A New Measurement of the Stellar Mass Density at z~5: Implications for the Sources of Cosmic Reionization

We present a new measurement of the integrated stellar mass per comoving volume at redshift 5 determined via spectral energy fitting drawn from a sample of 214 photometrically-selected galaxies with z'<26.5 in the southern GOODS field. Following procedures introduced by Eyles et al. (2005), we estimate stellar masses for various sub-samples for which reliable and unconfused Spitzer IRAC detections are available. A spectroscopic sample of 14 of the most luminous sources with =4.92 provides a firm lower limit to the stellar mass density of 1e6 Msun/Mpc^3. Several galaxies in this sub-sample have masses of order 10^11 Msun implying significant earlier activity occurred in massive systems. We then consider a larger sample whose photometric redshifts in the publicly-available GOODS-MUSIC catalog lie in the range 4.4 <z 5.6. Before adopting the GOODS-MUSIC photometric redshifts, we check the accuracy of their photometry and explore the possibility of contamination by low-z galaxies and low-mass stars. After excising probable stellar contaminants and using the z'-J color to exclude any remaining foreground red galaxies, we conclude that 196 sources are likely to be at z~5. The implied mass density from the unconfused IRAC fraction of this sample, scaled to the total available, is 6e6 Msun/Mpc^3. We discuss the uncertainties as well as the likelihood that we have underestimated the true mass density. Including fainter and quiescent sources the total integrated density could be as high as 1e7 Msun/Mpc^3. Using the currently available (but highly uncertain) rate of decline in the star formationhistory over 5 <z< 10, a better fit is obtained for the assembled mass at z~5 if we admit significant dust extinction at early times or extend the luminosity function to very faint limits. [abridged]Comment: Accepted for Publication in ApJ, 39 page

The Stellar Initial Mass Function at the Epoch of Reionization

I provide estimates of the ultraviolet and visible light luminosity density at z~6 after accounting for the contribution from faint galaxies below the detection limit of deep Hubble and Spitzer surveys. I find the rest-frame V-band luminosity density is a factor of ~2-3 below the ultraviolet luminosity density at z~6. This implies that the maximal age of the stellar population at z~6, for a Salpeter initial mass function, and a single, passively evolving burst, must be <100 Myr. If the stars in z~6 galaxies are remnants of the star-formation that was responsible for ionizing the intergalactic medium, reionization must have been a brief process that was completed at z<7. This assumes the most current estimates of the clumping factor and escape fraction and a Salpeter slope extending up to 200 M_{\sun} for the stellar initial mass function (IMF; dN/dM \propto M^{\alpha}, \alpha=-2.3). Unless the ratio of the clumping factor to escape fraction is less than 60, a Salpeter slope for the stellar IMF and reionization redshift higher than 7 is ruled out. In order to maintain an ionized intergalactic medium from redshift 9 onwards, the stellar IMF must have a slope of \alpha=-1.65 even if stars as massive as ~200 M_{\sun} are formed. Correspondingly, if the intergalactic medium was ionized from redshift 11 onwards, the IMF must have \alpha~-1.5. The range of stellar mass densities at z~6 straddled by IMFs which result in reionization at z>7 is 1.3+/-0.4\times10^{7} Msun/Mpc^3.Comment: 25 pages, 2 tables, 6 figures, ApJ, in press, v680 n

Using deuterated PAH amendments to validate chemical extraction methods to predict PAH bioavailability in soils

Validating chemical methods to predict bioavailable fractions of polycyclic aromatic hydrocarbons (PAHs) by comparison with accumulation bioassays is problematic. Concentrations accumulated in soil organisms not only depend on the bioavailable fraction but also on contaminant properties. A historically contaminated soil was freshly spiked with deuterated PAHs (dPAHs). dPAHs have a similar fate to their respective undeuterated analogues, so chemical methods that give good indications of bioavailability should extract the fresh more readily available dPAHs and historic more recalcitrant PAHs in similar proportions to those in which they are accumulated in the tissues of test organisms. Cyclodextrin and butanol extractions predicted the bioavailable fraction for earthworms (Eisenia fetida) and plants (Lolium multiflorum) better than the exhaustive extraction. The PAHs accumulated by earthworms had a larger dPAH:PAH ratio than that predicted by chemical methods. The isotope ratio method described here provides an effective way of evaluating other chemical methods to predict bioavailability

