Michael D. Gregg

A Survey of z > 5.7 Quasars in the Sloan Digital Sky Survey. II. Discovery of Three Additional Quasars at z > 6*

We present the discovery of three new quasars at z > 6 in ~ 1300 deg2 of Sloan Digital Sky Survey imaging data, J114816.64+525150.3 (z = 6.43), J104845.05+463718.3 (z = 6.23), and J163033.90+401209.6 (z = 6.05). The first two objects have weak Lyα emission lines; their redshifts are determined from the positions of the Lyman break. They are only accurate to ~0.05 and could be affected by the presence of broad absorption line systems. The last object has a Lyα strength more typical of lower redshift quasars. Based on a sample of six quasars at z > 5.7 that cover 2870 deg2 presented in this paper and in Paper I, we estimate the comoving density of luminous quasars at z ~ 6 and M1450 5.7 quasars and high-resolution, ground-based images (seeing ~04) of three additional z > 5.7 quasars show that none of them is gravitationally lensed. The luminosity distribution of the high-redshift quasar sample suggests the bright-end slope of the quasar luminosity function at z ~ 6 is shallower than Ψ ∝ L-3.5 (2 σ), consistent with the absence of strongly lensed objects.

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

We have used the Very Large Array (VLA) FIRST survey and the Automated Plate Measuring Facility (APM) catalog of the Palomar Observatory Sky Survey I (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 Lac objects can be identified with 60% selection efficiency; the efficiency increases to 70% for objects fainter than 17 mag. 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 (FBQS), with a catalog of 636 quasars distributed over 2682 deg2. The quasar sample is characterized and all spectra are displayed. The FBQS detects both radio-loud and radio-quiet quasars out to 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.

Properties of Radio-selected Broad Absorption Line Quasars from the First Bright Quasar 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 2, 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 to be inconsistent with simple unified models in which BAL quasars constitute a subset of quasars seen edge-on.

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 A. 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 lines. 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 proved to be valuable tools for a host of applications, and in that spirit we make these publicly available via the FIRST survey web page.

Ultracompact dwarf galaxies in the Fornax Cluster

By utilizing the large multiplexing advantage of the Two-degree Field spectrograph on the Anglo-Australian Telescope, we have been able to obtain a complete spectroscopic sample of all objects in a predefined magnitude range, 16.5 < bj < 19.7, regardless of morphology, in an area toward the center of the Fornax Cluster of galaxies. Among the unresolved or marginally resolved targets, we have found five objects that are actually at the redshift of the Fornax Cluster; i.e., they are extremely compact dwarf galaxies or extremely large star clusters. All five have absorption-line spectra. With intrinsic sizes of less than 11 HWHM (corresponding to approximately 100 pc at the distance of the cluster), they are more compact and significantly less luminous than other known compact dwarf galaxies, yet much brighter than any globular cluster. In this paper we present new ground-based optical observations of these enigmatic objects. In addition to having extremely high central surface brightnesses, these objects show no evidence of any surrounding low surface brightness envelopes down to much fainter limits than is the case for, e.g., nucleated dwarf elliptical galaxies. Thus, if they are not merely the stripped remains of some other type of galaxy, then they appear to have properties unlike any previously known type of stellar system.

MACHO Alert 95-30: First Real-Time Observation of Extended Source Effects in Gravitational Microlensing

We present analysis of MACHO Alert 95-30, a dramatic gravitational microlensing event toward the Galactic bulge whose peak magnification departs significantly from the standard point-source microlensing model. Alert 95-30 was observed in real time by the Global Microlensing Alert Network (GMAN), which obtained densely sampled photometric and spectroscopic data throughout the event. We interpret the light-curve fine structure as indicating transit of the lens across the extended face of the source star. This signifies resolution of a star several kiloparsecs distant. We find a lens angular impact parameter θmin/θsource = 0.715 ± 0.003. This information, along with the radius and distance of the source, provides an additional constraint on the lensing system. Spectroscopic and photometric data indicate the source is a M4 III star of radius 61 ± 12 R☉, located on the far side of the bulge at ~9 kpc. We derive a lens angular velocity, relative to the source, of 21.5 ± 2.9 km s-1 kpc-1, where the error is dominated by uncertainty in the angular size of the source star. Likelihood analysis yields a median lens mass of 0.67 -->+ 2.53−0.46 M☉, located with 80% probability in the Galactic bulge at a distance of 6.93 -->+ 1.56−2.25 kpc. If the lens is a main-sequence star, we can include constraints on the lens luminosity. This modifies our estimates to Mlens=0.53 -->+ 0.52−0.35 M☉ and Dlens=6.57 -->+ 0.99−2.25 kpc. Spectra taken during the event show that the absorption-line equivalent widths of Hα and the TiO bands near 6700 A vary, as predicted for microlensing of an extended source. This is most likely due to center-to-limb variation in the stellar spectral lines. The observed spectral changes further support our microlensing interpretation. These data demonstrate the feasibility of using microlensing limb crossings as a tool to probe stellar atmospheres directly.

