Showing papers in "The Astronomical Journal in 1996"

A Catalog of Parameters for Globular Clusters in the Milky Way

Abstract: A database of parameters for globular star clusters in the Milky Way is described which is available in electronic form through the WorldWideWeb. The information in the catalog includes up-to-date measurements for cluster distance, reddening, luminosity, colors and spectral types, velocity, structural and dynamical parameters, horizontal branch morphology, metallicity, and other quantities. This catalog will be updated regularly and maintained in electronic form for widest possible accessibility. Associated Articles Source Paper Catalog Description Catalog Description

The Sloan digital sky survey photometric system

Abstract: This paper describes the Sloan Digital Sky Survey photometric system, a new five-color (u' g' r' i' z') wide-band CCD system with wavelength coverage from 3000 to 11 000 A. The zero points will be based on an updated version of the spectrophotometric ABv system. This updated calibration, designated as AB95, is presented in this paper.

New Insight on Galaxy Formation and Evolution From Keck Spectroscopy of the Hawaii Deep Fields

Abstract: We present the results of spectroscopic studies with the LRIS spectrograph on Keck of two of the Hawaii deep survey fields. The 393 objects observed cover an area of 26.2 square arcmin and constitute a nearly complete sample down to K = 20, I = 23, and B = 24.5. The rest-frame K-band luminosity function and its evolution with redshift are described. Comparisons are made with other optically selected (B and I) samples in the literature, and the corresponding rest-frame B-band luminosity function evolution is presented. The B-band counts near B = 24 are shown to be a mixture of normal galaxies at modest redshifts and galaxies undergoing rapid star formation, which have a wide range of masses and which are spread over the redshift interval from z = 0.2 to beyond z = 1.7. The luminosity functions, number counts, and color distributions at optical and IR wavelengths are discussed in terms of a consistent picture of the star-forming history of the galaxy sample. [OII] emission-line diagnostics or rest-frame ultra-violet--infrared color information are used in combination with rest-frame absolute K magnitudes to construct a ``fundamental plane'' in which the evolution of the global star-formation rate with redshift can be shown, and we find that the maximum rest-frame K luminosity of galaxies undergoing rapid star formation has been declining smoothly with decreasing redshift from a value near L* at z > 1. This smooth decrease in the characteristic luminosity of galaxies dominated by star formation can simultaneously account for the high B-band galaxy counts at faint magnitudes and the redshift distribution at z < 1 in both the B- and K-selected samples. Finally, the overall K-band light density evolution is discussed as a tracer of the baryonic mass in stars and compared with the rate of star formation.

The Hubble Deep Field: Observations, data reduction, and galaxy photometry

Abstract: The Hubble Deep Field (HDF) is a Director’s Discretionary program on HST in Cycle 5 to image an undistinguished field at high Galactic latitude in four passbands as deeply as reasonably possible. These images provide the most detailed view to date of distant field galaxies and are likely to be important for a wide range of studies in galaxy evolution and cosmology. In order to optimize observing in the time available, a field in the northern continuous viewing zone was selected and images were taken for ten consecutive days, or approximately 150 orbits. Shorter 1-2 orbit images were obtained of the fields immediately adjacent to the primary HDF in order to facilitate spectroscopic follow-up by ground-based telescopes. The observations were made from 18 to 30 December 1995, and both raw and reduced data have been put in the public domain as a community service. We present a summary of the criteria for selecting the field, the rationale behind the filter selection and observing times in each band, and the strategies for planning the observations to maximize the exposure time while avoiding earth-scattered light. Data reduction procedures are outlined, and images of the combined frames in each band are presented. Objects detected in these images are listed in a catalog with their basic photometric parameters.

The Absolute Luminosities of the Calan/Tololo Type IA Supernovae

Abstract: We examine the absolute luminosities of 29 SNe Ia in the Calan/Tololo survey. We confirm a relation between the peak luminosity of the SNe and the decline rate as measured by the light curve, as suggested by Phillips (1993). We derive linear slopes to this magnitude-decline rate relation in BV(I)kc colors, using a sample with Bmax-Vmax < 0.2 mag. The scatter around this linear relation (and thus the ability to measure SNe Ia distances) ranges from 0.13 mag (in the I band) to 0.17 mag (in the B band). We also find evidence for significant correlations between the absolute magnitudes or the decline rate of the light curve, and the morphological type of the host galaxy.

