We present new measurements of the evolution in the Lyman break galaxy (LBG) population between z ≃ 4 and z ≃ 6. By utilizing the extensive multiwavelength datasets available in the GOODS fields, we identify 2443 B, 506 V, and 137 i ′ -band dropout galaxies likely to be at z ≈ 4, 5, & 6. For the subset of dropouts for which reliable Spitzer IRAC photometry is feasible (roughly 35% of the sample), we estimate luminosity-weighted ages and stellar masses. With the goal of understanding the duration of typical star formation episodes in galaxies at z ∼ > 4, we examine the distribution of stellar masses and ages as a function of cosmic time. We find that at a fixed rest-UV luminosity, the average stellar masses and ages of galaxies do not increase significantly between z ≃ 6 and 4. In order to maintain this near equilibrium in the average properties of high redshift LBGs, we argue that there must be a steady flux of young, newly-luminous objects at each successive redshift. When considered along with the short duty cycles inferred from clustering measurements, these results may suggest that galaxies are undergoing star formation episodes lasting only several hundred million years. In contrast to the unchanging relationship between the average stellar mass and rest-UV luminosity, we find that the number density of massive galaxies increases considerably with time over 4 ∼ < z ∼ < 6. Given this rapid increase of UV luminous massive galaxies, we explore the possibility that a significant fraction of massive (10 11 M ⊙ ) z ≃ 2−3 distant red galaxies (DRGs) were in part assembled in an LBG phase at earlier times. Integrating the growth in the stellar mass function of actively forming LBGs over 4 ∼ < z ∼ < 6 down to z ≃ 2, we find that z ∼ > 3 LBGs could have contributed significantly to the quiescent DRG population, indicating that the intense star-forming systems probed by sub-millimeter observations are not the only route toward the assembly of DRGs at z ≃ 2.

We present the first results of a new Keck spectroscopic survey of UV faint Lyman break galaxies in the redshift range 3 < z < 7. Combined with earlier Keck and published European Southern Observatory (ESO) VLT data, our spectroscopic sample contains more than 600 dropouts offering new insight into the nature of sub-L * sources typical of those likely to dominate the cosmic reionization process. In this first paper, in a series discussing these observations, we characterize the fraction of strong Lyα emitters within the continuum-selected dropout population. By quantifying how the 'Lyα fraction', x Lyα , varies with redshift, we seek to constrain changes in Lyα transmission associated with reionization. In order to distinguish the effects of reionization from other factors which affect the Lyα fraction [e.g. dust, interstellar medium (ISM) kinematics], we study the luminosity and redshift-dependence of the Lyα fraction over 3 z 6, when the intergalactic medium (IGM) is known to be ionized. These results reveal that low-luminosity galaxies show strong Lyα emission much more frequently (x Lyα = 0.47 ± 0.16 at M UV = −19) than luminous systems (x Lyα = 0.08 ± 0.02 at M UV = −21), and that at fixed luminosity, the prevalence of strong Lyman α emission increases moderately with redshift over 3 < z < 6 (d x Lyα /d z = 0.05 ± 0.03). Based on the bluer mean UV slopes of the strong Lyα emitting galaxies in our data set ( β Lyα − β no Lyα = −0.33 ± 0.09 at M UV = −20.5) we argue that the Lyα fraction trends are governed by redshift and luminosity-dependent variations in the dust obscuration, with likely additional contributions from trends in the kinematics and covering fraction of neutral hydrogen. Using the limited infrared spectroscopy of candidate z 7 galaxies, we find a tentative decrease in the Lyα fraction by a factor of >1.9 with respect to the predicted z 7 value, a result which, if confirmed with future surveys, would suggest an increase in the neutral fraction by this epoch. Given the abundant supply of z and Y drops now available from deep Hubble WFC3/IR surveys, we show it will soon be possible to significantly improve estimates of the Lyα fraction using optical and near-infrared multi-object spectrographs, thereby extending the study conducted in this paper to 7 z 8.

