arXiv subjects

R. H. Becker

Publications and source records attributed to R. H. Becker.

At least 37 records · Page 2Linked to original sources

arXiv2003

Uncovering High-z Clusters Using Wide-Angle Tailed Radio Sources

The morphologies of wide-angle tailed (WAT) radio sources (edge-darkened, C-shaped, FR I radio sources) are the result of confinement and distortion of the radio lobes by the dense X-ray-emitting gas in clusters or groups of galaxies. These radio sources are easily seen at high redshifts (z~1) in short-exposure images from the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey. Using a sample of approximately 400 WAT sources from the FIRST survey, we have discovered a number of high-z clusters. Here, we present the highest-z cluster found so far using this method: 1137+3000 at z=0.96. We include photometric and spectroscopic results. Ten galaxies are confirmed at the cluster redshift, with a line-of-sight velocity dispersion of 530 +190/-90 km/s, typical of an Abell richness class 0 cluster.

astro-ph↗

arXiv2002

Advanced Camera for Surveys Observations of Young Star Clusters in the Interacting Galaxy UGC 10214

We present the first Advanced Camera for Surveys (ACS) observations of young star clusters in the colliding/merging galaxy UGC 10214. The observations were made as part of the Early Release Observation (ERO) program for the newly installed ACS during service mission SM3B for the Hubble Space Telescope (HST). Many young star clusters can be identified in the tails of UGC 10214, with ages ranging from ~3 Myr to 10 Myr. The extreme blue V-I (F606W-F814W) colors of the star clusters found in the tail of UGC 10214 can only be explained if strong emission lines are included with a young stellar population. This has been confirmed by our Keck spectroscopy of some of these bright blue stellar knots. The most luminous and largest of these blue knots has an absolute magnitude of M_V = -14.45, with a half-light radius of 161 pc, and if it is a single star cluster, would qualify as a super star cluster (SSC). Alternatively, it could be a superposition of multiple scaled OB associations or clusters. With an estimated age of ~ 4-5 Myr, its derived mass is < 1.3 x 10^6 solar masses. Thus the young stellar knot is unbound and will not evolve into a normal globular cluster. The bright blue clusters and associations are much younger than the dynamical age of the tail, providing strong evidence that star formation occurs in the tail long after it was ejected. UGC 10214 provides a nearby example of processes that contributed to the formation of halos and intra-cluster media in the distant and younger Universe.

astro-ph↗

arXiv2002

The Reddest Quasars II. A gravitationally-lensed FeLoBAL quasar

We report the discovery of a z=2.65 low-ionization iron broad absorption line quasar, FIRST J100424.9+122922, which is gravitationally-lensed by a galaxy at z~0.95. The object was discovered as part of a program to find very red quasars by matching the FIRST radio survey with the 2-MASS near-infrared survey. J100424.9+122922 is the second lensed system to be found in this program, suggesting that many gravitational lenses are probably missed from conventional optical quasar surveys. We have made a simple lens model and a rough estimate of the reddening in the immediate environment of the quasar which suggests that the quasar is intrinsically very luminous and is accreting at close to the Eddington limit of its ~10^9 M_sun black hole. The lensing galaxy has a small amount of dust which is responsible for some excess reddening observed in the fainter image of the quasar, but is otherwise a fairly typical massive elliptical galaxy. We model the selection effects working against the detection of red quasars in both lensed and unlensed samples. We show that these selection effects are very effective at removing even lightly-reddened high redshift quasars from magnitude-limited samples, whether they are lensed or not. This suggests that the red quasar population in general could be very large, and in particular the class of iron broad absorption line quasars of which J100424.9+122922 is a member may be much larger than their rarity in magnitude-limited samples would suggest.

astro-ph↗

arXiv2001

The Optical, Infrared and Radio Properties of Extragalactic Sources Observed by SDSS, 2MASS and FIRST Surveys

