Preprints | Papers | Research Pursuits | Proposals | Paraphernalia | PhD Thesis
This page describes my activities in Astronomy. You can get much the same information via a
hypertext copy of my Curriculum Vitae.

Preprints with Pop Science Writeups


See my hypertext Curriculum Vitae.

Research Pursuits

Observing Proposals

PHIIRS -- Pat Hall's Infrared Imaging Reduction Software

This is a suite of IRAF tasks for the reduction of infrared imaging data. It was spawned from original tasks by Doug Williams & Anne Turner and including many cannibalized parts from the DIMSUM package of Eisenhardt et al. which, like PHIIRS, is also available from the IRAF contributed software web site.

PHAT -- Pat Hall's Add-on Tasks

PHIST -- Pat Hall's Interactive Sloan Tasks

PHUC -- Pat Hall's Utility Compilation

PHXS -- Pat Hall's X-ray Software

Miscellaneous tasks

Teaching Resources

Here are some links to various on-line teaching resources: Ned Wright's Cosmology Tutorial | Pasachoff On-Line, featuring Astronomy, From the Earth to the Universe | Nick Strobel's Introductory Astronomy Notes | The IDEA Program: Initiative to Develop Education through Astronomy | AAS Education Initiative | The Jason Project | Astro Camp | The American Physical Society's Forum on Education | The HTML Astronomy Classroom Project (HACP)


The CNOC2 Field Galaxy Redshift Survey

Once upon a time, I was a postdoc working with Howard Yee & Ray Carlberg on the Canadian Network for Observational Cosmology Field Galaxy Redshift Survey (CNOC2). This database includes redshifts, UBVRIJK magnitudes and spectral information for ~6200 galaxies (not all published yet). It would be interesting to add morphologies for many and dynamical masses for some to link the current description of color-dependent LF and clustering evolution with the physical processes behind the evolution of individual galaxies (that was my 1999 Hubble Fellowship proposal).

My other personal research interests in CNOC2 have been:

Measuring Omega_M in the Near-IR

I am leading a re-measurement of the CNOC1 Cluster Galaxy Redshift Survey Omega_M=0.19+-0.06 result using the same velocities but K-band luminosities from KPNO instead of the original r-band luminosities (Hall et al. 2002, in preparation). These data will also provide good measurements of the K-band cluster and field galaxy LFs at z=0.35. Get a copy of the proposal.

My Thesis: An Optical-Infrared Study of Quasar Environments

See my thesis page for the abstract and text of my thesis, data, color pictures, and papers derived from it.

At z<0.5, very few RLQs reside in moderately rich clusters, but at z=0.6-0.7, about 30% of them do so (see Yee & Ellingson 1993, ApJ 411, 43, and references therein). This indicates that the evolution of quasars is tied to their environments. The outstanding feature of quasar evolution is the steep decline in space density and/or luminosity since z~2, but little was known about quasar environments at z>0.7 when I began my thesis.

Under the direction of Richard F. Green (NOAO), I used radio-loud quasars (RLQs) at z=0.6-2.0 as signposts around which to search for high redshift clusters of galaxies. This investigation targeted the properties of high redshift galaxies and clusters, the environments of RLQs to high redshift, and the link between associated quasar absorption lines and galaxies. With the Steward 61" and 90" telescopes, I obtained K-band images of z=0.6-1 RLQ fields with existing optical data. I also used the Steward 90" and KPNO 4-meter to obtain r and K band images, respectively, of z=1-2 RLQ fields in two subsamples: with and without associated absorption in their spectra. At such redshifts, k-corrections for early-type galaxies make them very red, so K imaging was used to reach faint enough to detect galaxies at the quasar redshifts, and r imaging to separate very red galaxies from the field population. Clusters were identified through analysis of the galaxy number count and color distributions in the quasar fields.