The FIRST Radio-loud Broad Absorption Line QSO and Evidence for a Hidden Population of Quasars

We have discovered two low-ionization broad absorption line quasars in programs to obtain optical spectra for radio-selected quasar candidates from the VLA FIRST Survey (Becker, White, & Helfand 1995). Both belong to the extremely rare class of BAL QSOs that exhibit narrow absorption lines from metastable excited levels of Fe II and Fe III. Until now, there was just a single object in this class, 0059-2735 (Hazard et al. 1987). In addition, one of our new objects is the first known radio-loud BAL QSO. The properties of these three unusual objects suggest a trend of increasing radio luminosity with the amount of absorption to the quasar, and are perhaps transition objects between radio-loud and radio-quiet quasars. The two new objects are from a radio-selected sample comprising less than 200 quasars; one is heavily attenuated at optical wavelengths in the observed frame. These objects would be easily overlooked by most optical QSO searches; their abundance in the radio sample suggests that they may be representatives of a largely undetected component of the quasar population, perhaps as numerous as ordinary low-ionization BAL QSOs which constitute 1-2% of all QSOs.

The Reddest Quasars

In a survey of quasar candidates selected by matching the Faint Images of the Radio Sky at Twenty cm (FIRST) and Two Micron All-Sky Survey catalogs, we have found two extraordinarily red quasars. FIRST J013435.7-093102 is a 1 Jy source at z = 2.216 and has B-K 10, while FIRST J073820.1+275045 is a 2.5 mJy source at z = 1.985 with B-K ≈ 8.4. FIRST J073820.1+275045 has strong absorption lines of Mg II and C IV in the rest frame of the quasar and is highly polarized in the rest-frame ultraviolet, strongly favoring the interpretation that its red spectral energy distribution is caused by dust reddening local to the quasar. FIRST J073820.1+275045 is thus one of the few low radio luminosity, highly dust-reddened quasars known. The available observational evidence for FIRST J013435.7-093102 leads us to conclude that it too is reddened by dust. We show that FIRST J013435.7-093102 is gravitationally lensed, increasing the number of known lensed, extremely dust-reddened quasars to at least three, including MG 0414-0534 and PKS 1830-211. We discuss the implications of whether these objects are reddened by dust in the host or lensing galaxies. If reddened by their local environment, then we estimate that between 10% and 20% of the radio-loud quasar population is reddened by dust in the host galaxy. The discovery of FIRST J073820.1+275045 and objects now emerging from X-ray surveys suggests the existence of an analogous radio-quiet red quasar population. Such objects would be entirely missed by standard radio or optical quasar surveys. If dust in the lensing galaxies is primarily responsible for the extreme redness of the lensed quasars, then an untold number of gravitationally lensed quasars are being overlooked.

A gravitationally lensed quasar with quadruple images separated by 14.62 arcseconds.

Gravitational lensing is a powerful tool for the study of the distribution of dark matter in the Universe. The cold-dark-matter model of the formation of large-scale structures (that is, clusters of galaxies and even larger assemblies) predicts the existence of quasars gravitationally lensed by concentrations of dark matter so massive that the quasar images would be split by over 7 arcsec. Numerous searches for large-separation lensed quasars have, however, been unsuccessful. All of the roughly 70 lensed quasars known, including the first lensed quasar discovered, have smaller separations that can be explained in terms of galaxy-scale concentrations of baryonic matter. Although gravitationally lensed galaxies with large separations are known, quasars are more useful cosmological probes because of the simplicity of the resulting lens systems. Here we report the discovery of a lensed quasar, SDSS J1004 + 4112, which has a maximum separation between the components of 14.62 arcsec. Such a large separation means that the lensing object must be dominated by dark matter. Our results are fully consistent with theoretical expectations based on the cold-dark-matter model.

Substructure and Dynamics of the Fornax Cluster

We present the first dynamical analysis of a galaxy cluster to include a large fraction of dwarf galaxies. Our sample of 108 Fornax Cluster members measured with the UK Schmidt Telescope FLAIR-II spectrograph contains 55 dwarf galaxies (15.5 > b(j) > 18.0 or -16 > M-B > -13.5). H alpha emission shows that of the dwarfs are star forming, twice the fraction implied by morphological classifications. The total sample has a mean velocity of 1493 +/- 36 kms s(-1) and a velocity dispersion of 374 +/- 26 km s(-1). The dwarf galaxies form a distinct population: their velocity dispersion (429 +/- 41 km s(-1)) is larger than that of the giants () at the 98% confidence level. This suggests that the dwarf population is dominated by infalling objects whereas the giants are virialized. The Fornax system has two components, the main Fornax Cluster centered on NGC 1399 with cz = 1478 km s(-1) and sigma (cz) = 370 km s(-1) and a subcluster centered 3 degrees to the southwest including NGC 1316 with cz = 1583 km s(-1) and sigma (cz) = 377 km s(-1). This partition is preferred over a single cluster at the 99% confidence level. The subcluster, a site of intense star formation, is bound to Fornax and probably infalling toward the cluster core for the first time. We discuss the implications of this substructure for distance estimates of the Fornax Cluster. We determine the cluster mass profile using the method of Diaferio, which does not assume a virialized sample. The mass within a projected radius of 1.4 Mpc is (7 +/- 2) x 10(13) M-., and the mass-to-light ratio is 300 +/- 100 M-./L-.. The mass is consistent with values derived from the projected mass virial estimator and X-ray measurements at smaller radii.