The Dust Depletion and Extinction of the GRB 020813 Afterglow

The Keck optical spectrum of the GRB 020813 afterglow is the best ever obtained for GRBs. Its large spectral range and very high S/N ratio allowed for the first time the detection of a vast variety of absorption lines, associated with the circumburst medium or interstellar medium of the host. The remarkable similarity of the relative abundances of 8 elements with the dust depletion pattern seen in the Galactic ISM suggests the presence of dust. The derived visual dust extinction A_V=0.40+/-0.06 contradicts the featureless UV spectrum of the afterglow, very well described by a unreddened power law. The forthcoming Swift era will open exciting opportunities to explain similar phenomena in other GRB afterglows.Comment: To be published in "Il Nuovo Cimento", Proceedings of the 4th Rome Workshop on Gamma-Ray Bursts in the Afterglow Era, eds. L. Piro, L. Amati, S. Covino, B. Gendr

Bulgeless Galaxies and their Angular Momentum Problem

The specific angular momentum of Cold Dark Matter (CDM) halos in a $\Lambda$CDM universe is investigated. Their dimensionless specific angular momentum $\lambda'=\frac{j}{\sqrt{2}V_{vir} R{vir}}$ with $V_{vir}$ and $R_{vir}$ the virial velocity and virial radius, respectively depends strongly on their merging histories. We investigate a set of $\Lambda$CDM simulations and explore the specific angular momentum content of halos formed through various merging histories. Halos with a quiet merging history, dominated by minor mergers and accretion until the present epoch, acquire by tidal torques on average only 2% to 3% of the angular momentum required for their rotational support ($\lambda'=0.02$). This is in conflict with observational data for a sample of late-type bulgeless galaxies which indicates that those galaxies reside in dark halos with exceptionally high values of $\lambda' \approx 0.06-0.07$. Minor mergers and accretion preserve or slowly increase the specific angular momentum of dark halos with time. This mechanism is however not efficient enough in order to explain the observed spin values for late-type dwarf galaxies. Energetic feedback processes have been invoked to solve the problem that gas loses a large fraction of its specific angular momentum during infall. Under the assumption that dark halos hosting bulgeless galaxies acquire their mass via quiescent accretion, our results indicate yet another serious problem: the specific angular momentum gained during the formation of these objects is not large enough to explain their observed rotational properties,even if no angular momentum would be lost during gas infall.Comment: 4 pages, 3 figures. To appear in September 1, 2004, issue of ApJ Letter

The Angular Momentum Distribution of Gas and Dark Matter in Galactic Halos

(Abridged) We report results of a series of non radiative N-body/SPH simulations in a LCDM cosmology. We find that the spin of the baryonic component is on average larger than that of the dark matter (DM) component and we find this effect to be more pronounced at lower redshifts. A significant fraction f of gas has negative angular momentum and this fraction is found to increase with redshift. We describe a toy model in which the tangential velocities of particles are smeared by Gaussian random motions. This model is successful in explaining some of the angular momentum properties. We compare and contrast various techniques to determine the angular momentum distributions (AMDs). We show that broadening of velocity dispersions is unsuitable for making comparisons between gas and DM. We smooth the angular momentum of the particles over a fixed number of neighbors. We find that an analytical function based on gamma distribution can be used to describe a wide variety of profiles, with just one parameter \alpha. The distribution of the shape parameter $\alpha$ for both gas and DM follows roughly a log-normal distribution. The mean and standard deviation of log(\alpha) for gas is -0.04 and 0.11 respectively. About 90-95% of halos have \alpha<1.3, while exponential disks in NFW halos would require 1.3<\alpha<1.6. This implies that a typical halo in simulations has an excess of low angular momentum material as compared to that of observed exponential disks, a result which is consistent with the findings of earlier works. \alpha for gas is correlated with that of DM but they have a significant scatter =1.09 \pm 0.2. \alpha_Gas is also biased towards slightly higher values compared to \alpha_DM.Comment: 19 pages, 32 figures (replaced to correct a typo in the authors field in the above line, paper unchanged

WFPC2 Observations of Star Clusters in the Magellanic Clouds: I. The LMC Globular Cluster Hodge 11

