12,706 research outputs found
Luminosity Profiles of Merger Remnants
Using published luminosity and molecular gas profiles of the late-stage
mergers NGC 3921, NGC 7252 and Arp 220, we examine the expected luminosity
profiles of the evolved merger remnants, especially in light of the massive CO
complexes that are observed in their nuclei. For NGC 3921 and NGC 7252 we
predict that the resulting luminosity profiles will be characterized by an
r^{1/4} law. In view of previous optical work on these systems, it seems likely
that they will evolve into normal ellipticals as regards their optical
properties. Due to a much higher central molecular column density, Arp 220
might not evolve such a ``seamless'' light profile. We conclude that
ultraluminous infrared mergers such as Arp 220 either evolve into ellipticals
with anomalous luminosity profiles, or do not produce many low-mass stars out
of their molecular gas complexes.Comment: Final refereed version. Note new title. 4 pages, 2 encapsulated color
figures, uses emulateapj.sty. Accepted to ApJL. Also available at
http://www.cv.nrao.edu/~jhibbard/Remnants/remnants.htm
Radio and Far-Infrared Emission as Tracers of Star Formation and AGN in Nearby Cluster Galaxies
We have studied the radio and far-infrared (FIR) emission from 114 galaxies
in the 7 nearest clusters (<100 Mpc) with prominent X-ray emission to
investigate the impact of the cluster environment on the star formation and AGN
activity in the member galaxies. The X-ray selection criterion is adopted to
focus on the most massive and dynamically relaxed clusters. A large majority of
cluster galaxies show an excess in radio emission over that predicted from the
radio-FIR correlation, the fraction of sources with radio excess increases
toward cluster cores, and the radial gradient in the FIR/radio flux ratio is a
result of radio enhancement. Of the radio-excess sources, 70% are early-type
galaxies and the same fraction host an AGN. The galaxy density drops by a
factor of 10 from the composite cluster center out to 1.5 Mpc, yet galaxies
show no change in FIR properties over this region, and show no indication of
mass segregation. We have examined in detail the physical mechanisms that might
impact the FIR and radio emission of cluster galaxies. While collisional
heating of dust may be important for galaxies in cluster centers, it appears to
have a negligible effect on the observed FIR emission for our sample galaxies.
The correlations between radio and FIR luminosity and radius could be explained
by magnetic compression from thermal ICM pressure. We also find that simple
delayed harassment cannot fully account for the observed radio, FIR, and mid-IR
properties of cluster galaxies.Comment: 21 pages, 15 figures, Accepted by Ap
Radial Gas Flows in Colliding Galaxies: Connecting Simulations and Observations
(abridged) We investigate the detailed response of gas to the formation of
transient and long-lived dynamical structures induced in the early stages of a
disk-disk collision, and identify observational signatures of radial gas inflow
through a detailed examination of the collision simulation of an equal mass
bulge dominated galaxy. Stars respond to the tidal interaction by forming both
transient arms and long lived m=2 bars, but the gas response is more transient,
flowing directly toward the central regions within about 10^8 years after the
initial collision. The rate of inflow declines when more than half of the total
gas supply reaches the inner few kpc, where the gas forms a dense nuclear ring
inside the stellar bar. The average gas inflow rate to the central 1.8 kpc is
\~7 Msun/yr with a peak rate of 17 Msun/yr. The evolution of gas in a bulgeless
progenitor galaxy is also discussed, and a possible link to the ``chain
galaxy'' population observed at high redshifts is inferred. The evolution of
the structural parameters (the asymmetry and concentration) of both stars and
gas are studied in detail. Further, a new structural parameter (the compactness
parameter K) that traces the evolution of the size scale of the gas relative to
the stellar disk is introduced. Non-circular gas kinematics driven by the
perturbation of the non-axisymmetric structure can produce distinct emission
features in the "forbidden velocity quadrants'' of the position-velocity
diagram (PVD). The dynamical mass calculated using the rotation curve derived
from fitting the emission envelope of the PVD can determine the true mass to
within 20% to 40%. The evolution of the molecular fraction $M_H2/M_(H2 + HI)
and the compactness (K) are potential tracers to quantitatively assign the age
of the interaction.Comment: 52 pages, 20 figures (9 jpgs), accepted for publication in ApJ
Version with all figures at http://cfa-www.harvard.edu/~diono/ms.ps.g
Color Magnitude Relation and Morphology of Low-Redshift ULIRGs in SDSS
We present color-magnitude and morphological analysis of 54 low-redshift
ULIRGs, a subset of the IRAS 1Jy sample (Kim & Sanders, 1998), in the SDSS. The
ULIRGs are on average 1 magnitude brighter in M0.1r than the SDSS galaxies
within the same redshift range. The majority of the ULIRGs (~87%) have the
colors typical of the blue cloud, and only 4 sources (~7%) are located in the
red sequence. While ULIRGs are popularly thought to be precursors to a QSO
phase, we find few (~6%) in the "green valley" where the majority of the X-ray
and IR selected AGNs are found, and none of which harbors an AGN. For the 14
previously spectroscopic identified AGNs (~28%), we perform PSF subtractions
and find that on average the central point sources contribute less than one
third to the total luminosity, and that their high optical luminosities and
overall blue colors are apparently the result of star formation activity of the
host galaxies. Visual inspection of the SDSS images reveals a wide range of
disturbed morphologies. A detailed morphology analysis using Gini and M20
coefficients shows that slightly less than one half (~42% in g band) of the
ULIRGs are located in the region where most local mergers are found. The
heterogeneous distribution of ULIRGs in the G-M20 space is qualitatively
consistent with the results found by numerical simulations of disk-disk
mergers. Our study also shows that the measured morphological parameters are
systematically affected by the SNR and thus the merging galaxies can appear at
various regions in the G-M20 space. In general, our results reinforce the view
that ULIRGs contain young stellar populations and are mergers in progress. Our
study provides a uniform comparison sample for studying ULIRGs at higher
redshifts such as Spitzer mid-IR selected ULIRGs at z=1~2 and submm galaxies.Comment: 42 pages, 11 figures, ApJ accepte
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