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NASA NTRS1989

Contributions of low-luminosity sources to the X-ray background

We have exploited the HEAO 1 A-2 data base to determine hard X-ray fluxes, or upper limits, for complete optically selected samples of active and normal galaxies, in order to estimate or to constrain their local X-ray luminosity functions. We find that the luminosity function of Seyfert nuclei must flatten down drastically shortly below the observational limit of Piccinotti et al.; little can be added by dwarf active nuclei hidden in otherwise normal galaxies. We also analyze the possible contribution to the X-ray background from galaxies with strong star formation activity, in the light of the possibility of a substantial cosmological evolution suggested by recent radio and IRAS data.

Nuclei galaxies↗
NASA NTRS2002

Distant Cluster Hunting: A Comparison of X-Ray and Optical Cluster Detection Techniques and Catalogs from the ROSAT Optical X-Ray Survey - II

We present and analyze the optical and X-ray catalogs of moderate-redshift cluster candidates from the ROSA TOptical X-Ray Survey, or ROXS. The survey covers the sky area contained in the fields of view of 23 deep archival ROSA T PSPC pointings, 4.8 square degrees. The cross-correlated cluster catalogs were con- structed by comparing two independent catalogs extracted from the optical and X-ray bandpasses, using a matched-filter technique for the optical data and a wavelet technique for the X-ray data. We cross-identified cluster candidates in each catalog. As reported in Paper 1, the matched-filter technique found optical counter- parts for at least 60% (26 out of 43) of the X-ray cluster candidates; the estimated redshifts from the matched filter algorithm agree with at least 7 of 1 1 spectroscopic confirmations (Az 5 0.10). The matched filter technique. with an imaging sensitivity of ml N 23, identified approximately 3 times the number of candidates (155 candidates, 142 with a detection confidence >3 u) found in the X-ray survey of nearly the same area. There are 57 X-ray candidates, 43 of which are unobscured by scattered light or bright stars in the optical images. Twenty-six of these have fairly secure optical counterparts. We find that the matched filter algorithm, when applied to images with galaxy flux sensitivities of mI N 23, is fairly well-matched to discovering z 5 1 clusters detected by wavelets in ROSAT PSPC exposures of 8000-60,000 s. The difference in the spurious fractions between the optical and X-ray (30%) and IO%, respectively) cannot account for the difference in source number. In Paper I, we compared the optical and X-ray cluster luminosity functions and we found that the luminosity functions are consistent if the relationship between X-ray and optical luminosities is steep (Lx o( L&f). Here, in Paper 11, we present the cluster catalogs and a numerical simulation of the ROXS. We also present color-magnitude plots for several of the cluster candidates, and examine the prominence of the red sequence in each. We find that the X-ray clusters in our survey do not all have a prominent red sequence. We conclude that while the red sequence may be a distinct feature in the color-magnitude plots for virialized massive clusters, it may be less distinct in lower mass clusters of galaxies at even moderate redshifts. Multiple, complementary methods of selecting and defining clusters may be essential, particularly at high redshift where all methods start to run into completeness limits, incomplete understanding of physical evolution, and projection effects.

Donahue, Megan↗
NASA NTRS1990

Fanaroff-Riley I galaxies as the parent population of BL Lacertae objects. I - X-ray constraints

The hypothesis that BL Lacertae objects are a special subset of Fanaroff-Riley type I radio galaxies, namely, those dominated by beamed emission from a relativistic jet aligned with the line of sight, is examined by looking at the relative number densities. The calculation depends primarily on observed quantities, including the X-ray luminosity function of Fanaroff-Riley type I galaxies. The model predicts the X-ray luminosity function of BL Lacertae objects, the shape of the X-ray counts below currently observed fluxes, and the velocity of the X-ray emitting jet. With Fanaroff-Riley type I galaxies as the parent population the flat luminosity function, the observed X-ray number counts, and the partial redshift distribution of BL Lacertae objects can be explained.