Spitzer Constraints on the z=6.56 Galaxy Lensed by Abell 370

We report on Spitzer IRAC observations of the spectroscopically confirmed z=6.56 lensed Ly-alpha emitting source HCM 6A which was found behind the cluster Abell 370. Detection of the source at 3.6 and 4.5 microns, corresponding to rest-frame optical emission, allows us to study the stellar population of this primeval galaxy. The broadband flux density at 4.5 microns is enhanced compared to the continuum at other wavelengths, likely due to the presence of strong H-alpha in emission. The derived H-alpha line flux corresponds to a star-formation rate of around 140 M_{sun}/yr, more than an order of magnitude larger than estimates from the ultraviolet continuum and Ly-alpha emission line. The dust extinction required to explain the discrepancy is A_V of about 1 mag. The inference of dust at such high redshifts is surprising and implies that the first epoch of star-formation in this galaxy occurred at z~20.Comment: 11 pages, 3 figures, ApJ Letters in pres

Spitzer IRAC confirmation of z_850-dropout galaxies in the Hubble Ultra Deep Field: stellar masses and ages at z~7

Using Spitzer IRAC mid-infrared imaging from the Great Observatories Origins Deep Survey, we study z_850-dropout sources in the Hubble Ultra Deep Field. After carefully removing contaminating flux from foreground sources, we clearly detect two z_850-dropouts at 3.6 micron and 4.5 micron, while two others are marginally detected. The mid-infrared fluxes strongly support their interpretation as galaxies at z~7, seen when the Universe was only 750 Myr old. The IRAC observations allow us for the first time to constrain the rest-frame optical colors, stellar masses, and ages of the highest redshift galaxies. Fitting stellar population models to the spectral energy distributions, we find photometric redshifts in the range 6.7-7.4, rest-frame colors U-V=0.2-0.4, V-band luminosities L_V=0.6-3 x 10^10 L_sun, stellar masses 1-10 x 10^9 M_sun, stellar ages 50-200 Myr, star formation rates up to ~25 M_sun/yr, and low reddening A_V<0.4. Overall, the z=7 galaxies appear substantially less massive and evolved than Lyman break galaxies or Distant Red Galaxies at z=2-3, but fairly similar to recently identified systems at z=5-6. The stellar mass density inferred from our z=7 sample is rho* = 1.6^{+1.6}_{-0.8} x 10^6 M_sun Mpc^-3 (to 0.3 L*(z=3)), in apparent agreement with recent cosmological hydrodynamic simulations, but we note that incompleteness and sample variance may introduce larger uncertainties. The ages of the two most massive galaxies suggest they formed at z>8, during the era of cosmic reionization, but the star formation rate density derived from their stellar masses and ages is not nearly sufficient to reionize the universe. The simplest explanation for this deficiency is that lower-mass galaxies beyond our detection limit reionized the universe.Comment: 4 pages, 3 figures, emulateapj, Accepted for publication in ApJ Letter

The Stellar Population of Lyman-alpha Emitting Galaxies at z ~ 5.7

We present a study of three Lyman-alpha emitting galaxies (LAEs), selected via a narrow-band survey in the GOODS northern field, and spectroscopically confirmed to have redshifts of z ~ 5.65. Using HST ACS and Spitzer IRAC data, we constrain the rest-frame UV-to-optical spectral energy distributions (SEDs) of the galaxies. Fitting stellar population synthesis models to the observed SEDs, we find best-fit stellar populations with masses between ~ 10^9 - 10^10 M_sun and ages between ~ 5 - 100 Myr, assuming a simple starburst star formation history. However, stellar populations as old as 700 Myr are admissible if a constant star formation rate model is considered. Very deep near-IR observations may help to narrow the range of allowed models by providing extra constraints on the rest-frame UV spectral slope. Our narrow-band selected objects and other IRAC-detected z ~ 6 i'-dropout galaxies have similar 3.6 um magnitudes and z' - [3.6] colors, suggesting that they posses stellar populations of similar masses and ages. This similarity may be the result of a selection bias, since the IRAC-detected LAEs and i'-dropouts probably only sample the bright end of the luminosity function. On the other hand, our LAEs have blue i' - z' colors compared to the i'-dropouts, and would have been missed by the i'-dropout selection criterion. A better understanding of the overlap between the LAE and the i'-dropout populations is necessary in order to constrain the properties of the overall high-redshift galaxy population, such as the total stellar mass density at z ~ 6.Comment: 10 pages, 8 figures. Accepted for publication in Ap

