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NASA NTRS1997

Internal Luminosity Distribution of Bright Gamma-Ray Bursts and its Relation to Duration and Spectral Hardness

We present first results from a comprehensive investigation into the distribution of luminosity within the 50 brightest cosmic gamma-ray bursts detected by the Burst and Transient Source Experiment (BATSE). The internal luminosity function psi(L) is defined such that the quantity psi(L)dL represents the fraction of total emission time during which the burst possesses a luminosity between L and L + dL. For these brightest bursts, the psi(L) functions are quasi-power-law-like and decrease in amplitude with increasing luminosity. Through investigation of both individual psi(L) distributions and data from the ensemble of bursts, we demonstrate a high probability for correlation between the shape of the internal luminosity function as measured by the average logarithmic slope and the burst duration as measured by the T(sub 90) parameter and, with lower significance, between the shape of psi(L) and the burst photon-fluence spectral index. We furthermore demonstrate a correlation between burst hardness ratio and duration in these brightest bursts which is opposite to that of the entire gamma-ray burst ensemble.

Horack, John M.↗
NASA NTRS1977

The correlation function for density perturbations in an expanding universe. II - Nonlinear theory

A formalism is developed to find the two-point and higher-order correlation functions for a given distribution of sizes and shapes of perturbations which are randomly placed in three-dimensional space. The perturbations are described by two parameters such as central density and size, and the two-point correlation function is explicitly related to the luminosity function of groups and clusters of galaxies

Mcclelland, J.↗
NASA NTRS1993

High redshift QSOs and the x ray background

ROSAT pointed observations were made of 9 QSO's from the Large Bright Quasar Survey (LBQS). The LBQS is based on machine measurement of objective prism plates taken with the UK Schmidt Telescope. Software has been used to select QSO's by both color and by the presence of spectral features and continuum breaks. The probability of detection can be calculated as a function of magnitude, redshift and spectral features, and the completeness of the survey can be accurately estimated. Nine out of 1040 QSO's in the LBQS have z greater than 3. The observations will provide an important data point in the X-ray luminosity function of QSO's at high redshift. The QSO's with z greater than 3 span less than a magnitude in M(sub B), so can be combined as a homogeneous sample. This analysis is only possible with a sample drawn from a large and complete catalog such as the LBQS. Four of the 9 QSO's that were observed with the ROSAT PSPC for this proposal were detected, including one of the most luminous X-ray sources ever observed. The April 1992 version of the PROS DETECT package was used to reduce the data. The results have been used to search for evolution of the X-ray properties of QSO's in redshift. The 9 QSO's lie in the range -28.7 less than M(sub B) less than -27.8. When combined with data for 16 QSO's in a similar luminosity range at lower redshift correlations with luminosity and redshift can be separated out. The LBQS sample also yields a new constraint on the contribution of high redshift QSO's to the X-ray background. An initial requirement is knowledge of the X-ray properties (alpha(sub OX)) as a function of redshift. Integration over the evolving luminosity function of the LBQS then gives the QSO contribution to the source counts.

Impey, Chris↗
NASA NTRS2016

Local starburst galaxies and their descendants. Statistics from the Sloan Digital Sky Survey

Despite strong interest in the starburst phenomenon in extragalactic astronomy, the concept remains ill-defined. Here we use a strict definition of starburst to examine the statistical properties of starburst galaxies in the local universe. We also seek to establish links between starburst galaxies, post-starburst (hereafter postburst) galaxies, and active galaxies. Data were selected from the Sloan Digital Sky Survey DR7. We applied a novel method of treating dust attenuation and derive star formation rates, ages, and stellar masses assuming a two-component stellar population model. Dynamical masses are calculated from the width of the H-alpha line. These masses agree excellently with the photometric masses. The mass (gas+stars) range is approximately 10( exp 9) - 10(exp 11.5) solar mass. As a selection criterion for starburst galaxies, we use, the birthrate parameter, b = SFR/SFR, requiring that b is greater than 3. For postburst galaxies, we use, the equivalent width of Hdelta in absorption with the criterion EW (sub Hdelta_abs) is greater than 6 A. Results. We find that only 1% of star-forming galaxies are starburst galaxies. They contribute 3-6% to the stellar production and are therefore unimportant for the local star formation activity. The median starburst age is 70 Myr roughly independent of mass, indicating that star formation is mainly regulated by local feedback processes. The b-parameter strongly depends on burst age. Values close to b = 60 are found at ages approximately 10 Myr, while almost no starbursts are found at ages greater than 1 Gyr. The median baryonic burst mass fraction of sub-L galaxies is 5% and decreases slowly towards high masses. The median mass fraction of the recent burst in the postburst sample is 5-10%. A smaller fraction of the postburst galaxies, however, originates in non-bursting galaxies. The age-mass distribution of the postburst progenitors (with mass fractions is greater than 3%) is bimodal with a break at logM(solar mass ) 10.6, above which the ages are doubled. The starburst and postburst luminosity functions (LFs) follow each other closely until M(sub r ) (is) approximately -21, when active galactic nuclei (AGNs) begin to dominate. The postburst LF continues to follow the AGN LF, while starbursts become less significant. This suggests that the number of luminous starbursts is underestimated by about one dex at high luminosities, because of having large amounts of dust and/or being outshone by an AGN. It also indicates that the starburst phase preceded the AGN phase. Finally, we look at the conditions for global gas outflow caused by stellar feedback and find that massive starburst galaxies are susceptible to such outflows.

