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Register a new userThe Brightest Galaxies in the Dark Ages: Galaxies Dust Continuum Emission During the Reionization Era
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Though half of cosmic starlight is absorbed by dust and reradiated at long wavelengths (3$mu$m-3mm), constraints on the infrared through millimeter galaxy luminosity function (the `IRLF) are poor in comparison to the rest-frame ultraviolet and optical galaxy luminosity function, particularly at z>2.5. Here we present a backward evolution model for interpreting number counts, redshift distributions, and cross-band flux density correlations in the infrared and submillimeter sky, from 70$mu$m-2mm, using a model for the IRLF out to the epoch of reionization. Mock submillimeter maps are generated by injecting sources according to the prescribed IRLF and flux densities drawn from model spectral energy distributions that mirror the distribution of SEDs observed in $0<z<5$ dusty star-forming galaxies (DSFGs). We explore two extreme hypothetical case-studies: a dust-poor early Universe model, where DSFGs contribute negligibly ($<$10%) to the integrated star-formation rate density at $z>4$, and an alternate dust-rich early Universe model, where DSFGs dominate $sim$90% of $z>4$ star-formation. We find that current submm/mm datasets do not clearly rule out either of these extreme models. We suggest that future surveys at 2mm will be crucial to measuring the IRLF beyond $zsim4$. The model framework developed in this paper serves as a unique tool for the interpretation of multiwavelength IR/submm extragalactic datasets and will enable more refined constraints on the IRLF than can be made from direct measurements of individual galaxies integrated dust emission.
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Motivated by the stellar fossil record of Local Group (LG) dwarf galaxies, we show that the star-forming ancestors of the faintest ultra-faint dwarf galaxies (UFDs; ${rm M}_{rm V}$ $sim -2$ or ${rm M}_{star}$ $sim 10^{2}$ at $z=0$) had ultra-violet (UV) luminosities of ${rm M}_{rm UV}$ $sim -3$ to $-6$ during reionization ($zsim6-10$). The existence of such faint galaxies has substantial implications for early epochs of galaxy formation and reionization. If the faint-end slopes of the UV luminosity functions (UVLFs) during reionization are steep ($alphalesssim-2$) to ${rm M}_{rm UV}$ $sim -3$, then: (i) the ancestors of UFDs produced $>50$% of UV flux from galaxies; (ii) galaxies can maintain reionization with escape fractions that are $>$2 times lower than currently-adopted values; (iii) direct HST and JWST observations may detect only $sim10-50$% of the UV light from galaxies; (iv) the cosmic star formation history increases by $gtrsim4-6$ at $zgtrsim6$. Significant flux from UFDs, and resultant tensions with LG dwarf galaxy counts, are reduced if the high-redshift UVLF turns over. Independent of the UVLF shape, the existence of a large population of UFDs requires a non-zero luminosity function to ${rm M}_{rm UV}$ $sim -3$ during reionization.
We study the morphologies and sizes of galaxies at z>5 using high-resolution cosmological zoom-in simulations from the Feedback In Realistic Environments project. The galaxies show a variety of morphologies, from compact to clumpy to irregular. The simulated galaxies have more extended morphologies and larger sizes when measured using rest-frame optical B-band light than rest-frame UV light; sizes measured from stellar mass surface density are even larger. The UV morphologies are usually dominated by several small, bright young stellar clumps that are not always associated with significant stellar mass. The B-band light traces stellar mass better than the UV, but it can also be biased by the bright clumps. At all redshifts, galaxy size correlates with stellar mass/luminosity with large scatter. The half-light radii range from 0.01 to 0.2 arcsec (0.05-1 kpc physical) at fixed magnitude. At z>5, the size of galaxies at fixed stellar mass/luminosity evolves as (1+z)^{-m}, with m~1-2. For galaxies less massive than M_star~10^8 M_sun, the ratio of the half-mass radius to the halo virial radius is ~10% and does not evolve significantly at z=5-10; this ratio is typically 1-5% for more massive galaxies. A galaxys observed size decreases dramatically at shallower surface brightness limits. This effect may account for the extremely small sizes of z>5 galaxies measured in the Hubble Frontier Fields. We provide predictions for the cumulative light distribution as a function of surface brightness for typical galaxies at z=6.
