2,228 research outputs found
Multiple minor mergers: formation of elliptical galaxies and constraints for the growth of spiral disks
Multiple, sequential mergers are unavoidable in the hierarchical build-up
picture of galaxies, in particular for the minor mergers that are frequent and
highly likely to have occured several times for most present-day galaxies.
However, the effect of repeated minor mergers on galactic structure and
evolution has not been studied systematically so far. We present a numerical
study of multiple, subsequent, minor galaxy mergers, with various mass ratios
ranging from 4:1 to 50:1. The N-body simulations include gas dynamics and star
formation. We study the morphological and kinematical properties of the
remnants, and show that several so-called "minor" mergers can lead to the
formation of elliptical-like galaxies that have global morphological and
kinematical properties similar to that observed in real elliptical galaxies.
The properties of these systems are compared with those of elliptical galaxies
produced by the standard scenario of one single major merger. We thus show that
repeated minor mergers can theoretically form elliptical galaxies without major
mergers, and can be more frequent than major mergers, in particular at moderate
redshift. This process must then have formed some elliptical galaxies seen
today, and could in particular explain the high boxiness of massive
ellipticals, and some fundamental relations observed in ellipticals. In
addition, because repeated minor mergers, even at high mass ratios, destroy
disks into spheroids, these results indicate that spiral galaxies cannot have
grown only by a succession of minor mergers.Comment: A&A in press, final version to be published with minor corrections
and updated reference lis
A Lyman-alpha blob in the GOODS South field: evidence for cold accretion onto a dark matter halo
We report on the discovery of a z = 3.16 Lyman-alpha emitting blob in the
GOODS South field. The blob has a total Ly-alpha luminosity of ~ 10^(43) erg
s^(-1) and a diameter larger than 60 kpc. The available multi-wavelength data
in the GOODS field consists of 13 bands from X-rays (Chandra) to infrared
(Spitzer). Unlike other discovered Ly-alpha blobs, this blob shows no obvious
continuum counter-part in any of the broad-bands. In particular, no optical
counter-parts are found in the deep HST/ACS imaging available. For previously
published blobs, AGN (Active Galactic Nuclei) or 'superwind' models have been
found to provide the best match with the data. We here argue that the most
probable origin of the extended Ly-alpha emission from the blob in the GOODS
South field is cold accretion onto a dark matter halo.Comment: 4 pages, 2 tables, 2 figures, Accepted to A&A Letters, minor changes
to tex
On the Origin of the Galaxy Star-Formation-Rate Sequence: Evolution and Scatter
We use a semi-analytic model for disk galaxies to explore the origin of the
time evolution and small scatter of the galaxy SFR sequence -- the tight
correlation between star-formation rate (SFR) and stellar mass (M_star). The
steep decline of SFR from z~2 to the present, at fixed M_star, is a consequence
of the following: First, disk galaxies are in a steady state with the SFR
following the net (i.e., inflow minus outflow) gas accretion rate. The
evolution of the SFR sequence is determined by evolution in the cosmological
specific accretion rates, \propto (1+z)^{2.25}, but is found to be independent
of feedback. Although feedback determines the outflow rates, it shifts galaxies
along the SFR sequence, leaving its zero point invariant. Second, the
conversion of accretion rate to SFR is materialized through gas density, not
gas mass. Although the model SFR is an increasing function of both gas mass
fraction and gas density, only the gas densities are predicted to evolve
significantly with redshift. Third, star formation is fueled by molecular gas.
Since the molecular gas fraction increases monotonically with increasing gas
density, the model predicts strong evolution in the molecular gas fractions,
increasing by an order of magnitude from z=0 to z~2. On the other hand, the
model predicts that the effective surface density of atomic gas is ~10 M_sun
pc^{-2}, independent of redshift, stellar mass or feedback. Our model suggests
that the scatter in the SFR sequence reflects variations in the gas accretion
history, and thus is insensitive to stellar mass, redshift or feedback. The
large scatter in halo spin contributes negligibly, because it scatters galaxies
along the SFR sequence. An observational consequence of this is that the
scatter in the SFR sequence is independent of the size (both stellar and
gaseous) of galaxy disks.Comment: 24 pages, 19 figures, accepted to MNRAS, minor changes to previous
versio
Investigating the properties of AGN feedback in hot atmospheres triggered by cooling-induced gravitational collapse
Radiative cooling may plausibly cause hot gas in the centre of a massive
galaxy, or galaxy cluster, to become gravitationally unstable. The subsequent
collapse of this gas on a dynamical timescale can provide an abundant source of
fuel for AGN heating and star formation. Thus, this mechanism provides a way to
link the AGN accretion rate to the global properties of an ambient cooling
flow, but without the implicit assumption that the accreted material must have
flowed onto the black hole from 10s of kiloparsecs away. It is shown that a
fuelling mechanism of this sort naturally leads to a close balance between AGN
heating and the radiative cooling rate of the hot, X-ray emitting halo.
