arXiv subjects

Richard F. Green

Publications and source records attributed to Richard F. Green.

At least 37 records · Page 2Linked to original sources

arXiv2001

HST STIS Observations of PG 0946+301: the Highest Quality UV Spectrum of a BALQSO

We describe deep (40 orbits) HST/STIS observations of the BALQSO PG 0946+301 and make them available to the community. These observations are the major part of a multi-wavelength campaign on this object aimed at determining the ionization equilibrium and abundances (IEA) in broad absorption line (BAL) QSOs. We present simple template fits to the entire data set, which yield firm identifications for more than two dozen BALs from 18 ions and give lower limits for the ionic column densities. We find that the outflow's metalicity is consistent with being solar, while the abundance ratio of phosphorus to other metals is at least ten times solar. These findings are based on diagnostics that are not sensitive to saturation and partial covering effects in the BALs, which considerably weakened previous claims for enhanced metalicity. Ample evidence for these effects is seen in the spectrum. We also discuss several options for extracting tighter IEA constraints in future analyses, and present the significant temporal changes which are detected between these spectra and those taken by the HST/FOS in 1992.

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arXiv1998

Compact Nuclei in Galaxies at Moderate Redshift:II. Their Nature and Implications for the AGN Luminosity Function

This study explores the space density and properties of active galaxies to z=0.8. We have investigated the frequency and nature of unresolved nuclei in galaxies at moderate redshift as indicators of nuclear activity such as Active Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged galaxies with multi-component models using maximum likelihood estimate techniques to determine the best model fit. We select those galaxies requiring an unresolved point-source component in the galaxy nucleus, in addition to a disk and/or bulge component, to adequately model the galaxy light. We have searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies containing compact nuclei. In our survey of 1033 galaxies, the fraction containing an unresolved nuclear component greater than 5% of the total galaxy light is 9+/-1% corrected for incompleteness. In this second of two papers in this series, we discuss the nature of the compact nuclei and their hosts. We present the upper limit luminosity function (LF) for low-luminosity AGN (LLAGN) in two redshift bins to z=0.8. Mild number density evolution is detected for nuclei at -18 < M_B < -14. The LFs appear to flatten at M_B > -16 and this flatness, combined with the increase in number density, is inconsistent with pure luminosity evolution. Based on the amount of density evolution observed for these objects, we find that almost all present-day spiral galaxies could have hosted a LLAGN at some point in their lives. We also comment on the likely contribution of these compact nuclei to the soft X-ray background.

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arXiv1998

Compact Nuclei in Galaxies at Moderate Redshift: I. Imaging and Spectroscopy

This study explores the space density and properties of active galaxies to z=0.8. We have investigated the frequency and nature of unresolved nuclei in galaxies at moderate redshift as indicators of nuclear activity such as Active Galactic Nuclei (AGN) or starbursts. Candidates are selected by fitting imaged galaxies with multi-component models using maximum likelihood estimate techniques to determine the best model fit. We select those galaxies requiring an unresolved, point source component in the galaxy nucleus, in addition to a disk and/or bulge component, to adequately model the galaxy light. We have searched 70 WFPC2 images primarily from the Medium Deep Survey for galaxies containing compact nuclei. In our survey of 1033 galaxies, the fraction containing an unresolved nuclear component greater than 3% of the total galaxy light is 16+/-3% corrected for incompleteness and 9+/-1% for nuclei greater than 5% of the galaxy light. Spectroscopic redshifts have been obtained for 35 of our AGN/starburst candidates and photometric redshifts are estimated to an accuracy of sigma_z=0.1 for the remaining sample. In this paper, the first of two in this series, we present the selected HST imaged galaxies having unresolved nuclei and discuss the selection procedure. We also present the ground-based spectroscopy for these galaxies as well as the photometric redshifts estimated for those galaxies without spectra.

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arXiv1998

The Very Highly Ionized Broad Absorption Line System of the QSO SBS1542+541

We have analyzed the broad absorption line system of the bright (V=16.5) high-redshift (z=2.361) QSO SBS1542+541 using UV spectra from the HST FOS along with optical data from the MMT and the Steward Observatory 2.3m telescope. These spectra give continuous wavelength coverage from 1200 to 8000 Angstroms, corresponding to 340 to 2480 Angstroms in the QSO rest frame. This object therefore offers a rare opportunity to study broad absorption lines in the rest-frame extreme UV. We find that the absorption system is dominated by very high-ionization species, including O VI, NeVIII, and SiXII. We also identify apparently saturated broad Lyman-series lines of order Ly-gamma and higher. There is strong evidence for partial occultation of the QSO emission source, particularly from the higher-order Lyman lines which indicate a covered fraction less than 0.2. Overall, the data suggest a correlation between a larger covered fraction and a higher state of ionization. We suggest that the different covered fractions can be explained by either a special line of sight through a disk-like geometry or by the existence of density fluctuations of a factor >2 in the BAL gas. Our photoionization models of the system indicate a large column density and high ionization state similar to that found in X-ray ``warm absorbers''.

