Publications 2007

• Rossa, J., Laine, S., van der Marel, R. P., Mihos, J. C., Hibbard,
  J. E., Boker, T., Zabludoff, A. I. 2007, The Toomre Sequence Revisited
  with HST NICMOS: Nuclear Brightness Profiles and Colors of Interacting
  and Merging Galaxies, AJ, 134, 2124. (Extragalactic)
  
  Gonzalez, A. H., Zaritsky, D., Zabludoff, A. I. 2007, ApJ, A Census
  of Baryons in Galaxy Clusters and Groups, 666, 147.  (Extragalactic)
  
  Gonzalez, A. H., Zaritsky, D., Zabludoff, A. I. 2007, A Census of
  Baryons in Galaxy Clusters and Groups, in Dark Galaxies and Lost
  Baryons, Proceedings of the International Astronomical Union, IAU
  Symposium, Volume 244, p. 167-175.  (Extragalactic)
  
  Christlein, D., Zabludoff, A. 2007, Disentangling Star Formation,
  Environment, and Morphology in Galaxy Evolution in Island Universes,
  Astrophysics and Space Science Proceedings, ISBN 978-1-4020-5572-0,
  Springer, p. 17.  (Extragalactic)
  
  Momcheva, I., Williams, K., Zabludoff, A., Keeton, C. 2007, Poor
  Groups Around Strong Gravitational Lenses, in Galaxy Evolution Across
  the Hubble Time, Edited by F. Combes and J. Palous, Proceedings of the
  International Astronomical Union 2, IAU Symposium #235, held 14-17
  August, 2006 in Prague, Czech Republic. Cambridge: Cambridge
  University Press, pp.230-230.  (Extragalactic)
  
  Zabludoff, A. 2007, Groups: The Rich, the Poor and the Destitute, in
  Groups of Galaxies in the Nearby Universe, Proceedings of the ESO
  Workshop held at Santiago de Chile, 5-9 December 2005, Edited by
  I. Saviane, V.D. Ivanov, J. Borissova, ESO ASTROPHYSICS SYMPOSIA, ISBN
  978-3-540-71172-8. Springer-Verlag, p. 249.  (Extragalactic)
  
  

Ongoing Research

D. Zaritsky (Steward), A. Zabludoff (Steward), and A. Gonzalez
(Florida) find that all classes of galaxies, ranging from disks to
spheroids and from dwarf spheroidals to brightest cluster galaxies,
lie on a two dimensional surface within the space defined by the
logarithms of the half-light radius, r_e, mean surface brightness
within r_e, I_e, and internal velocity, V^2 = (1/2)v_c^2 + sigma^2,
where v_c is the rotational velocity and sigma is the velocity
dispersion. If these quantities are expressed in terms of kpc,
L_solar/pc^2, and km/s, then log r_e - log V^2 + log I_e + log
Upsilon_e + 0.8 = 0, where they provide a fitting function for
Upsilon_e, the mass-to-light ratio within r_e in units of
M_solar/L_solar, that depends only on V and I_e. The scatter about
this surface for their heterogeneous sample of 1925 galaxies is small
(< 0.1 dex) and could be as low as ~ 0.05 dex, or 10%. This small
scatter has three possible implications for how gross galactic
structure is affected by internal factors, such as stellar orbital
structure, and by external factors, such as environment. These factors
either 1) play no role beyond generating some of the observed scatter,
2) move galaxies along the surface, or 3) balance each other to
maintain this surface as the locus of galactic structure
equilibria. They cast the behavior of Upsilon_e in terms of the
fraction of baryons converted to stars, eta, and the concentration of
those stars within the dark matter halo, xi = R_{200}/r_e. They derive
eta = 1.9 x 10^{-5} (L/L^*) Upsilon_* V^{-3} and xi = 1.4
V/r_e. Finally, they present and discuss the distributions of eta and xi
for the full range of galaxies. For systems with internal velocities
comparable to that of the Milky Way (149 < V < 163 km/s), eta = 0.14
+- 0.05, and xi is, on average, ~ 5 times greater for spheroids than
for disks.  (Extragalactic, Theoretical Astrophysics)