Padovani, P.↗
NASA NTRS1988

M dwarfs from the Einstein extended medium sensitivity survey

A complete sample of X-ray selected M dwarfs are presented which were serendipitously detected in the extended Einstein Observatory Medium Sensitivity Survey (MSS). The MSS detected only early M dwarfs (type MO through M5), 84 percent of which were emission stars (i.e., type Me V). It was calculated that the X-ray luminosity function for early M dwarfs with a log Lx greater than 27.6 and, by comparison to optical luminosity functions, find that about 25 percent of all early M dwarfs emit X-rays at a level greater than log Lx = 27.6. This result is compared to luminosity function estimates from optically selected samples and discussed in light of the sample's kinematic and mean age. Also presented is H alpha and Ca II K line fluxes for each member of the sample and it is shown that the surface H alpha and Ca II K luminosities do indeed correlate with Lx. The significance of this effect to theories of chromospheric and coronal heating is explained.

Fleming, Thomas A.↗
NASA NTRS1985

Limits to the burster repetition rate as deduced from the 2nd catalog of the interplanetary network

The burster repetition rate is an important parameter in many gamma ray burst models. The localizations of the interplanetary network, which have a relatively small combined surface area, may be used to estimate the average repetition rate. The method consists of (1) estimating the number of random overlaps between error boxes expected in the catalog and comparing this number to that actually observed; (2) modeling the response of the detectors in the network, so that the probability of detecting a burst can be estimated; and (3) simulating the arrival of bursts at the network assuming that burster repetition is governed by a Poisson process. The application of this method for many different burster luminosity functions shows that (1) the lower limit to the burster repetition rate depends strongly upon the assumed luminosity function; (2) the best lower limit to the repetition period obtainable from the data of the network is about 100 months; and (3) that a luminosity function for all bursters similar to that of the 1979 Mar 5 burster is inconsistent with the data.

Atteia, J. L.↗
NASA NTRS1984

Very low luminosity active galaxies and the X-ray background

The properties of very low luminosity active galactic nuclei are not well studied, and, in particular, their possible contribution to the diffuse X-ray background is not known. In the present investigation, an X-ray luminosity function for the range from 10 to the 39th to 10 to the 42.5th ergs/s is constructed. The obtained X-ray luminosity function is integrated to estimate the contribution of these very low luminosity active galaxies to the diffuse X-ray background. The construction of the X-ray luminosity function is based on data obtained by Keel (1983) and some simple assumptions about optical and X-ray properties.

Elvis, M.↗
NASA NTRS1998

NGST: Exploring the Fossil Record of Galaxy Formation

During this grant period the investigator has accomplished the following: developed and researched the science case for high resolution optical imaging with NGST (Next Generation Space Telescope); presented to the ASWG (Ad-Hoc Science Working Group) on the use of NGST to image and measure the proper motions of white dwarf stars that could account for the MACHO events toward the LMC (Large Magellanic Cloud); wrote proposals for the Design Reference Mission. Three proposals have been written on the following topics: Measuring the faint end of the white dwarf luminosity function to get an independent measure of the age of the oldest stars, measurement of the stellar luminosity function over the full range of age, abundance, and population type, and deep imaging of Local Group halos to measure the relative ages of the oldest stars in the Milky Way and other galaxies; he has introduced simulations of crowded field stellar photometry on a hypothetical population in the M31 halo. Using a physically correct luminosity function and a surface brightness of 24.5 mag/sq. arcsec, NGST imaging in 10 hours easily measures the turnoff and reaches 3 mags down the main sequence in the halo of M31; and has delivered talks at the NGST workshop in Liege, and at the meeting in Paris entitled "Connecting the Distant Universe with the Local Fossil Record".

Rich, R. Michael↗
NASA NTRS1993

Resolved stars in nearby galaxies: Ground-based photometry of M81

Using the Canada-France-Hawaii Telescope (CFHT) we have obtained three closely spaced epochs of calibrated Blue Violet Red Infrared (BVRI) CCD imaging of two fields in M81, each known to contain a thirty-day Cepheid. Calibrated BVRI photometry of the brightest stars in these fields is presented. The slope of the luminosity function from the brightest 3-4 mag of the main-sequence blue plume is consistent with similar determinations of the apparent luminosity function in other resolved galaxies, thereby removing the one potential deviation from universality noted by Freedman in a photographic study of luminosity functions in nearby resolved galaxies. Under the assumption that the two Cepheids are representative, a reddening-law fit to the multiwavelength BVRI period-luminosity moduli give a true distance modulus of (m-M)sub 0 = 27.79 mag for M81, corresponding to a linear distance of 3.6 Mpc. An error analysis shows that the derived true distance modulus has a random error of +/- 0.28 mag (due to the photometric uncertainties in the BVRI data), with a systematic uncertainty of +/- 0.10 mag (accounting for the combined effects of unknown phasing of the data points, and the unknown positioning of these particular stars within the Cepheid instabiliy strip).