We positionally match sources observed by the Sloan Digital Sky Survey (SDSS), the Two Micron All Sky Survey (2MASS), and the Faint Images of the Radio Sky at Twenty-cm (FIRST) survey. Practically all 2MASS sources are matched to an SDSS source within 2 arcsec; ~11% of them are optically resolved galaxies and the rest are dominated by stars. About 1/3 of FIRST sources are matched to an SDSS source within 2 arcsec; ~80% of these are galaxies and the rest are dominated by quasars. Based on these results, we project that by the completion of these surveys the matched samples will include about 10^7 stars and 10^6 galaxies observed by both SDSS and 2MASS, and about 250,000 galaxies and 50,000 quasars observed by both SDSS and FIRST. Here we present a preliminary analysis of the optical, infrared and radio properties for the extragalactic sources from the matched samples. In particular, we find that the fraction of quasars with stellar colors missed by the SDSS spectroscopic survey is probably not larger than ~10%, and that the optical colors of radio-loud quasars are ~0.05 mag. redder (with 4-sigma significance) than the colors of radio-quiet quasars.

astro-ph↗

arXiv2001

Extreme BAL Quasars from the Sloan Digital Sky Survey

The Sloan Digital Sky Survey has discovered a population of broad absorption line quasars with various extreme properties. Many show absorption from metastable states of FeII with varying excitations; several objects are almost completely absorbed bluewards of MgII; at least one shows stronger absorption from FeIII than FeII, indicating temperatures T>35000 K in the absorbing region; and one object even seems to have broad H-beta absorption. Many of these extreme BALs are also heavily reddened, though `normal' BALs (particularly LoBALs) from SDSS also show evidence for internal reddening.

astro-ph↗

arXiv2001

The radio-luminosity - black hole mass correlation for quasars from the FIRST Bright Quasar Survey, and a ``unification scheme'' for radio-loud and radio-quiet quasars

Several independent lines of evidence now point to a correlation between black hole mass, M_bh, and radio-luminosity. In this paper we discuss the correlation for quasars from the FIRST Bright Quasar Survey (FBQS), using black hole mass estimates from Hbeta linewidths. The FBQS objects fill in the gap between the radio-loud and radio-quiet quasars in the radio-luminosity - optical-luminosity plane, and we find that they fill the corresponding gap in the M_bh - radio luminosity correlation. There is thus a continuous variation of radio luminosity with M_bh, and no evidence for a ``switch'' at some set of critical parameter values which turns on powerful radio jets. By combining the FBQS data with that for quasars from the Palomar-Green survey we find evidence for a dependence of radio-luminosity on accretion rate relative to the Eddington limit, L/L_Edd, as well as on M_bh, consistent with the well-known radio-optical correlation for radio-loud quasars. We therefore suggest a new scheme to ``unify'' radio-loud and radio-quiet objects in which radio luminosity scales proportional to M_bh^{1.9+/-0.2}(L/L_Edd)^{1.0} for L/L_Edd~0.1, with an apparently weaker accretion rate dependence at low L/L_Edd. The scatter about this relation is +/-1.1 dex, and may well hide significant contributions from other physical effects, such as black hole spin and radio source environment.

astro-ph↗

arXiv2001

The First Observations of the Second Brightest Quasar

We report the discovery of a new bright quasar PHL 1811 as part of a followup program to identify bright quasars in the FIRST radio survey. With B=13.9, R=13.9, and z=0.192, this quasar is now the second brightest quasar (in apparent magnitude) known beyond z=0.1. Optically classified as a Narrow-line Seyfert 1 galaxy (NLS1), PHL 1811 is unusual for an object in this class in that it was not detected in X-rays in the ROSAT All Sky survey or in any previous X-ray survey. A follow-up BeppoSAX observation confirms that it is deficient in X-rays compared with other quasars, but poor signal to noise prevents a definitive characterization of the X-ray spectrum. We consider three alternative hypotheses to explain why PHL 1811 is a weak X-ray source: 1.) it is a BALQSO and suffers X-ray absorption; 2.) like several other luminous NLS1s, it exhibits high amplitude X-ray variability, and has been observed only when it is in an X-ray quiescent state; 3.) it is intrinsically weak because it simply lacks an X-ray emitting region.

astro-ph↗

arXiv2001

The Environments of a Complete, Moderate-Redshift Sample of FIRST Bent-Double Radio Sources

