Nimish P. Hathi

Spectroscopic Discovery of the Supernova 2003dh Associated with GRB 030329

We present early observations of the afterglow of GRB 030329 and the spectroscopic discovery of its associated supernova SN 2003dh. We obtained spectra of the afterglow of GRB 030329 each night from March 30.12 (0.6 days after the burst) to April 8.13 (UT) (9.6 days after the burst). The spectra cover a wavelength range of 350-850 nm. The early spectra consist of a power-law continuum (Fν ν-0.9) with narrow emission lines originating from H II regions in the host galaxy, indicating a low redshift of z = 0.1687. However, our spectra taken after 2003 April 5 show broad peaks in flux characteristic of a supernova. Correcting for the afterglow emission, we find that the spectrum of the supernova is remarkably similar to the Type Ic hypernova SN 1998bw. While the presence of supernovae has been inferred from the light curves and colors of gamma-ray burst afterglows in the past, this is the first direct, spectroscopic confirmation that a subset of classical gamma-ray bursts originate from supernovae.

CANDELS: Constraining the AGN-Merger Connection with Host Morphologies at z 2

Using Hubble Space Telescope/WFC3 imaging taken as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, we examine the role that major galaxy mergers play in triggering active galactic nucleus (AGN) activity at z ~ 2. Our sample consists of 72 moderate-luminosity (L X ~ 1042-44 erg s–1) AGNs at 1.5 < z < 2.5 that are selected using the 4 Ms Chandra observations in the Chandra Deep Field South, the deepest X-ray observations to date. Employing visual classifications, we have analyzed the rest-frame optical morphologies of the AGN host galaxies and compared them to a mass-matched control sample of 216 non-active galaxies at the same redshift. We find that most of the AGNs reside in disk galaxies (51.4+5.8 – 5.9%), while a smaller percentage are found in spheroids (27.8+5.8 – 4.6%). Roughly 16.7+5.3 – 3.5% of the AGN hosts have highly disturbed morphologies and appear to be involved in a major merger or interaction, while most of the hosts (55.6+5.6 – 5.9%) appear relatively relaxed and undisturbed. These fractions are statistically consistent with the fraction of control galaxies that show similar morphological disturbances. These results suggest that the hosts of moderate-luminosity AGNs are no more likely to be involved in an ongoing merger or interaction relative to non-active galaxies of similar mass at z ~ 2. The high disk fraction observed among the AGN hosts also appears to be at odds with predictions that merger-driven accretion should be the dominant AGN fueling mode at z ~ 2, even at moderate X-ray luminosities. Although we cannot rule out that minor mergers are responsible for triggering these systems, the presence of a large population of relatively undisturbed disk-like hosts suggests that the stochastic accretion of gas plays a greater role in fueling AGN activity at z ~ 2 than previously thought.

The Evolution of the Galaxy Rest-Frame Ultraviolet Luminosity Function Over the First Two Billion Years