Madore, Barry F.↗
NASA NTRS1986

Far-infrared investigation of the Taurus star-forming region using the IRAS database

The Infrared Astronomical Satellite (IRAS) has given us the first completely unbiased sky-survey in the far-infrared with wavebands centered at 12, 25, 60 and 100 microns. The Taurus-Auriga complex was selected as the first molecular cloud to be investigated in this study. The Taurus clouds were defined as lying between 04h and 05h in R.A. and +16 to +31 degrees in Dec., then the IRAS point-source catalogue was searched for sources with good or moderate quality fluxes in all three of the shortest IRAS bands. The sources which were selected in this way were then classified into subgroups according to their IRAS colors. Taurus is generally believed to be an area of low-mass star formation, having no luminous O-B associations within or near to the cloud complex. Once field stars, galaxies and planetary nebulae had been removed from the sample only the molecular cloud cores, T Tauri stars and a few emission-line A and B stars remained. The great majority of these objects are pre-main sequence in nature and, as stated by Chester (1985), main sequence stars without excess far-infrared emission would only be seen in Taurus if their spectral types were earlier than about A5 and then not 25 microns. By choosing our sample in this way we are naturally selecting the hotter and thus more evolved sources. To counteract this, the molecular cloud core-criterion was applied to soruces with good or moderate quality flux at 25, 60 and 100 microns, increasing the core sample by about one third. The candidate protostar B335 is only detected by IRAS at 60 and 100 microns while Taurus is heavily contaminated by cirrus at 100 microns. This means that detection at 25 microns is also required with those at 60 and 100 microns to avoid confusing a ridge of cirrus with a genuine protostar. The far-infrared luminosity function of these sources is then calculated and converted to the visual band by a standard method to compare with the field star luminosity function of Miller and Scalo. The eventual aim of this work is to obtain far-infrared luminosity functions for a number of molecular clouds which are known to be forming low-mass stars and to investigate how the slope is affected by changes in the density and turbulence of material.

Hughes, J. D.↗
NASA NTRS1990

HII regions in dwarf irregular galaxies of the local group

Deep, narrowband H alpha Charge Coupled Device (CCD) surveys of HII regions were carried out in several dwarf irregular galaxies in and near the local group. Data are now complete for these galaxies: NGC 6822, GR 8, IC 10, IC 1613, Sextans A, Sextans B, and Sag Irr. Observations are complete for DDO 47, 53, 167, 168 and 187. Details of some of the results for the surveys completed so far are discussed. For NGC 6822, CCD survey at H alpha resulted in the detection of 145 HII regions in the local group irregular galaxy NGC 6822. Most of them are newly detected, faint surface-brightness objects. Positions, maps and dimensions are being published elsewhere. For GR 8, a deep narrowband H alpha imaging of the nearby dwarf irregular galaxy GR 8 revealed a total of 32 HII regions. Positions, H alpha luminosities, and sizes of these objects were determined. The H alpha luminosity function has the same shape as that for more luminous galaxies, except for size of sample effects. Most HII regions detected are at the very low luminosity end of the general luminosity function. For IC 10, a deep CCD narrowband H alpha imaging of the local group dwarf irregular galaxy IC 10 revealed a total of 144 HII regions. Positions, H alpha luminosities, and sizes of these objects were determined. The H alpha luminosity function has the same shape as that for more luminous galaxies.

Hodge, Paul↗
NASA NTRS1983

Redshift modifications to HEAO A-1 cluster X-ray luminosities

New redshift measurements for the 24 Abell galactic clusters are presented. The optical observations are described and applied to the cluster X-ray luminosity function, and the results are used to delimit the cluster contribution to the diffuse X-ray background (DXRB). The evolution of the X-ray luminosity function is then investigated. It is found that while simple two-parameter analytic forms do not fit this function very well, three-parameter forms such as the exponential times power law do fit it very well. Future redshift observations will probably not change the volume emissivity significantly, but they may in a statistically significant way affect the parameters of analytic fits which define the shape of the luminosity function. The contribution of rich clusters to the DXRB in the 2-6 keV range is 5.2 percent. Future redshift observations will modify this result by no more than a factor of about 1.1.