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arXiv1998

A Deep Multicolor Survey. IV. The Electronic Stellar Catalog

We make available in electronic form the stellar catalog of 19,494 objects from the Deep Multicolor Survey (DMS). The DMS is based on CCD imaging with the Mayall 4-m telescope in U,B,V,R',I75,I86 and covers 0.83 sq. deg. in six fields at high galactic latitude. The survey reached 5 sigma limiting magnitudes of 22.1 in I86 to 23.8 in B. The catalog gives positions, magnitudes and error estimates, and classification codes in the six filter bands for all the objects. We present tables that summarize the spectroscopic results for the 55 quasars, 44 compact narrow emission-line galaxies, and 135 stars in the DMS that we have confirmed to date. We also make available illustrations of all the spectra. The catalog and spectra can be obtained from the World Wide Web at http://www.astronomy.ohio-state.edu/~posmer/DMS/.

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arXiv1998

An Optical/Near-Infrared Study of Radio-Loud Quasar Environments II. Imaging Results

We use optical and near-IR imaging to examine the properties of the significant excess population of K>=19 galaxies found in the fields of 31 z=1-2 radio-loud quasars by Hall, Green & Cohen (1998). The excess occurs on two spatial scales: a component at <40'' from the quasars significant compared to the galaxy surface density at >40'' in the same fields, and a component roughly uniform to ~100'' significant compared to the galaxy surface density seen in random-field surveys in the literature. The r-K color distributions of the excess galaxy populations are indistinguishable and are significantly redder than the color distribution of the field population. The excess galaxies are consistent with being predominantly early-type galaxies at the quasar redshifts, and there is no evidence that they are associated with intervening MgII absorption systems. The average excess within 0.5 Mpc (~65'') of the quasars corresponds to Abell richness class ~0 compared to the galaxy surface density at >0.5 Mpc from the quasars, and to Abell richness class ~1.5 compared to that from the literature. We discuss the spectral energy distributions (SEDs) of galaxies in fields with data in several passbands. Most candidate quasar-associated galaxies are consistent with being 2-3 Gyr old early-types at the quasar redshifts of z~1.5. However, some objects have SEDs consistent with being 4-5 Gyr old at z~1.5, and a number of others are consistent with ~2 Gyr old but dust-reddened galaxies at the quasar redshifts. These potentially different galaxy types suggest there may be considerable dispersion in the properties of early-type cluster galaxies at z~1.5. There is also a population of galaxies whose SEDs are best modelled by background galaxies at z>2.5.

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arXiv1998

An Optical/Near-Infrared Study of Radio-Loud Quasar Environments I. Methods and z=1-2 Observations

We have conducted an optical/near-infrared study of the environments of radio-loud quasars (RLQs) at redshifts z=0.6-2. In this paper we discuss the sample selection and observations for the z=1-2 subsample and the reduction and cataloguing techniques used. We discuss technical issues at some length, since few detailed descriptions of near-IR data reduction and multicolor object cataloguing are currently available in single literature references. Our sample of 33 RLQs contains comparable numbers of flat- and steep- radio spectrum sources and sources of various radio morphologies, and spans a similar range of absolute magnitude and radio power, allowing us to disentangle dependence of environment on such properties from redshift evolution. We use the standard ``shift-and-stare'' method of creating deep mosaiced images where the exposure time (and thus the RMS noise) at each pixel is not constant across the mosaic. An unusual feature of our reduction procedure is the creation of images with constant RMS noise from such mosaics. We adopted this procedure to enable use of the FOCAS detection package over almost the entire mosaic instead of only in the area of deepest observation where the RMS noise is constant, thereby roughly doubling our areal coverage. We correct the object counts in our fields for stellar contamination using the SKY model of Cohen (1995) and compare the galaxy counts to those in random fields. Even after accounting for possible systematic magnitude offsets we find a significant excess of K>=19 galaxies. Analysis and discussion of this excess population is presented by Hall & Green (1998).