I. Momcheva (Steward), K. Williams (Steward), C. Keeton (Rutgers U.),
and A. Zabludoff (Steward) have conducted the first major survey of
the environments of strong gravitational lensing galaxies.
Previously, Keeton and Zabludoff predicted that the effects of
environment, particularly when a hard-to-detect group or cluster lies
at the lens redshift, lead to significant biases in the lens models.
These biases often exceed the quoted random errors on important
quantities derived from lens models, such as the Hubble constant, dark
energy density, and distribution of dark matter in lens galaxy halos.
Using extensive photometric and spectroscopic data acquired from more
than 30 nights of 6.5m telescope time and 25 nights of 4m telescope
time, Momcheva and her collaborators have now found that the majority
of lenses lie in overdense environments that can significantly affect
lens models.  In addition, they show that many lenses are perturbed by
line-of-sight structures over a wide range of spatial and redshift
offsets from the lens. They are currently modeling the contribution of
the hot, X-ray emitting, intragroup medium -- a large component of the
total group mass that is often displaced from the lens -- to the lens
potential.  Ultimately, they will develop the first error budget for
strong lensing analyses, quantifying the most important systematics
both to guide future observations and to set realistic limits on the
errors of lensing-derived quantities.  (Extragalactic, Theoretical
Astrophysics)

I. Momcheva (Steward), K. Williams (Steward), C. Keeton (Rutgers U.),
and A. Zabludoff (Steward) are now using the groups of galaxies that
they have discovered around lenses at intermediate redshifts to study
directly the evolution of galaxies.  Few other samples of poor groups
exist over those redshifts, and all have complex selection biases.
Momcheva et al. are investigating how the star formation rates, galaxy
morphologies, AGN properties, and X-ray-optical scaling relations for
groups change from z = 0.3-0.8 and how they compare with groups at the
present epoch.  Understanding how galaxies evolve in groups is
important for three reasons: 1) groups are the most common
environments for galaxies, 2) groups are simpler environments than
clusters, in that only galaxy-galaxy interactions and mergers are
likely to drive the evolution of group galaxies, whereas additional
evolutionary drivers such as ram-pressure stripping or "galaxy
harassment" may operate in clusters, 3) groups are the building blocks
of clusters, so understanding how group galaxies evolve provides a
control for any changes arising from later exposure to a cluster
environment.  (Extragalactic)

Motivated by their theoretical work, Y. Yang (Steward), A. Zabludoff
(Steward), R. Dave (Steward), and D.J. Eisenstein (Steward) are the
first to search for major cooling lines (Lyalpha, Halpha, HeII) from
the gas surrounding forming galaxies to find and study galaxies
forming via the process of gas accretion.  The best candidates for
gas-accreting galaxies are extended Lyalpha sources, the so-called
Lyalpha blobs.  However, there is controversy over their true
origin(s) for several reasons.  First, the few existing samples of
Lyalpha blobs are small and drawn from non-random, clustered fields.
As a consequence, it is difficult to assess how representative these
blobs are and to compare them directly with gas accretion simulations,
which are best at modeling random patches.  Second, there are many
intrinsic ambiguities in interpreting Lyalpha emission, and the
absence of observations of optically thin diagnostic lines -- for
example, Halpha -- has slowed progress further.  To resolve these
issues, Yang and his collaborators have developed an ambitious new
approach to resolve the nature of Lya blobs, one that could culminate
in the first unambiguous detection of a galaxy forming via gas
accretion.  First, they have conducted a large-volume, blind survey
using a custom narrow-band filter and wide-field imagers on telescopes
as small as 2.3m to discover blobs at z = 2.3.  They have now
discovered 16 blobs and hundreds of Lya emitters across four fields.
These blobs are highly clustered --- 14 lie in only one of the survey
fields, Chandra Deep Field South --- suggesting that they are the
sites of on-going, massive galaxy formation, and perhaps the
precursors of brightest cluster galaxies.  Second, motivated by the
results of their cosmological simulations (see below) they are using
multi-wavelength archival data, and ground-based optical and near-IR
spectroscopy, to constrain the source of blob emission from the
kinematics and energetics of each blob.  (Extragalactic)