We determine the abundance of i -band dropouts in the recently released HST/ACS Hubble Ultra-Deep Field (UDF). Because the majority of these sources are likely to be z ≈ 6 galaxies whose flux decrement between the F775W i -band and F850LP z -band arises from Lyman-α absorption, the number of detected candidates provides a valuable upper limit to the unextincted star formation rate at this redshift. We demonstrate that the increased depth of UDF enables us to reach an 8 σ limiting magnitude of z AB = 28.5 (equivalent to 1.5 h −2 70 M yr −1 at z = 6.1, or 0.1 L UV for the z ≈ 3 U-drop population), permitting us to address earlier ambiguities arising from the unobserved form of the luminosity function. We identify 54 galaxies (and only one star) at z AB < 28.5 with (i − z ) AB > 1.3 over the deepest 11-arcmin 2 portion of the UDF. The characteristic luminosity (L ) is consistent with values observed at z ≈ 3. The faint end slope (α) is less well constrained, but is consistent with only modest evolution. The main change appears to be in the number density ( * ). Specifically, and regardless of possible contamination from cool stars and lower-redshift sources, the UDF data support our previous result that the star formation rate at z ≈ 6 was approximately six times less than at z ≈ 3. This declining comoving star formation rate [0.005 h 70 M yr −1 Mpc −3 at z ≈ 6 at L UV > 0.1 L for a Salpeter initial mass function (IMF)] poses an interesting challenge for models which suggest that L UV > 0.1 L star-forming galaxies at z 6 reionized the Universe. The short-fall in ionizing photons might be alleviated by galaxies fainter than our limit, or a radically different IMF. Alternatively, the bulk of reionization might have occurred at z 6.

Original article can be found at: http://www.iop.org/EJ/journal/apj Copyright American Astronomical Society [Full text of this article is not available in the UHRA

We report on observations of GRB 080503, a short gamma-ray burst with very bright extended emission (about 30 times the gamma-ray fluence of the initial spike) in conjunction with a thorough comparison to other short Swift events. In spite of the prompt-emission brightness, however, the optical counterpart is extraordinarily faint, never exceeding 25 mag in deep observations starting at ∼1 hr after the BAT trigger. The optical brightness peaks at ∼ 1 day and then falls sharply in a manner similar to the predictions of Li & Paczyński (1998) for supernova-like emission following compactbinary mergers. However, a shallow spectral index and similar evolution in X-rays inferred from Chandra observations are more consistent with an afterglow interpretation. The extreme faintness of this probable afterglow relative to the bright gamma-ray emission argues for a very low-density medium surrounding the burst (a "naked" GRB), consistent with the lack of a coincident host galaxy down to 28.5 mag in deep Hubble Space Telescope imaging. The late optical and X-ray peak could be explained by a slightly off-axis jet or by a refreshed shock. Our observations reinforce the notion that short gamma-ray bursts generally occur outside regions of active star formation, but demonstrate that in some cases the luminosity of the extended prompt emission can greatly exceed that of the short spike, which may constrain theoretical interpretation of this class of events. This extended emission is not the onset of an afterglow, and its relative brightness is probably either a viewing-angle effect or intrinsic to the central engine itself. Because most previous BAT short bursts without observed extended emission are too faint for this signature to have been detectable even if it were present at typical level, conclusions based solely on the observed presence or absence of extended emission in the existing Swift sample are premature.

We present results from near-infrared spectroscopy of 26 emission-line galaxies at z ∼ 2.2 and z ∼ 1.5 obtained with the Folded-port InfraRed Echellette (FIRE) spectrometer on the 6.5-meter Magellan Baade telescope. The sample was selected from the WFC3 Infrared Spectroscopic Parallels (WISP) survey, which uses the near-infrared grism of the Hubble Space Telescope Wide Field Camera 3 to detect emission-line galaxies over 0.3 z 2.3. Our FIRE follow-up spectroscopy (R∼5000) over 1.0-2.5 µm permits detailed measurements of physical properties of the z ∼ 2 emission-line galaxies. Dust-corrected star formation rates for the sample range from ∼5-100 M ⊙ yr −1 with a mean of 29 M ⊙ yr −1 . We derive a median metallicity for the sample of 12 + log(O/H) = 8.34 or ∼0.45 Z ⊙ . The estimated stellar masses range from ∼10 8.5 − 10 9.5 M ⊙ , and a clear positive correlation between metallicity and stellar mass is observed. The average ionization parameter measured for the sample, log U ≈ −2.5, is significantly higher than what is found for most star-forming galaxies in the local universe, but similar to the values found for other star-forming galaxies at high redshift. We derive composite spectra from the FIRE sample, from which we infer typical nebular electron densities of ∼100-400 cm −3 . Based on the location of the galaxies and composite spectra on BPT diagrams, we do not find evidence for significant AGN activity in the sample. Most of the galaxies as well as the composites are offset in the BPT diagram toward higher [O iii]/Hβ at a given [N ii]/Hα, in agreement with other observations of z 1 star-forming galaxies, but composite spectra derived from the sample do not show an appreciable offset from the local star-forming sequence on the [O iii]/Hβ versus [S ii]/Hα diagram. We infer a high nitrogen-to-oxygen abundance ratio from the composite spectrum, which may contribute to the offset of the high-redshift galaxies from the local star-forming sequence in the [O iii]/Hβ versus [N ii]/Hα diagram. We speculate that the elevated nitrogen abundance could result from substantial numbers of Wolf-Rayet stars in starbursting galaxies at z ∼ 2.