Spectroscopy of Lyman Break Galaxies in the Hubble Deep Field

Abstract: We report on the initial results of a spectroscopic investigation of galaxies in the Hubble Deep Field which exhibit spectral discontinuities between the F450W and F300W passbands, indicative of the presence of the Lyman continuum break in the redshift range 2.4 3 galaxies we have previously identified in other fields, characterized by very compact cores (some with multiple components) with half--light radii of 0.2-0.3 arcseconds, often surrounded by more diffuse and asymmetric ``halos''. A few of the brighter HDF Lyman break galaxies, however, have particularly unusual morphologies.

Hubble Space Telescope WFPC2 Imaging of M16: Photoevaporation and Emerging Young Stellar Objects

Abstract: We present Hubble Space Telescope WFPC2 images of elephant trunks in the H II region M16. There are three principle results of this study. First, the morphology and stratified ionization structure of the interface between the dense molecular material and the interior of the H II region is well understood in terms of photoionization of a photoevaporative flow. Photoionization models of an empirical density profile capture the essential features of the observations, including the extremely localized region of [S II] emission at the interface and the observed offset between emission peaks in lower and higher ionization lines. The details of this structure are found to be a sensitive function both of the density profile of the interface and of the shape of the ionizing continuum. Interpretation of the interaction of the photoevaporative flow with gas in the interior of the nebula supports the view that much of the emission from H II regions may arise in such flows. Photoionization of photoevaporative flows may provide a useful paradigm for interpreting a wide range of observations of H II regions. Second, we report the discovery of a population of small cometary globules that are being uncovered as the main bodies of the elephant trunks are dispersed. Several lines of evidence connect these globules to ongoing star formation, including the association of a number of globules with stellar objects seen in IR images of M16 or in the continuum HST images themselves. We refer to these structures as evaporating gaseous globules, or "EGGs." These appear to be the same type of object as the nebular condensations seen previously in M42. The primary difference between the two cases is that in M16 we are seeing the objects from the side, while in M42 the objects are seen more nearly face-on against the backdrop of the ionized face of the molecular cloud. We find that the "evaporating globule" interpretation naturally accounts for the properties of objects in both nebulae, while avoiding serious difficulties with the competing "evaporating disk" model previously applied to the objects in M42. More generally, we find that disk-like structures are relatively rare in either nebula. Third, the data indicate that photoevaporation may have uncovered many EGGs while the stellar objects in them were still accreting mass, thereby freezing the mass distribution of the protostars at an early stage in their evolution. We conclude that the masses of stars in the cluster environment in M16 are generally determined not by the onset of stellar winds, as in more isolated regions of star formation, but rather by disruption of the star forming environment by the nearby O stars.

Metal enrichment and ionization balance in the Lyman alpha forest at z = 3

Abstract: The recent discovery of carbon in close to half of the low neutral hydrogen column density [$N({\rm H~I}) > 3\ten{14}\cm2$] Lyman forest clouds toward $z \sim 3$ quasars has challenged the widely held view of this forest as a chemically pristine population uniformly distributed in the intergalactic medium, but has not eliminated the possibility that a primordial population might be present as well. Using extremely high signal-to-noise observations of a sample of quasars we now show that \ion{C}{4} can be found in 75% of clouds with $N({\rm H~I}) > 3\ten{14}\cm2$ and more than 90% of those with $N({\rm H~I}) > 1.6\ten{15}\cm2$. Clouds with $N({\rm H~I}) > 10^{15}\cm2$ show a narrow range of ionization ratios, spanning less than an order of magnitude in \ion{C}{4}/\ion{H}{1}, \ion{C}{2}/\ion{C}{4}, \ion{Si}{4}/\ion{C}{4} and \ion{N}{5}/\ion{C}{4}, and their line widths require that they be photoionized rather than collisionally ionized. This in turn implies that the systems have a spread of less than an order of magnitude in both volume density and metallicity. Carbon is seen to have a typical abundance of very approximately $10^{-2}$ of solar and Si/C about three times solar, so that the chemical abundances of these clouds are very similar to those of Galactic halo stars. \ion{Si}{4}/\ion{C}{4} decreases rapidly with redshift from high values ($> 0.1$) at $z > 3.1$, a circumstance which we interpret as a change in the ionizing spectrum as the intergalactic medium becomes optically thin to He$^+$\ ionizing photons. Weak clustering is seen in the \ion{C}{4} systems for $\Delta v < 250\kms$, which we argue provides an upper limit to the clustering of \ion{H}{1} clouds. If the clouds are associated with galaxies, this requires a rapid evolution in galaxy clustering between $z = 3$ and $z = 0$.

