380 research outputs found
Formation of the Galactic globular clusters with He-rich stars in low-mass halos virialized at high redshift
Recent observations have reported that the Galactic globular clusters (GCs)
with unusually extended horizontal-branch (EHB) morphologies show a
significantly lower velocity dispersion compared with that of the entire
Galactic GC system. We consider that the observed distinctive kinematics of GCs
with EHB has valuable information on the formation epochs of GCs and
accordingly discuss this observational result based on cosmological N-body
simulations with a model of GC formation. We assume that GCs in galaxies were
initially formed in low-mass halos at high redshifts and we investigate final
kinematics of GCs in their host halos at . We find that GCs formed in
halos virialized at z>10 show lower velocity dispersions on average than those
formed at z>6 for halos with GCs at z=0. We thus suggest that the origin of the
observed lower velocity dispersion for the Galactic GCs with EHBs is closely
associated with earlier formation epochs (z>10) of halos initially hosting the
GCs in the course of the Galaxy formation. Considering that the origin of EHBs
can be due to the presence of helium-enhanced second-generation stars in GCs,
we discuss the longstanding second parameter problem of GCs in the context of
different degrees of chemical pollution in GC-forming gas clouds within
low-mass halos virialized at different redshifts.Comment: 5 pages, 3 figures, accepted by MNRAS Letter
On the Origin of Mass--Metallicity Relations, Blue Tilts, and Scaling Relations for Metal-poor Globular Cluster Systems
We investigate formation processes and physical properties of globular
cluster systems (GCSs) in galaxies based on high-resolution cosmological
simulations with globular clusters. We focus on metal-poor clusters (MPCs) and
correlations with their host galaxies by assuming that MPC formation is
truncated at a high redshift (z_trun > 6). We find that the correlation between
mean metallicities (Z_gc) of MPCs and their host galaxy luminosities (L)
flattens from z=z_trun to z=0. We also find that the observed relation (Z_gc ~
L^0.15) in MPCs can be reproduced well in the models with Z_gc ~ L^0.5 at
z=z_trun when z_trun ~ 10, if mass-to-light-ratios are assumed to be constant
at z=z_trun. However, better agreement with the observed relation is found for
models with different mass-to-light-ratios between z=z_trun and z=0. It is also
found that the observed color-magnitude relation of luminous MPCs (i.e., ``blue
tilts'') may only have a small contribution from the stripped stellar nuclei of
dwarf galaxies, which have nuclei masses that correlate with their total mass
at z=z_trun. The simulated blue tilts are found to be seen more clearly in more
massive galaxies, which reflects the fact that more massive galaxies at z=0 are
formed from a larger number of dwarfs with stellar nuclei formed at z>z_trun.
The half-number radii (R_e) of GCSs, velocity dispersions of GCSs (sigma), and
their host galaxy masses (M_h) are found to be correlated with one another such
that R_e ~ M_h^{0.57} and sigma ~ M_h^{0.32}.Comment: 15 pages, 20 figures, accepted by MNRA
The origin of globular cluster systems from cosmological simulations
We investigate the structural, kinematical, and chemical properties of
globular cluster systems (GCSs) in galaxies of different Hubble types in a
self-consistent manner based on high-resolution cosmological N-body simulations
combined with semi-analytic models of galaxy and globular cluster (GC)
formation. We focus on correlations between the physical properties of GCSs and
those of their host galaxies for about 10^5 simulated galaxies located at the
centres of dark matter halos (i.e. we do not consider satellite galaxies in
sub-halos). Our principal results, which can be tested against observations,
are as follows. The majority (about 90%) of GCs currently in halos are formed
in low-mass galaxies at redshifts greater than 3 with mean formation redshifts
of z = 5.7 (12.7 Gyrs ago) and 4.3 (12.3 Gyrs ago) for metal-poor GCs (MPC) and
metal-rich GCs (MRCs), respectively. About 52 % of galaxies with GCs show clear
bimodality in their metallicity distribution functions, though less luminous
galaxies with M_B fainter than -17 are much less likely to show bimodality