Cruddace, R. G.↗
NASA NTRS1995

White dwarfs and mass segregation in the core of 47 Tucanae

The core of the galactic globular cluster (GC) 47 Tucanae was observed by the Faint Object Camera (FOC) on the refurbished Hubble Space Telescope (HST) for the first time through medium-band F253M and F346M filters centered at 255 and 348 nm. A total of 442 stars down to the detection limit at m(sub 346) approximately equal to 24 in the 7 sec x 7 sec field were accurately placed on a m(sub 346) versus m(sub 253) - M(sub 346) magnitude diagram to characterize the stellar populations in the core. Approximately 100 objects above and to the right of the main-sequence (MS) turnoff (TO) are the same as those classified earlier with the aberrated HST, but the rest below it are new objects seen now for the first time thanks to the substantial improvement in sensitivity. The new objects form a well-defined main sequence whose luminosity function (LF) turns over dramatically at m(sub 346) approximately equal to 19 well before the completeness limit and well before the end of the slowly increasing luminosity function for the outer fields measured from the ground by Hesser et al. We interpret this premature drop in the number of stars below approximately 0.8 solar masses in the core as the effect of mass segregation due to two-body relaxation. The position of nine objects in the range 20 less than m(sub 346) less than 24 and -1.5 less than m(sub 253) - m(sub 346) less than 0 is consistent with the expected location of a approx. a half solar mass white dwarf cooling sequence and luminosity function.

Paresce, Francesco↗
NASA NTRS1993

X-ray emission from a complete sample of Abell clusters of galaxies

The ROSAT All-Sky Survey (RASS) is used to investigate the X-ray properties of a complete sample of Abell clusters with measured redshifts and accurate positions. The sample comprises the 145 clusters within a 561 square degree region at high galactic latitude. The mean redshift is 0.17. This sample is especially well suited to be studied within the RASS since the mean exposure time is higher than average and the mean galactic column density is very low. These together produce a flux limit of about 4.2 x 10(exp -13) erg/sq cm/s in the 0.5 to 2.5 keV energy band. Sixty-six (46%) individual clusters are detected at a significance level higher than 99.7% of which 7 could be chance coincidences of background or foreground sources. At redshifts greater than 0.3 six clusters out of seven (86%) are detected at the same significance level. The detected objects show a clear X-ray luminosity -- galaxy count relation with a dispersion consistent with other external estimates of the error in the counts. By analyzing the excess of positive fluctuations of the X-ray flux at the cluster positions, compared with the fluctuations of randomly drawn background fields, it is possible to extend these results below the nominal flux limit. We find 80% of richness R greater than or = 0 and 86% of R greater than or = 1 clusters are X-ray emitters with fluxes above 1 x 10(exp -13) erg/sq cm/s. Nearly 90% of the clusters meeting the requirements to be in Abell's statistical sample emit above the same level. We therefore conclude that almost all Abell clusters are real clusters and the Abell catalog is not strongly contaminated by projection effects. We use the Kaplan-Meier product limit estimator to calculate the cumulative X-ray luminosity function. We show that the shape of the luminosity functions are similiar for different richness classes, but the characteristic luminosities of richness 2 clusters are about twice those of richness 1 clusters which are in turn about twice those of richness 0 clusters. This result is another manifestation of the luminosity -- richness elation for Abell clusters.

Briel, Ulrich G.↗
NASA NTRS1987

A study of a flux-limited sample of IRAS galaxies

Redshift data and accurate four-color infrared photometry are presented for a complete IRAS sample of galaxies brighter than 2 Jy at 60 microns. A simple power law provides a good fit to the distribution for luminosities from 10 to the 10th to 10 to the 12th solar. There is no indication of an exponential dropoff in the luminosity function at high energies. A flattening of the luminosity function occurs at L less than 10 to the 20th solar. The highest luminosity galaxies typically are found in multiple, possibly interacting, systems and exhibit marginally narrower infrared spectral energy distributions than the isolated spirals which predominate at low luminosities. Infrared-bright galaxies come from a different population than the majority of optically bright galaxies. In particular, galaxies of low blue luminosity are not strong infrared emitters.

