420 research outputs found
Kinematic Effects of Tidal Interaction on Galaxy Rotation Curves
We use self-consistent N-body models, in conjunction with models of test
particles moving in galaxy potentials, to explore the initial effects of
interactions on the rotation curves of spiral galaxies. Using nearly
self-consistent disk/bulge/halo galaxy models (Kuijken & Dubinski 1995), we
simulate the first pass of galaxies on nearly parabolic orbits; we vary orbit
inclinations, galaxy halo masses and impact parameters. For each simulation, we
mimic observed rotation curves of the model galaxies. Transient
interaction-induced features of the curves include distinctly rising or falling
profiles at large radii and pronounced bumps in the central regions. Remarkably
similar features occur in our statistical sample of optical emission-line
rotation curves of spiral galaxies in tight pairs and n-tuples.Comment: 9 pages, 2 figures, accepted for publication in ApJ Letter
SAURON's Challenge for the Major Merger Scenario of Elliptical Galaxy Formation
The intrinsic anisotropy delta and flattening epsilon of simulated merger
remnants is compared with elliptical galaxies that have been observed by the
SAURON collaboration, and that were analysed using axisymmetric Schwarzschild
models. Collisionless binary mergers of stellar disks and disk mergers with an
additional isothermal gas component, neglecting star formation cannot reproduce
the observed trend delta = 0.55 epsilon (SAURON relationship). An excellent fit
of the SAURON relationship for flattened ellipticals with epsilon >= 0.25 is
however found for merger simulations of disks with gas fractions >= 20%,
including star formation and stellar energy feedback. Massive black hole
feedback does not strongly affect this result. Subsequent dry merging of merger
remnants however does not generate the slowly-rotating SAURON ellipticals which
are characterized by low ellipticities epsilon < 0.25 and low anisotropies.
This indicates that at least some ellipticals on the red galaxy sequence did
not form by binary mergers of disks or early-type galaxies. We show that
stellar spheroids resulting from multiple, hierarchical mergers of
star-bursting subunits in a cosmological context are in excellent agreement
with the low ellipticities and anisotropies of the slowly rotating SAURON
ellipticals and their observed trend of delta with epsilon. The numerical
simulations indicate that the SAURON relation might be a result of strong
violent relaxation and phase mixing of multiple, kinematically cold stellar
subunits with the angular momentum of the system determining its location on
the relation.Comment: 13 pages, 3 figures, submitted to Ap
NGC 4138 - A Case Study in Counterrotating Disk Formation
The Sa(r) galaxy NGC 4138 has been recently found to contain an extensive
counterrotating disk which appears to be still forming. Up to a third of the
stars in the disk system may be on retrograde orbits. A counterrotating ring of
H II regions, along with extended counterrotating H I gas, suggests that the
retrograde material has been recently acquired in the gas phase and is still
trickling in. Using numerical simulations, we have attempted to model the
process by which the counterrotating mass has been accreted by this galaxy. We
investigate two possibilities: continuous retrograde infall of gas, and a
retrograde merger with a gas-rich dwarf galaxy. Both processes are successful
in producing a counterrotating disk of the observed mass and dimensions without
heating up the primary significantly. Contrary to our experience with a
fiducial cold, thin primary disk, the gas-rich merger works well for the
massive, compact primary disk of NGC 4138 even though the mass of the dwarf
galaxy is a significant fraction of the mass of the primary disk. Although we
have restricted ourselves mainly to coplanar infall and mergers, we report on
one inclined infall simulation as well. We also explore the possibility that
the H-alpha ring seen in the inner half of the disk is a consequence of
counterrotating gas clouds colliding with corotating gas already present in the
disk and forming stars in the process.Comment: To appear in ApJ, 21 pages, LaTeX (aaspp4) format, 17 figs (gzipped
tar file) also available at ftp://bessel.mps.ohio-state.edu/pub/thakar/cr2/
or at http://www-astronomy.mps.ohio-state.edu/~thakar
Color Gradients and Surface Brightness Profiles of Galaxies in the Hubble Deep Field-North
We fit elliptical isophotes to the Hubble Deep Field-North WFPC-2 and NICMOS
data to study the rest-frame UV_{218}-U_{300} color profiles and rest-frame B
surface brightness profiles of 33 intermediate redshift galaxies (0.5 <= z <=
1.2) with I_{814} < 25 and 50 high redshift galaxies (2.0 <= z <= 3.5) with
H_{160}< 27. From the weighted least-squares fit to the color profiles we find
that, at intermediate redshifts, the galaxies possess negative color gradients
indicating a reddening towards the center of the profile similar to local
samples whereas, at high redshifts, the galaxies possess positive color
gradients. This indicates that star formation is more centrally concentrated in
the distant galaxy sample which differs from the prevalent mode of extended
disk star formation that we observe in the local universe. Additionally, we
