVB is within 2-4 times that of Haardt & Madau (2012), which is fair agreement given the uncertainties. Nonetheless, we use a calibration in post-processing to account for Lyman-alpha forest measurements while preserving the predicted spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way that it always exceeds the volume average in regions where metals are found. This implies both that a spatially-uniform UVB is a poor approximation and that metal absorption is not sensitive to the epoch when HII regions overlap globally even at column densites of 10^{12} cm^{-2}. We find, consistent with observations, that the CII mass fraction drops to low redshift while CIV rises owing the combined effects of a growing UVB and continued addition of carbon in low-density regions. This is mimicked in absorption statistics, which broadly agree with observations at z=6-3 while predicting that the absorber column density distributions rise steeply to the lowest observable columns. Our model reproduces the large observed scatter in the number of low-ionization absorbers per sightline, implying that the scatter does not indicate a partially-neutral Universe at z=6.Comment: 16 pages, 14 figures, accepted to MNRA

Galactic Outflows and Photoionization Heating in the Reionization Epoch

We carry out a new suite of cosmological radiation hydrodynamic simulations and explore the relative impacts on reionization-epoch star formation of galactic outflows and photoionization heating. By itself, an extragalactic ultraviolet background (EUVB) suppresses the luminosity function by less than 50% at z=6, overproducing the observed galaxy abundance by a factor of 3-5. Galactic outflows restore agreement with observations without preventing Population II star formation from reionizing the Universe by z=6. The resulting EUVB suppresses star formation in halos with virial temperatures below 10^5K but has a weaker impact in more massive halos. Nonetheless, the low-mass halos contribute up to 50% of all ionizing photons owing to the EUVB's inhomogeneity. Overall, star formation rate scales as halo mass M_h to the 1.3-1.4 in halos with M_h=10^{8.2--10.2}\msun. This is a steeper dependence than is often assumed in reionization models, boosting the expected power spectrum of 21 centimeter fluctuations on large scales. The luminosity function rises steeply to at least M_1600=-13, indicating that reionization was driven by faint galaxies (M_1600 >= -15) that have not yet been observed. Our models cannot simultaneously explain observations of galaxies, the cosmic microwave background, and the intergalactic medium. Increased dynamic range will alleviate the existing discrepancies, but observations may still require additional physics such as a variable ionizing escape fraction (abridged).Comment: 23 pages, 15 Figures, accepted to Ap

The nature of submillimetre galaxies in cosmological hydrodynamic simulations

We study the nature of rapidly star-forming galaxies at z= 2 in cosmological hydrodynamic simulations, and compare their properties to observations of submillimetre galaxies (SMGs). We identify simulated SMGs as the most rapidly star-forming systems that match the observed number density of SMGs. In our models, SMGs are massive galaxies sitting at the centres of large potential wells, being fed by smooth infall and gas-rich satellites at rates comparable to their star formation rates (SFRs). They are not typically undergoing major mergers that significantly boost their quiescent SFR, but they still often show complex gas morphologies and kinematics. Our simulated SMGs have stellar masses of M*∼ 1011−11.7 M⊙, SFRs of ∼180–500 M⊙ yr−1, a clustering length of ∼10 h−1 Mpc and solar metallicities. The SFRs are lower than those inferred from far-infrared (far-IR) data by ∼×3, which we suggest may owe to one or more systematic effects in the SFR calibrations. SMGs at z= 2 live in ∼1013 M⊙ haloes, and by z= 0 they mostly end up as brightest group galaxies in ∼1014 M⊙ haloes. We predict that higher M* SMGs should have on average lower specific SFRs, less disturbed morphologies and higher clustering. We also predict that deeper far-IR surveys will smoothly join SMGs on to the massive end of the SFR–M* relationship defined by lower mass z∼ 2 galaxies. Overall, our simulated rapid star-formers provide as good a match to available SMG data as merger-based scenarios, offering an alternative scenario that emerges naturally from cosmological simulations

A fundamental problem in our understanding of low-mass galaxy evolution

Recent studies have found a dramatic difference between the observed number density evolution of low-mass galaxies and that predicted by semi-analytic models. Whilst models accurately reproduce the z= 0 number density, they require that the evolution occurs rapidly at early times, which is incompatible with the strong late evolution found in observational results. We report here the same discrepancy in two state-of-the-art cosmological hydrodynamical simulations, which is evidence that the problem is fundamental. We search for the underlying cause of this problem using two complementary methods. First, we consider a narrow range in stellar mass of log (Mstar/(h−2M_)) = 9–9.5 and look for evidence of a different history of today’s low-mass galaxies in models and observations. We find that the exclusion of satellite galaxies from the analysis brings the median ages and star formation rates of galaxies into reasonable agreement. However, the models yield too few young, strongly star-forming galaxies. Secondly, we construct a toy model to link the observed evolution of specific star formation rates with the evolution of the galaxy stellar mass function. We infer from this model that a key problem in both semi-analytic and hydrodynamical models is the presence of a positive instead of a negative correlation between specific star formation rate and stellar mass. A similar positive correlation is found between the specific dark matter halo accretion rate and the halo mass, indicating that model galaxies are growing in a way that follows the growth of their host haloes too closely. It therefore appears necessary to find a mechanism that decouples the growth of low-mass galaxies, which occurs primarily at late times, from the growth of their host haloes, which occurs primarily at early times. We argue that the current form of star formation-driven feedback implemented in most galaxy formation models is unlikely to achieve this goal, owing to its fundamental dependence on host halo mass and time