I found a significant excess of K>19 galaxies around a sample of 31 z=1--2 quasars (Hall et al. 1998a, 1998b). Part of the excess is at <40" from the quasars; the rest extends to 100". The excess galaxies have an r-K color distribution significantly redder than in random fields and are consistent with predominantly early-type galaxies at the quasar redshifts in structures of Abell richness 0 or greater. Multicolor studies showed that many candidate quasar-associated galaxies are well fit as 2--3~Gyr old quiescent galaxies, but some are better fit as 4--5~Gyr old (which requires a high minimum age for the universe at large lookback times), and a number are consistent with 2~Gyr old dust-reddened galaxies. This suggests there may be considerable dispersion in early-type cluster galaxy properties at z=1.5. I also found a few objects well fit only as relatively old and/or dusty background galaxies at z>=2.5 (see below).

Galaxies, Galaxy Clusters, and Extremely Red Objects at z>1

Hall et al. (2001) presented further observations and analyses of selected fields from my thesis. The existence of such `J-dropout' galaxies also points to the feasibility of searching for clusters at z=2-3 using wide-field RJK imaging. Identifying galaxies 3 Gyr old at such redshifts would place extremely strong constraints on the formation timescales of early-type cluster galaxies and on the cosmological model. This idea - which I'd been kicking around since 1997 - will finally become reality in January 2003, when I'll start using the ISPI 11'x11' field-of-view imager at the CTIO 4m to survey 1 deg^2 of sky to K=21 as part of a larger Catolica-U.Chile-Yale collaboration. Get a copy of the continuing proposal.
The Red-Sequence Cluster Survey

I am also collaborating on The Red-Sequence Cluster Survey (RCS), a definitive z=0.5-1.4 galaxy cluster search using ~85 deg^2 of deep R and Z data from CFHT and CTIO (Yee & Gladders 2001). Clusters are identified and their redshifts estimated using the putative red sequence of early type galaxies. I am also using the RCS to search for z>5.5 quasars as well as L and T dwarfs, by obtaining follow-up near-IR snapshots of objects with very red R-Z colors.

Quasar Surveys, Properties, and Evolution

Myself, Pat Osmer, Julia Kennefick, Richard Green, and Steve Warren have mostly completed the spectroscopic followup for a multicolor CCD survey for quasars, the Deep Multicolor Survey. Spectroscopic mopping-up of the remaining quasar candidates is an ongoing `background job' of ours.

Paper I and Paper II (along with an erratum to correct printer's errors) were published in 1996, Paper III in 1997, and Paper IV in 1998. A few minor additional errors in Paper II have come to light: Figures 16b and 16c were incorrectly switched with each other in the published version; Quasar #33 in Table 1 should have a declination of +49 52 00.75; and some of the uncertainties in Tables 3 and 4 were computed incorrectly, though the differences are small and do not change the conclusions, and in any case those Tables have been superseded by Tables 3 and 4 of Paper III.

The sole z>4 quasar discovered in this survey has been confirmed to have z=4.36 and the counterpart to the z=4.10 DLA system seen in its spectrum has been spectroscopically confirmed, as a ~0.5L* (R~26) galaxy with SFR of ~0.7 solar masses per year.


The DMS serves as a useful comparison with a new sample of 42 spectroscopically indentified serendipitous AGN from CNOC2 (Hall et al. 2000). Color selection based on the DMS recovers ~81% of the non-color-selected CNOC2 AGN. This is evidence that color selection of AGN (e.g. 2dF, Sloan) introduces no strong biases into a magnitude limited sample, although it does not address the issue of dust-extincted quasars. Also, some of the more interesting objects are not easily color selected, including one with strong double-peaked MgII emission and one at z=3? with a very strange spectrum. Color selection does recover object with only absorption in the rest UV, but broad H-beta and H-alpha in an IR spectrum. It may be similar to the handful of composite starbursts/quasars known,though it is not as red (dusty) or as absorbed as some of them (Hall et al. 1997).

This sample also has a higher incidence of associated MgII and CIV absorption than previous optically selected samples, and weaker CIV and Ly-alpha Baldwin effects than lower redshift samples. The former may be due to the lack of color selection, a luminosity correlation, or associated dust, while the latter may be due to a steepening of the slope of the Baldwin effect with cosmic time, due to evolution of AGN SEDs or BELR ionization parameters. Wider wavelength coverage spectroscopy is being obtained to extend these conclusions.