We present an optical spectroscopic and imaging study of the environments of a complete sample of moderate-redshift, bent-double radio sources. More than half of the forty radio galaxies in the sample are associated with Abell richness class 0 or greater clusters at z<0.4. Most of the remaining objects are associated with groups, although a few appear to be hosted by nearly isolated elliptical galaxies. For the bent doubles appearing in poor environments, either dense gas must be associated with the systems to provide the ram pressure to bend the lobes, or alternative bending mechanisms will have to be invoked to explain the radio morphologies. Correlation with the ROSAT All Sky Survey Bright and Faint Source Catalogs reveals the majority of the z<0.2 objects in our sample that we classify optically as clusters are also X-ray sources.

astro-ph↗

arXiv2000

High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data III: A Color Selected Sample at i^*<20 in the Fall Equatorial Stripe

This is the third paper in a series aimed at finding high-redshift quasars from five-color (u'g'r'i'z') imaging data taken along the Celestial Equator by the SDSS during its commissioning phase. In this paper, we first present the observations of 14 bright high-redshift quasars (3.66<z<4.77, i^*<20) discovered in the SDSS Fall Equatorial Stripe, and the SDSS photometry of two previously known high-redshift quasars in the same region of the sky. Combined with the quasars presented in previous papers, we define a color-selected flux-limited sample of 39 quasars at 3.6 < z < 5.0 and i^*<20, covering a total effective area of 182 deg^2. From this sample, we estimate the average spectral power law slope in the rest-frame ultraviolet for quasars at z~4 to be -0.79 with a standard deviation of 0.34, and the average rest-frame equivalent width of the Ly alpha+N V emission line to be 69 A with a standard deviation of 18 A. The selection completeness of this multicolor sample is determined from the model colors of high-redshift quasars, taking into account the distributions of emission line strengths, intrinsic continuum slope, the line and continuum absorption from intervening material, and the effects of photometric errors. The average completeness of this sample is about 75%. The selection function calculated in this paper will be used to correct the incompleteness of this color-selected sample and to derive the high-redshift quasar luminosity function in a subsequent paper. In the Appendix, we present the observations of an additional 18 faint quasars (3.57<z<4.80, 20.1<i^*<20.8) discovered in the region on the sky that has been imaged twice. Several quasars presented in this paper exhibit interesting properties, including a radio-loud quasar at z=4.77, and a narrow-line quasar (FWHM = 1500 km s^-1) at z=3.57.

astro-ph↗

arXiv2000

High-Redshift Quasars Found in Sloan Digital Sky Survey Commissioning Data IV: Luminosity Function from the Fall Equatorial Stripe Sampl

This is the fourth paper in a series aimed at finding high-redshift quasars from five-color imaging data taken along the Celestial Equator by the SDSS. during its commissioning phase. In this paper, we use the color-selected sample of 39 luminous high-redshift quasars presented in Paper III to derive the evolution of the quasar luminosity function over the range of 3.6<z<5.0, and -27.5<M_1450<-25.5 (Omega=1, H_0=50 km s^-1 Mpc^-1). We use the selection function derived in Paper III to correct for sample incompleteness. The luminosity function is estimated using three different methods: (1) the 1/V_a estimator; (2) a maximum likelihood solution, assuming that the density of quasars depends exponentially on redshift and as a power law in luminosity and (3) Lynden-Bell's non-parametric C^- estimator. All three methods give consistent results. The luminous quasar density decreases by a factor of ~ 6 from z=3.5 to z=5.0, consistent with the decline seen from several previous optical surveys at z<4.5. The luminosity function follows psi(L) ~ L^{-2.5} for z~4 at the bright end, significantly flatter than the bright end luminosity function psi(L) \propto L^{-3.5} found in previous studies for z<3, suggesting that the shape of the quasar luminosity function evolves with redshift as well, and that the quasar evolution from z=2 to 5 cannot be described as pure luminosity evolution. Possible selection biases and the effect of dust extinction on the redshift evolution of the quasar density are also discussed.

astro-ph↗

arXiv2000

Composite Spectra from the FIRST Bright Quasar Survey

We present a very high signal-to-noise ratio composite spectrum created using 657 radio-selected quasars from the FIRST Bright Quasar Survey. The spectrum spans rest-frame wavelengths 900 - 7500 Angstroms. Additionally we present composite spectra formed from subsets of the total data set in order to investigate the spectral dependence on radio loudness and the presence of broad absorption. In particular, radio-loud quasars are red compared to radio-quiet quasars, and quasars showing low-ionization broad absorption lines are red compared to other quasars. We compare our composites with those from the Large Bright Quasar Survey. Composite quasar spectra have proven to be valuable tools for a host of applications, and in that spirit we make these publically available via the FIRST survey web page.

