48 research outputs found
Formation of a spiral galaxy in a major merger
We use numerical simulations to examine the structure of merger remnants
resulting from collisions of gas-rich spiral galaxies. When the gas fraction of
the progenitors is small, the remnants structurally and kinematically resemble
elliptical galaxies, in agreement with earlier work. However, if the progenitor
disks are gas-dominated, new types of outcomes are possible. In fact, we show
that a prominent disk may survive in certain cases. To illustrate this
scenario, we analyze an extreme example with progenitor galaxies consisting of
dark matter halos, pure gas disks, and no bulges, as might be appropriate for
mergers at high redshifts. While rapid star formation triggered by tidal
torques during the merger forms a central, rotating bulge in the remnant, not
all the gas is consumed in the burst. The remaining gas cools very quickly and
settles into an extended star-forming disk, yielding an object similar to a
spiral galaxy, and not an early type galaxy. This is contrary to the usual view
that major mergers invariably destroy disks. The morphological evolution of
galaxies can therefore be more complicated than often assumed, and in
particular, theoretical constraints based on the fragility of spiral disks need
to be reevaluated.Comment: submitted to ApJL, 4 figure
A Parallel Tree-SPH code for Galaxy Formation
We describe a new implementation of a parallel Tree-SPH code with the aim to
simulate Galaxy Formation and Evolution. The code has been parallelized using
SHMEM, a Cray proprietary library to handle communications between the 256
processors of the Silicon Graphics T3E massively parallel supercomputer hosted
by the Cineca Supercomputing Center (Bologna, Italy). The code combines the
Smoothed Particle Hydrodynamics (SPH) method to solve hydro-dynamical equations
with the popular Barnes and Hut (1986) tree-code to perform gravity calculation
with a NlogN scaling, and it is based on the scalar Tree-SPH code developed by
Carraro et al(1998)[MNRAS 297, 1021]. Parallelization is achieved distributing
particles along processors according to a work-load criterion. Benchmarks, in
terms of load-balance and scalability, of the code are analyzed and critically
discussed against the adiabatic collapse of an isothermal gas sphere test using
20,000 particles on 8 processors. The code results balanced at more that 95%
level. Increasing the number of processors, the load-balance slightly worsens.
The deviation from perfect scalability at increasing number of processors is
almost negligible up to 32 processors. Finally we present a simulation of the
formation of an X-ray galaxy cluster in a flat cold dark matter cosmology,
using 200,000 particles and 32 processors, and compare our results with Evrard
(1988) P3M-SPH simulations. Additionaly we have incorporated radiative cooling,
star formation, feed-back from SNae of type II and Ia, stellar winds and UV
flux from massive stars, and an algorithm to follow the chemical enrichment of
the inter-stellar medium. Simulations with some of these ingredients are also
presented.Comment: 19 pages, 14 figures, accepted for publication in MNRA
Observing the Sunyaev-Zel'dovich Effect Closer to Home
Hot gas trapped in a dark matter halo will produce a decrement in the surface
brightness of the microwave background, the Sunyaev-Zel'dovich (SZ) effect.
While massive clusters produce the strongest central SZ decrements, we point
out that a local galaxy halo, specifically the halo of M31, may be one of the
brightest integrated SZ sources in the sky. For various realistic gas
distributions consistent with current X-ray limits, we show that the integrated
SZ decrement from M31 will be comparable to decrements already detected in more
distant sources, provided its halo contains an appreciable quantity of hot gas.
A measurement of this decrement would provide direct information on the mass,
spatial distribution and thermodynamic state of hot gas in a low-mass halo, and
could place important constraints on current models of galaxy formation.
Detecting such an extended (~ 10 degree), low-amplitude signal will be
challenging, but should be possible with all-sky SZ maps from satellite
missions such as the Wilkinson Microwave Anisotropy Probe or the Planck
Surveyor.Comment: 5 pages, 3 figures; submitted to MNRA
Automatic quantitative morphological analysis of interacting galaxies
The large number of galaxies imaged by digital sky surveys reinforces the
need for computational methods for analyzing galaxy morphology. While the
morphology of most galaxies can be associated with a stage on the Hubble
sequence, morphology of galaxy mergers is far more complex due to the
combination of two or more galaxies with different morphologies and the
interaction between them. Here we propose a computational method based on
unsupervised machine learning that can quantitatively analyze morphologies of
galaxy mergers and associate galaxies by their morphology. The method works by
first generating multiple synthetic galaxy models for each galaxy merger, and
then extracting a large set of numerical image content descriptors for each
galaxy model. These numbers are weighted using Fisher discriminant scores, and
then the similarities between the galaxy mergers are deduced using a variation
of Weighted Nearest Neighbor analysis such that the Fisher scores are used as
weights. The similarities between the galaxy mergers are visualized using
phylogenies to provide a graph that reflects the morphological similarities
between the different galaxy mergers, and thus quantitatively profile the
morphology of galaxy mergers.Comment: Astronomy & Computing, accepte
Coevolution of the galactic cores and spiral galaxies
Using high-resolution N-body/SPH simulations with particles,
we investigate the evolution of stellar and gaseous galactic cores during the
hierarchical formation of a spiral galaxy. We find that the galactic core ( pc) coevolves with the host galaxy. The average mass ratio between the
baryonic core and the halo is nearly constant, 0.04 from to
. However, there are several `rapid-growing phases' during the
evolution, in which the rate of mass accretion to the central sub-kpc region is
ten times higher ( yr pc) than the average accretion
rate. The rapid growth of the inner core is associated with the major merger
events with a time-delay. We also find that the spin-axis of the gas core
frequently changes. As a result, the angular momentum vector of the central
part of the galaxy is independent of the rotation of the outer part. Our
results suggest that if a constant fraction of the baryonic mass in the central
several 100 pc of a galaxy is converted into a massive black hole, the black
hole mass should correlate with the total mass of the galaxies.Comment: 4 pages, 4 figures, Accepted for publication in ApJ
The Recent Star Formation History of NGC 5102
We present Hubble Space Telescope photometry of young stars in NGC 5102, a
nearby gas-rich post-starburst S0 galaxy with a bright young stellar nucleus.
