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Register a new userThe assembly of dusty galaxies at $z geq 4$: statistical properties
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The recent discovery of high redshift dusty galaxies implies a rapid dust enrichment of their interstellar medium (ISM). To interpret these observations, we run a cosmological simulation in a 30$h^{-1}$ cMpc/size volume down to $z approx 4$. We use the hydrodynamical code dustyGadget, which accounts for the production of dust by stellar populations and its evolution in the ISM. We find that the cosmic dust density parameter ($Omega_{rm d}$) is mainly driven by stellar dust at $z gtrsim 10$, so that mass- and metallicity-dependent yields are required to assess the dust content in the first galaxies. At $z lesssim 9$ the growth of grains in the ISM of evolved systems (Log$(M_{star}/M_{odot})>8.5$) significantly increases their dust mass, in agreement with observations in the redshift range $4 lesssim z < 8$. Our simulation shows that the variety of high redshift galaxies observed with ALMA can naturally be accounted for by modeling the grain-growth timescale as a function of the physical conditions in the gas cold phase. In addition, the trends of dust-to-metal (DTM) and dust-to-gas (${cal D}$) ratios are compatible with the available data. A qualitative investigation of the inhomogeneous dust distribution in a representative massive halo at $z approx 4$ shows that dust is found from the central galaxy up to the closest satellites along polluted filaments with $rm Log({cal D}) leq -2.4$, but sharply declines at distances $d gtrsim 30$ kpc along many lines of sight, where $rm Log({cal D}) lesssim -4.0$.
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We exploit the continuity equation approach and the `main sequence star-formation timescales to show that the observed high abundance of galaxies with stellar masses > a few 10^10 M_sun at redshift z>4 implies the existence of a galaxy population featuring large star formation rates (SFRs) > 10^2 M_sun/yr in heavily dust-obscured conditions. These galaxies constitute the high-redshift counterparts of the dusty star-forming population already surveyed for z<3 in the far-IR band by the Herschel space observatory. We work out specific predictions for the evolution of the corresponding stellar mass and SFR functions out to z~10, elucidating that the number density at z<8 for SFRs >30 M_sun/yr cannot be estimated relying on the UV luminosity function alone, even when standard corrections for dust extinction based on the UV slope are applied. We compute the number counts and redshift distributions (including galaxy-scale gravitational lensing) of this galaxy population, and show that current data from AzTEC-LABOCA, SCUBA-2 and ALMA-SPT surveys are already digging into it. We substantiate how an observational strategy based on a color preselection in the far-IR or (sub-)mm band with Herschel and SCUBA-2, supplemented by photometric data via on-source observations with ALMA, can allow to reconstruct the bright end of the SFR functions out to z~8. In parallel, such a challenging task can be managed by exploiting current UV surveys in combination with (sub-)mm observations by ALMA and NIKA2 and/or radio observations by SKA and its precursors.
We investigate quasar outflows at $z geq 6$ by performing zoom-in cosmological hydrodynamical simulations. By employing the SPH code GADGET-3, we zoom in the $2 R_{200}$ region around a $2 times 10^{12} M_{odot}$ halo at $z = 6$, inside a $(500 ~ {rm Mpc})^3$ comoving volume. We compare the results of our AGN runs with a control simulation in which only stellar/SN feedback is considered. Seeding $10^5 M_{odot}$ BHs at the centers of $10^{9} M_{odot}$ halos, we find the following results. BHs accrete gas at the Eddington rate over $z = 9 - 6$. At $z = 6$, our most-massive BH has grown to $M_{rm BH} = 4 times 10^9 M_{odot}$. Fast ($v_{r} > 1000$ km/s), powerful ($dot{M}_{rm out} sim 2000 M_{odot}$/yr) outflows of shock-heated low-density gas form at $z sim 7$, and propagate up to hundreds kpc. Star-formation is quenched over $z = 8 - 6$, and the total SFR (SFR surface density near the galaxy center) is reduced by a factor of $5$ ($1000$). We analyse the relative contribution of multiple physical process: (i) disrupting cosmic filamentary cold gas inflows, (ii) reducing central gas density, (iii) ejecting gas outside the galaxy; and find that AGN feedback has the following effects at $z = 6$. The inflowing gas mass fraction is reduced by $sim 12 %$, the high-density gas fraction is lowered by $sim 13 %$, and $sim 20 %$ of the gas outflows at a speed larger than the escape velocity ($500$ km/s). We conclude that quasar-host galaxies at $z geq 6$ are accreting non-negligible amount of cosmic gas, nevertheless AGN feedback quenches their star formation dominantly by powerful outflows ejecting gas out of the host galaxy halo.