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arXiv1998

An Optical Multicolor System For Measuring Galaxy Redshifts And Spectral Types

A method of obtaining approximate redshifts and spectral types of galaxies using a photometric system of six broad-bandpass filters is developed. The technique utilizes a smallest maximum difference approach rather than a least-squares approach, and does not consider a galaxy's apparent magnitude in the determination of its redshift. In an evalution of its accuracy using two distinct galaxy samples, the photometric redshifts are found to have a root mean square deviation of $\pm$0.05 from spectroscopically determined redshifts. Possible systematic errors of the method are investigated, including the effects of post-starburst (``E+A'') galaxies and attempts to measure redshifts with incomplete color information. Applications of the technique are discussed.

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arXiv1998

Luminosity Functions And Evolution Of Blue Galaxies In A Deep Multicolor CCD Field Survey

A complete sample of 659 field galaxies with 17.0$<$U$\leq$21.1, each with U-B-V-R-I7500-I8600 photometry, has been selected from a deep field survey which covers 0.83 deg$^2$ along six lines of sight (Hall et al. 1996a). Each galaxy's spectral type and redshift has been estimated using a multicolor photometric technique (Liu & Green 1998). Total number counts of the galaxies in the U-band give a count slope d(logN)/dM = 0.55 $\pm$ 0.05, consistent with previous studies. The 545 galaxies in the sample classified as spectral type Sbc or bluer are analyzed for signs of evolution with redshift, and for unusual star formation histories. The U-band luminosity function of these blue galaxies at 0.02$<$z$<$0.15 has a steep $α$ $\simeq$ -1.85 down to M(B)$\simeq$-14. The luminosity functions at 0.15$\leq$z$<$0.3 and 0.3$\leq$z$\leq$0.5 show significant evolution in M* and $ϕ*$, at levels consistent with those found in the Canada-France and Autofib Redshift Surveys. A significant population of very blue (rest frame U-B$<$-0.35) galaxies, with spectral energy distributions indicating strong starburst activity, is observed at z$\gtrsim$0.3 but not at z$<$0.3. This population is confirmed via spectroscopy of part of the sample. These may be galaxies temporarily brightened by global starbursts, which subsequently fade and redden at lower redshifts.

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arXiv1997

A Deep Multicolor Survey. III. Additional Spectroscopy and Implications for the Number Counts of Faint Quasars

We have made spectroscopic identifications of 39 additional quasar candidates from the Deep Multicolor Survey (DMS) of Hall et al. (1996, ApJ, 462, 614, astro-ph/9512052). We have identified 9 new quasars with 0.3 < z < 2.8 and 16.8 < B < 21.6, all from the group of candidates with ultraviolet excess (UVX). No new quasars with z > 3 were found among the observed candidates selected due to their red (B-R) and (V-R) colors. As a result, there are now 55 confirmed quasars in the survey: 42 with 0.3 < z < 2, nine with 2 < z < 3, three with 3 < z < 4, and 1 at z = 4.3. One new quasar, DMS 0059-0055, is very bright with B=16.8 and z=0.3, making its detection by our survey very unexpected. Including this new spectroscopy, the results of the DMS are converging with the predicted space densities of other surveys. In particular, we no longer find an excess of quasars with z < 2.3 and B < 21 in the survey over predictions based on models by Koo & Kron. Also, the excess in the number of quasars seen at z > 3 over predictions based on models by Warren, Hewett, & Osmer is less than previously suggested. We also demonstrate the success of our quasar color modeling which is important in assessing the completeness of our survey.

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arXiv1997

I ZW 18 -- a New Wolf-Rayet Galaxy

We report the discovery of broad Wolf-Rayet emission lines in the Multiple Mirror Telescope (MMT) spectrum of the NW component of I Zw 18, the lowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad Wolf-Rayet (W-R) bumps at the wavelengths $λ$4650 and $λ$5800 are detected indicating the presence of WN and WC stars. The total numbers of WN and WC stars inferred from the luminosities of the broad He II $λ$4686 and C IV $λ$5808 lines are equal to 17(+/-)4 and 5(+/-)2, respectively. The W-R to O stars number ratio is equal to about 0.02, in satisfactory agreement with the value predicted by massive stellar evolution models with enhanced mass loss rates. The WC stars in the northwest component of I Zw 18 can be responsible for the presence of the nebular He II $λ$4686 emission line, however the observed intensity of this line is several times larger than model predictions, and other sources of ionizing radiation at wavelengths shorter than 228Åare necessary.