Statistical distributions↗
NASA NTRS1986

On the completeness of the Medium Sensitivity Survey quasar sample

The Medium Sensitivity Survey (MSS) quasar sample is being used more and more to study quasar properties in general and to analyze the relationship between X-ray-selected and optically selected quasars in particular. These studies have recently shown that current knowledge of the properties of optically selected quasars (luminosity function, evolution, X-ray to optical luminosity ratio) leads to the prediction that many more X-ray-selected quasars should have been detected than are actually observed. Prompted by this fact, a detailed examination of possible causes of incompleteness of the MSS quasar sample has been undertaken, paying particular attention to the problem of photoelectric absorption due to the interstellar medium within the Galaxy. It is found that there is no evidence of a loss of sources due to the effects considered, and that the MSS quasar sample is statistically complete.

Maccacaro, T.↗
NASA NTRS1987

Primeval galaxies and cold dark matter

In the context of the cold dark matter theory for the large-scale matter distribution, the onset of galaxy formation is a gradual process, with star formation being initiated at z = about 10 and reaching a peak for luminous galaxies at z = about 1. The mass function of galaxy cores matches the observed quasar luminosity function at z = 2-3. Primeval galaxies are envisaged as a collection of many interacting and merging clumps, attaining a peak luminosity that is an order of magnitude below that achieved in models in which galaxy formation is initiated abruptly. Hence, ongoing searches for primeval galaxies would not necessarily have been successful unless they are designed to find moderately low-luminosity, low-surface-brigtness extended objects at low redshift.

Silk, Joseph↗
NASA NTRS1986

Continuum radiation from active galactic nuclei: A statistical study

The physics of the continuum spectrum of active galactic nuclei (AGNs) was examined using a large data set and rigorous statistical methods. A data base was constructed for 469 objects which include radio selected quasars, optically selected quasars, X-ray selected AGNs, BL Lac objects, and optically unidentified compact radio sources. Each object has measurements of its radio, optical, X-ray core continuum luminosity, though many of them are upper limits. Since many radio sources have extended components, the core component were carefully selected out from the total radio luminosity. With survival analysis statistical methods, which can treat upper limits correctly, these data can yield better statistical results than those previously obtained. A variety of statistical tests are performed, such as the comparison of the luminosity functions in different subsamples, and linear regressions of luminosities in different bands. Interpretation of the results leads to the following tentative conclusions: the main emission mechanism of optically selected quasars and X-ray selected AGNs is thermal, while that of BL Lac objects is synchrotron; radio selected quasars may have two different emission mechanisms in the X-ray band; BL Lac objects appear to be special cases of the radio selected quasars; some compact radio sources show the possibility of synchrotron self-Compton (SSC) in the optical band; and the spectral index between the optical and the X-ray bands depends on the optical luminosity.

Isobe, T.↗
NASA NTRS1991

A hydrodynamic approach to cosmology - Texture-seeded cold dark matter and hot dark matter cosmogonies

Hydrodynamical simulations of galaxy formation in a texture-seeded cosmology are presented, with attention given to Omega = 1 galaxies dominated by both hot dark matter (HDM) and cold dark matter (CDM). The simulations include both gravitational and hydrodynamical physics with a detailed treatment of collisional and radiative thermal processes, and use a cooling criterion to estimate galaxy formation. Background radiation fields and Zel'dovich-Sunyaev fluctuations are explicitly computed. The derived galaxy mass function is well fitted by the observed Schechter luminosity function for a baryonic M/L of 3 and total M/L of 60 in galaxies. In both HDM and CDM texture scenarios, the 'galaxies' and 'clusters' are significantly more strongly correlated than the dark matter due to physical bias processes. The slope of the correlation function in both cases is consistent with observations. In contrast to Gaussian models, peaks in the dark matter density distributrion are less correlated than average.

Cen, R. Y.↗
NASA NTRS1992

Galaxies and gas in a cold dark matter universe

We use a combined gravity/hydrodynamics code to simulate the formation of structure in a random 22 Mpc cube of a cold dark matter universe. Adiabatic compression and shocks heat much of the gas to temperatures of 10 exp 6 - 10 exp 7 K, but a fraction of the gas cools radiatively to about 10 exp 4 K and condenses into discrete, highly overdense lumps. We identify these lumps with galaxies. The high-mass end of their baryonic mass function fits the form of the observed galaxy luminosity function. They retain independent identities after their dark halos merge, so gravitational clustering produces groups of galaxies embedded in relatively smooth envelopes of hot gas and dark matter. The galaxy correlation function is approximately an r exp -2.1 power law from separations of 35 kpc to 7 Mpc. Galaxy fluctuations are biased relative to dark matter fluctuations by a factor b about 1.5. We find no significant 'velocity bias' between galaxies and dark matter particles. However, virial analysis of the simulation's richest group leads to an estimated Omega of about 0.3, even though the simulation adopts Omega = 1.