astro-ph↗

arXiv2000

QSO 2359-1241: A Bright, Highly Polarized, Radio-Moderate, Reddened, Low-Ionization Broad Absorption Line Quasar

We report the discovery of a bright quasar (E=15.8, z=0.868) associated with the flat spectrum radio source NVSS J235953-124148. This quasar we designate QSO 2359-1241 possesses a rare combination of extreme properties that make it of special interest. These properties include: intrinsic high-velocity outflow seen in absorption for both high and low-ionization species, high optical polarization (about 5%), significant radio emission, and dust reddening. The dereddened absolute magnitude of QSO 2359-1241 places it among the three most optically luminous quasars known at z<1. High-resolution spectroscopy and a detailed analysis of the optical/ultraviolet absorption features will be given in a companion paper (Arav et al 2000).

astro-ph↗

arXiv2000

The Discovery of a Luminous z=5.80 Quasar from the Sloan Digital Sky Survey

We present observations of SDSSp J104433.04--012502.2, a luminous quasar at z=5.80 discovered from Sloan Digital Sky Survey (SDSS) multicolor imaging data. This object was selected as an i'-band dropout object, with i*=21.8 +/- 0.2, z*=19.2 +/- 0.1. It has an absolute magnitude M1450 = -27.2 (H_0 =50 km/s/Mpc, q0 = 0.5). The spectrum shows a strong and broad Ly alpha emission line, strong Ly alpha forest absorption lines with a mean continuum decrement D_A = 0.91, and a Lyman Limit System at z=5.72. The spectrum also shows strong OI and SiIV emission lines similar to those of quasars at z<= 5, suggesting that these metals were produced at redshift beyond six. The lack of a Gunn-Peterson trough in the spectrum indicates that the universe is already highly ionized at z ~ 5.8. Using a high-resolution spectrum in the Ly alpha forest region, we place a conservative upper limit of the optical depth due to the Gunn-Peterson effect of tau < 0.5 in regions of minimum absorption. The Ly alpha forest absorption in this object is much stronger than that in quasars at z<= 5. The object is unresolved in a deep image with excellent seeing, implying that it is unlensed. The black hole mass of this quasar is ~3 x 10^9 M_solar if we assume that it is radiating at the Eddington luminosity and no lensing amplification, implying that it resides in a very massive dark matter halo. The discovery of one quasar at M_1450 < -27 in a survey area of 600 deg^2 is consistent with an extrapolation of the observed luminosity function at lower redshift. The abundance and evolution of such quasars can provide sensitive tests of models of quasar and galaxy formation.

astro-ph↗

arXiv2000

Five High-Redshift Quasars Discovered in Commissioning Imaging Data of the Sloan Digital Sky Survey

We report the discovery of five quasars with redshifts of 4.67 - 5.27 and z'-band magnitudes of 19.5-20.7 M_B ~ -27. All were originally selected as distant quasar candidates in optical/near-infrared photometry from the Sloan Digital Sky Survey (SDSS), and most were confirmed as probable high-redshift quasars by supplementing the SDSS data with J and K measurements. The quasars possess strong, broad Lyman-alpha emission lines, with the characteristic sharp cutoff on the blue side produced by Lyman-alpha forest absorption. Three quasars contain strong, broad absorption features, and one of them exhibits very strong N V emission. The amount of absorption produced by the Lyman-alpha forest increases toward higher redshift, and that in the z=5.27 object (D_A ~ 0.7) is consistent with a smooth extrapolation of the absorption seen in lower redshift quasars. The high luminosity of these objects relative to most other known objects at z >~ 5 makes them potentially valuable as probes of early quasar properties and of the intervening intergalactic medium.

astro-ph↗

arXiv2000

Search for the Identification of 3EG J1835+5918: Evidence for a New Type of High-Energy Gamma-ray Source