Furthermore, AGN powered by cooling-induced gravitational instability would
exhibit characteristic duty cycles (delta) which are redolent of recent
observational findings: delta is proportional to L_X/sigma_{*}^{3}, where L_X
is the X-ray luminosity of the hot atmosphere, and sigma_{*} is the central
stellar velocity dispersion of the host galaxy. Combining this result with
well-known scaling relations, we deduce a duty cycle for radio AGN in
elliptical galaxies that is approximately proportional to M_{BH}^{1.5}, where
M_{BH} is the central black hole mass. Outburst durations and Eddington ratios
are also given. Based on the results of this study, we conclude that
gravitational instability could provide an important mechanism for supplying
fuel to AGN in massive galaxies and clusters, and warrants further
investigation.Comment: Accepted for publication in MNRAS. 8 page
The Rise and Fall of Galaxy Activity in Dark Matter Haloes
We use a SDSS galaxy group catalogue to study the dependence of galaxy
activity on stellar mass, halo mass, and group hierarchy (centrals vs.
satellites). We split our galaxy sample in star-forming galaxies, galaxies with
optical AGN activity and radio sources. We find a smooth transition in halo
mass as the activity of central galaxies changes from star formation to optical
AGN activity to radio emission. Star-forming centrals preferentially reside in
haloes with M<10^{12} Msun, central galaxies with optical-AGN activity
typically inhabit haloes with M \sim 10^{13} Msun, and centrals emitting in the
radio mainly reside in haloes more massive than 10^{14} Msun. Although this
seems to suggest that the environment (halo mass) determines the type of
activity of its central galaxy, we find a similar trend with stellar mass:
central star formers typically have stellar masses below 10^{10} Msun, while
optical-AGN hosts and central radio sources have characteristic stellar masses
of 10^{10.8} Msun and 10^{11.6} Msun, respectively. Since more massive haloes
typically host more massive centrals, it is unclear whether the activity of a
central galaxy is causally connected to its stellar mass or to its halo mass.
In general, satellite galaxies have their activity suppressed wrt central
galaxies of the same stellar mass. At fixed stellar mass, we find that the
activity of satellite galaxies depends only weakly on halo mass. In fact, for
satellite galaxies the dependence of galaxy activity on halo mass is more than
four times weaker than the dependence on stellar mass. As we discuss, all these
results are consistent with a picture in which low mass haloes accrete cold
gas, while massive haloes have coronae of hot gas that promote radio activity
of their central galaxies. [Abridged]Comment: 17 pages, 13 figures. Submitted for publication in MNRA
Enriched haloes at redshift with no star-formation: Implications for accretion and wind scenarios
[Abridged] In order to understand which process (e.g. galactic winds, cold
accretion) is responsible for the cool (T~10^4 K) halo gas around galaxies, we
embarked on a program to study the star-formation properties of galaxies
selected by their MgII absorption signature in quasar spectra. Specifically, we
searched for the H-alpha line emission from galaxies near very strong z=2 MgII
absorbers (with rest-frame equivalent width EW>2 \AA) because these could be
the sign-posts of outflows or inflows. Surprisingly, we detect H-alpha from
only 4 hosts out of 20 sight-lines (and 2 out of the 19 HI-selected
sight-lines), despite reaching a star-formation rate (SFR) sensitivity limit of
2.9 M/yr (5-sigma) for a Chabrier initial mass function. This low success rate
is in contrast with our z=1 survey where we detected 66%\ (14/21) of the MgII
hosts. Taking into account the difference in sensitivity between the two
surveys, we should have been able to detect >11.4 of the 20 z=2 hosts whereas
we found only 4 galaxies. Interestingly, all the z=2 detected hosts have
observed SFR greater than 9 M/yr, well above our sensitivity limit, while at
z=1 they all have SFR less than 9 M/yr, an evolution that is in good agreement
with the evolution of the SFR main sequence. Moreover, we show that the z=2
undetected hosts are not hidden under the quasar continuum after stacking our
data and that they also cannot be outside our surveyed area. Hence, strong MgII
absorbers could trace star-formation driven winds in low-mass halos (Mhalo <
10^{10.6} Msun). Alternatively, our results imply that z=2 galaxies traced by
strong MgII absorbers do not form stars at a rate expected (3--10 M/yr) for
their (halo or stellar) masses, supporting the existence of a transition in
accretion efficiency at Mhalo ~ 10^{11} Msun. This scenario can explain both
the detections and the non-detections.Comment: 14 pages, 4 fig.; MNRAS in press, minor corrections to match proof
Tomato protoplast DNA transformation: physical linkage and recombination of exogenous DNA sequences
Tomato protoplasts have been transformed with plasmid DNA's, containing a chimeric kanamycin resistance gene and putative tomato origins of replication. A calcium phosphate-DNA mediated transformation procedure was employed in combination with either polyethylene glycol or polyvinyl alcohol. There were no indications that the tomato DNA inserts conferred autonomous replication on the plasmids. Instead, Southern blot hybridization analysis of seven kanamycin resistant calli revealed the presence of at least one kanamycin resistance locus per transformant integrated in the tomato nuclear DNA. Generally one to three truncated plasmid copies were found integrated into the tomato nuclear DNA, often physically linked to each other. For one transformant we have been able to use the bacterial ampicillin resistance marker of the vector plasmid pUC9 to 'rescue' a recombinant plasmid from the tomato genome. Analysis of the foreign sequences included in the rescued plasmid showed that integration had occurred in a non-repetitive DNA region. Calf-thymus DNA, used as a carrier in transformation procedure, was found to be covalently linked to plasmid DNA sequences in the genomic DNA of one transformant. A model is presented describing the fate of exogenously added DNA during the transformation of a plant cell. The results are discussed in reference to the possibility of isolating DNA sequences responsible for autonomous replication in tomato.
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