292 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
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
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
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
Interaction-induced star formation in a complete sample of 10^5 nearby star-forming galaxies
We investigate the clustering properties of a complete sample of 10^5
star-forming galaxies drawn from the SDSS DR4. On scales less than 100 kpc, the
amplitude of the correlation function exhibits a strong dependence on the
specific star formation rate of the galaxy. We interpret this as the signature
of enhanced star formation induced by tidal interactions. We then explore how
the average star formation rate in a galaxy is enhanced as the projected
separation r_p between the galaxy and its companions decreases. We find that
the enhancement depends strongly on r_p, but very weakly on the relative
luminosity of the companions. The enhancement is also stronger in low mass
galaxies than in high mass galaxies. In order to explore whether a tidal
interaction is not only sufficient, but also necessary to trigger enhanced star
formation in a galaxy, we compute background subtracted neighbour counts for
the galaxies in our sample. The average number of close neighbours around
galaxies with low to average values of SFR/M* is close to zero. At the highest
specific star formation rates, however, more than 40% of the galaxies in our
sample have a companion within a projected radius of 100 kpc. Visual inspection
of the highest SFR/M* galaxies without companions reveals that more than 50% of
these are clear interacting or merging systems. We conclude that tidal
interactions are the dominant trigger of enhanced star formation in the most
strongly star-forming systems. Finally, we find clear evidence that tidal
interactions not only lead to enhanced star formation in galaxies, but also
cause structural changes such as an increase in concentration.Comment: v1: 13 pages, 12 figures, submitted for publication in Monthly
Notices; v2: 15 pages, 14 figures, accepted for publication, a new analysis
(sec. 6 and figs 13 and 14) is done in order to address the effect of rich
environment
Social technologies for online learning: theoretical and contextual issues
Three exemplars are presented of social technologies deployed in educational contexts: wikis; a photo-sharing environment; and a social bookmarking tool. Students were found to engage with the technologies selectively, sometimes rejecting them, in the light of their prior conceptions of education. Some students (a minority in all the studies) were unsympathetic to the educational philosophy underpinning the technology’s adoption. The paper demonstrates, through an examination of in-context use, the importance of socio-cultural factors in relation to education, and the non-deterministic nature of educational technology. The academic study of technology has increasingly called into question the deterministic views which are so pervasive in popular discourse and among policy makers. Instead, socio-cultural factors play a crucial role in shaping and defining technology and educational technology is no exception, as the examples in the paper show. The paper concludes by drawing out some implications of the examples for the use of social technologies in education
The COBE Diffuse Infrared Background Experiment Search for the Cosmic Infrared Background: IV. Cosmological Implications
In this paper we examine the cosmological constraints of the recent DIRBE and
FIRAS detection of the extragalactic background light between 125-5000 microns
on the metal and star formation histories of the universe.Comment: 38 pages and 9 figures. Accepted for publications in The
Astrophysical Journa
Tidal tails in CDM cosmologies
We study the formation of tidal tails in pairs of merging disk galaxies with
structural properties motivated by current theories of cold dark matter (CDM)
cosmologies. In a recent study, Dubinski, Mihos & Hernquist (1996) showed that
the formation of prominent tidal tails can be strongly suppressed by massive
and extended dark haloes. For the large halo-to-disk mass ratio expected in CDM
cosmologies their sequence of models failed to produce strong tails like those
observed in many well-known pairs of interacting galaxies. In order to test
whether this effect can constrain the viability of CDM cosmologies, we
construct N-body models of disk galaxies with structural properties derived in
analogy to the analytical work of Mo, Mao & White (1998). With a series of
self-consistent collisionless simulations of galaxy-galaxy mergers we
demonstrate that even the disks of very massive dark haloes have no problems
developing long tidal tails, provided the halo spin parameter is large enough.
We show that the halo-to-disk mass ratio is a poor indicator for the ability to
produce tails. Instead, the relative size of disk and halo, or alternatively,
the ratio of circular velocity to local escape speed at the half mass radius of
the disk are more useful criteria. This result holds in all CDM cosmologies.
The length of tidal tails is thus unlikely to provide useful constraints on
such models.Comment: 17 pages, mn.sty, 13 included eps-figures, submitted to MNRA
A Multiple Merger Model for the Origin of Ultraluminous Infrared Galaxies
It has been often considered that the dissipative collapse caused by a merger
of two gas-rich galaxies is responsible for the intense nuclear starbursts or
the nonthermal quasar activity in ultraluminous infrared galaxies due to the
efficient fueling induced by it. It is also widely known that Ultraluminous
Infrared Galaxies (ULIGs) are often found in merging systems. Some ULIGs, such
as Arp 220, show two compact starburst regions which are considered to be
associated with two galactic nuclei in the process of merging. However, since a
merger between two galaxies may make only one compact starburst region, we
suggest the possibility that double-nucleus ULIGs are composed of two merging
nuclei, each of which contains a couple of galactic nuclei.Comment: 13 pages with two figures. To appear in THE ASTROPHYSICAL JOURNAL
(Letters
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