BVRI Light Curves for 29 Type Ia Supernovae

Abstract: This paper was possible thanks to grant 92/0312 from Fondo Nacional de Ciencias y Tecnologia (FONDECYT- -Chile). M.H. acknowledges support provided for this work by the National Science Foundation through grant number GF-1002-96 from the Association of Universities for Research in Astronomy, Inc., under NSF Cooperative Agree ment No. AST-8947990 and from Fundacion Andes under project C-12984. J.M. and M.H. acknowledge support by Catedra Presidencial de Ciencias 1996-1997.

The 0.12-2.5 micron Absolute Flux Distribution of the Sun for Comparison With Solar Analog Stars

Abstract: An absolute ux calibrated reference spectrum of the Sun covering the 0.12 to 2.5 m wavelength range is presented. The ultraviolet and optical spectrum is based on absolute ux measurements from satellites and from the ground. The near-infrared spectrum is based on measurements using the NASA CV-990 aircraft and on a model spectrum. The synthetic optical and near-infrared magnitudes of the absolute calibrated solar reference spectrum agree with published values to 0.01 0.03 magnitudes, i.e. within the uncertainties of the measurements. The absolute ux of the reference spectrum over the optical and near-infrared 0.4 to 2.5 m range is known with an uncertainty of 5%, or better. In the blue and ultraviolet, especially for wavelengths in the 0.12 to 0.2 m interval, the uncertainty increases up to about 20% due to the variability of the solar energy output at these wavelengths. The absolute ux spectrum of the Sun presented here will help to establish the absolute calibration of NICMOS, the HST near-infrared camera and Multi-object Spectrograph.

The global rate and efficiency of star formation in spiral galaxies as a function of morphology and environment

Abstract: CCD images of Ha: and R-band emission in 120 spiral galaxies were obtained using the now-retired No. 1-0.9 m telescope of Kitt Peak National Observatory. These images were used to derive the distribution and total flux of continuum-subtracted Ha: line emission, and therefore the Ha: surface brightnesses and high mass star formation rates in these galaxies. We find a small but significant variation in the mean Ha surface brightness for spiral galaxies along the Hubble sequence; the Sd-Ir galaxies exhibit a mean Ha surface brightness 1.4 times higher than the Sbc-Scd galaxies, and 2-3 times higher than the Sa-Sb galaxies. Estimates for the total formation rate for high mass stars have been compared with global molecular gas masses to determine the global efficiency of high mass star formation as a function of morphological type and environment. We find that the mean efficiency of high mass star formation in this sample of spiral galaxies shows little dependence on morphological type for galaxies of type Sa through Scd, although there is a wide range in star formation efficiencies within each type. Galaxies in disturbed environments (i.e., strongly interacting systems) are found to have a mean star formation efficiency ~4 times higher than in isolated spiral galaxies, uncorrected for extinction. This confirms previous findings (Young et al 1986a,b; Sanders et al 1986; Solomon & Sage 1988; Tinney et al 1990), based on the far-inffared luminosity rather than the Ha luminosity to trace the rate of high mass star formation, that the mean star formation efficiency among isolated galaxies is significantly lower than that among interacting systems. This result provides further confirmation that the rate of high mass star formation is reasonably well traced by both the Ha and the IR luminosity in spiral galaxies. © 1996 American Astronomical

Expanding the Realm of Microlensing Surveys with Difference Image Photometry

Abstract: We present a new technique for monitoring microlensing activity even in highly crowded fields, and use this technique to place limits on low-mass MACHOs in the haloes of M31 and the Galaxy. Unlike present Galactic microlensing surveys, we employ a technique in which a large fraction of the stellar sample is compressed into a single CCD field, rather than spread out in a way requiring many different telescope pointings. We implement the suggestion by Crotts (1992) that crowded fields can be monitored by searching for changes in flux of variable objects by subtracting images of the same field, taken in time sequence, positionally registered, photometrically normalized, then subtracted from one another (or a sequence average). The present work tackles the most difficult part of this task, the adjustment of the point spread function among images in the sequence so that seeing variations play an insignificant role in determining the residual after subtraction. The interesting signal following this process consists of positive and negative point sources due to variable sources. The measurement of changes in flux determined in this way we dub "difference image photometry" (also called "pixel lensing" [Gould 1996]). - The matching of the image point spread function (PSF) is accomplished by a division of PSFs in Fourier space to produce a convolution kernel, in a manner explored for other reasons by Phillips & Davis (1995). In practice, we find the application of this method is difficult in a typical telescope and wide field imaging camera due to a subtle interplay between the spatial variation of the PSF associated with the optical design and the inevitable time variability of the telescope focus. Such effects lead to complexities...(abstract continues)

Radii and Effective Temperatures for G, K, and M Giants and Supergiants

Abstract: Interferometrically determined angular diameters obtained at the Palomar Testbed Interferometer (PTI) for 80 giant and supergiant stars are presented in this paper.