We present a robust measurement and analysis of the rest-frame ultraviolet (UV) luminosity functions at z = 4-8. We use deep Hubble Space Telescope imaging over the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey/GOODS fields, the Hubble Ultra Deep Field, and the Hubble Frontier Field deep parallel observations near the Abell 2744 and MACS J0416.1-2403 clusters. The combination of these surveys provides an effective volume of 0.6-1.2 x 10(6) Mpc(3) over this epoch, allowing us to perform a robust search for faint (M-UV = -18) and bright (Muv \textless -21) high-redshift galaxies. We select candidate galaxies using a well-tested photometric redshift technique with careful screening of contaminants, finding a sample of 7446 candidate galaxies at 3.5 \textless z \textless 8.5, with \textgreater1000 galaxies at z approximate to 6-8. We measure both a stepwise luminosity function for candidate galaxies in our redshift samples, and a Schechter function, using a Markov Chain Monte Carlo analysis to measure robust uncertainties. At the faint end, our UV luminosity functions agree with previous studies, yet we find a higher abundance of UV-bright candidate galaxies at z \textgreater= 6. Our best-fit value of the characteristic magnitude MN is consistent with -21 at z \textgreater=, 5, which is different than that inferred based on previous trends at lower redshift, and brighter at similar to 2 sigma significance than previous measures at z = 6 and 7. At z = 8, a single power law provides an equally good fit to the UV luminosity function, while at z = 6 and 7 an exponential cutoff at the bright end is moderately preferred. We compare our luminosity functions to semi-analytical models, and find that the lack of evolution in M-UV(*) is consistent with models where the impact of dust attenuation on the bright end of the luminosity function decreases at higher redshift, although a decreasing impact of feedback may also be possible. We measure the evolution of the cosmic star-formation rate (SFR) density by integrating our observed luminosity functions to M-UV = -17, correcting for dust attenuation, and find that the SFR density declines proportionally to (1 +z)(-4.3 +/- 0 5) at z \textgreater 4, which is consistent with observations at z \textgreater= 9. Our observed luminosity functions are consistent with a reionization history that starts at z greater than or similar to 10, completes at z \textgreater 6, and reaches a midpoint (xH = 0.5) at 6.7 \textless z \textless9.4. Finally, using a constant cumulative number density selection and an empirically derived rising star-formation history, our observations predict that the abundance of bright z = 9 galaxies is likely higher than previous constraints, although consistent with recent estimates of bright z similar to 10 galaxies.

CANDELS: The Progenitors of Compact Quiescent Galaxies at z 2

We combine high-resolution Hubble Space Telescope/WFC3 images with multi-wavelength photometry to track the evolution of structure and activity of massive (M > 1010 M ?) galaxies at redshifts z = 1.4-3 in two fields of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey. We detect compact, star-forming galaxies (cSFGs) whose number densities, masses, sizes, and star formation rates (SFRs) qualify them as likely progenitors of compact, quiescent, massive galaxies (cQGs) at z = 1.5-3. At z 2, cSFGs present SFR = 100-200 M ? yr?1, yet their specific star formation rates (sSFR ~ 10?9?yr?1) are typically half that of other massive SFGs at the same epoch, and host X-ray luminous active galactic nuclei (AGNs) 30?times (~30%) more frequently. These properties suggest that cSFGs are formed by gas-rich processes (mergers or disk-instabilities) that induce a compact starburst and feed an AGN, which, in turn, quench the star formation on dynamical timescales (few 108?yr). The cSFGs are continuously being formed at z = 2-3 and fade to cQGs down to z ~ 1.5. After this epoch, cSFGs are rare, thereby truncating the formation of new cQGs. Meanwhile, down to z = 1, existing cQGs continue to enlarge to match local QGs in size, while less-gas-rich mergers and other secular mechanisms shepherd (larger) SFGs as later arrivals to the red sequence. In summary, we propose two evolutionary tracks of QG formation: an early (z 2), formation path of rapidly quenched cSFGs fading into cQGs that later enlarge within the quiescent phase, and a late-arrival (z 2) path in which larger SFGs form extended QGs without passing through a compact state.

