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Register a new userRadiation-pressure-driven dust transport to galaxy halos at $zsim 10$
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The origin of dust in galaxy halos or in the circum-galactic medium (CGM) is still a mystery. We investigate if the radiation pressure in high-redshift ($zsim 10$) galaxies can efficiently transport dust to halos. To clarify the first dust enrichment of galaxy halos in the early Universe, we solve the motion of a dust grain considering radiation pressure, gas drag, and gravity in the vertical direction of the galactic disc. Radiation pressure is estimated in a consistent manner with the stellar spectra and dust extinction. As a consequence, we find that dust grains with radii $asim 0.1~mu$m successfully escape from the galactic disc if the ongoing star formation episode converts more than 15 per cent of the baryon content into stars and lasts $gtrsim 30$ Myr, while larger and smaller grains are trapped in the disc because of gravity and gas drag, respectively. We also show that grain charge significantly enhances gas drag at a few--10 scale heights of the galactic disc, where the grain velocities are suppressed to $sim 1$ km s$^{-1}$. There is an optimum dust-to-gas ratio ($sim 10^{-3}$) in the galactic disc and an optimum virial mass $sim 10^{10}$--$10^{11}$ M$_{odot}$ for the transport of $asim 0.1~mu$m grains to the halo. We conclude that early dust enrichment of galaxy halos at $zgtrsim 10$ is important for the origin of dust in the CGM.
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We investigate the abundance and properties (especially, grain size) of dust in galaxy halos using available observational data in the literature. There are two major sets of data. One is (i) the reddening curves at redshifts $zsim 1$ and 2 derived for Mg II absorbers, which are assumed to trace the medium in galaxy halos. The other is (ii) the cosmic extinction up to $zsim 2$ mainly traced by distant background quasars. For (i), the observed reddening curves favor a grain radius of $asim 0.03~mu$m for silicate, while graphite is not supported because of its strong 2175 AA bump. Using amorphous carbon improves the fit to the reddening curves compared with graphite if the grain radius is $alesssim 0.03~mu$m. For (ii), the cosmic extinction requires $etagtrsim 10^{-2}$ ($eta$ is the ratio of the halo dust mass to the stellar mass; the observationally suggested value is $etasim 10^{-3}$) for silicate if $asim 0.03~mu$m as suggested by the reddening curve constraint. Thus, for silicate, we do not find any grain radius that satisfies both (i) and (ii) unless the halo dust abundance is much larger than suggested by the observations. For amorphous carbon, in contrast, a wide range of grain radius ($asim 0.01$--0.3~$mu$m) is accepted by the cosmic extinction; thus, we find that a grain radius range of $asim 0.01$--0.03 $mu$m is supported by combining (i) and (ii). We also discuss the origin of dust in galaxy halos, focusing on the importance of grain size in the physical mechanism of dust supply to galaxy halos.
We present the first statistical analysis of kinematically-resolved, spatially-extended Ly$alpha$ emission around $z = 2-3$ galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies ($z_mathrm{med} = 2.29$) comprises the subset with typical KCWI integration times of ~5 hours and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high resolution images were used to evaluate the azimuthal dependence of the diffuse Ly$alpha$ emission with respect to the stellar continuum within projected galactocentric distances of $lesssim 30$ proper kpc. We introduce cylindrically-projected 2D spectra (CP2D) that map the averaged Ly$alpha$ spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of Ly$alpha$ resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended Ly$alpha$ emission along the galaxy principal axes are statistically indistinguishable, with residual asymmetry of $le$ 2% ($sim 2 sigma$) of the integrated Ly$alpha$ emission. The symmetry implies that the Ly$alpha$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of Ly$alpha$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak Ly$alpha$ emission. We speculate that this weak directional dependence of Ly$alpha$ emission becomes discernible only when the Ly$alpha$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with Ly$alpha$ radiative transfer modeling.
A proto-cluster core is the most massive dark matter halo (DMH) in a given proto-cluster. To reveal the galaxy formation in core regions, we search for proto-cluster cores at $zsim 2$ in $sim 1.5, mathrm{deg}^{2}$ of the COSMOS field. Using pairs of massive galaxies ($log(M_{*}/M_{odot})geq11$) as tracers of cores, we find 75 candidate cores, among which 54% are estimated to be real. A clustering analysis finds that these cores have an average DMH mass of $2.6_{-0.8}^{+0.9}times 10^{13}, M_{odot}$, or $4.0_{-1.5}^{+1.8}, times 10^{13} M_{odot}$ after contamination correction. The extended Press-Schechter model shows that their descendant mass at $z=0$ is consistent with Fornax-like or Virgo-like clusters. Moreover, using the IllustrisTNG simulation, we confirm that pairs of massive galaxies are good tracers of DMHs massive enough to be regarded as proto-cluster cores. We then derive the stellar mass function (SMF) and the quiescent fraction for member galaxies of the 75 candidate cores. We find that the core galaxies have a more top-heavy SMF than field galaxies at the same redshift, showing an excess at $log(M_{*}/M_{odot})gtrsim 10.5$. The quiescent fraction, $0.17_{-0.04}^{+0.04}$ in the mass range $9.0leq log(M_{*}/M_{odot})leq 11.0$, is about three times higher than that of field counterparts, giving an environmental quenching efficiency of $0.13_{-0.04}^{+0.04}$. These results suggest that stellar mass assembly and quenching are accelerated as early as at $zsim 2$ in proto-cluster cores.
We investigate the attenuation law in $zsim 6$ quasars by combining cosmological zoom-in hydrodynamical simulations of quasar host galaxies, with multi-frequency radiative transfer calculations. We consider several dust models differing in terms of grain size distributions, dust mass and chemical composition, and compare the resulting synthetic Spectral Energy Distributions (SEDs) with data from bright, early quasars. We show that only dust models with grain size distributions in which small grains ($a < 0.1~mu$m, corresponding to $approx 60%$ of the total dust mass) are selectively removed from the dusty medium provide a good fit to the data. Removal can occur if small grains are efficiently destroyed in quasar environments and/or early dust production preferentially results in large grains. Attenuation curves for these models are close to flat, and consistent with recent data; they correspond to an effective dust-to-metal ratio $f_d simeq 0.38$, i.e. close to the Milky Way value.
The absence of high Eddington ratio, obscured Active Galactic Nuclei (AGN) in local ($zlesssim0.1$) samples of moderate luminosity AGN has generally been explained to result from radiation pressure on the dusty gas governing the level of nuclear ($lesssim10$pc) obscuration. However, very high accretion rates are routinely reported among obscured quasars at higher luminosities, and may require a different feedback mechanism. We compile constraints on obscuration and Eddington ratio for samples of X-ray, optical, infrared, and submm selected AGN at quasar luminosities. Whereas moderate luminosity, obscured AGN in the local universe have a range of lower Eddington ratios ($f_{rm Edd} sim 0.001-0.1$), the most luminous ($L_{rm bol} gtrsim 10^{46} $erg s$^{-1}$) IR/submm-bright, obscured quasars out to $zsim3$ commonly have very high Eddington ratios ($f_{rm Edd} sim 0.1-1$). This apparent lack of radiation pressure feedback in luminous obscured quasars is likely coupled with AGN timescales, such that a higher fraction of luminous obscured quasars are seen due to the short timescale for which quasars are most luminous. Adopting quasar evolutionary scenarios, extended ($sim10^{2-3}$pc) obscuration may work together with the shorter timescales to explain the observed fraction of obscured, luminous quasars, while outflows driven by radiation pressure will slowly clear this material over the AGN lifetime.
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