Mark Lacy | NRAO - Academia.edu (original) (raw)

Papers by Mark Lacy

The Astrophysical …, Jan 1, 2006

We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) qu... more We present an analysis of the mid-infrared (MIR) and optical properties of type 1 (broad-line) quasars detected by the Spitzer Space Telescope. The MIR color-redshift relation is characterized to z ∼ 3, with predictions to z = 7. We demonstrate how combining MIR and optical colors can yield even more efficient selection of active galactic nuclei (AGN) than MIR or optical colors alone. Composite spectral energy distributions (SEDs) are constructed for 259 quasars with both Sloan Digital Sky Survey and Spitzer photometry, supplemented by near-IR, GALEX, VLA and ROSAT data where available. We discuss how the spectral diversity of quasars influences the determination of bolometric luminosities and accretion rates; assuming the mean SED can lead to -2errors as large as a factor of 2 for individual quasars. Finally, we show that careful consideration of the shape of the mean quasar SED and its redshift dependence leads to a lower estimate of the fraction of reddened/obscured AGNs missed by optical surveys as compared to estimates derived from a single mean MIR to optical flux ratio.

Monthly Notices of the …, Jan 1, 1999

We have used narrow emission line data from the new 7C Redshift Survey to investigate correlation... more We have used narrow emission line data from the new 7C Redshift Survey to investigate correlations between the narrow-line luminosities and the radio properties of radio galaxies and steep-spectrum quasars. The 7C Redshift Survey is a lowfrequency (151 MHz) selected sample with a flux-density limit about 25-times fainter than the 3CRR sample. By combining these samples, we can for the first time distinguish whether the correlations present are controlled by 151 MHz radio luminosity L 151 or redshift z. We find unequivocal evidence that the dominant effect is a strong positive correlation between narrow line luminosity L NLR and L 151 , of the form L NLR ∝ L 0.79±0.04

Monthly Notices of …, Jan 1, 2001

We measure the radio luminosity function (RLF) of steep-spectrum radio sources using three redshi... more We measure the radio luminosity function (RLF) of steep-spectrum radio sources using three redshift surveys of flux-limited samples selected at low (151 & 178 MHz) radio frequency, low-frequency source counts and the local RLF. The redshift surveys used are the new 7C Redshift Survey (7CRS) and the brighter 3CRR and 6CE surveys totalling 356 sources with virtually complete redshift z information. This yields unprecedented coverage of the radio luminosity versus z plane for steep-spectrum sources, and hence the most accurate measurements of the steep-spectrum RLF yet made. We find that a simple dual-population model for the RLF fits the data well, requiring differential density evolution (with z) for the two populations. The low-luminosity population can be associated with radio galaxies with weak emission lines, and includes sources with both FRI and FRII radio structures; its comoving space density ρ rises by about one dex between z ∼ 0 and z ∼ 1 but cannot yet be meaningfully constrained at higher redshifts. The high-luminosity population can be associated with radio galaxies and quasars with strong emission lines, and consists almost exclusively of sources with FRII radio structure; its ρ rises by nearly three dex between z ∼ 0 and z ∼ 2. These results mirror the situation seen in X-ray and optically-selected samples of AGN where: (i) low luminosity objects exhibit a gradual rise in ρ with z which crudely matches the rises seen in the rates of global star formation and galaxy mergers; and (ii) the density of high luminosity objects rises much more dramatically. The integrated radio luminosity density of the combination of the two populations is controlled by the value of ρ at the low-luminosity end of the RLF of the high-luminosity population, a quantity which has been directly measured at z ∼ 1 by the 7CRS. We argue that robust determination of this quantity at higher redshifts requires a new redshift survey based on a large (∼ 1000 source) sample about five times fainter than the 7CRS.

Arxiv preprint astro-ph/ …, Jan 1, 2005

Supermassive black holes underwent periods of exponential growth during which we seem them as qua... more Supermassive black holes underwent periods of exponential growth during which we seem them as quasars in the distant Universe. The summed emission from these quasars generates the cosmic X-ray background, the spectrum of which has been used to argue that most black-hole growth is obscured [1−2] .