CANDELS: THE EVOLUTION OF GALAXY REST-FRAME ULTRAVIOLET COLORS FROM z = 8 TO 4

We study the evolution of galaxy rest-frame ultraviolet (UV) colors in the epoch 4 z 8. We use new wide-field near-infrared data in the Great Observatories Origins Deep Survey-South field from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Hubble Ultra Deep Field (HUDF) 2009, and Early Release Science programs to select galaxies via photometric redshift measurements. Our sample consists of 2812 candidate galaxies at z 3.5, including 113 at z 7-8. We fit the observed spectral energy distribution to a suite of synthetic stellar population models and measure the value of the UV spectral slope (?) from the best-fit model spectrum. We run simulations to show that this measurement technique results in a smaller scatter on ? than other methods, as well as a reduced number of galaxies with catastrophically incorrect ? measurements (i.e., ?? > 1). We find that the median value of ? evolves significantly from ?1.82+0.00 ? 0.04 at z?= 4 to ?2.37+0.26 ? 0.06 at z?= 7. Additionally, we find that faint galaxies at z?= 7 have ? = ?2.68+0.39 ? 0.24 (~ ?2.4 after correcting for observational bias); this is redder than previous claims in the literature and does not require exotic stellar populations (e.g., very low metallicities or top-heavy initial mass functions) to explain their colors. This evolution can be explained by an increase in dust extinction, from low amounts at z?= 7 to A V ~ 0.5?mag at z?= 4. The timescale for this increase is consistent with low-mass asymptotic giant branch stars forming the bulk of the dust. We find no significant (<2?) correlation between ? and M UV when measuring M UV at a consistent rest-frame wavelength of 1500??. This is particularly true at bright magnitudes, though our results do show evidence for a weak correlation at faint magnitudes when galaxies in the HUDF are considered separately, hinting that dynamic range in sample luminosities may play a role. We do find a strong correlation between ? and the stellar mass at all redshifts, in that more massive galaxies exhibit redder colors. The most massive galaxies in our sample have similarly red colors at each redshift, implying that dust can build up quickly in massive galaxies and that feedback is likely removing dust from low-mass galaxies at z ? 7. Thus, the stellar-mass?metallicity relation, previously observed up to z ~ 3, may extend out to z?= 7-8.

Photometry and Spectroscopy of GRB 030329 and Its Associated Supernova 2003dh: The First Two Months

We present extensive optical and infrared photometry of the afterglow of gamma-ray burst (GRB) 030329 and its associated supernova (SN) 2003dh over the first two months after detection (2003 March 30-May 29 UT). Optical spectroscopy from a variety of telescopes is shown and, when combined with the photometry, allows an unambiguous separation between the afterglow and SN contributions. The optical afterglow of the GRB is initially a power-law continuum but shows significant color variations during the first week that are unrelated to the presence of an SN. The early afterglow light curve also shows deviations from the typical power-law decay. An SN spectrum is first detectable ~7 days after the burst and dominates the light after ~11 days. The spectral evolution and the light curve are shown to closely resemble those of SN 1998bw, a peculiar Type Ic SN associated with GRB 980425, and the time of the SN explosion is close to the observed time of the GRB. It is now clear that at least some GRBs arise from core-collapse SNe.

Smooth(er) Stellar Mass Maps in CANDELS: Constraints on the Longevity of Clumps in High-redshift Star-forming Galaxies

We perform a detailed analysis of the resolved colors and stellar populations of a complete sample of 323 star-forming galaxies (SFGs) at 0.5 < z < 1.5 and 326 SFGs at 1.5 < z < 2.5 in the ERS and CANDELS-Deep region of GOODS-South. Galaxies were selected to be more massive than 10(10) M-circle dot and have specific star formation rates (SFRs) above 1/t(H). We model the seven-band optical ACS + near-IR WFC3 spectral energy distributions of individual bins of pixels, accounting simultaneously for the galaxy-integrated photometric constraints available over a longer wavelength range. We analyze variations in rest-frame color, stellar surface mass density, age, and extinction as a function of galactocentric radius and local surface brightness/density, and measure structural parameters on luminosity and stellar mass maps. We find evidence for redder colors, older stellar ages, and increased dust extinction in the nuclei of galaxies. Big star-forming clumps seen in star formation tracers are less prominent or even invisible in the inferred stellar mass distributions. Off-center clumps contribute up to similar to 20% to the integrated SFR, but only 7% or less to the integrated mass of all massive SFGs at z similar to 1 and z similar to 2, with the fractional contributions being a decreasing function of wavelength used to select the clumps. The stellar mass profiles tend to have smaller sizes and M20 coefficients, and higher concentration and Gini coefficients than the light distribution. Our results are consistent with an inside-out disk growth scenario with brief (100-200 Myr) episodic local enhancements in star formation superposed on the underlying disk. Alternatively, the young ages of off-center clumps may signal inward clump migration, provided this happens efficiently on the order of an orbital timescale.