We consider the capabilities of current and future large facilities operating at 2,mm to 3,mm wavelength to detect and image the [CII] 158,$mu$m line from galaxies into the cosmic dark ages ($z sim 10$ to 20). The [CII] line may prove to be a powerful tool in determining spectroscopic redshifts, and galaxy dynamics, for the first galaxies. We emphasize that the nature, and even existence, of such extreme redshift galaxies, remains at the frontier of open questions in galaxy formation. In 40,hr, ALMA has the sensitivity to detect the integrated [CII] line emission from a moderate metallicity, active star-forming galaxy [$Z_A = 0.2,Z_{odot}$; star formation rate (SFR) = 5,$M_odot$,yr$^{-1}$], at $z = 10$ at a significance of 6$sigma$. The next-generation Very Large Array (ngVLA) will detect the integrated [CII] line emission from a Milky-Way like star formation rate galaxy ($Z_{A} = 0.2,Z_{odot}$, SFR = 1,$M_odot$,yr$^{-1}$), at $z = 15$ at a significance of 6$sigma$. Imaging simulations show that the ngVLA can determine rotation dynamics for active star-forming galaxies at $z sim 15$, if they exist. Based on our very limited knowledge of the extreme redshift Universe, we calculate the count rate in blind, volumetric surveys for [CII] emission at $z sim 10$ to 20. The detection rates in blind surveys will be slow (of order unity per 40,hr pointing). However, the observations are well suited to commensal searches. We compare [CII] with the [OIII] 88$mu$m line, and other ancillary information in high $z$ galaxies that would aid these studies.
We have undertaken a search for the infrared emission from the intracluster dust in the Coma cluster of galaxies by the Multiband Imaging Photometer for Spitzer. Our observations yield the deepest mid and far-infrared images of a galaxy cluster ever achieved. In each of the three bands, we have not detected a signature of the central excess component in contrast to the previous report on the detection by Infrared Space Observatory (ISO). We still find that the brightness ratio between 70 and 160 microns shows a marginal sign of the central excess, in qualitative agreement with the ISO result. Our analysis suggests that the excess ratio is more likely due to faint infrared sources lying on fluctuating cirrus foreground. Our observations yield the 2 sigma upper limits on the excess emission within 100 kpc of the cluster center as 5 x 10^-3 MJy/sr, 6 x 10^-2 MJy/sr, and 7 x 10^-2 MJy/sr, at 24, 70, and 160 microns, respectively. These values are in agreement with those found in other galaxy clusters and suggest that dust is deficient near the cluster center by more than 3 orders of magnitude compared to the interstellar medium.
The highly neutral inter-galactic medium (IGM) during the Epoch of Reionization (EoR) is expected to suppress Ly$alpha$ emission with damping-wing absorption, causing nearly no Ly$alpha$ detection from star-forming galaxies at $z{sim}8$. However, spectroscopic observations of the 4 brightest galaxies (${rm H}_{160}{sim}25$ mag) at these redshifts do reveal prominent Ly$alpha$ line, suggesting locally ionised IGM. In this paper, we explore the Ly$alpha$ IGM transmission and environment of bright galaxies during the EoR using the Meraxes semi-analytic model. We find brighter galaxies to be less affected by damping-wing absorption as they are effective at ionizing surrounding neutral hydrogen. Specifically, the brightest sources (${rm H}_{160}{lesssim}25.5$ mag) lie in the largest ionized regions in our simulation, and have low attenuation of their Ly$alpha$ from the IGM (optical depth ${<}1$). Fainter galaxies (25.5 mag${<}{rm H}_{160}{<}27.5$ mag) have transmission that depends on UV luminosity, leading to a lower incidence of Ly$alpha$ detection at fainter magnitudes. This luminosity-dependent attenuation explains why Ly$alpha$ has only been observed in the brightest galaxies at $z{sim}8$. Follow-up observations have revealed counterparts in the vicinity of these confirmed $z{sim}8$ Ly$alpha$ emitters. The environments of our modelled analogues agree with these observations in the number of nearby galaxies, which is a good indicator of whether Ly$alpha$ can be detected among fainter galaxies. At the current observational limit, galaxies with ${ge}2$--5 neighbours within $2{times}2$ are ${sim}2$--3 times more likely to show Ly$alpha$ emission. JWST will discover an order of magnitude more neighbours, revealing ${gtrsim}50$ galaxies in the largest ionizing bubbles and facilitating direct study of reionization morphology.
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