276 research outputs found
Modeling the High-z Universe: Probing Galaxy Formation
We discuss how the conditions at high redshift differ from those at low
redshift, and what the impact is on the galaxy population. We focus in
particular on the role of gaseous dissipation and its impact on sustaining high
star formation rates as well as on driving star-bursts in mergers. Gas
accretion onto galaxies at high redshifts occurs on a halo dynamical time
allowing for very efficiently sustained star formation. In addition cold
accretion flows are able to drive turbulence in high redshift disks at the
level observed if at least 20% of the accretion energy is converted into random
motion in the gaseous disk. In general we find that the fraction of gas
involved in galaxy mergers is a strong function of time and increases with
redshift. A model combining the role of dissipation during mergers and
continued infall of satellite galaxies allows to reproduce the observed
size-evolution of early-type galaxies with redshift. Furthermore we investigate
how the evolution of the faint-end of the luminosity function can be explained
in terms of the evolution of the underlying dark matter evolution.Comment: To appear in "Reviews in Modern Astronomy", volume 21, Proceedings of
JENAM 2008, Vienn
Gravity at Work: How the Build-Up of Environments Shape Galaxy Properties
We present results on the heating of the inter-cluster medium (ICM) by
gravitational potential energy from in-falling satellites. We calculate the
available excess energy of baryons once they are stripped from their satellite
and added to the ICM of the hosting environment. this excess energy is a strong
function of environment and we find that it can exceed the contribution from
AGNs or supernovae (SN) by up to two orders of magnitude in the densest
environments/haloes. Cooling by radiative losses is in general fully
compensated by gravitational heating in massive groups and clusters with hot
gas temperature > 1 keV. The reason for the strong environment dependence is
the continued infall of substructure onto dense environments during their
formation in contrast to field-like environments. We show that gravitational
heating is able to reduce the number of too luminous galaxies in models and to
produce model luminosity functions in agreement with observations.Comment: 8 pages, 3 figures. To be published in Proceedings of JENAM 2010,
Symposium 2: "Environment and the formation of galaxies: 30 years later
A simple model for the size-evolution of elliptical galaxies
We use semi-analytical modelling of galaxy formation to predict the
redshift-size-evolution of elliptical galaxies. Using a simple model in which
relative sizes of elliptical galaxies of a given mass correlate with the
fraction of stars formed in a star burst during a major merger event, we are
able to reproduce the observed size redshif t evolution. The size evolution is
a result of the amount of cold gas available during the major merger. Mergers
at high redshifts are gas-rich and produce ellipticals with smaller sizes. In
particular we find a power-law relation between the sizes at different
redshifts, with the power-law index giving a measure of the relative amount of
dissipation during the mergers that lead to the formation of an elliptical. The
size evolution is found to be stronger for more massive galaxies as they
involve more gas at high redshifts when they form compared to less massive
ellipticals. Local ellipticals more massive than M
will be approximately 4 times larger than their counterparts at . Our
results indicate that the scatter in the size of similar massive present day
elliptical galaxies is a result of their formation epoch, with smaller
ellipticals being formed earlier.Comment: 4 pages, submitted to ApJ Letters, replaced by accepted versio
The Evolving Faint-End of the Luminosity Function
We investigate the evolution of the faint-end slope of the luminosity
function, , using semi-analytical modeling of galaxy formation. In
agreement with observations, we find that the slope can be fitted well by
, with a=-1.13 and b=-0.1. The main driver for the evolution
in is the evolution in the underlying dark matter mass function.
Sub-L_* galaxies reside in dark matter halos that occupy a different part of
the mass function. At high redshifts, this part of the mass function is steeper
than at low redshifts and hence is steeper. Supernova feedback in
general causes the same relative flattening with respect to the dark matter
mass function. The faint-end slope at low redshifts is dominated by field
galaxies and at high redshifts by cluster galaxies. The evolution of
in each of these environments is different, with field galaxies
having a slope b=-0.14 and cluster galaxies b=-0.05. The transition from
cluster-dominated to field-dominated faint-end slope occurs roughly at a
redshift , and suggests that a single linear fit to the overall
evolution of might not be appropriate. Furthermore, this result
indicates that tidal disruption of dwarf galaxies in clusters cannot play a
significant role in explaining the evolution of at z< z_*. In
addition we find that different star formation efficiencies a_* in the
Schmidt-Kennicutt-law and supernovae-feedback efficiencies generally
do not strongly influence the evolution of .Comment: 4 pages, replaced with version accepted to ApJL, minor changes to
figure
Merger History of Galaxies and Disk+Bulge Formation
We discuss the transitions of galaxy morphologies within the CDM paradigm
under the assumption of bulge formation in mergers and disk growth via cooling
of gas and subsequent star formation. Based on the relative importance of these
two competing processes it is possible to make predictions on the expected
morphological mix of galaxies. In particular we here discuss the generation of
massive disk galaxies with low bulge-to-total mass ratios. Our results indicate
that it is difficult to generate enough massive disk galaxies with B/T
via major mergers and subsequent disk re-growth, if during the major merger
progenitor disks get disrupted completely. On average low B/T galaxies must
have had there last major merger at . The main limiting factor is the
ability to re-grow massive disks at late times after the last major merger of a
galaxy. Taking into account the contribution from minor mergers (, ) to the formation of bulges, we recover the right
fraction of massive low B/T disk galaxies, indicating that minor mergers play
an important role in the formation of massive low B/T disk galaxies.Comment: To appear in 'Galaxy Evolution: Emerging Insights and Future
Challenges' ASP Conference Series, 2009. Editors: Shardha Jogee, Lei Hao,
Guillermo Blanc & Irina Marinov
How galaxies lose their angular momentum
The processes are investigated by which gas loses its angular momentum during
the protogalactic collapse phase, leading to disk galaxies that are too compact
with respect to the observations. High-resolution N-body/SPH simulations in a
cosmological context are presented including cold gas and dark matter. A halo
with quiet merging activity since z~3.8 and with a high spin parameter is
analysed that should be an ideal candidate for the formation of an extended
galactic disk. We show that the gas and the dark matter have similar specific
angular momenta until a merger event occurs at z~2 with a mass ratio of 5:1.
