The Stellar Masses and Star Formation Histories of Galaxies at z ≈ 6: Constraints from Spitzer Observations in the Great Observatories Origins Deep Survey

Using the deep Spitzer Infrared Array Camera (IRAC) observations of the Great Observatories Origins Deep Survey (GOODS), we study the stellar masses and star formation histories of galaxies at z approx 6 based on the i_(775)-band dropout sample selected from the GOODS fields. In total, we derive stellar masses for 53 i_(775)-band dropouts that have robust IRAC detections. These galaxies have typical stellar masses of ~10^(10) M_⊙ and typical ages of a couple of hundred million years, consistent with earlier results based on a smaller sample of z ≈ 6 galaxies. The existence of such massive galaxies at z ≈ 6 can be explained by at least one set of N-body simulations of the hierarchical paradigm. We also study 79 i_(775)-band dropouts that are invisible in the IRAC data and find that they are typically less massive by a factor of 10. These galaxies are much bluer than those detected by the IRAC, indicating that their luminosities are dominated by stellar populations with ages ≾ 40 Myr. Based on our mass estimates, we derive a lower limit to the global stellar mass density at z ≈ 6, which is 1.1-6.7 × 10^6 M_⊙ Mpc^(-3). The prospect of detecting the progenitors of the most massive galaxies at yet higher redshifts is explored. We also investigate the implication of our results for reionization and find that the progenitors of the galaxies comparable to those in our sample, even in the most optimized (probably unrealistic) scenario, cannot sustain the reionization for a period longer than ~2 Myr. Thus most of the photons required for reionization must have been provided by other sources, such as the progenitors of the dwarf galaxies that are far below our current detection capability

GOODS Spitzer IRAC Observations of High-z Galaxies

The Great Observatory Origins Deep Survey (GOODS) Spitzer Legacy Program has obtained ~23.2 hours of IRAC observations in the HDF-N and the CDF-S (~330 arcmin^2 in total), among which a sub-area of ~50 arcmin^2 in the HDF-N has been observed to a depth of ~72 hours. Utilizing these deep IRAC data, we study the stellar masses and star formation histories of galaxies at z ≈ 6. While some of the i-band dropouts selected by the GOODS ACS observations are detected by these IRAC observations, some of them are not. We find that the i_(775)-dropouts in the IRAC-detected sample typically have stellar masses of ~10^(10)M_⊙ and ages of a few hundred million years old, consistent with our earlier results based on a smaller sample in the HUDF. On the other hand, the i-dropouts in the IRAC-invisible sample are significantly younger and less massive. Using these results, we derive the lower limit of the global stellar density at z ≈ 6, and find that at least 0.2–1.1% of the stellar mass in our local universe has been locked in stars by z ≈ 6

Hubble Space Telescope Spectroscopy of Brown Dwarfs Discovered with the Wide-field Infrared Survey Explorer

We present a sample of brown dwarfs identified with the {\it Wide-field Infrared Survey Explorer} (WISE) for which we have obtained {\it Hubble Space Telescope} ({\it HST}) Wide Field Camera 3 (WFC3) near-infrared grism spectroscopy. The sample (twenty-two in total) was observed with the G141 grism covering 1.10$-$1.70 $\mu$m, while fifteen were also observed with the G102 grism, which covers 0.90$-$1.10 $\mu$m. The additional wavelength coverage provided by the G102 grism allows us to 1) search for spectroscopic features predicted to emerge at low effective temperatures (e.g.\ ammonia bands) and 2) construct a smooth spectral sequence across the T/Y boundary. We find no evidence of absorption due to ammonia in the G102 spectra. Six of these brown dwarfs are new discoveries, three of which are found to have spectral types of T8 or T9. The remaining three, WISE J082507.35$+$280548.5 (Y0.5), WISE J120604.38$+$840110.6 (Y0), and WISE J235402.77$+$024015.0 (Y1) are the nineteenth, twentieth, and twenty-first spectroscopically confirmed Y dwarfs to date. We also present {\it HST} grism spectroscopy and reevaluate the spectral types of five brown dwarfs for which spectral types have been determined previously using other instruments.Comment: Accepted for publication in the Astrophysical Journal. 20 pages, 18 figures, 7 table

Optical and Infrared Non-detection of the z=10 Galaxy Behind Abell 1835

Gravitational lensing by massive galaxy clusters is a powerful tool for the discovery and study of high redshift galaxies, including those at z>=6 likely responsible for cosmic re-ionization. Pello et al. recently used this technique to discover a candidate gravitationally magnified galaxy at z=10 behind the massive cluster lens Abell 1835 (z=0.25). We present new Keck (LRIS) and Spitzer Space Telescope (IRAC) observations of the z=10 candidate (hereafter #1916) together with a re-analysis of archival optical and near-infrared imaging from the Hubble Space Telescope and VLT respectively. Our analysis therefore extends from the atmospheric cut-off at lambda_obs=0.35um out to lambda_obs=5um with Spitzer/IRAC. The z=10 galaxy is not detected in any of these data, including an independent reduction of Pello et al.'s discovery H- and K-band imaging. We conclude that there is no statistically reliable evidence for the existence of #1916. We also assess the implications of our results for ground-based near-infrared searches for gravitationally magnified galaxies at z>~7. The broad conclusion is that such experiments remain feasible, assuming that space-based optical and mid-infrared imaging are available to break the degeneracy with low redshift interlopers (e.g. z~2-3) when fitting spectral templates to the photometric data.Comment: Published in ApJ, 636, 575-58

