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 $z=0$. 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 ($\nu$GC), based on a semi-analytic model of galaxy formation combined with high-resolution N-body simulations in a $\Lambda$-dominated flat cold dark matter ($\Lambda$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

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)

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

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