Smith, Beverly J.↗
NASA NTRS1980

The Hercules supercluster. II - Analysis

The almost complete sample of more than 150 redshifts for galaxies brighter than m sub p = 15.8 mag in a 28 square degree field in Hercules by Tarenghi et al. is used to examine the cosmography of a 60,000 cubic Mpc conical volume of space. It contains a supercluster of Abell clusters with a depth of about 100 Mpc, a vacant 'void' also with a depth of about 100 Mpc, and a structure wider than 12 Mpc which could be part of a more extensive system of small groups of galaxies. Properties of individual rich clusters in the Hercules supercluster are derived, but they are uncertain because the clusters merge into one another, so that it is difficult to identify them and their member galaxies unambiguously. However, those regions in Hercules containing the largest proportion of early-type galaxies show the least evidence for gravitational relaxation. A difference may exist between the luminosity function of clusters of galaxies and the luminosity function of the foreground sheet.

Tarenghi, M.↗
NASA NTRS1983

Photometry of resolved galaxies. II - Sextans A

Luminosity functions and color-magnitude diagrams are given for 652 stars in the Sextans A dwarf irregular galaxy, and they are compared with other Local Group galaxies. The brightest red and blue stars presently determined are in agreement with those of Sandage and Carlson (1982), and a group of red stars is identified which may be analogous to the bright carbon star population recently found in the Magellanic Clouds. While the similarity of Sextans A's luminosity function with those of the LMC and the Milky Way is in keeping with a universal luminosity function, stellar mass function and birthrate resemble those of the SMC. Per unit gas mass, Sextans A has a birthrate almost identical to the SMC, suggesting that the mean age of stars in Sextans A is significantly less than of those in the Magellanic Clouds.

Hoessel, J. G.↗
NASA NTRS1994

Counts of galaxies in a merger model

A model for the photometric evolution of galaxies has been developed and has been applied to the problem of galaxy counts. The integrated colors of galaxies are calculated using the most recently computed evolutionary tracks from Maeder and collaborators complemented with evolutionary tracks derived by other authors. The asymptotic giant branch lifetime is left as a free parameter. A series of cosmological models using different values of the cosmological constant, lambda(sub 0), and the density parameter, omega(sub 0), have been computed. The universality hypothesis of the luminosity function of galaxies has been abandoned. The influence of galaxy merging on the counts has been considered in a simple manner by assuming that the number of strongly interacting galaxies in a comoving volume increases with redshift as a power law given by (1 + z)(exp 3.8). Taking a Schechter parametrization for the luminosity function of the different types of galaxies, we are able to reproduce the observations reasonably well. We have also considered models with a Gaussian distribution for the luminosity function of the brighter galaxies that provide a poorer fit to the observations. It is shown that galaxy count data are not yet able to make unambiguous cosmological statements since evolutionary assumptions are critical. In particular, an omega(sub 0) = 1, lambda(sub 0) = 0 cosmology is shown to be consistent with the data.

Colin, P.↗
NASA NTRS1993

The detectability of brown dwarfs - Predictions and uncertainties

In order to determine the likelihood for the detection of isolated brown dwarfs in ground-based observations as well as in future spaced-based astronomy missions, and in order to evaluate the significance of any detections that might be made, we must first know the expected surface density of brown dwarfs on the celestial sphere as a function of limiting magnitude, wavelength band, and Galactic latitude. It is the purpose of this paper to provide theoretical estimates of this surface density, as well as the range of uncertainty in these estimates resulting from various theoretical uncertainties. We first present theoretical cooling curves for low-mass stars that we have computed with the latest version of our stellar evolution code. We use our evolutionary results to compute theoretical brown-dwarf luminosity functions for a wide range of assumed initial mass functions and stellar birth rate functions. The luminosity functions, in turn, are utilized to compute theoretical surface density functions for brown dwarfs on the celestial sphere. We find, in particular, that for reasonable theoretical assumptions, the currently available upper bounds on the brown-dwarf surface density are consistent with the possibility that brown dwarfs contribute a substantial fraction of the mass of the Galactic disk.

Nelson, L. A.↗