We study the luminosity function and the correlation function of about 1200 z~4 Lyman break galaxies (LBGs) with i<26 that are photometrically selected from deep BRi imaging data of a 618 arcmin^2 area in the Subaru/XMM-Newton Deep Field taken with Subaru Prime Focus Camera. The contamination and completeness of our LBG sample are evaluated, on the basis of the Hubble Deep Field-North (HDF-N) objects, to be 17% and 45%, respectively. We derive the UV (rest 1700A) luminosity functions (LFs) and find a large population of UV-luminous galaxies at z~4. The LFs of the red and blue subsamples imply that the bright LBGs are redder in the UV continuum than the average color of the LBGs. Then we calculate the correlation function over theta = 2-1000 and find that it is fitted fairly well by a power law, omega(theta)=A_omega theta^(-0.8), with A_omega=0.71 +/- 0.26. We estimate the correlation length r_0 (in comoving units) of the two-point spatial correlation function xi(r) = (r/r_0)^(-1.8) to be r_0=2.7 +0.5/-0.6 h^(-1) Mpc (Omega_m=0.3 and Omega_Lambda=0.7). The correlation function shows an excess of omega (theta) on small scales (theta < 5), departing from the power-law fit at > 3 sigma significance level. Interpreting this as being due to galaxy mergers, we evaluate the fraction of galaxies undergoing mergers to be 3.0 +/- 0.9%, which is significantly smaller than those of galaxies at intermediate redshifts.
We report new deep ALMA observations aimed at investigating the [CII]158um line and continuum emission in three spectroscopically confirmed Lyman Break Galaxies at 6.8<z<7.1, i.e. well within the re-ionization epoch. With Star Formation Rates of SFR ~ 5-15 Msun/yr these systems are much more representative of the high-z galaxy population than other systems targeted in the past by millimeter observations. For the galaxy with the deepest observation we detect [CII] emission at redshift z=7.107, fully consistent with the Lyalpha redshift, but spatially offset by 0.7 (4 kpc) from the optical emission. At the location of the optical emission, tracing both the Lyalpha line and the far-UV continuum, no [CII] emission is detected in any of the three galaxies, with 3sigma upper limits significantly lower than the [CII] emission observed in lower reshift galaxies. These results suggest that molecular clouds in the central parts of primordial galaxies are rapidly disrupted by stellar feedback. As a result, [CII] emission mostly arises from more external accreting/satellite clumps of neutral gas. These findings are in agreement with recent models of galaxy formation. Thermal far-infrared continuum is not detected in any of the three galaxies. However, the upper limits on the infrared-to-UV emission ratio do not exceed those derived in metal- and dust-poor galaxies.
Until recently, only a handful of dusty, star-forming galaxies (DSFGs) were known at $z>4$, most of them significantly amplified by gravitational lensing. Here, we have increased the number of such DSFGs substantially, selecting galaxies from the uniquely wide 250-, 350- and 500-$mu$m Herschel-ATLAS imaging survey on the basis of their extremely red far-infrared colors and faint 350- and 500-$mu$m flux densities - ergo they are expected to be largely unlensed, luminous, rare and very distant. The addition of ground-based continuum photometry at longer wavelengths from the JCMT and APEX allows us to identify the dust peak in their SEDs, better constraining their redshifts. We select the SED templates best able to determine photometric redshifts using a sample of 69 high-redshift, lensed DSFGs, then perform checks to assess the impact of the CMB on our technique, and to quantify the systematic uncertainty associated with our photometric redshifts, $sigma=0.14,(1+z)$, using a sample of 25 galaxies with spectroscopic redshifts, each consistent with our color selection. For Herschel-selected ultrared galaxies with typical colors of $S_{500}/S_{250}sim 2.2$ and $S_{500}/S_{350}sim 1.3$ and flux densities, $S_{500}sim 50,$mJy, we determine a median redshift, $hat{z}_{rm phot}=3.66$, an interquartile redshift range, 3.30$-$4.27, with a median rest-frame 8$-$1000-$mu$m luminosity, $hat{L}_{rm IR}$, of $1.3times 10^{13},$L$_odot$. A third lie at $z>4$, suggesting a space density, $rho_{z>4}$, of $approx 6 times 10^{-7},$Mpc$^{-3}$. Our sample contains the most luminous known star-forming galaxies, and the most over-dense cluster of starbursting proto-ellipticals yet found.
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