Big Faint Quasar Survey

I am interested in continuing the study of faint quasar populations, particularly at high redshift, via larger and/or fainter surveys. The benchmark quasar survey, the Large Bright Quasar Survey, contains 1000 quasars with z<3 and B<19. Even larger optically selected bright quasar samples are now becoming available: 10,000 of an eventual ~25,000 quasars with z<2.8 and B<21 from the 2QZ survey, and 4,000 or an eventual ~100,000 quasars with i<20 from the SDSS.

However, these surveys will not completely sample the redshift-luminosity plane. Surveys for low-luminosity quasars at high (z>3) redshift are needed to understand the dependence of quasar properties on redshift and luminosity. With the advent of large CCD mosaics (beginning with the Big Throughput Camera at CTIO) and ~1 square degree multifiber spectrographs on 4-m class telescopes, it is feasible to assemble a sample of ~200 faint (20 < R < 23) z > 3 color-selected quasars over ~8 deg^2 in a reasonable amount of telescope time, and larger numbers of z<3 quasars to R=23 via wide-field U-band imaging. A `Big Faint Quasar Survey' of this sort can be performed in conjunction with almost any high galactic latitude imaging project, as long as data is obtained in several filters. (For example, the NOAO Deep Wide-Field Survey will be an excellent complement to the BFQS for z>3 quasars, while the smaller ESO Imaging Survey includes U-band data that will make it sensitive to the more numerous z<3 quasars.)

We successfully proposed to do exactly this with the BTC at CTIO. We performed deep B imaging in fields with existing R and/or I data to search for faint quasars at z=3.3-5.0. About 7 deg^2 was covered to the necessary depth during runs in June 1997 and March 1998, and the data are being catalogued and quasar candidates selected (Hall et al. 2002, in preparation). The first two candidates observed turned out to be M stars, but they were selected from a very small subset of the full area. Get a copy of the latest observing proposal..

Clustering studies of very faint quasars and AGN discovered in this survey (and the BTC40 survey described below) will complement similar studies with brighter z<3 quasars and extend them to z~5. The shape and evolution of the faint end of the quasar luminosity function has implications for the origin of the UV and x-ray backgrounds, the initial fluctuation power spectrum of the universe, and galaxy (and massive black hole) formation and chemical evolution at high z. Also, comparison with other quasar samples will allow the redshift and luminosity dependencies of quasar emission line properties, the BAL fraction, radio emission and other properties to be disentangled.

BTC40: The CTIO/BTC 40-square-degree Survey

This collaboration imaged 40 deg^2 (the original goal was 50 deg^2) to B=24.7, V=25, I=23.8 (5-sig, point source) to search for gravitationally lensed quasars, distant clusters of galaxies, and z>5 quasars (my interest). Spectroscopic followup of z>5 quasar candidates began in April 1998, at which time it was realized that Z band data was necessary for efficient discrimination of quasars from late type stars. Z band data was then obtained for the full 40 deg^2 Spectroscopy of most I<22 candidates yielded quasars at z=4.6 and 4.8 (Monier et al. 2002, in press), but none at z>5.

z>5.5 Quasars in the Red-Sequence Cluster Survey (and Brown Dwarfs Too)

Quasars at z>3.3 are easily selected as B-band dropouts, z>4.5 quasars as V-band dropouts, and z>5.5 quasars as R band dropouts. The area and depth of the Red-Sequence Cluster Survey -- 85 deg^2 to R=25.5 (3-sigma) and z=23.3 (8-sigma) -- make it unique in its ability to select z>5.5 quasars. Unfortunately, just as M stars contaminate z>4 quasar surveys, the even cooler L and T dwarfs (which are mostly brown dwarfs) contaminate this survey. Fortunately, J-band snapshot imaging of R-Z>4 objects cleanly separates quasars, L dwarfs, and T dwarfs. The full RCS should yield at least 40 T dwarfs and 400 L dwarfs -- down to 2.5 magnitudes fainter than SDSS -- and up to several dozen z>5.5 quasars. To date we have obtained IR imaging for several dozen candidates, and spectroscopy for a handful of the brightest (all dwarf stars). We are currently digesting the data in order to refine our selection criteria (now that we have 85 square degrees to choose candidates from!) so that efficient spectroscopic followup of quasar candidates can begin in earnest in 2002.