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arXiv1997

The UV Properties of the Narrow Line Quasar I Zwicky 1

I Zw 1 is the prototype narrow line quasar. We report here the results of our study of the UV emission of I Zw 1 using a high S/N (50-120) spectrum obtained with the HST FOS. The following main new results are obtained: 1. The Mg II and Al III doublets are partially/fully resolved. The measured doublet ratios verify theoretical predictions that the lines are thermalized in the BLR. 2. A weak associated UV absorption system is detected in N~V, and possibly also in C IV and Lya, suggesting an outflow with a velocity of 1870 km/s and velocity dispersion <300 km/s. 3. Lines from ions of increasing ionization level show increasing excess blue wing flux, and an increasing line peak velocity shift, reaching a maximum blueshift of about 2000 km/s for He II 1640. This may indicate an out-flowing component in the BLR, where the ionization level increases with velocity, and which is visible only in the approaching direction. The highest velocity part of this outflow may produce the associated UV absorption system. 4. The small C III] 1909 EW, and the small C III] 1909/Lya and C III] 1909/Si III] 1892 flux ratios indicate a typical BLR density of 10^11, i.e. about an order of magnitude larger than implied by C III] 1909 in most quasars. A BLR component of a higher density is implied by the EW and doublet ratio of the Al III 1857 doublet. 5. Prominent Fe II UV 191 emission is seen, together with weaker line emission at 1294 and 1871 A. These three features have been proposed as evidence for significant Lya pumping of the 8-10 eV levels of Fe II. 6. Significant Fe III emission is present. The Fe III UV 34 and UV 48 multiplets are clearly resolved, and Fe III UV 1, UV 47, UV 50, and UV 68 may also be present. (Shortened version)

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arXiv1996

X-ray Emission from the Host Clusters of Powerful AGN

(Abridged) We report the detection of X-ray emission from the host cluster of the radio-quiet quasar H1821+643 with the ROSAT HRI, and the non-detection of the host cluster of the radio-loud quasar 3C206 using the EINSTEIN HRI. CL1821+643 has a rest-frame 0.1-2.4 keV luminosity of 3.74$\pm$0.57 h$_{50}^{-2}$ 10^45 ergs/sec, 38% from a barely resolved cooling flow component, which places it among the most X-ray luminous clusters known. The cluster emission complicates interpretation of previous X-ray spectra of this field; in particular, the observed FeK$α$ emission can probably be attributed entirely to the cluster, and either the quasar is relatively X-ray quiet for its optical luminosity or the cluster has a relatively low temperature for its luminosity. We combine these data with the recent detection of X-ray emission from the host cluster of the `buried' radio-quiet quasar IRAS 09104+4109, our previous upper limits for the host clusters of two z$\sim$0.7 RLQs, and literature data on FR II radio galaxies and compare to the predictions of three models for the presence and evolution of powerful AGN in clusters: the low-velocity-dispersion model, the low-ICM-density model, and the cooling flow model. Neither of the latter two models can explain all the observations. We suggest that strong interactions with gas-containing galaxies may be the only mechanism needed to explain the presence and evolution of powerful AGN in clusters, consistent with the far-IR and optical properties of the host galaxies studied here. However, the cooling flow model cannot be ruled out for at least some objects, and it is likely that both processes are at work. Each scenario makes predictions for future X-ray and optical observations which can test their relative importance.

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arXiv1996

The HST Spectrum of I Zw 1: Implications of the C III^* λ1176 Emission Line

I Zw 1 is a well known narrow line quasar with very strong Fe II emission. High S/N spectra obtained with the HST FOS show a remarkably rich emission line spectrum. The C III$^* λ1176$ line is clearly detected in emission for the first time in AGNs. This line arises from radiative decay to the $2s2p^3P^o_{0,1,2}$ metastable levels of C III. The observed flux is 50 larger than expected from collisional excitation, or dielectronic recombination, in photoionized gas. The most plausible mechanism for the large enhancement in the C III^* λ1176 flux is resonance scattering of continuum photons by C III^* ions. This mechanism requires large velocity gradients 1000 km/s within each emitting cloud in the BLR. Such large velocity gradients can be induced by forces external to the gas in the BLR clouds, such as tidal disruption, or radiation pressure.