Y. Yang (Steward), A. Zabludoff (Steward), and R. Dave (Steward) are
using new high resolution cosmological simulations to test their
previous theoretical work on the nature of the Lya, Halpha, and HeII
emission arising from gas accretion by forming galaxies.  Although it
may not be possible to fully discriminate observationally between
outflows and gravitational infall under some scenarios, the study of
the energy deposited by outflows is in of itself extremely interesting
and valuable.  It is possible that the known Lyalpha blobs represent a
direct observation of galactic or AGN feedback in action.  Such early
energetic input is a favored explanation for the enhanced entropy seen
in intracluster media, i.e., the so-called ``pre-heating".  Observing
a random patch of the universe -- as they are now doing, see above --
rather than a clustered region makes it much more straightforward to
constrain the amount of feedback energy being deposited into
surrounding intergalactic gas, by simply measuring the difference
between the observed emission and simulations that only include
cooling radiation.  This approach would constrain the most popular
outflow models.  Now that the simulations include superwinds and,
soon, AGN, Yang and his collaborators can make direct predictions for
how much the extended emission of Lyalpha blobs is due to infall and
how much is due to non-gravitational energy input.  Quantifying the
fraction of Lyalpha emission arising from gravitational versus
non-gravitational processes will provide valuable insights into
outflows that shape galaxies and their surrounding intergalactic
medium.  (Theoretical Astrophysics)

Suresh Sivanadam (Steward), Ann Zabludoff (Steward), Dennis Zaritsky
(Steward), Anthony Gonzalez (U of Florida), and Daniel Kelson (OCIW)
determine the relative contributions of galactic and intracluster
stars to the enrichment of the intracluster medium (ICM) using X-ray
surface brightness, temperature, and Fe abundance profiles for a set
of twelve galaxy clusters with existing optical photometry of the
brightest cluster galaxy (BCG), intracluster stars (ICS), and cluster
galaxies. Assuming a standard IMF and simple chemical evolution model
scaled to match the present-day cluster early-type SN Ia rate, the
BCG+ICS component generates 31+11,-9, on average, of the observed ICM
Fe within r_500 (~0.6 times r_200, the virial radius).  Because the
BCG typically contributes only 20% of the BCG+ICS metals, they conclude
that the ICS is a significant, yet often neglected, contributor to the
ICM Fe within r_500.  With the combination of the intracluster and
galactic stars, they can account for all the observed metals.  However,
these models require signficant, and perhaps unrealistic, metal loss
from the galaxies (~85%).  The SN Ia rates, especially as a function
of galaxy environment and redshift, remain a significant source of
uncertainty.  Increasing the SN Ia rate for present-day cluster
early-type galaxies by 1.8x -- to just within the 2sigma error
of the measured value -- generates all the ICM metals with a smaller
galactic metal loss fraction (~35%) that is consistent with other,
independent observations.  (Extragalactic)

D. Kelson (OCIW), A. Zabludoff (Steward), and A. Gonzalez (U. of
Florida) have obtained deep, long-slit spectroscopy along the major
axis of the central, dominant elliptical in seven groups and clusters
in order to measure the kinematics of the intracluster stars out to
~100 kpc. In the first system they have studied, NGC 6166 in Abell
2199, the velocity dispersion initially decreases from the central
value of 300 km/s to 200 km/s within a few kpc then steadily rises to
to 660 km/s at a radius of 60 kpc, nearly reaching the velocity
dispersion of the cluster (775 km/s). These data suggest that the
stars in the halo of the cD trace the potential of the cluster and
that the kinematics of these intracluster stars can be used to
constrain the mass profile of the cluster. In addition, they find
evidence for systematic rotation (V/sigma ~ 0.3) in the intracluster
stars beyond 20 kpc. Such rotation is not seen in the kinematics of
the cluster members.  The observed major-axis kinematics can be
reproduced only if the halo, parameterized by a generalized-NFW
profile, has a soft core, i.e., alpha < 1 (a generalized-NFW halo with
alpha=1 is excluded due to low implied stellar mass-to-light
ratios). This result is inconsistent with the predictions of current
N-body simulations for dark matter halos.  Analysis of the other six
systems will yield the first empirical determination of the *scatter*
in alpha, providing a new and higher-order constraint on the models.
(Extragalactic)