A CRITICAL ASSESSMENT OF PHOTOMETRIC REDSHIFT METHODS: A CANDELS INVESTIGATION

We present results from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) photometric redshift methods investigation. In this investigation, the results from 11 participants, each using a different combination of photometric redshift code, template spectral energy distributions (SEDs), and priors, are used to examine the properties of photometric redshifts applied to deep fields with broadband multi-wavelength coverage. The photometry used includes U-band through mid-infrared filters and was derived using the TFIT method. Comparing the results, we find that there is no particular code or set of template SEDs that results in significantly better photometric redshifts compared to others. However, we find that codes producing the lowest scatter and outlier fraction utilize a training sample to optimize photometric redshifts by adding zero-point offsets, template adjusting, or adding extra smoothing errors. These results therefore stress the importance of the training procedure. We find a strong dependence of the photometric redshift accuracy on the signal-to-noise ratio of the photometry. On the other hand, we find a weak dependence of the photometric redshift scatter with redshift and galaxy color. We find that most photometric redshift codes quote redshift errors (e.g., 68% confidence intervals) that are too small compared to that expected from the spectroscopic control sample. We find that all codes show a statistically significant bias in the photometric redshifts. However, the bias is in all cases smaller than the scatter; the latter therefore dominates the errors. Finally, we find that combining results from multiple codes significantly decreases the photometric redshift scatter and outlier fraction. We discuss different ways of combining data to produce accurate photometric redshifts and error estimates.

VERY STRONG EMISSION-LINE GALAXIES IN THE WFC3 INFRARED SPECTROSCOPIC PARALLEL SURVEY AND IMPLICATIONS FOR HIGH-REDSHIFT GALAXIES* , **

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 A. 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.

Faint AGNs at z > 4 in the CANDELS GOODS-S Field: Looking for Contributors to the Reionization of the Universe

Context. Establishing the number of faint active galactic nuclei (AGNs) at z = 4-6 is crucial to understanding their cosmological importance as main contributors to the reionization of the Universe. Aims. In order to derive the AGN contribution to the cosmological ionizing emissivity we have selected faint AGN candidates at z \textgreater 4 in the CANDELS GOODS-South field, which is one of the deepest fields with extensive multiwavelength coverage from Chandra, HST, Spitzer, and various ground-based telescopes. Methods. We have adopted a relatively novel criterion. As a first step, high redshift galaxies are selected in the NIR H band down to very faint levels (H \textless= 27) using reliable photometric redshifts. At z \textgreater 4 this corresponds to a selection criterion based on the galaxy rest-frame UV flux. AGN candidates are then picked up from this parent sample if they show X-ray fluxes above a threshold of F-X similar to 1.5 x 10(-17) erg cm(-2) s(-1) (0.5-2 keV), corresponding to a probability of spurious detections of 2 x 10(-4) in the deep X-ray 4 Ms Chandra image. Results. We have found 22 AGN candidates at z \textgreater 4 and we have derived the first estimate of the UV luminosity function in the redshift interval 4 \textless z \textless 6.5 and absolute magnitude interval -22.5 less than or similar to M-1450 less than or similar to -18.5 typical of local Seyfert galaxies. The faint end of the derived luminosity function is about two to four magnitudes fainter at z similar to 4-6 than that derived from previous UV surveys. We estimated ionizing emissivities and hydrogen photoionization rates in the same redshift interval under reasonable assumptions and after discussion of possible caveats, the most important being the large uncertainties involved in the estimate of photometric redshift for sources with featureless, almost power-law SEDs and/or low average escape fraction of ionizing photons from the AGN host galaxies. Both effects could, in principle, significantly reduce the estimated average volume densities and/or ionizing emissivities, especially at the highest redshifts. Conclusions. At z = 4-6.5 we argue that, under reasonable evaluations of possible biases, the probed AGN population can produce photoionization rates consistent with that required to keep the intergalactic medium observed in the Lyman-alpha forest of high redshift QSO spectra highly ionized, providing an important contribution to the cosmic reionization.