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arXiv1996

Compact Nuclei in Moderately Redshifted Galaxies

The Hubble Space Telescope WFPC2 is being used to obtain high-resolution images in the V and I bands for several thousand distant galaxies as part of the Medium Deep Survey (MDS). An important scientific aim of the MDS is to identify possible AGN candidates from these images in order to measure the faint end of the AGN luminosity function as well as to study the host galaxies of AGNs and nuclear starburst systems. We are able to identify candidate objects based on morphology. Candidates are selected by fitting bulge+disk models and bulge+disk+point source nuclei models to HST imaged galaxies and determining the best model fit to the galaxy light profile. We present results from a sample of MDS galaxies with I less than 21.5 mag that have been searched for AGN/starburst nuclei in this manner. We identify 84 candidates with unresolved nuclei in a sample of 825 galaxies. For the expected range of galaxy redshifts, all normal bulges are resolved. Most of the candidates are found in galaxies displaying exponential disks with some containing an additional bulge component. 5% of the hosts are dominated by an r^-1/4 bulge. The V-I color distribution of the nuclei is consistent with a dominant population of Seyfert-type nuclei combined with an additional population of starbursts. Our results suggest that 10% +/- 1% of field galaxies at z less than 0.6 may contain AGN/starburst nuclei that are 1 to 5 magnitudes fainter than the host galaxies.

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arXiv1995

A Deep Multicolor Survey II. Initial Spectroscopy and Comparison With Expected Quasar Number Counts

We have used the KPNO 4-meter Mayall telescope to image 0.83 square degrees of sky in six fields at high galactic latitude in six filters spanning 3000-10000Å to magnitude limits ranging from 22.1 to 23.8. As a first use of this database, we have conducted a multicolor survey for quasars. We discuss various methods of selecting outliers in different color-color diagrams and multicolor space that have been used to identify quasars at all redshifts from their colors alone. We discuss the initial results of our program of spectroscopic identification which has so far resulted in the identification of over forty faint quasars, including one at z$>$4, a similar number of compact narrow-emission-line galaxies, and a number of unusual and potentially interesting stars. We use these spectroscopic results, along with extensive simulations of quasar spectra, to study the efficiency of our candidate selection procedures. Finally, we compare the number counts of our quasars and quasar candidates to the expected numbers based on previous studies of the quasar luminosity function. The agreement of our observations with these expectations is good in most cases. However, we do estimate that our survey contains more quasars with B$<$21 and z$<$2.3 than expected from the results of Koo \& Kron (1988) and more z$>$3 quasars than expected from the results of Warren, Hewett, \& Osmer (1994), both at the 3$σ$\ level. Additional spectroscopic observations will be required to confirm or refute these excesses.

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arXiv1995

A Deep Multicolor Survey I. Imaging Observations and Catalog of Stellar Objects

We have used the KPNO 4-meter Mayall telescope to image 0.83 square degrees of sky in six fields at high galactic latitude in six filters spanning 3000-10000Å to magnitude limits ranging from 22.1 to 23.8. We have assembled a catalog of 21,375 stellar objects detected in the fields for use primarily in conducting a multicolor search for quasars. This paper describes the data reduction techniques used on the CCD data, the methods used to construct the stellar object catalog, and the simulations performed to understand its completeness and contamination.

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arXiv1995

Requirements for Investigating the Connection Between Lyman Alpha Absorption Clouds and the Large-Scale Distribution of Galaxies

We model the requirements on observational data that would allow an accurate determination of the degree of association between Lyman alpha absorbers and peaks in the redshift distribution of galaxies (large-scale structures like clusters of galaxies). We compare simulated distributions of low-redshift Lyman alpha absorption systems, constrained to be consistent with the distribution observed with HST, with the large-scale distribution of galaxies determined from pencil-beam redshift surveys. We estimate the amount of observational data required from catalogues of Lyman alpha absorbers and galaxies to allow a statistically significant test of the association of absorbers with large-scale structures of galaxies. We find that for each line-of-sight observed for Lyman alpha absorption lines (assuming that the entire redshift range out to z=0.4 is observable), redshifts must be obtained for at least 18 galaxies brighter than Mb=-18 and having redshifts between 0.2 and 0.4. Based on the redshift surveys used in this study, a search radius of 10' from the quasar line-of-sight is required. This will ensure that all peaks in the galaxy redshift distribution are represented by at least one galaxy in the observed sample. If Lyman alpha absorbers are intrinsically uncorrelated with galaxies, we find that 8 lines-of-sight must be observed to show that the distributions are different at the 95% confidence level. However, if a fraction of the Lyman alpha absorbers are distributed with the peaks in the galaxy distribution, 38 lines-of-sight must be mapped for the distribution of both Lyman alpha absorbers and galaxies in order to determine the fraction of absorbers distributed with the peaks of the galaxy distribution to an accuracy of 10%.

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