The Role of the Dust in Primeval Galaxies: A Simple Physical Model for Lyman Break Galaxies and Lyman Alpha Emitters

We explore the onset of star formation in the early Universe, exploiting the observations of high-redshift Lyman-break galaxies (LBGs) and Lyman alpha emitters (LAEs), in the framework of the galaxy formation scenario elaborated by Granato et al. (2004) already successfully tested against the wealth of data on later evolutionary stages. Complementing the model with a simple, physically plausible, recipe for the evolution of dust attenuation in metal poor galaxies we reproduce the luminosity functions (LFs) of LBGs and of LAEs at different redshifts. This recipe yields a much faster increase with galactic age of attenuation in more massive galaxies, endowed with higher star formation rates. These objects have therefore shorter lifetimes in the LAE and LBG phases, and are more easily detected in the dusty submillimeter bright phase. The short UV bright lifetimes of massive objects strongly mitigate the effect of the fast increase of the massive halo density with decreasing redshift, thus accounting for the weaker evolution of the LBG LF, compared to that of the halo mass function, and the even weaker evolution between z~6 and z~3 of the LAE LF. LAEs are on the average expected to be younger, with lower stellar masses, and associated to less massive halos than LBGs. Finally, we show that the intergalactic medium can be completely reionized at redshift z~6-7 by massive stars shining in protogalactic spheroids with halo masses from a few 10^10 to a few 10^11 M_sun, showing up as faint LBGs with magnitude in the range -17<M_1350<-20, without resorting to any special stellar initial mass function.Comment: 13 pages, 8 figures, uses REVTeX 4 + emulateapj.cls and apjfonts.sty. Title changed and text revised following referee's comments. Accepted by Ap

Finding LoTSS of hosts for GRBs: a search for galaxy - gamma-ray burst coincidences at low frequencies with LOFAR

The LOFAR Two-Metre Sky Survey (LoTSS) is an invaluable new tool for investigating the properties of sources at low frequencies and has helped to open up the study of galaxy populations in this regime. In this work, we perform a search for host galaxies of gamma-ray bursts (GRBs). We use the relative density of sources in Data Release 2 of LoTSS to define the probability of a chance alignment, $P_{\rm chance}$, and find 18 sources corresponding to 17 GRBs which meet a $P_{\rm chance}$<1% criterion. We examine the nature and properties of these radio sources using both LOFAR data and broadband information, including their radio spectral index, star formation rate estimates and any contributions from active galactic nucleus emission. Assuming the radio emission is dominated by star formation, we find that our sources show high star formation rates ($10^1$-$10^3$ $M_{\odot}$ yr$^{-1}$) compared with both a field galaxy sample and a sample of core-collapse supernova hosts, and the majority of putative hosts are consistent with ultraluminous infrared galaxy (ULIRG) classifications. As a result of our analyses, we define a final sample of eight likely GRB host candidates in the LoTSS DR2 survey.Comment: 15 pages, 9 figures and 6 tables. Accepted by MNRA

Relative proportions of polycyclic aromatic hydrocarbons differ between accumulation bioassays and chemical methods to predict bioavailability

Chemical methods to predict the bioavailable fraction of organic contaminants are usually validated in the literature by comparison with established bioassays. A soil spiked with polycyclic aromatic hydrocarbons (PAHs) was aged over six months and subjected to butanol, cyclodextrin and tenax extractions as well as an exhaustive extraction to determine total PAH concentrations at several time points. Earthworm (Eisenia fetida) and rye grass root (Lolium multiflorum) accumulation bioassays were conducted in parallel. Butanol extractions gave the best relationship with earthworm accumulation (r2 ≤ 0.54, p ≤ 0.01); cyclodextrin, butanol and acetone–hexane extractions all gave good predictions of accumulation in rye grass roots (r2 ≤ 0.86, p ≤ 0.01). However, the profile of the PAHs extracted by the different chemical methods was significantly different (p < 0.01) to that accumulated in the organisms. Biota accumulated a higher proportion of the heavier 4-ringed PAHs. It is concluded that bioaccumulation is a complex process that cannot be predicted by measuring the bioavailable fraction alone. The ability of chemical methods to predict PAH accumulation in Eisenia fetida and Lolium multiflorum was hindered by the varied metabolic fate of the different PAHs within the organisms