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Register a new userAn Increasing Stellar Baryon Fraction in Bright Galaxies at High Redshift
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Recent observations have shown that the characteristic luminosity of the rest-frame ultraviolet (UV) luminosity function does not significantly evolve at 4 < z < 7 and is approximately M*_UV ~ -21. We investigate this apparent non-evolution by examining a sample of 178 bright, M_UV < -21 galaxies at z=4 to 7, analyzing their stellar populations and host halo masses. Including deep Spitzer/IRAC imaging to constrain the rest-frame optical light, we find that M*_UV galaxies at z=4-7 have similar stellar masses of log(M/Msol)=9.6-9.9 and are thus relatively massive for these high redshifts. However, bright galaxies at z=4-7 are less massive and have younger inferred ages than similarly bright galaxies at z=2-3, even though the two populations have similar star formation rates and levels of dust attenuation. We match the abundances of these bright z=4-7 galaxies to halo mass functions from the Bolshoi Lambda-CDM simulation to estimate the halo masses. We find that the typical halo masses in ~M*_UV galaxies decrease from log(M_h/Msol)=11.9 at z=4 to log(M_h/Msol)=11.4 at z=7. Thus, although we are studying galaxies at a similar mass across multiple redshifts, these galaxies live in lower mass halos at higher redshift. The stellar baryon fraction in units of the cosmic mean Omega_b/Omega_m rises from 5.1% at z=4 to 11.7% at z=7; this evolution is significant at the ~3-sigma level. This rise does not agree with simple expectations of how galaxies grow, and implies that some effect, perhaps a diminishing efficiency of feedback, is allowing a higher fraction of available baryons to be converted into stars at high redshifts.
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We present Herschel-PACS observations of rest-frame mid-infrared and far-infrared spectral line emissions from two lensed, ultra-luminous infrared galaxies at high redshift: MIPS J142824.0+352619 (MIPS J1428), a starburst-dominated system at z = 1.3, and IRAS F10214+4724 (F10214), a source at z = 2.3 hosting both star-formation and a luminous AGN. We have detected [OI]63 micron and [OIII]52 micron in MIPS J1428, and tentatively [OIII]52 micron in F10214. Together with the recent ZEUS-CSO [CII]158 micron detection in MIPS J1428 we can for the first time combine [OI], [CII] and far-IR (FIR) continuum measurements for photo-dissociation (PDR) modeling of an ultra-luminous (L_IR > 10^12 L_sun) star forming galaxy at the peak epoch of cosmic star formation. We find that MIPS J1428, contrary to average local ULIRGs, does not show a deficit in [OI] relative to FIR. The combination of far-UV flux G_0 and gas density n (derived from the PDR models), as well as the star formation efficiency (derived from CO and FIR) is similar to normal or starburst galaxies, despite the high infrared luminosity of this system. In contrast, F10214 has stringent upper limits on [OIV] and [SIII], and an [OIII]/FIR ratio at least an order of magnitude lower than local starbursts or AGN, similar to local ULIRGs.
We use high-resolution ($approx 10$ pc), zoom-in simulations of a typical (stellar mass $M_starsimeq10^{10}M_odot$) Lyman Break Galaxy (LBG) at $zsimeq 6$ to investigate the stellar populations of its six dwarf galaxy satellites, whose stellar [gas] masses are in the range $log (M_star/M_odot) simeq 6-9$ [$log (M_{gas}/M_odot) simeq4.3-7.75$]. The properties and evolution of satellites show no dependence on the distance from the central massive LBG ($< 11.5$ kpc). Instead, their star formation and chemical enrichment histories are tightly connected their stellar (and sub-halo) mass. High-mass dwarf galaxies ($rm M_star gtrsim 5times 10^8 M_odot$) experience a long history of star formation, characterised by many merger events. Lower-mass systems go through a series of short star formation episodes, with no signs of mergers; their star formation activity starts relatively late ($zapprox 7$), and it is rapidly quenched by internal stellar feedback. In spite of the different evolutionary patterns, all satellites show a spherical morphology, with ancient and more metal-poor stars located towards the inner regions. All six dwarf satellites experienced high star formation rate ($rm >5,M_odot yr ^{-1}$) bursts, which can be detected by JWST while targeting high-$z$ LBGs.
Star formation happens in two types of environment: ultraviolet-bright starbursts (like 30 Doradus and HII galaxies at low redshift and Lyman-break galaxies at high redshift) and infrared-bright dust-enshrouded regions (which may be moderately star-forming like Orion in the Galaxy or extreme like the core of Arp 220). In this work I will estimate how many of the stars in the local Universe formed in each type of environment, using observations of star-forming galaxies at all redshifts at different wavelengths and of the evolution of the field galaxy population.
We perform joint modeling of the composite rest-frame far-UV (FUV) and optical spectra of redshift 1.85<z<3.49 star-forming galaxies to deduce key properties of the massive stars, ionized ISM, and neutral ISM, with the aim of investigating the principal factors affecting the production and escape of Ly-alpha (Lya) photons. Our sample consists of 136 galaxies with deep Keck/LRIS and MOSFIRE spectra covering, respectively, Ly-beta through CIII] 1907, 1909; and [OII], [NeIII], H-beta, [OIII], H-alpha, [NII], and [SII]. Spectral and photoionization modeling indicate that the galaxies are uniformly consistent with stellar population synthesis models that include the effects of stellar binarity. Over the dynamic range of our sample, there is little variation in stellar and nebular abundance with Lya equivalent width, W(Lya), and only a marginal anti-correlation between age and W(Lya). The inferred range of ionizing spectral shapes is insufficient to solely account for the variation in W(Lya). Rather, the covering fraction of optically-thick HI appears to be the principal factor modulating the escape of Lya, with most of the Lya photons in down-the-barrel observations of galaxies escaping through low-column-density or ionized channels in the ISM. Our analysis shows that a high star-formation-rate surface density, Sigma_SFR, particularly when coupled with a low galaxy potential (i.e., low stellar mass), can aid in reducing the covering fraction and ease the escape of Lya photons. We conclude with a discussion of the implications of our results for the escape of ionizing radiation at high redshift.
Simulations of nearby (0.015 < z < 0.025) SDSS galaxies have been used to reproduce as accurately as possible the appearance that they would have on COSMOS ACS images if they had been observed at z ~ 0.7 and z ~ 1.2. By adding the SDSS galaxies to random locations in the COSMOS images, we simulate the effects of chance superpositions of high redshift galaxies with unrelated foreground or background objects. We have used these simulated images, together with those of real COSMOS galaxies at these same redshifts, to undertake a blind morphological classification of galaxies to identify those that appear to be undergoing mergers and thus to estimate the change in merger fraction with redshift. We find that real mergers are harder to recognize at high redshift, and also that the chance superposition of unrelated galaxies often produces the appearance of mergers where in reality none exists. In particular, we estimate that 1.5 - 2.0% of objects randomly added to ACS images are misclassified as mergers due to projection with unrelated objects, and as a result, that 40% of the apparent mergers in COSMOS at z=0.7 are likely to be spurious. We find that the fraction of galaxies undergoing mergers increases as (1+z)^3.8+/-1.2 to z ~ 0.7 and that this trend appears to continue to z = 1.2. Merger candidates at z ~ 0.7 are bluer than the parent population, especially when the statistical effects of the chance projections are accounted for. Merger candidates are more asymmetric than the population as a whole, and are often associated with irregular morphology. Nevertheless, the majority (~60%) of the merger candidates appear to be associated with spiral galaxies although in this case we cannot correct for the effects of chance projections.
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