owing to little or no MRCs. The number fraction of MRCs does not depend on
Hubble type but is generally smaller for less luminous galaxies. The specific
frequencies (S_ N) of GCSs are typically higher in ellipticals (S_ N ~ 4.0)
than in spirals (S_ N ~ 1.8), and higher again (S_N ~ 5.0) for galaxies located
at the centers of clusters of galaxies. The total number of GCs per unit halo
mass does not depend strongly on M_B or Hubble type of the host galaxy. The
mean metallicities of MPCs and MRCs depend on M_B such that they are higher in
more luminous galaxies, though the dependence is significantly weakerfor MPCs.Comment: 19 pages, 29 figures, accepted in MNRA
Revisiting the Cosmic Star Formation History: Caution on the Uncertainties in Dust Correction and Star Formation Rate Conversion
The cosmic star formation rate density (CSFRD) has been observationally
investigated out to redshift z~10. However, most of theoretical models for
galaxy formation underpredict the CSFRD at z>1. Since the theoretical models
reproduce the observed luminosity functions (LFs), luminosity densities (LDs),
and stellar mass density at each redshift, this inconsistency does not simply
imply that theoretical models should incorporate some missing unknown physical
processes in galaxy formation. Here, we examine the cause of this inconsistency
in UV wavelengths by using a mock catalog of galaxies generated by a
semi-analytic model of galaxy formation. We find that this inconsistency is due
to two observational uncertainties: dust obscuration correction and conversion
from UV luminosity to star formation rate (SFR). The methods for correction of
obscuration and SFR conversion used in observational studies result in the
overestimation of CSFRD by ~ 0.1-0.3 dex and ~ 0.1-0.2 dex, respectively,
compared to the results obtained directly from our mock catalog. We present new
empirical calibrations for dust attenuation and conversion from observed UV LFs
and LDs into CSFRD.Comment: 12 pages including 11 figures. matches the published version (ApJ
2013 Jan. 20 issue
Numerical Galaxy Catalog -I. A Semi-analytic Model of Galaxy Formation with N-body simulations
We construct the Numerical Galaxy Catalog (GC), based on a semi-analytic
model of galaxy formation combined with high-resolution N-body simulations in a
-dominated flat cold dark matter (CDM) cosmological model.
The model includes several essential ingredients for galaxy formation, such as
merging histories of dark halos directly taken from N-body simulations,
radiative gas cooling, star formation, heating by supernova explosions
(supernova feedback), mergers of galaxies, population synthesis, and extinction
by internal dust and intervening HI clouds. As the first paper in a series
using this model, we focus on basic photometric, structural and kinematical
properties of galaxies at present and high redshifts. Two sets of model
parameters are examined, strong and weak supernova feedback models, which are
in good agreement with observational luminosity functions of local galaxies in
a range of observational uncertainty. Both models agree well with many
observations such as cold gas mass-to-stellar luminosity ratios of spiral
galaxies, HI mass functions, galaxy sizes, faint galaxy number counts and
photometric redshift distributions in optical pass-bands, isophotal angular
sizes, and cosmic star formation rates. In particular, the strong supernova
feedback model is in much better agreement with near-infrared (K'-band) faint
galaxy number counts and redshift distribution than the weak feedback model and
our previous semi-analytic models based on the extended Press-Schechter
formalism. (Abridged)Comment: 26 pages including 27 figures, accepted for publication in ApJ,
full-resolution version is available at
http://grape.astron.s.u-tokyo.ac.jp/~yahagi/nugc
VLT photometry in the Antlia Cluster: the giant ellipticals NGC 3258 and NGC 3268 and their globular cluster systems
We present a deep VLT photometry in the regions surrounding the two dominant
galaxies of the Antlia cluster, the giant ellipticals NGC 3258 and NGC 3268. We
construct the luminosity functions of their globular cluster systems (GCSs) and
determine their distances through the turn-over magnitudes. These distances are
in good agreement with those obtained by the SBF method. There is some, but not
conclusive, evidence that the distance to NGC 3268 is larger by several Mpc.
