Stephen C. Odewahn

The optical counterpart to the γ-ray burst GRB970508

Understanding the nature of the γ-ray burst phenomenon is one of the outstanding problems of modern astrophysics. The identification of counterparts at optical wavelengths is considered a crucial factor for determining the origin of these events. Here we report the detection and temporal properties of a variable optical source, which has been identified, as the counterpart of the X-ray transient associated with the γ-ray burst GRB970508 (ref. 3). The initial optical images were obtained only 5.8 hours after the initial γ-ray burst, after which the optical source was observed to brighten over the next two days before declining in luminosity with a t−1 power law. The decline in brightness follows a form predicted by many relativistic fireball models for γ-ray bursts, although the initial rise does not appear to be compatible with the simplest of these models. The observed fluence of the source at visible wavelengths over the period spanned by our observations is ⩾4.6 × 10−8 erg cm−2, about 3% of the fluence of the γ-ray burst itself.

The Host Galaxy of GRB 990123

We present deep images of the field of GRB 990123 obtained in a broadband UV/visible bandpass with the Hubble Space Telescope (HST) and deep near-infrared images obtained with the Keck I 10 m telescope. The HST image reveals that the optical transient (OT) is offset by 0farcs67 (5.8 kpc in projection) from an extended, apparently interacting galaxy. This galaxy, which we conclude is the host galaxy of GRB 990123, is the most likely source of the absorption lines of metals at a redshift of z=1.6 seen in the spectrum of the OT. With magnitudes of Gunn-r = 24.5 ± 0.2 and K = 22.1±0.3 mag, this corresponds to an L ~ 0.5L

UB V R AND HST MID-UV AND NEAR-IR SURFACE PHOTOMETRY AND RADIAL COLOR GRADIENTS OF LATE-TYPE, IRREGULAR, AND PECULIAR GALAXIES.

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A systematic UBVRIJHK survey of nearby galaxies to classify faint galaxies from deep HST surveys

galaxy, assuming that it is located at z = 1.6. The estimated unobscured star formation rate is ≈4 M_☉ yr^(−1), which is typical for normal galaxies at comparable redshifts. There is no evidence for strong gravitational lensing magnification of this burst, and some alternative explanation for its remarkable energetics (such as beaming) may therefore be required. The observed offset of the OT from the nominal host center, the absence of broad absorption lines in the afterglow spectrum, and the relatively blue continuum of the host do not support the notion that gamma-ray bursts (GRBs) originate from active galactic nuclei or massive black holes. Rather, the data are consistent with models of GRBs that involve the death and/or merger of massive stars. Indeed, the HST image suggests an intimate connection between GRB 990123 and a star-forming region.

GRB 990123: discovery of the probable host galaxy.

We introduce a data set of 142 mostly late-type spiral, irregular, and peculiar (interacting or merging) nearby galaxies observed in UBVR at the Vatican Advanced Technology Telescope (VATT), and we present an analysis of their radial color gradients. We confirm that nearby elliptical and early- to mid-type spiral galaxies show either no or only small color gradients, becoming slightly bluer with radius. In contrast, we find that late-type spiral, irregular, peculiar, and merging galaxies become on average redder with increasing distance from the center. The scatter in radial color gradient trends increases toward later Hubble type. As a preliminary analysis of a larger data set obtained with the Hubble Space Telescope (HST), we also analyze the color gradients of six nearby galaxies observed with NICMOS in the near-IR (H) and with WFPC2 in the mid-UV (F300W) and red (F814W). We discuss the possible implications of these results on galaxy formation and compare our nearby galaxy color gradients to those at high redshift. We present examples of images and UBVR radial surface brightness and color profiles, as well as of the tables of measurements; the full atlas and tables are published in the electronic edition only.

The Luminosity Function Normalization and the Faint Galaxy Counts

To quantify the changes of galaxy parameters with rest-frame wavelength (and redshift), we present a large systematic survey in UBVRIJHK of about 400 nearby galaxies of all inclinations, Hubble types, and colors. These galaxies come from samples already imaged in (a subset of) the filters UBVRIJHK by de Jong et al. [6], Frei et al. [9], and Frogel et al. [10]. We present the first part of a large U-band imaging project with the new high throughput wide-field camera at the Vatican Advanced Technology Telescope (VATT) on Mt. Graham, AZ. The combined UBVRIJHK sample will allow us to quantify local galaxy parameters in the range 3000u2009A−2.2u2009μm. A modeling analysis applied to faint field galaxies seen with Hubble Space Telescope (HST) will allow us to address the relative importance of bulge versus disk dominated galaxies, of late-type versus irregular galaxies, and of low surface brightness (SB) galaxies versus sub-galactic clumps as a function of redshift.