find that it is critical to correct for PSF effects when evaluating the surface
brightness profiles since at small scale lengths and faint magnitudes, an
r^{1/4} profile can be smoothed out substantially to become consistent with an
exponential profile. After correcting for PSF effects, we find that at higher
look-back time, the fraction of galaxies possessing exponential profiles have
slightly decreased while the fraction of galaxies possessing r^{1/4} profiles
have slightly increased. Our results also suggest a statistically insignificant
increase in the fraction of peculiar/irregular type galaxies. We compare our
results with recent semi-analytical models which treat galaxy formation and
evolution following the cold dark matter hierarchical framework.Comment: 31 pages, 10 JPEG figures. To be published in AJ Vol. 124, October
200
Kinematic Structure of Merger Remnants
We use numerical simulations to study the kinematic structure of remnants
formed from mergers of equal-mass disk galaxies. In particular, we show that
remnants of dissipational mergers, which include the radiative cooling of gas,
star formation, feedback from supernovae, and the growth of supermassive black
holes, are smaller, rounder, have, on average, a larger central velocity
dispersion, and show significant rotation compared to remnants of
dissipationless mergers. The increased rotation speed of dissipational remnants
owes its origin to star formation that occurs in the central regions during the
galaxy merger. We have further quantified the anisotropy, three-dimensional
shape, minor axis rotation, and isophotal shape of each merger remnant, finding
that dissipational remnants are more isotropic, closer to oblate, have the
majority of their rotation along their major axis, and are more disky than
dissipationless remnants. Individual remnants display a wide variety of
kinematic properties. A large fraction of the dissipational remnants are oblate
isotropic rotators. Many dissipational, and all of the dissipationless, are
slowly rotating and anisotropic. The remnants of gas-rich major mergers can
well-reproduce the observed distribution of projected ellipticities, rotation
parameter (V/\sigma)*, kinematic misalignments, Psi, and isophotal shapes. The
dissipationless remnants are a poor match to this data. Our results support the
merger hypothesis for the origin of low-luminosity elliptical galaxies provided
that the progenitor disks are sufficiently gas-rich, however our remnants are a
poor match to the bright ellipticals that are slowly rotating and uniformly
boxy.Comment: 22 pages, 17 figures, accepted to Ap
Clustering Analyses of 300,000 Photometrically Classified Quasars--I. Luminosity and Redshift Evolution in Quasar Bias
Using ~300,000 photometrically classified quasars, by far the largest quasar
sample ever used for such analyses, we study the redshift and luminosity
evolution of quasar clustering on scales of ~50 kpc/h to ~20 Mpc/h from
redshifts of z~0.75 to z~2.28. We parameterize our clustering amplitudes using
realistic dark matter models, and find that a LCDM power spectrum provides a
superb fit to our data with a redshift-averaged quasar bias of b_Q =
2.41+/-0.08 () for . This represents a better
fit than the best-fit power-law model (; ). We find b_Q increases with redshift.
This evolution is significant at >99.6% using our data set alone, increasing to
>99.9999% if stellar contamination is not explicitly parameterized. We measure
the quasar classification efficiency across our full sample as a = 95.6 +/-
^{4.4}_{1.9}%, a star-quasar separation comparable with the star-galaxy
separation in many photometric studies of galaxy clustering. We derive the mean
mass of the dark matter halos hosting quasars as MDMH=(5.2+/-0.6)x10^{12}
M_solar/h. At z~1.9 we find a deviation from luminosity-independent
quasar clustering; this suggests that increasing our sample size by a factor of
1.8 could begin to constrain any luminosity dependence in quasar bias at z~2.
Our results agree with recent studies of quasar environments at z < 0.4, which
detected little luminosity dependence to quasar clustering on proper scales >50
kpc/h. At z < 1.6, our analysis suggests that b_Q is constant with luminosity
to within ~0.6, and that, for g < 21, angular quasar autocorrelation
measurements are unlikely to have sufficient statistical power at z < 1.6 to
detect any luminosity dependence in quasars' clustering.Comment: 13 pages, 9 figures, 2 tables; uses amulateapj; accepted to Ap
Statistical properties of ultraluminous IRAS galaxies from an HST imaging survey
We perform photometric measurements on a large HST snapshot imaging survey
sample of 97 ultraluminous infrared galaxies (ULIRGs). We select putative
nuclei from bright clumps in all the sample targets, mainly based on a
quantitative criterion of I-band luminosity as well as the global and local
morphological information. All the sources are then classified into three
categories with multiple, double and single nucleus/nuclei. The resultant
fractions of multiple, double and single nucleus/nuclei ULIRGs are 18%, 39% and
43%, respectively. This supports the multiple merger scenario as a possible
origin of ULIRGs, in addition to the commonly-accepted pair merger model.