The GCSs colour distributions are bimodal but the brightest globular clusters
(GCs) show a unimodal distribution with an intermediate colour peak. The radial
distributions of both GCSs are well fitted by de Vaucouleurs laws up to 5
arcmin. Red GCs present a steeper radial density profile than the blue GCs, and
follow closely the galaxies' brightness profiles. Total GC populations are
estimated to be about 6000+/-150 GCs in NGC 3258 and 4750+/-150 GCs in NGC
3268. We discuss the possible existence of GCs in a field located between the
two giant galaxies (intracluster GCs). Their luminosity functions and number
densities are consistent with the two GCSs overlapping in projection.Comment: 13 pages, 16 figures. Accepted for publication in MNRA
Some improvements to the spherical collapse model
I study the joint effect of dynamical friction, tidal torques and
cosmological constant on clusters of galaxies formation I show that within
high-density environments, such as rich clusters of galaxies, both dynamical
friction and tidal torques slows down the collapse of low-? peaks producing an
observable variation in the time of collapse of the perturbation and, as a
consequence, a reduction in the mass bound to the collapsed perturbation
Moreover, the delay of the collapse produces a tendency for less dense regions
to accrete less mass, with respect to a classical spherical model, inducing a
biasing of over-dense regions toward higher mass I show how the threshold of
collapse is modified if dynamical friction, tidal torques and a non-zero
cosmological constant are taken into account and I use the Extended Press
Schecter (EPS) approach to calculate the effects on the mass function Then, I
compare the numerical mass function given in Reed et al (2003) with the
theoretical mass function obtained in the present paper I show that the barrier
obtained in the present paper gives rise to a better description of the mass
function evolution with respect to other previous models (Sheth & Tormen 1999,
MNRAS, 308, 119 (hereafter ST); Sheth & Tormen 2002, MNRAS, 329, 61 (hereafter
ST1)
On the cosmological mass function theory
This paper provides, from one side, a review of the theory of the
cosmological mass function from a theoretical point of view, starting from the
seminal paper of Press & Shechter (1974) to the last developments (Del Popolo &
Gambera (1998, 1999), Sheth & Tormen 1999 (ST), Sheth, Mo & Tormen 2001 (ST1),
Jenkins et al. 2001 (J01), Shet & Tormen 2002 (ST2), Del Popolo 2002a, Yagi et
al. 2004 (YNY)), and from another side some improvements on the multiplicity
function models in literature. ...Comment: Astronomy Reports, in prin
Internal properties of ultracompact dwarf galaxies in the Virgo cluster
We present new imaging and spectroscopic observations of six ultracompact dwarf (UCD) galaxies in the Virgo Cluster, along with reanalyzed data for five Fornax Cluster UCDs. These are the most luminous UCDs: -14 mag < M-V < -12 mag. Our Hubble Space Telescope imaging shows that most of the UCDs have shallow or steep cusps in their cores; only one UCD has a flat "King'' core. None of the UCDs show tidal cutoffs down to our limiting surface brightness. Spectroscopic analysis shows that Virgo UCDs are old ( older than 8 Gyr) and have metallicities in the range from [Z/H] = -1.35 to +0.35 dex. Five Virgo UCDs have supersolar [alpha/Fe] abundance ratios, and one Virgo UCD has a solar abundance ratio. The supersolar [alpha/Fe] abundances are typical of old stellar populations found in globular clusters and elliptical galaxies. We find that Virgo UCDs have structural and dynamical properties similar to Fornax UCDs. The Virgo and Fornax UCDs all have masses approximate to(2-9) x 10(7) M-circle dot and mass-to-light ratios approximate to(3-5) M-circle dot/L-circle dot,(V). The dynamical mass-to-light ratios for Virgo UCDs are consistent with simple stellar population model predictions: the Virgo UCDs do not require dark matter to explain their mass-to-light ratios. We conclude that the internal properties of Virgo UCDs are consistent with them being the high-mass/high-luminosity extreme of known globular cluster populations. We refrain from any firm conclusions on Fornax UCD origins until accurate age, metallicity, and alpha-abundance estimates are obtained for them. Some of our results, notably the fundamental plane projections, are consistent with the formation of UCDs by the simple removal of the halo from the nuclei of nucleated dwarf galaxies. However, the ages, metallicities, and abundances for Virgo UCDs are not consistent with this simple stripping model. It might be consistent with more sophisticated models of the stripping process that include the effects of gas removal on the chemical evolution of the nuclei
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