The centers of early-type galaxies with HST. III. Non-parametric recovery of stellar luminosity distributions

Abstract: We have non-parametrically determined the luminosity density profiles and their logarithmic slopes for 42 early-type galaxies observed with HST. Assuming that the isodensity contours are spheroidal, then the luminosity density is uniquely determined from the surface brightness data through the Abel equation. For nearly all the galaxies in our sample, the logarithmic slope of the luminosity density measured at 0.1" (the innermost reliable measurement with the uncorrected HST) is significantly different from zero; i.e. most elliptical galaxies have cusps. There are only two galaxies for which an analytic core cannot be excluded. The distribution of logarithmic slopes at 0.1" appears to be bimodal, confirming the conclusion of Lauer et al. (1995) that early-type galaxies can be divided into two types based on their surface-brightness profiles; i.e., those with cuspy cores and those whose steep power-law profiles continue essentially unchanged in to the resolution limit. The peaks in the slope distribution occur at -0.8 and -1.9. More than half of the galaxies have slopes steeper than -1.0. Taken together with the recent theoretical work of Merritt & Fridman, these results suggest that many (and maybe most) elliptical galaxies are either nearly axisymmetric or spherical near the center, or slowly evolve due to the influence of stochastic orbits.

The Hubble diagram of the Calan/Tololo type IA supernovae and the value of HO

Abstract: The Calan/Tololo supernova survey has discovered ~30 Type Ia supernovae out to z~0.1. Using BVI data for these objects and nearby SNe Ia, we have shown that there exists a significant dispersion in the intrinsic luminosities of these objects. We have devised a robust chisquare minimization technique simultaneously fitting the BVI light curves to parametrize the SN event as a function of (tb,m, m15(B)) where tb is the time of B maximum, m is the peak BVI magnitude corrected for luminosity variations, and m15(B) is a single parameter describing the whole light curve morphology. When properly corrected for m15(B), SNe Ia prove to be high precision distance indicators,yielding relative distances with errors 7-10%. The corrected peak magnitudes are used to construct BVI Hubble diagrams (HD), and with Cepheid distances recently measured with the HST to four nearby SNe Ia (37C, 72E, 81B, 90N) we derive a value of the Hubble constant of 63.1+/-3.4 (internal) km/s/Mpc. This value is ~10-15% larger than the value obtained by assuming that SNe Ia are perfect standard candles. As we have shown in Paper V, there is now strong evidence that galaxies with younger stellar population appear to host the slowest-declining, and therefore most luminous SNe Ia. Hence, the use of Pop I objects such as Cepheids to calibrate the zero point of the SNe Ia HD can easily bias the results toward luminous SNe Ia, unless the absolute magnitude-decline relation is taken into account.

The Ursa Major Cluster of Galaxies.I.Cluster Definition and Photometric Data

Abstract: The Ursa Major Cluster has received remarkably little attention, although it is as near as the Virgo Cluster and contains a comparable number of H I-rich galaxies. In this paper, criteria for group membership are discussed and data are presented for 79 galaxies identified with the group. Of these, all 79 have been imaged at B,R,I bands with CCDs, 70 have been imaged at K' with a HgCdTe array detector, and 70 have been detected in the H I 21 cm line. A complete sample of 62 galaxies brighter than M(B)=-16.5 is identified. Images and gradients in surface brightness and color are presented at a common linear scale. As has been seen previously, the galaxies with the reddest global colors are reddest at the centers and get bluer at large radii. However, curiously, among the galaxies with the bluest global colors there are systems with very blue cores that get redder at large radii. (C) 1996 American Astronomical Society.

The morphology of type ia supernovae light curves

Abstract: We present a family of six BVI template light curves for SNe Ia for days -5 and +80, based on high-quality data gathered at CTIO. These templates display a wide range of light curve morphologies, with initial decline rates of their B light curves between m15(B)=0.87 mag and 1.93 mag. We use these templates to study the general morphology of SNe Ia light curves. We find that several of the main features of the BVI templates correlate tightly with m15(B). In particular, the V light curves, which are probably a reasonably good approximation of the bolometric light curves, display an orderly progression in shapes between the most-luminous, slowest-declining events and the least-luminous, fastest-declining SNe. This supports the idea that the observed spectroscopic and photometric sequences of SNe Ia are due primarily to one parameter. Nevertheless, SNe with very similar initial decline rates do show significant differences in their light curve properties when examined in detail, suggesting the influence of one or more secondary parameters.