8,190 research outputs found
How Do Disks Survive Mergers?
We develop a physical model for how galactic disks survive and/or are
destroyed in interactions. Based on dynamical arguments, we show gas primarily
loses angular momentum to internal torques in a merger. Gas within some
characteristic radius (a function of the orbital parameters, mass ratio, and
gas fraction of the merging galaxies), will quickly lose angular momentum to
the stars sharing the perturbed disk, fall to the center and be consumed in a
starburst. A similar analysis predicts where violent relaxation of the stellar
disks is efficient. Our model allows us to predict the stellar and gas content
that will survive to re-form a disk in the remnant, versus being violently
relaxed or contributing to a starburst. We test this in hydrodynamic
simulations and find good agreement as a function of mass ratio, orbital
parameters, and gas fraction, in simulations spanning a wide range in these
properties and others, including different prescriptions for gas physics and
feedback. In an immediate sense, the amount of disk that re-forms can be
understood in terms of well-understood gravitational physics, independent of
details of ISM gas physics or feedback. This allows us to explicitly quantify
the requirements for such feedback to (indirectly) enable disk survival, by
changing the pre-merger gas content and distribution. The efficiency of disk
destruction is a strong function of gas content: we show how and why
sufficiently gas-rich major mergers can, under general conditions, yield
systems with small bulges (B/T<0.2). We provide prescriptions for inclusion of
our results in semi-analytic models.Comment: 32 pages, 16 figures, accepted to ApJ (minor revisions to match
accepted version
Quasar Feedback: More Bang for Your Buck
We propose a two-stage model for the effects of feedback from a bright quasar
on the cold gas in a galaxy. It is difficult for feedback from near the
accretion disk to directly impact dense molecular clouds at ~kpc. But if such
feedback can drive a weak wind or outflow in the hot, diffuse ISM (a relatively
'easy' task), then in the wake of such an outflow passing over a cold cloud, a
combination of instabilities will drive the cloud material to effectively
expand in the direction perpendicular to the outflow. Such expansion
dramatically increases the effective cross section of the cloud material and
makes it more susceptible to ionization and momentum coupling from absorption
of the incident quasar radiation field. Even a moderate effect of this nature
can dramatically alter the ability of clouds at large radii to be fully ionized
and driven into a secondary outflow by radiation pressure. Since the amount of
momentum and volume which can be ionized by observed quasar radiation field is
more than sufficient to affect the entire cold gas supply once it has been
altered in this manner (and the 'initial' feedback need only initiate a
moderate wind in the low-density hot gas), this reduces by an order of
magnitude the required energy budget for feedback to affect a host galaxy.
Instead of ~5% of the radiated energy (~100% momentum) needed if the initial
feedback must directly heat or blow out the galactic gas, if only ~0.5% of the
luminosity (~10% momentum) can couple to drive the initial hot outflow, this
mechanism could be efficient. This amounts to hot gas outflow rates from near
the accretion disk of only 5-10% of the BH accretion rate.Comment: 9 pages, 2 figures, accepted to MNRAS (revised to match published
version, methodology expanded
A Simple Model for Quasar Demographics
We present a simple model for the relationship between quasars, galaxies, and
dark matter halos from 0.5<z<6. In the model, black hole (BH) mass is linearly
related to galaxy mass, and galaxies are connected to dark matter halos via
empirically constrained relations. A simple "scattered" light bulb model for
quasars is adopted, wherein BHs shine at a fixed fraction of the Eddington
luminosity during accretion episodes, and Eddington ratios are drawn from a
lognormal distribution that is redshift-independent. This model has two free,
physically meaningful parameters at each redshift: the normalization of the
Mbh-Mgal relation and the quasar duty cycle; these parameters are fit to the
observed quasar luminosity function (LF) over the interval 0.5<z<6. This simple
model provides an excellent fit to the LF at all epochs, and also successfully
predicts the observed projected two-point correlation of quasars from
0.5<z<2.5. It is significant that a single quasar duty cycle at each redshift
is capable of reproducing the extant observations. The data are therefore
consistent with a scenario wherein quasars are equally likely to exist in
galaxies, and therefore dark matter halos, over a wide range in masses. The
knee in the quasar LF is a reflection of the knee in the stellar mass-halo mass
relation. Future constraints on the quasar LF and quasar clustering at high
redshift will provide strong constraints on the model. In the model, the
autocorrelation function of quasars becomes a strong function of luminosity
only at the very highest luminosities, and will be difficult to observe because
such quasars are so rare. Cross-correlation techniques may provide useful
constraints on the bias of such rare objects.Comment: 12 pages, 12 figures, ApJ accepte
Star Formation in Galaxy Mergers with Realistic Models of Stellar Feedback & the Interstellar Medium
We use simulations with realistic models for stellar feedback to study galaxy
mergers. These high resolution (1 pc) simulations follow formation and
destruction of individual GMCs and star clusters. The final starburst is
dominated by in situ star formation, fueled by gas which flows inwards due to
global torques. The resulting high gas density results in rapid star formation.