The EGRET source 3EG J1835+5918 is the brightest and most accurately positioned of the as-yet unidentified high-energy gamma-ray sources at high Galactic latitude (l,b=89,25). We present a multiwavelength study of the region around it, including X-ray, radio, and optical imaging surveys, as well as optical spectroscopic classification of most of the active objects in this area. The identified X-ray sources in or near the EGRET error ellipse are radio-quiet QSOs, a galaxy cluster, and coronal emitting stars. The radio sources inside the error ellipse are all fainter than 4 mJy at 1.4 GHz. In addition there are no flat-spectrum radio sources in the vicinity. Since no blazar-like or pulsar-like candidate has been found as a result of these searches, 3EG J1835+5918 must be lacking one or more of the physically essential attributes of these known classes of gamma-ray emitters. If it is an AGN it lacks the beamed radio emission of blazars by at least a factor of 100 relative to identified EGRET blazars. If it is an isolated neutron star, it lacks the steady thermal X-rays from a cooling surface and the magnetospheric non-thermal X-ray emission that is characteristic of all EGRET pulsars. If a pulsar, 3EG J1835+5918 must be either older or more distant than Geminga, and probably an even more efficient or beamed gamma-ray engine.

astro-ph↗

arXiv2000

Properties of Radio-Selected Broad Absorption-Line Quasars from the FIRST Survey

In a spectroscopic follow-up to the VLA FIRST survey, the FIRST Bright Quasar Survey (FBQS) has found 29 radio-selected broad absorption line (BAL) quasars. This sample provides the first opportunity to study the properties of radio-selected BAL quasars. Contrary to most previous studies, we establish that a significant population of radio-loud BAL quasars exists. Radio-selected BAL quasars display compact radio morphologies and possess both steep and flat radio spectra. Quasars with low-ionization BALs have a color distribution redder than that of the FBQS sample as a whole. The frequency of BAL quasars in the FBQS is significantly greater, perhaps by as much as a factor of two, than that inferred from optically selected samples. The frequency of BAL quasars appears to have a complex dependence on radio-loudness. The properties of this sample appear inconsistent with simple unified models in which BAL quasars constitute a subset of quasars seen edge-on.

astro-ph↗

arXiv1999

The FIRST Bright Quasar Survey. II. 60 Nights and 1200 Spectra Later

We have used the VLA FIRST survey and the APM catalog of the POSS-I plates as the basis for constructing a new radio-selected sample of optically bright quasars. This is the first radio-selected sample that is competitive in size with current optically selected quasar surveys. Using only two basic criteria, radio-optical positional coincidence and optical morphology, quasars and BL Lacs can be identified with 60% selection efficiency; the efficiency increases to 70% for objects fainter than magnitude 17. We show that a more sophisticated selection scheme can predict with better than 85% reliability which candidates will turn out to be quasars. This paper presents the second installment of the FIRST Bright Quasar Survey with a catalog of 636 quasars distributed over 2682 square degrees. The quasar sample is characterized and all spectra are displayed. The FBQS detects both radio-loud and radio-quiet quasars out to a redshift z>3. We find a large population of objects of intermediate radio-loudness; there is no evidence in our sample for a bimodal distribution of radio characteristics. The sample includes ~29 broad absorption line quasars, both high and low ionization, and a number of new objects with remarkable optical spectra.

astro-ph↗

arXiv1999

FIRST Bent-Double Radio Sources: Tracers of High-Redshift Clusters

Bent-double radio sources can act as tracers for clusters of galaxies. We present imaging and spectroscopic observations of the environments surrounding ten of these sources (most of them Wide Angle Tails (WATs)) selected from the VLA FIRST survey. Our results reveal a previously unknown cluster associated with eight of the radio sources with redshifts in the range 0.33 < z < 0.85; furthermore, we cannot rule out that the other two bent doubles may be associated with clusters at higher redshift. Richness measurements indicate that these clusters are typical of the majority of those found in the Abell (1958) catalog, with a range of Abell richness classes from 0 to 2. The line-of-sight velocity dispersions are very different from cluster to cluster, ranging from approximately 300 to 1100 km/s. At the upper end of these intervals, we may be sampling some of the highest-redshift massive clusters known. Alternatively, the large velocity dispersions measured in some of the clusters may indicate that they are merging systems with significant substructure, consistent with recent ideas concerning WAT formation (Burns et al. 1994).

astro-ph↗