Yujin Yang (Steward), Christy Tremonti (Steward), Ann Zabludoff
(Steward), and Dennis Zaritsky (Steward) are using HST WFPC2 and ACS
observations to study the detailed morphologies of post-merger,
post-starburst (aka "E+A" or "k+a") galaxies, which are currently
evolving from late to early-type galaxies.  They find that E+As have
profiles qualitatively like those of normal power-law early-type
galaxies, but have higher surface brightnesses. This result provides
the first direct evidence supporting the hypothesis that power-law
ellipticals form via gas-rich mergers.  These authors detect compact
sources, possibly young star clusters, associated with the
galaxies. These sources are much brighter (M_R ~ -13) than Galactic
globular clusters, have luminosities consistent with the brightest
clusters in nearby starburst galaxies, and have blue colors consistent
with the ages estimated from the E+A galaxy spectra (several 10^8 yr).
These authors are now using the stellar ages of these young clusters
to determine the duration of the E+A phase, which, when coupled with
the number density of E+As observed in the nearby Universe, suggests
that many early-type galaxies in the field could have evolved through
the E+A phase.  More interestingly, these authors identify at least
four E+A galaxies with blue cores, which are revealed by follow-up
spectroscopy to have LINER spectra.  If LINERs are low-luminosity AGN,
their presence in E+As suggests that any rapid growth phase of the
central AGN has ended in rough concert with the star formation and
therefore that the evolution of the black hole is tied to that of the
galactic bulge.  What is not clear from this work is whether the
coupling between AGN and bulge evolution is causal, as is suggested by
some theoretical models incorporating AGN feedback. The study of a
large sample of E+As (see below), including an investigation of any
correlation between AGN strength and the time elapsed since the
starburst, could provide a test of the AGN-feedback hypothesis, as
those models predict that the black hole accretion rate peaks shortly
after the starburst and declines quickly as the merger remnant ages.
(Extragalactic)

Yujin Yang (Steward), Christy Tremonti (Steward), and Ann Zabludoff
(Steward) have now compiled a unique sample of 241 post-starburst
galaxies from the SDSS.  Using GALEX UV photometry and SDSS optical
spectroscopy and photometry, they have developed a new technique to
date (to within uncertainties of only 50-100 Myr) the time elapsed
since the starbursts ended, thereby breaking the degeneracy between
the strength of the burst and its age for the first time.  Their
preliminary results are compelling. Although the post-starburst
selection criteria bias their sample against strong AGN, they search
for low-luminosity AGN by subtracting models of the spectral continuum
and measuring any weak emission lines.  This technique reveals LINERs
or Seyfert 2s in 70% of the galaxies.  This fraction is significantly
higher than for normal early-types (30-40%), the likely endpoints of
these galaxies' evolution.  Furthermore, these galaxies' Eddington
ratios, which characterize the black hole accretion rate relative to
the theoretical maximum, do not decline with post-starburst age, and a
handful of ratios exceed 0.25, the mean of luminous AGN.  Perhaps most
intriguing is that while the star formation continues at most 250 Myr
after the burst, the LINER fraction is still rising at 400-500 Myr.
Taken at face value, these results suggest that 1) the AGN may
continue to grow (albeit slowly) after the era of bulge-building has
ended and 2) it is possible to place quantitative constraints on
theories of a causal nucleus-bulge relationship. (Extragalactic)