All the gas involved in the merger loses a substantial fraction of its specific
angular momentum due to tidal torques and falls quickly into the center.
Dynamical friction plays a minor role,in contrast to previous claims. In fact,
after this event a new extended disk begins to form from gas that was not
involved in the 5:1 merger event and that falls in subsequently. We argue that
the angular momentum problem of disk galaxy formation is a merger problem: in
cold dark matter cosmology substantial mergers with mass ratios of 1:1 to 6:1
are expected to occur in almost all galaxies. We suggest that energetic
feedback processes could in principle solve this problem, however only if the
heating occurs at the time or shortly before the last substantial merger event.
Good candidates for such a coordinated feedback would be a merger-triggered
star burst or central black hole heating. If a large fraction of the low
angular momentum gas would be ejected as a result of these processes, late-type
galaxies could form with a dominant extended disk component, resulting from
late infall, a small bulge-to-disk ratio and a low baryon fraction, in
agreement with observations.Comment: 7 pages, 5 figures, submitted to MNRAS. Request for high resolution
figures to the author
The First Billion Years project: gamma-ray bursts at z>5
Long gamma-ray burst's (LGRB's) association to the death of massive stars
suggest they could be used to probe the cosmic star formation history (CSFH)
with high accuracy, due to their high luminosities. We utilise cosmological
simulations from the First Billion Years project to investigate the biases
between the CSFH and the LGRB rate at z>5, assuming various different models
and constraints on the progenitors of LGRBs. We populate LGRBs using a
selection based on environmental properties and demonstrate that the LGRB rate
should trace the CSFH to high redshifts. The measured LGRB rate suggests that
LGRBs have opening angles of theta_jet=0.1 deg, although the degeneracy with
the progenitor model cannot rule out an underlying bias. We demonstrate that
proxies that relate the LGRB rate with global LGRB host properties do not
reflect the underlying LGRB environment, and are in fact a result of the host
galaxy's spatial properties, such that LGRBs can exist in galaxies of solar
metallicity. However, we find a class of host galaxies that have low stellar
mass and are metal-rich, and that their metallicity dispersions would not allow
low-metallicity environments. Detection of hosts with this set of properties
would directly reflect the progenitor's environment. We predict that 10% of
LGRBs per year are associated with this set of galaxies that would have
forbidden line emission that could be detected by instruments on the James Webb
Space Telescope. Such a discovery would place strong constraints on the
collapsar model and suggest other avenues to be investigated.Comment: 13 pages, 8 figures, 1 table, accepted for publication in MNRA
Adding Environmental Gas Physics to the Semi-Analytic Method for Galaxy Formation: Gravitational Heating
We present results of an attempt to include more detailed gas physics
motivated from hydrodynamical simulations within semi-analytic models (SAM) of
galaxy formation, focusing on the role that environmental effects play. The
main difference to previous SAMs is that we include 'gravitational' heating of
the intra-cluster medium (ICM) by the net surplus of gravitational potential
energy released from gas that has been stripped from infalling satellites.
Gravitational heating appears to be an efficient heating source able to prevent
cooling in environments corresponding to dark matter halos more massive than
M. The energy release by gravitational heating can
match that by AGN-feedback in massive galaxies and can exceed it in the most
massive ones. However, there is a fundamental difference in the way the two
processes operate. Gravitational heating becomes important at late times, when
the peak activity of AGNs is already over, and it is very mass dependent. This
mass dependency and time behaviour gives the right trend to recover down-sizing
in the star-formation rate of massive galaxies. Abridged...Comment: replaced by accepted version to ApJ, some sections have been dropped
and text has been added to others to include the referee's comments, several
typos have been correcte
Recent star formation in high-redshift early-type galaxies: insights from the rest-frame UV
We combine deep UBVRIzJK photometry from the MUSYC survey with redshifts from
the COMBO-17 survey to study the rest-frame ultraviolet (UV) properties of 674
high-redshift (0.5<z<1) early-type galaxies, drawn from the Extended Chandra
Deep Field South (E-CDFS). Galaxy morphologies are determined through visual
inspection of Hubble Space Telescope (HST) images taken from the GEMS survey.
We harness the sensitivity of the UV to young (<1 Gyrs old) stars to quantify
the recent star formation history of the early-type population. We find
compelling evidence that early-types of all luminosities form stars over the
lifetime of the Universe, although the bulk of their star formation is already
complete at high redshift. Luminous (-23<M(V)<-20.5) early-types form 10-15
percent of their mass after z=1, while their less luminous (M(V)>-20.5)
counterparts form 30-60 percent of their mass in the same redshift range.Comment: To appear in the proceedings of the IAU 245, eds. M. Bureau, E.
Athanassoula, and B. Barbu
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