Further statistical studies indicate that the AGN fraction increases from
multiple (36%) to double (65%) and then to single (80%) nucleus/nuclei ULIRGs.
For the single nucleus category, there is a high luminosity tail in the
luminosity distribution, which corresponds to a Seyfert 1/QSO excess. This
indicates that active galactic nuclei tend to appear at final merging stage.
For multiple/double nuclei galaxies, we also find a high fraction of very close
nucleus pairs (e.g., 3/4 for those separated by less than 5 kpc). This
strengthens the conclusion that systems at late merging phase preferentially
host ULIRGs.Comment: 38 pages, 21 figures, to be published in A
A gas-rich nuclear bar fuelling a powerful central starburst in NGC 2782
We present evidence that the peculiar interacting starburst galaxy NGC 2782
(Arp 215) harbors a gas-rich nuclear stellar bar feeding an M82-class powerful
central starburst, from a study based on OVRO CO (J=1->0) data, WIYN BVR &
Halpha observations, along with available NIR images, a 5 GHz RC map and HST
images. NGC 2782 harbors a clumpy, bar-like CO feature of radius ~ 7.5'' (1.3
kpc) which leads a nuclear stellar bar of similar size. The nuclear CO bar is
massive: it contains ~2.5x10**9 M_sun of molecular gas, which makes up ~ 8 % of
the dynamical'mass present within a 1.3 kpc radius. Within the CO bar, emission
peaks in two extended clumpy lobes which lie on opposite sides of the nucleus,
separated by ~ 6'' (1 kpc). Between the CO lobes, in the inner 200 pc radius,
resides a powerful central starburst which is forming stars at a rate of 3 to 6
M_sun yr-1. While circular motions dominate the CO velocity field, the CO lobes
show weak bar-like streaming motions on the leading side of the nuclear stellar
bar, suggestive of gas inflow. We estimate semi-analytically the gravitational
torque from the nuclear stellar bar on the gas, and suggest large gas inflow
rates from the CO lobes into the central starburst. These observations, which
are amongst the first ones showing a nuclear stellar bar fuelling molecular gas
into an intense central starburst, are consistent with simulations and theory
which suggest that nuclear bars provide an efficient way of transporting gas
closer to the galactic center to fuel central activity. Furthermore, several
massive clumps are present at low radii, and dynamical friction might produce
further gas inflow. We suggest that the nuclear molecular gas bas and central
activity will be very short-lived, likely disappearing within 5x10**8 years.Comment: Accepted by the Astrophysical Journal, 10 pages, Latex with
emulateapj.sty, apjfonts.sty, 10 postscript & 2 gif figure
The Importance of Satellite Quenching for the Build-Up of the Red Sequence of Present Day Galaxies
In the current paradigm, red sequence galaxies are believed to have formed as
blue disk galaxies that subsequently had their star formation quenched. Since
red-sequence galaxies typically have an early-type morphology, the transition
from the blue to the red sequence also involves a morphological transformation.
In this paper we study the impact of transformation mechanisms that operate
only on satellite galaxies, such as strangulation, ram-pressure stripping and
galaxy harassment. Using a large galaxy group catalogue constructed from the
SDSS, we compare the colors and concentrations of satellites galaxies to those
of central galaxies of the same stellar mass, adopting the hypothesis that the
latter are the progenitors of the former. On average, satellites are redder and
more concentrated than central galaxies of the same stellar mass.
Central-satellite pairs that are matched in both stellar mass and color,
however, show no average concentration difference, indicating that the
transformation mechanisms affect color more than morphology. The color and
concentration differences of matched central-satellite pairs are completely
independent of the halo mass of the satellite galaxy, indicating that
satellite-specific transformation mechanisms are equally efficient in haloes of
all masses. This strongly favors strangulation as the main quenching mechanism
for satellite galaxies. Finally, we determine the relative importance of
satellite quenching for the build-up of the red sequence. We find that roughly
70 percent of red sequence satellite galaxies with a stellar mass of 10^9 Msun
had their star formation quenched as satellites. This drops rapidly to zero
with increasing stellar mass, indicating that a significant fraction of red
satellites were already quenched before they became a satellite.Comment: 14 pages, 10 figures. Submitted for publication in MNRA
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