Near-IR Surveys for AGN

All the surveys above select quasars in the optical, which translates to ultraviolet wavelengths in the quasar rest frames. Thus these surveys could all be insensitive to a population of dust-reddened quasars. I am studying this issue using SDSS, 2MASS, and FIRST.

The SDSS color-outlier selection is sensitive to reddened quasars, and in Richards & Hall et al. (2002, submitted) we define and study a sample of `anomalously red quasars'. Dust reddening is clearly responsible for the majority of such SDSS quasars, but a simple screen model of reddening cannot explain the differences between normal- and reddened-quasar composite spectra. Also, the SDSS cannot determine with certainty the maximum reddening present around quasars, since with sufficient reddening, any given object can be pushed fainter than the SDSS spectroscopic magnitude limit.

To determine the maximum dust reddening affecting quasars, IR selection is needed - soon this will mean the far-IR (with SIRTF), but for now it means the near-IR (with 2MASS). However, Wayne Barkhouse and I showed in Barkhouse & Hall 2001 that infrared-infrared color selection is not as sensitive to dust-obscured quasars as optical-infrared color selection. Using optical-infrared color selection criteria based on the work presented in that paper, I have begun a spectroscopic survey for AGN using 2MASS, the Two-Micron All-Sky Survey. Two of every three targets turn out to be AGN, and even the non-AGN are interesting (see Hall 2002a, my ApJ Letter on the discovery in this survey of the first L dwarf star with strong H-alpha emission, and Hall 2002b (ApJL, submitted), which confirms its proper motion and suggests how to distinguish flares from disk/binary accretion as the emission mechanism in the handful of persistent H-alpha emitters known among late-type dwarfs). To date the survey has emphasized objects at $K$$<$13.5, and thus has yielded mostly $z$$<$0.4 AGN (Hall 2002, in preparation), in part due to the k-correction for the host galaxy light at such redshifts. The properties of such an infrared-selected quasar sample are of great interest (cf. the work of Cutri, Smith et al.), but in order to determine the dust reddening bias in optical samples, higher redshifts must be probed.

Galaxy contamination makes it tough to probe fainter in search of higher redshifts using the 2MASS dataset alone. Fainter IR quasar selection over a usefully large area will soon be possible using BRIK imaging from the NOAO Deep Wide-Field Survey. This survey will be unique in that its IR data will enable the first wide-field IR selection of quasars of average luminosity out to z=5, directly determining the fraction of radio-quiet quasars optical surveys miss due to dust extinction.

I am also using another approach to efficiently hunt for quasars at K>13.5 in the 2MASS dataset, in collaboration with Bob Becker & Michael Gregg. We have combined 2MASS with the FIRST radio survey to isolate radio sources for followup. Star and normal galaxy contamination is greatly reduced in this subsample. Thus it is feasible to obtain targeted IR imaging to separate extended radio galaxies from unresolved quasar candidates, and spectroscopic followup on only the latter. We have spectroscopically confirmed at least one reddened quasar to date, and are analyzing the IR imaging in hand for the remaining candidates (Hall et al. 2002, in preparation).

Unusual Broad Absorption Line Quasars

Broad Absorption Line quasars show absorption from gas with blueshifted outflow velocities of typically 0.1c. About 10% of quasars exhibit BAL troughs, but this is usually attributed to an orientation effect. Most quasars probably have BAL outflows covering ~30% of the sky as seen from the quasar, with mass loss rates possibly comparable to the accretion rates required to power the quasar. Many or perhaps even all young quasars seem to experience a phase of close to 100% covering by BAL outflows. Therefore an understanding of BAL outflows is required for an understanding of quasars as a whole.