The WFC3 Infrared Spectroscopic Parallel Survey uses the Hubble Space Telescope (HST) infrared grism capabilities to obtain slitless spectra of thousands of galaxies over a wide redshift range including the peak of star formation history of the universe. We select a population of very strong emission-line galaxies with rest-frame equivalent widths (EWs) higher than 200 Å. A total of 176 objects are found over the redshift range 0.35 < z < 2.3 in the 180 arcmin 2 area that we have analyzed so far. This population consists of young and low-mass starbursts with high specific star formation rates (sSFR). After spectroscopic follow-up of one of these galaxies with Keck/Low Resolution Imaging Spectrometer, we report the detection at z = 0.7 of an extremely metal-poor galaxy with 12 + log(O/H) = 7.47 ± 0.11. After estimating the active galactic nucleus fraction in the sample, we show that the high-EW galaxies have higher sSFR than normal star-forming galaxies at any redshift. We find that the nebular emission lines can substantially affect the total broadband flux density with a median brightening of 0.3 mag, with some examples of line contamination producing brightening of up to 1 mag. We show that the presence of strong emission lines in low-z galaxies can mimic the color-selection criteria used in the z ∼ 8 dropout surveys. In order to effectively remove low-redshift interlopers, deep optical imaging is needed, at least 1 mag deeper than the bands in which the objects are detected. Without deep optical data, most of the interlopers cannot be ruled out in the wide shallow HST imaging surveys. Finally, we empirically demonstrate that strong nebular lines can lead to an overestimation of the mass and the age of galaxies derived from fitting of their spectral energy distribution (SED). Without removing emission lines, the age and the stellar mass estimates are overestimated by a factor of 2 on average and up to a factor of 10 for the high-EW galaxies. Therefore, the contribution of emission lines should be systematically taken into account in SED fitting of star-forming galaxies at all redshifts.

The KMOS Redshift One Spectroscopic Survey (KROSS) is an ESO guaranteed time survey of 795 typical star-forming galaxies in the redshift range z = 0.8−1.0 with the KMOS instrument on the VLT. In this paper we present resolved kinematics and star formation rates for 584 z ∼ 1 galaxies. This constitutes the largest near-infrared Integral Field Unit survey of galaxies at z ∼ 1 to date. We demonstrate the success of our selection criteria with 90% of our targets found to be Hα emitters, of which 81% are spatially resolved. The fraction of the resolved KROSS sample with dynamics dominated by ordered rotation is found to be 83 ± 5%. However, when compared with local samples these are turbulent discs with high gas to baryonic mass fractions, ∼ 35%, and the majority are consistent with being marginally unstable (Toomre Q ∼ 1). There is no strong correlation between galaxy averaged velocity dispersion and the total star formation rate, suggesting that feedback from star formation is not the origin of the elevated turbulence. We postulate that it is the ubiquity of high (likely molecular) gas fractions and the associated gravitational instabilities that drive the elevated star-formation rates in these typical z ∼ 1 galaxies, leading to the ten-fold enhanced starformation rate density. Finally, by comparing the gas masses obtained from inverting the starformation law with the dynamical and stellar masses, we infer an average dark matter to total mass fraction within 2.2 r e (9.5 kpc) of 65 ± 12%, in agreement with the results from hydrodynamic simulations of galaxy formation.