The gas is self gravitating, and forms massive (~10^10 M_sun) GMCs and
subsequent super-starclusters (masses up to 10^8 M_sun). However, in contrast
to some recent simulations, the bulk of new stars which eventually form the
central bulge are not born in superclusters which then sink to the center of
the galaxy, because feedback efficiently disperses GMCs after they turn several
percent of their mass into stars. Most of the mass that reaches the nucleus
does so in the form of gas. The Kennicutt-Schmidt law emerges naturally as a
consequence of feedback balancing gravitational collapse, independent of the
small-scale star formation microphysics. The same mechanisms that drive this
relation in isolated galaxies, in particular radiation pressure from IR
photons, extend over seven decades in SFR to regulate star formation in the
most extreme starbursts (densities >10^4 M_sun/pc^2). Feedback also drives
super-winds with large mass loss rates; but a significant fraction of the wind
material falls back onto the disks at later times, leading to higher
post-starburst SFRs in the presence of stellar feedback. Strong AGN feedback is
required to explain sharp cutoffs in star formation rate. We compare the
predicted relic structure, mass profile, morphology, and efficiency of disk
survival to simulations which do not explicitly resolve GMCs or feedback.
Global galaxy properties are similar, but sub-galactic properties and star
formation rates can differ significantly.Comment: 17 pages, 13 figures (+appendices), MNRAS accepted (matches
published). Movies of the simulations are available at
http://www.tapir.caltech.edu/~phopkins/Site/Movies_sbw_mgr.htm
Radiation Pressure Driven Galactic Winds from Self-Gravitating Discs
(Abridged) We study large-scale winds driven from uniformly bright
self-gravitating discs radiating near the Eddington limit. We show that the
ratio of the radiation pressure force to the gravitational force increases with
height above the disc surface to a maximum of twice the value of the ratio at
the disc surface. Thus, uniformly bright self-gravitating discs radiating at
the Eddington limit are fundamentally unstable to driving large-scale winds.
These results contrast with the spherically symmetric case, where
super-Eddington luminosities are required for wind formation. We apply this
theory to galactic winds from rapidly star-forming galaxies that approach the
Eddington limit for dust. For hydrodynamically coupled gas and dust, we find
that the asymptotic velocity of the wind is v_\infty ~ 1.5 v_rot and that
v_\infty SFR^{0.36}, where v_rot is the disc rotation velocity and SFR is the
star formation rate, both of which are in agreement with observations. However,
these results of the model neglect the gravitational potential of the
surrounding dark matter halo and an old passive stellar bulge or extended disc,
which act to decrease v_\infty. A more realistic treatment shows that the flow
can either be unbound, or bound, forming a "fountain flow" with a typical
turning timescale of t_turn ~ 0.1-1 Gyr. We provide quantitative criteria and
scaling relations for assessing whether or not a rapidly star-forming galaxy of
given properties can drive unbound flows via the mechanism described in this
paper. Importantly, we note that because t_turn is longer than the star
formation timescale in the rapidly star-forming galaxies and ULIRGs for which
our theory is most applicable, if rapidly star-forming galaxies are selected as
such, they may be observed to have strong outflows, even though their winds are
eventually bound on large scales.Comment: 10 pages, 6 figures, Accepted for publication in MNRA
Evidence for 1000 km/s Molecular Outflows in the Local ULIRG Population
The feedback from galactic outflows is thought to play an important role in
shaping the gas content, star formation history, and ultimately the stellar
mass function of galaxies. Here we present evidence for massive molecular
outflows associated with ultra-luminous infrared galaxies (ULIRGs) in the
coadded Redshift Search Receiver 12CO(1-0) spectrum. Our stacked spectrum of 27
ULIRGs at z = 0.043-0.11 (freq_rest = 110-120 GHz) shows broad wings around the
CO line with delta_V(FWZI)~2000 km/s. Its integrated line flux accounts for up
to 25+/-5% of the total CO line luminosity. When interpreted as a massive
molecular outflow wind, the associated mechanical energy can be explained by a
concentrated starburst with SFR \geq 100 M_sun/yr, which agrees well with their
SFR derived from the FIR luminosity. Using the high signal-to-noise stacked
composite spectrum, we also probe 13CO and 12CN emission in the sample and
discuss how the chemical abundance of molecular gas may vary depending on the
physical conditions of the nuclear region.Comment: 5 pages, 2 figures. Accepted for publication in ApJ
Stellar Population Gradients in ULIRGs: Implications for Gas Inflow Timescales
Using longslit, optical spectra of Ultraluminous Infrared Galaxies (ULIRGs),
we measure the evolution in the star-formation intensity during galactic
mergers. In individual galaxies, we resolve kpc scales allowing comparison of
the nucleus, inner disk, and outer disk. We find that the strength of the Hbeta
absorption line increases with the projected distance from the center of the
merger, typically reaching about 9 Angstrom around 10 kpc. At these radii, the
star formation intensity must have rapidly decreased about 300-400 Myr ago;
only stellar populations deficient in stars more massive than Type A produce
such strong Balmer absorption. In contrast, we find the star formation history
in the central kpc consistent with continuous star formation. Our measurements
indicate that gas depletion occurs from the outer disk inwards during major
mergers. This result is consistent with merger-induced gas inflow and
empirically constrains the gas inflow timescale. Numerical simulations
accurately calculate the total amount of infalling gas but often assume the
timescale for infall. These new measurements are therefore central to modeling
merger-induced star formation and AGN activity.Comment: Accepted by ApJ; 11 pages, 8 figures, 18 online-only figures that can
be found at http://physics.ucsb.edu/~ktsoto/online_figs/2009arXiv0909.2050S
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