Katz, Neal↗
NASA NTRS1992

BATSE analysis techniques for probing the GRB spatial and luminosity distributions

The Burst And Transient Source Experiment (BATSE) has measured homogeneity and isotropy parameters from an increasingly large sample of observed gamma-ray bursts (GRBs), while also maintaining a summary of the way in which the sky has been sampled. Measurement of both of these are necessary for any study of the BATSE data statistically, as they take into account the most serious observational selection effects known in the study of GRBs: beam-smearing and inhomogeneous, anisotropic sky sampling. Knowledge of these effects is important to analysis of GRB angular and intensity distributions. In addition to determining that the bursts are local, it is hoped that analysis of such distributions will allow boundaries to be placed on the true GRB spatial distribution and luminosity function. The technique for studying GRB spatial and luminosity distributions is direct. Results of BATSE analyses are compared to Monte Carlo models parameterized by a variety of spatial and luminosity characteristics.

Hakkila, Jon↗
NASA NTRS1983

X-ray studies of quasars with the Einstein Observatory. III The 3CR sample

Using X-ray emission detected by the Einstein Observatory from each of the 33 3CR quasars in Schmidt's complete sample, it is demonstrated that there is a relationship between the X-ray emission and centimeter wavelength radio emission even for steep-spectrum quasars, provided that only the radio emission from the central object is considered. The observations are summarized, and the estimation of the X-ray luminosity distribution, rather narrow compared to the radio and optical luminosity distributions, is presented. The relationship between X-ray and radio emission is discussed, as is the cosmological evolution of the X-ray luminosity function. The relationship between X-ray and optical luminosity provides an explanation for the finding that X-ray selected quasars have lower redshifts than optically selected quasars of the same optical magnitude.

Tananbaum, H.↗
NASA NTRS1994

Einstein X-ray observations of Herbig Ae/Be stars

We have investigated the X-ray emission from Herbig Ae/Be stars, using the full set of Einstein Imaging Proportional Counter (IPC) observations. Of a total of 31 observed Herbig stars, 11 are confidently identified with X-ray sources, with four additonal dubious identifications. We have used maximum likelihood luminosity functions to study the distribution of X-ray luminosity, and we find that Be stars are significantly brighter in X-rays than Ae stars and that their X-ray luminosity is independent of projected rotational velocity v sin i. The X-ray emission is instead correlated with stellar bolometric luminosity and with effective temperature, and also with the kinetic luminosity of the stellar wind. These results seem to exclude a solar-like origin for the X-ray emission, a possibility suggested by the most recent models of Herbig stars' structure, and suggest an analogy with the X-ray emission of O (and early B) stars. We also observe correlations between X-ray luminosity and the emission at 2.2 microns (K band) and 25 microns, which strengthen the case for X-ray emission of Herbig stars originating in their circumstellar envelopes.

Damiani, F.↗
NASA NTRS1991

On the derivation of selection functions from redshift survey data

A previously unrecognized effect is described in the derivation of luminosity functions and selection functions from existing redshift survey data, due to binning of quoted magnitudes and diameters. Corrections are made for this effect in the Center for Astrophysics (CfA) and Southern Sky (SSRS) Redshift Surveys. The correction makes subtle but systematic changes in the derived density fields of the CfA survey, especially within 2000 km/s of the Local Group. The effect on the density field of the SSRS survey is negligible.

Strauss, Michael A.↗
NASA NTRS1989

Soft X-ray observations of pre-main-sequence stars in the Chamaeleon dark cloud

X-ray observations of the Chamaeleon I cloud, a star-forming region, are reported. A total of 22 distinct X-ray sources, most associated with previously identified premain sequence stars, are found. The spatial distributions and HR diagrams of the stars are very similar, suggesting that they are coeval. Luminosity functions suggest that the stars have an average X-ray luminosity (Lx) several times that of the Pleiades dwarfs. The value of Lx is significantly correlated with optical magnitude, though no relation between X-ray emission and any photometric or emission line characteristic is present. It is suggested that a Skumanich-type power-law relation may be present over the entire range of stellar ages between 10 to the 6th and 10 to the 10th yr.

Feigelson, Eric D.↗
NASA NTRS1994

Star formation in Taurus

Data with the Two Micron All Sky Survey (2MASS) prototype camera were obtained in a 2.3 sq. deg region in Taurus containing Heiles Cloud 2, a region known from Infrared Astronomy Satellite (IRAS) observations to contain a number of very young solar type stars. Data at 1.25 (J), 1.65 (H), and 2.2 (K(sub s)) micrometers are presented. These data are representative of the type and quality of data expected from the planned near-IR surveys, 2MASS and Deep Near-Infrared Survey (DENIS). Near-IR surveys will be useful for determining the large scale variation of extinction with clouds, for determining the luminosity function in nearby clouds down to ranges of 0.1-1.0 solar luminosity, and for finding highly extincted T Tauri stars missed by IRAS because the bulk of their luminosity is emitted shortward of 12 micrometers.

Beichman, C. A.↗