The Physical and Photometric Properties of High-Redshift Galaxies in Cosmological Hydrodynamic Simulations

We study the physical and photometric properties of galaxies at z=4 in cosmological hydrodynamic simulations of a lambda-CDM universe. We focus on galaxies satisfying the GOODS "B-dropout" criteria. Our goals are: (1) to study the nature of high-redshift galaxies; (2) to test the simulations against published measurements of high-redshift galaxies; (3) to find relations between photometric measurements by HST/ACS (0.4 -- 1 micron) and Spitzer/IRAC (3.6 -- 8 micron) and the intrinsic physical properties of GOODS "B-dropouts" such as stellar mass, stellar age, dust reddening, and star-formation rate; and (4) to assess how representative the GOODS survey is at this epoch. Our simulations predict that high-redshift galaxies show strong correlations in star formation rate versus stellar mass, and weaker correlations versus environment and age, such that GOODS galaxies are predicted to be the most massive, most rapidly star-forming galaxies at that epoch, living preferentially in dense regions. The simulated rest-frame UV luminosity function (LF) and integrated luminosity density are in broad agreement with observations at z~4. The predicted rest-frame optical (observed 3.6 micron) LF is similar to the rest-frame UV LF, shifted roughly one magnitude brighter. We predict that GOODS detects less than 50% of the total stellar mass density formed in galaxies more massive than 10^8.7 M_sun by z=4, mainly because of brightness limits in the HST/ACS bands. The most rapidly star forming galaxies in our simulations have rates exceeding 1000 M_sun yr^-1, similar to observed sub-mm galaxies. The star formation rates of these galaxies show at most a mild excess (2--3x) over the rates that would be expected for their stellar mass. Whether these bright galaxies would be observable as LBGs depends on the uncertain effects of dust reddening.Comment: 27 pages, 15 figures, submitted to Ap

The Optical, Infrared and Radio Properties of Extragalactic Sources Observed by SDSS, 2MASS and FIRST Surveys

We positionally match sources observed by the Sloan Digital Sky Survey (SDSS), the Two Micron All Sky Survey (2MASS), and the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey. Practically all 2MASS sources are matched to an SDSS source within 2 arcsec; ~11% of them are optically resolved galaxies and the rest are dominated by stars. About 1/3 of FIRST sources are matched to an SDSS source within 2 arcsec; ~80% of these are galaxies and the rest are dominated by quasars. Based on these results, we project that by the completion of these surveys the matched samples will include about 10^7 stars and 10^6 galaxies observed by both SDSS and 2MASS, and about 250,000 galaxies and 50,000 quasars observed by both SDSS and FIRST. Here we present a preliminary analysis of the optical, infrared and radio properties for the extragalactic sources from the matched samples. In particular, we find that the fraction of quasars with stellar colors missed by the SDSS spectroscopic survey is probably not larger than ~10%, and that the optical colors of radio-loud quasars are ~0.05 mag. redder (with 4-sigma significance) than the colors of radio-quiet quasars.Comment: 10 pages, 6 color figures, presented at IAU Colloquium 184. AGN Survey

Understanding the Observed Evolution of the Galaxy Luminosity Function from z=6-10 in the Context of Hierarchical Structure Formation

Recent observations of the Lyman-break galaxy (LBG) luminosity function (LF) from z~6-10 show a steep decline in abundance with increasing redshift. However, the LF is a convolution of the mass function of dark matter halos (HMF)--which also declines sharply over this redshift range--and the galaxy-formation physics that maps halo mass to galaxy luminosity. We consider the strong observed evolution in the LF from z~6-10 in this context and determine whether it can be explained solely by the behavior of the HMF. From z~6-8, we find a residual change in the physics of galaxy formation corresponding to a ~0.5 dex increase in the average luminosity of a halo of fixed mass. On the other hand, our analysis of recent LF measurements at z~10 shows that the paucity of detected galaxies is consistent with almost no change in the average luminosity at fixed halo mass from z~8. The LF slope also constrains the variation about this mean such that the luminosity of galaxies hosted by halos of the same mass are all within about an order-of-magnitude of each other. We show that these results are well-described by a simple model of galaxy formation in which cold-flow accretion is balanced by star formation and momentum-driven outflows. If galaxy formation proceeds in halos with masses down to 10^8 Msun, then such a model predicts that LBGs at z~10 should be able to maintain an ionized intergalactic medium as long as the ratio of the clumping factor to the ionizing escape fraction is C/f_esc < 10.Comment: 15 pages, 2 figures; results unchanged; accepted by JCA