The most extreme examples of BAL quasars may be a help in this endeavor, as they illustrate the full range of parameter space spanned by BAL outflows. The FIRST bright quasar survey was the first to discover extremely unusual BAL quasars, such as the heavily absorbed low-ionization BAL 1556+3517. (Shortly after its discovery, it was shown to be extremely red, in Hall et al. 1997.) The Digitized POSS has also discovered several unusual BAL quasars (Brunner, Hall, Djorgovski et al. 2002, in preparation). But it is the SDSS which has confirmed that these are members of populations of unusual BAL quasars (Hall et al. 2002, ApJS 141, 267). This paper presents numerous examples of each class of objects illustrated in my recent conference proceedings contribution on the subject (Hall et al. 2001):

Ongoing followup work on these objects includes: The SDSS keeps churning out new examples every month, and one of the handful of BALs found by the ISO satellite is an unusual BAL, and a hyperluminous IR source to boot (Duc, Hall et al. 2002). There are no doubt more surprises in store from this class of objects over the next several years.

Spectroscopy and High-Resolution Imaging of Gravitational Lenses

I have always been interested in gravitational lensing, because it's cool. Since joining the SDSS collaboration as a Princeton/Catolica postdoc, I have obtained spectroscopy of candidate lensed quasars from the SDSS - I didn't find any lenses, but I did find a binary quasar and an emission-line galaxy companion to a z~1 quasar.

Also, I was able to use an SDSS spectrum to obtain a lensing galaxy redshift for the recently discovered, extremely reddened, gravitationally lensed z=2.2 quasar PMN J0134-0931, which has a complex, six-image radio morphology (Hall et al. 2002). HST imaging shows that some lensed images are significantly redder than others, meaning that at least some reddening occurs in the lensing galaxy (or galaxies). We outlined a model wherein lensing, interstellar scattering, and differential reddening by a pair of galaxies can entire explain the system; a more detailed study by Keeton (2002, submitted) confirms the feasibility of our model.

CLASS B1359+154

Observations of the radio lens CLASS B1359+154 (above) using the CFHT Adaptive Optics system (Rusin, Hall et al. 2000) confirm the prediction of multiple deflectors in this system; in fact, the lens may be the core of a small group. No simple model tied to the 4 or 5 possible IR or radio deflector positions reproduces the image splitting, and if the 3 or 4 observed deflectors are all distinct galaxies they must be undermassive, at z>1, or both. (HST imaging eventually showed that this is a six-image lens produced by a group of three lensing galaxies at z~1, the first galaxy-scale six-image lens known.)

HST 180746+45599

Another candidate group lens (an arc this time) is present in a sample of HST Medium Deep Survey candidate galaxy groups for which I obtained near-IR adaptive optics images (Hall & Nichol 2003). The goal of this project is to extend the pixel-by-pixel stellar population studies of Abraham et al. to the near-IR in a range of environments out to z=1; data in hand will provide an instructive first look at the usefulness of IR data in such analyses.

Candidate Spectroscopic Gravitational Lenses in CNOC2

Five of the ~6200 galaxies in the CNOC2 redshift survey have firm redshifts from multiple features, plus a discrepant but real emission line which matches no known or plausible line at that redshift (Hall et al. 2000). These objects are naturally understood as gravitational lenses consisting of the foreground CNOC2 target and a background Ly-alpha emitting galaxy at z=3-4 lensed into a partial or full Einstein Ring (cf. Warren et al. 1996, 1999). In collaboration with J. Willis, VLT+ISAAC spectroscopy is underway in 2001-2002 to confirm the lensed galaxy redshifts and study their rest-frame optical emission lines. CFHT integral-field spectroscopy has been obtained for one object, to study the lensing geometry, and similar work on the others may soon be proposed to 8m class telescopes.

IRAS FSC 10214+4724

I have used Steward Observatory's FASTTRAC infrared adaptive optics system and MMT spectroscopy with Laird Close and Charles Liu to image the `hyperluminous' object IRAS FSC 10214+4724. Our images showed that IRAS 10214+4724 is actually a gravitational lens, and our spectra show a tentative lensing galaxy redshift of z=0.43, as detailed in this ApJ Letter. Although IRAS 10214+4724 is a gravitational lens, as a population `HyLIRGs' may be young or `edge-on' quasars whose central AGN are hidden from direct view by large amounts of dust and gas in a surrounding torus and/or merging or disturbed host galaxy, and thus studying them with high-resolution imaging, spectroscopy, and spectropolarimetry in the optical and IR yields insight into the geometry of the central regions of quasars.