We use the IAC-pop/MinnIAC algorithm to derive the recent star formation
history in three fields in the bulge and disk of NGC 5102. In the disk fields,
the recent star formation rate has declined monotonically and is now barely
detectable, but a starburst is still in progress in the bulge and has added
about 2 percent to the mass of the bulge over the last 200 Myr. Other studies
of star formation in NGC 5102 indicate that about 20 percent of its stellar
mass was added over the past Gyr. If this is correct, then much of the stellar
mass of the bulge may have formed over this period. It seems likely that this
star formation was fueled by the accretion of a gas-rich system with HI mass of
about 2 x 10^9 Msol which has now been almost completely converted into stars.
The large mass of recently formed stars and the blue colours of the bulge
suggest that the current starburst, which is now fading, may have made a
significant contribution to build the bulge of NGC 5102.Comment: 36 pages, 16 figures, accepted in A
Formation of High-redshift (z>6) Quasars Driven by Nuclear Starbursts
Based on the physical model of a supermassive black hole (SMBH) growth via
gas accretion in a circumnuclear disk (CND) proposed by Kawakatu & Wada (2008),
we describe the formation of high- () quasars (QSOs) whose BH masses
are M_{BH}> 10^{9} M_{\odot}. We derive the necessary conditions to form QSOs
at z > 6 by only gas accretion: (i) A large mass supply with M_{sup} >
10^{10}M_{\odot} from host galaxies to CNDs, because the final BH mass is only
1-10% of the total supplied mass from QSO hosts. (ii) High star formation
efficiency for a rapid BH growth. We also find that if the BH growth is limited
by the Eddington accretion, the final BH mass is greatly suppressed. Thus, the
super-Eddington growth is required for the QSO formation. The evolution of the
QSO luminosity depends on the redshift z_{i} at which accretion onto a seed BH
is initiated. In other words, the brighter QSOs at z >6 favor the late growth
of SMBHs (i.e., z_{i}=10) rather than early growth (i.e., z_{i}=30). Moreover,
we predict the observable properties and the evolution of QSOs at z >6. In a
QSO phase, there should exist a stellar rich massive CND, whose gas mass is
about 10% of the dynamical mass inside 0.1-1 kpc}. On the other hand, in a
phase where the BH grows (i.e., a proto-QSO phase), the proto-QSO has a gas
rich massive CNDs whose gas mass is comparable to the dynamical mass
(abridged).Comment: 12 pages, 10 pages, accepted by Ap
Creating S0s with major mergers : a 3D view
Miguel Querejeta, Glenn van de Ven and Jesús Falcón-Barroso acknowledge financial support to the Detailed Anatomy of Galaxies (DAGAL) network from the People Programme (Marie Curie Actions) of the European Union’s Seventh Framework Programme FP7/2007- 2013/ under REA grant agreement number PITN-GA-2011-289313. M. Carmen Eliche-Moral acknowledges support from the Spanish Ministry of Economy and Competitiveness (MINECO) under projects AYA2012-31277 and AYA2013-48226-C3-1-P. Jairo Méndez-Abreu acknowledges support from the European Research Council Starting Grant (SEDmorph; P.I.V. Wild).A number of simulators have argued that major mergers can sometimes preserve discs, but the possibility that they could explain the emergence of lenticular galaxies (S0s) has been generally neglected. In fact, observations of S0s reveal a strong structural coupling between their bulges and discs, which seems difficult to reconcile with the idea that they come from major mergers. However, in our recent papers we have used N-body simulations of binary mergers to show that, under favourable conditions, discs are first destroyed but soon regrow out of the leftover debris, matching observational photometric scaling relations. Additionally, we have shown how the merger scenario agrees with the recent discovery that S0s and most spirals are not compatible in an angular momentum-concentration plane. This important result from CALIFA constitutes a serious objection to the idea that spirals transform into S0s mainly by fading (e.g., via ram-pressure stripping, as that would not explain the observed simultaneous change in λ Re and concentration), but our simulations of major mergers do explain that mismatch. From such a 3D comparison we conclude that mergers must be a relevant process in the build-up of the current population of S0s.Publisher PDFPeer reviewe
Energy input from quasars regulates the growth and activity of black holes and their host galaxies
In the early Universe, while galaxies were still forming, black holes as
massive as a billion solar masses powered quasars. Supermassive black holes are
found at the centers of most galaxies today, where their masses are related to
the velocity dispersions of stars in their host galaxies and hence to the mass
of the central bulge of the galaxy. This suggests a link between the growth of
the black holes and the host galaxies, which has indeed been assumed for a
number of years. But the origin of the observed relation between black hole
mass and stellar velocity dispersion, and its connection with the evolution of
galaxies have remained unclear. Here we report simulations that simultaneously
follow star formation and the growth of black holes during galaxy-galaxy
collisions. We find that in addition to generating a burst of star formation, a
merger leads to strong inflows that feed gas to the supermassive black hole and
thereby power the quasar. The energy released by the quasar expels enough gas
to quench both star formation and further black hole growth. This determines
the lifetime of the quasar phase (approaching 100 million years) and explains
the relationship between the black hole mass and the stellar velocity
dispersion.Comment: 15 pages, 3 figures, published in Nature, February 10, 2005. For
supplementary information, see http://web.phys.cmu.edu/~tiziana/NatureSI