R. van den Marel (STScI), T. Boeker (ESA), S. Laine (IPAC), J. Rossa
(STScI), C. Mihos (CWRU), J. Hibbard (NRAO), and A. Zabludoff
(Steward) have used HST to investigate the role of mergers in the
evolution of galaxies.  Galaxy mergers are believed responsible not
only for transforming some spiral galaxies into ellipticals, but also
for triggering starburst and AGN activity.  Ground-based observations
and numerical simulations have shed light on these issues, but have
not been able to adequately resolve the nuclei of merging galaxies.
It is these nuclei where most of the important physical processes
operate and where a direct study of the stellar and gaseous components
yields crucial insight on any picture of merger-driven galaxy
evolution.  Rossa et al. discuss the near-infrared (NIR) properties of
the nuclei in the 11 merging galaxies of the Toomre sequence, based on
high spatial resolution J, H, and K imaging data using NICMOS on board
the Hubble Space Telescope (HST). The observations are less affected
by dust extinction than their previous HST WFPC2 observations and
offer higher spatial resolution than existing ground-based NIR data.
They see a marginal trend for the nuclei to become bluer with
advancing merger stage, which they attribute to a dispersal of dust at
late times in the merging process.  Their data also indicate a
statistically significant trend for the nuclei in the sequence to
become more luminous, within an aperture of fixed physical size and
after correcting for dust extinction, with advancing merger stage. The
majority of the nuclei have steep profiles that can be characterized
as power-law type. In general, the Toomre-sequence galaxies tend to
have steeper profiles and higher central luminosity surface densities
than E/S0 galaxies. Their findings can be explained if the
Toomre-sequence galaxies have newly formed stars that are concentrated
toward their centers.  Overall, their results are consistent with the
generic predictions of N-body simulations of spiral galaxy mergers. If
left to evolve and fade for several gigayears, it is possible that the
properties of the Toomre-sequence nuclei would resemble the properties
of the nuclei of normal E/S0 galaxies. Their results therefore support
the view that mergers of spiral galaxies can lead to the formation of
early-type galaxies.  (Extragalactic)

J. Charlton (Penn State), Ann Zabludoff (Steward), and 18 others are
conducting an HST, Spitzer, GALEX, and ground-based survey of 12
nearby compact groups of galaxies.  In these dynamically evolving
environments, these authors expect to detect over 1000 young star
clusters, > 4000 old globular clusters in more than 40 giant galaxies,
about 15 AGN, numerous tidal features, and hot, intragroup gas.  These
data make possible a detailed study of stellar nurseries, dust, and
gas kinematics.  (Extragalactic)

J. van Gorkom (Columbia), B. Poggianti (Padua Obs.), R. Guhathakurta
(UCSC), E. Wilcots (U. Wisc.), M. Verheijen (NRAO), and A. Zabludoff
(Steward) are conducting a long-term survey with the Very Large Array
to obtain complete HI imaging data for 14 rich clusters and five poor
groups of galaxies covering a range of redshifts from ~0 to 0.2.
Combined with deep optical multi-color imaging and spectroscopy, this
program will provide the most complete information yet on the duty
cycle of star formation in nearby galaxies across a range of
environments.  This survey will also allow the first direct test of
how the efficiency of galaxy formation -- as indicated by the cold gas
content of the intergalactic medium -- may vary from one environment
to another.  The observations are spread over three C arrays and two B
arrays (~400 hours of time over three years).  (Extragalactic)

Ann Zabludoff (Steward) is a member of a large team (Destiny) selected
by NASA to develop a concept for the NASA/DOE Joint Dark Energy
Mission (JDEM).  Destiny would detect more than 3,000 Type Ia
supernovae over its two-year primary mission, followed by a year-long
survey of 1,000 square-degrees of the sky at near-infrared wavelengths
to measure how the large-scale distribution of matter in the universe
has evolved since the Big Bang. The data from these two surveys will
have 10 times the sensitivity of current ground-based projects to
explore the properties of dark energy.  The central concept is an
all-grism NIR survey camera. SN will be discovered by repeated imaging
of an area located at the North Ecliptic Pole (NEP). Grism spectra
with resolving power R ~ 100 will provide broad-band
spectrophotometry, redshifts, SN classification, as well as valuable
time-resolved diagnostic data for understanding the SN explosion
physics. (Extragalactic)