Arxiv preprint astro-ph/ …, Jan 1, 1997

A first sample of faint radio sources with virtually complete redshifts I. Infrared images, the H... more A first sample of faint radio sources with virtually complete redshifts I. Infrared images, the Hubble diagram, and the alignment effect

Monthly Notices of …, Jan 1, 2007

We measure the ages, stellar masses, and star formation histories of z ∼ 6 galaxies, observed wit... more We measure the ages, stellar masses, and star formation histories of z ∼ 6 galaxies, observed within 1 Gyr of the Big Bang. We use imaging from the Hubble Space Telescope (HST) and the Spitzer Space Telescope from the public "Great Observatories Origins Deep Survey" (GOODS), coupled with ground-based near-infrared imaging, to measure their spectral energy distributions (SEDs) from 0.8 − 5 µm, spanning the restframe UV and optical. From our sample of ≈ 50 'i ′ -drop' Lyman-break star-forming galaxies in GOODS-South with z ′ AB < 27, we focus on ≈ 30 with reliable photometric or spectroscopic redshifts. Half of these are confused with foreground sources at Spitzer resolution, but from the 16 with clean photometry we find that a surprisingly large fraction (40%) have evidence for substantial Balmer/4000Å spectral breaks. This indicates the presence of old underlying stellar populations that dominate the stellar masses. For these objects, we find ages of ∼ 200 − 700 Myr, implying formation redshifts of 7 ≤ z f ≤ 18, and large stellar masses in the range ∼ 1 − 3 × 10 10 M ⊙ . Analysis of seven i ′ -drops that are undetected at 3.6 µm indicates that these are younger, considerably less massive systems. We calculate that emission line contamination should not severely affect our photometry or derived results. Using SED fits out to 8 µm, we find little evidence for substantial intrinsic dust reddening in our sources. We use our individual galaxy results to obtain an estimate of the global stellar mass density at z ∼ 6. Correcting for incompleteness in our sample, we find the z ∼ 6 comoving stellar mass density to be 2.5 × 10 6 M ⊙ Mpc −3 . This is a lower limit, as post-starburst and dust-obscured objects, and also galaxies below our selection thresholds, are not accounted for. From our results, we are able to explore the star formation histories of our selected galaxies, and we suggest that the past global star formation rate may have been much higher than that observed at the z ∼ 6 epoch. The associated UV flux we infer at z > 7 could have played a major role in reionizing the universe.

The Astrophysical …, Jan 1, 2001

Several independent lines of evidence now point to a correlation between black hole mass, M bh , ... more 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 Hβ linewidths. The FBQS objects fill in the gap between the radio-loud and radio-quiet quasars in the radio-luminosityoptical-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 radioloud quasars. We therefore suggest a new scheme to "unify" radio-loud and radio-quiet objects in which radio luminosity scales ∝ M 1.9±0.2 bh (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.

The Astrophysical …, Jan 1, 2002

In a survey of quasar candidates selected by matching the FIRST and 2MASS catalogs, we have found... more In a survey of quasar candidates selected by matching the FIRST and 2MASS 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 MG0414−0534 and PKS1830−211. 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 will 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.

Monthly Notices of …, Jan 1, 2005

The Astrophysical …, Jan 1, 2003

We report deep Keck narrow-band Lyα images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.0... more We report deep Keck narrow-band Lyα images of the luminous z > 3 radio galaxies 4C 41.17, 4C 60.07, and B2 0902+34. The images show giant, 100−200 kpc scale emission line nebulae, centered on these galaxies, which exhibit a wealth of morphological structure, including extended low surface brightness emission in the outer regions, radially directed filaments, coneshaped structures and (indirect) evidence for extended Lyα absorption. We discuss these features within a general scenario where the nebular gas cools gravitationally in large Cold Dark Matter (CDM) halos, forming stars and multiple stellar systems. Merging of these "building" blocks triggers large scale starbursts, forming the stellar bulges of massive radio galaxy hosts, and feeds super-massive black holes which produce the powerful radio jets and lobes. The radio sources, starburst superwinds and AGN radiation then disrupt the accretion process limiting galaxy and black hole growth, and imprint the observed filamentary and cone-shaped structures of the Lyα nebulae.