The Formation of Massive Cluster Galaxies

We present composite 3.6 and 4.5 micron luminosity functions for cluster galaxies measured from the Spitzer Deep, Wide-Field Survey (SDWFS) for 0.3<z<2. We compare the evolution of m* for these luminosity functions to models for passively evolving stellar populations to constrain the primary epoch of star formation in massive cluster galaxies. At low redshifts (z < 1.3) our results agree well with models with no mass assembly and passively evolving stellar populations with a luminosity-weighted mean formation redshift zf=2.4 assuming a Kroupa initial mass function (IMF). We conduct a thorough investigation of systematic biases that might influence our results, and estimate systematic uncertainites of Delta zf=(+0.16-0.18) (model normalization), Delta zf=(+0.40-0.05) (alpha), and Delta zf=(+0.30-0.45) (choice of stellar population model). For a Salpeter type IMF, the typical formation epoch is thus strongly constrained to be z ~2-3. Higher formation redshifts can only be made consistent with the data if one permits an evolving IMF that is bottom-light at high redshift, as suggested by van Dokkum et al 2008. At high redshift (z > 1.3) we also witness a statistically significant (>5sigma) disagreement between the measured luminosity function and the continuation of the passive evolution model from lower redshifts. After considering potential systematic biases that might influence our highest redshift data points, we interpret the observed deviation as potential evidence for ongoing mass assembly at this epoch.Comment: 17 pages, 14 figures, accepted for publication in Ap

HST Imaging Polarimetry of the Gravitational Lens FSC10214+4724

We present imaging polarimetry of the extremely luminous, redshift 2.3 IRAS source FSC10214+4724. The observations were obtained with HST's Faint Object Camera in the F437M filter, which is free of strong emission lines. The 0.7 arcsec long arc is unresolved to 0.04 arcsec FWHM in the transverse direction, and has an integrated polarization of 28 +/- 3 percent, in good agreement with ground-based observations. The polarization position angle varies along the arc by up to 35 deg. The overall position angle is 62 +/- 3 deg east of north. No counterimage is detected to B = 27.5 mag ($3\sigma$), giving an observed arc to counterimage flux ratio greater than 250, considerably greater than the flux ratio of 100 measured previously in the I-band. This implies that the configuration of the object in the source plane at the B-band is different from that at I-band, and/or that the lensing galaxy is dusty.Comment: 17 pages, 3 figures. Accepted for publication in Astronomical Journal, February 199

Interferometric Follow-Up of WISE Hyper-Luminous Hot, Dust-Obscured Galaxies

WISE has discovered an extraordinary population of hyper-luminous dusty galaxies which are faint in the two bluer passbands ($3.4\, \mu$m and $4.6\, \mu$m) but are bright in the two redder passbands of WISE ($12\, \mu$m and $22\, \mu$m). We report on initial follow-up observations of three of these hot, dust-obscured galaxies, or Hot DOGs, using the CARMA and SMA interferometer arrays at submm/mm wavelengths. We report continuum detections at $\sim$ 1.3 mm of two sources (WISE J014946.17+235014.5 and WISE J223810.20+265319.7, hereafter W0149+2350 and W2238+2653, respectively), and upper limits to CO line emission at 3 mm in the observed frame for two sources (W0149+2350 and WISE J181417.29+341224.8, hereafter W1814+3412). The 1.3 mm continuum images have a resolution of 1-2 arcsec and are consistent with single point sources. We estimate the masses of cold dust are 2.0$\times 10^{8} M_{\odot}$ for W0149+2350 and 3.9$\times 10^{8} M_{\odot}$ for W2238+2653, comparable to cold dust masses of luminous quasars. We obtain 2$\sigma$ upper limits to the molecular gas masses traced by CO, which are 3.3$\times 10^{10} M_{\odot}$ and 2.3$\times 10^{10} M_{\odot}$ for W0149+2350 and W1814+3412, respectively. We also present high-resolution, near-IR imaging with WFC3 on the Hubble Space Telescope for W0149+2653 and with NIRC2 on Keck for W2238+2653. The near-IR images show morphological structure dominated by a single, centrally condensed source with effective radius less than 4 kpc. No signs of gravitational lensing are evident.Comment: 13 pages, 3 figures. ApJ in pres