Studies of the Intra-Cluster Medium in Quasar Host Clusters

With Erica Ellingson (CASA) and Richard Green (NOAO), I used the High Resolution Imager (HRI) on the American-German-British satellite ROSAT to observe quasars known to reside in rich clusters. The properties and evolution of the hot intra-cluster medium in quasar host clusters helps discriminate between theories of quasar creation and fueling, and also provides an interesting comparison to optically-, X-ray, and radio-galaxy-selected clusters at moderate redshifts.

Observations of two objects (3C 263 and PKS 2352-34) at z~0.7 were published in my first first-author paper (Hall et al. 1995); no host cluster emission was detected, demonstrating that these two RLQ host clusters are not overly X-ray luminous for their optical richnesses.

The H1821+643 observation, an archival EINSTEIN observation of the z=0.2 RLQ 3C206, and the observation of the `buried' RQQ IRAS 09104+4109 by Crawford & Fabian (1995) were analyzed and compared with radio galaxy host clusters and `normal' clusters in Paper 2. 3C206 is undetected at a level of ~10^44 erg/sec, while the two RQQ host clusters are not only detected, they are among the most luminous clusters of any type known, with soft X-ray luminosities of ~10^45 erg/sec and cooling flows of ~1000 Msun/yr.

The detection of X-ray emission from the host cluster of H1821+643 was later independently confirmed by Saxton et al. (1997). Also, ASCA spectra of this field (Yamashita et al. 1997), which are well fit by a simple power law plus Fe K-alpha emission line, suggest either that the cluster has an above-average temperature for its high luminosity (>11 keV), and that our cooling flow detection is erroneous, or that the ICM is a multi-phase, multi-temperature medium whose integrated emission can masquerade as a power law (as was the case for IRAS 09104+4109). We proposed Chandra observations of H1821+643 to test this hypothesis but were not granted time.

We consider three models for the formation and evolution of powerful FR II AGN in rich clusters in light of these observations: the low-velocity-dispersion model, the low-ICM-density model, and the cooling flow model. None of the models easily explain all the observations. The low ICM density model cannot account for the several AGN known or thought to have clusters with dense ICM, but is consistent with our nondetection of ICM emission from RLQ host clusters. The cooling flow model requires very strong cooling flows, and thus has difficulty accounting for objects without luminous cluster X-ray emission. The low velocity dispersion model has difficulty explaining 3C~295 and H1821+643, which do not seem to have low velocity dispersions. However, the mergers and interactions required in this model can still occur in high velocity dispersion clusters, albeit rarely. Although the cooling flow model may explain some objects, and cannot be completely ruled out in others, the strong interactions postulated by the low velocity dispersion model may be the only mechanism needed to explain the observations. If this is the case, the host galaxies of all AGN in clusters should show optical and/or far-IR evidence of interactions, and we find that all objects in our sample do. Since strong interactions will occur more frequently in low velocity dispersion clusters, a larger sample of AGN host cluster velocity dispersions should show a bias toward lower velocity dispersion.

Polarimetric Studies of AGN

With Paul Smith, in 1991 I participated in intensive optical photometric and polarimetric monitoring of the BL Lac PKS2155-304. The polarized flux varied more rapidly than the total flux, with a variability timescale comparable to that of soft X-rays, even as the spectral indices of total and polarized flux remained constant. This is strong evidence that synchrotron emission from a single compact emission region dominates the optical spectrum.

In 2001, I have once again entered the realm of polarimetry. Near-IR polarimetric observations of three unusual quasars from the SDSS have been obtained with VLT+ISAAC, and are being analyzed. Also, I am collaborating with Paul Smith, Gary Schmidt, and Dean Hines on polarimetry of samples of unusual and `normal' BALs from the SDSS.

Astronomical Resources

Pat Hall -- Astronomer -- pathall[at]