We present our analysis of Hubble Space Telescope Wide Field Planetary Camera 2 observations in F555W (broadband V) and F450W (broadband B) of the globular cluster Hodge 11 in the Large Magellanic Cloud galaxy. The resulting V vs. (B-V) color-magnitude diagram reaches 2.4 mag below the main-sequence turnoff (which is at V_TO = 22.65 +- 0.10 mag or M_V^TO = 4.00 +- 0.16 mag). Comparing the fiducial sequence of Hodge 11 with that of the Galactic globular cluster M92, we conclude that, within the accuracy of our photometry, the age of Hodge 11 is identical to that of M92 with a relative age-difference uncertainty ranging from 10% to 21%. Provided that Hodge 11 has always been a part of the Large Magellanic Cloud and was not stripped from the halo of the Milky Way or absorbed from a cannibalized dwarf spheroidal galaxy, then the oldest stars in the Large Magellanic Clouds and the Milky Way appear to have the same age.Comment: 14 pages (LaTeX+aaspp4.sty), 3 tables and 4 figures (Postscript, gzipped tar file). Postscript version of paper, tables, and full-resolution figures available at http://www.astro.columbia.edu/~mighell/hodge11.html To appear in the Astronomical Journa

Angular Momentum Profiles of Warm Dark Matter Halos

We compare the specific angular momentum profiles of virialized dark halos in cold dark matter (CDM) and warm dark matter (WDM) models using high-resolution dissipationless simulations. The simulations were initialized using the same set of modes, except on small scales, where the power was suppressed in WDM below the filtering length. Remarkably, WDM as well as CDM halos are well-described by the two-parameter angular momentum profile of Bullock et al. (2001), even though the halo masses are below the filtering scale of the WDM. Although the best-fit shape parameters change quantitatively for individual halos in the two simulations, we find no systematic variation in profile shapes as a function of the dark matter type. The scatter in shape parameters is significantly smaller for the WDM halos, suggesting that substructure and/or merging history plays a role producing scatter about the mean angular momentum distribution, but that the average angular momentum profiles of halos originate from larger-scale phenomena or a mechanism associated with the virialization process. The known mismatch between the angular momentum distributions of dark halos and disk galaxies is therefore present in WDM as well as CDM models. Our WDM halos tend to have a less coherent (more misaligned) angular momentum structure and smaller spin parameters than do their CDM counterparts, although we caution that this result is based on a small number of halos.Comment: 5 pages, 1 figure, Submitted to ApJ

Star Formation, Metallicity and Dust Properties Derived from the SAPM Galaxy Survey Spectra

We have derived star formation rates (SFRs), gas-phase oxygen abundances and effective dust absorption optical depths for a sample of galaxies drawn from the Stromlo-APM redshift survey using the new Charlot and Longhetti (2001; CL01) models, which provide a physically consistent description of the effects of stars, gas and dust on the integrated spectra of galaxies. Our sample consists of 705 galaxies with measurements of the fluxes and equivalent widths of Halpha, [OII], and one or both of [NII] and [SII]. For a subset of the galaxies, 60 and 100 micron IRAS fluxes are available. We compare the star formation rates derived using the models with those derived using standard estimators based on the Halpha, the [OII] and the far-infrared luminosities of the galaxies. The CL01 SFR estimates agree well with those derived from the IRAS fluxes, but are typically a factor of ~3 higher than those derived from the Halpha or the [OII] fluxes, even after the usual mean attenuation correction of A_Halpha=1 mag is applied to the data. We show that the reason for this discrepancy is that the standard Halpha estimator neglects the absorption of ionizing photons by dust in HII regions and the contamination of Halpha emission by stellar absorption. We also use our sample to study variations in star formation and metallicity as a function of galaxy absolute bJ magnitude. For this sample, the star formation rate per unit bJ luminosity is independent of magnitude. The gas-phase oxygen abundance does increase with bJ luminosity, although the scatter in metallicity at fixed magnitude is large.Comment: 17 pages, 8 figures, accepted for publication in MNRA

Modeling Luminosity-Dependent Galaxy Clustering Through Cosmic Time

We employ high-resolution dissipationless simulations of the concordance LCDM cosmology to model the observed luminosity dependence and evolution of galaxy clustering through most of the age of the universe, from z~5 to z~0. We use a simple, non-parametric model which monotonically relates galaxy luminosities to the maximum circular velocity of dark matter halos (V_max) by preserving the observed galaxy luminosity function in order to match the halos in simulations with observed galaxies. The novel feature of the model is the use of the maximum circular velocity at the time of accretion, V_max,acc, for subhalos, the halos located within virial regions of larger halos. We argue that for subhalos in dissipationless simulations, V_max,acc reflects the luminosity and stellar mass of the associated galaxies better than the circular velocity at the epoch of observation, V_max,now. The simulations and our model L-V_max relation predict the shape, amplitude, and luminosity dependence of the two-point correlation function in excellent agreement with the observed galaxy clustering in the SDSS data at z~0 and in the DEEP2 samples at z~1 over the entire probed range of projected separations, 0.1<r_p/(Mpc/h)<10.0. In particular, the small-scale upturn of the correlation function from the power-law form in the SDSS and DEEP2 luminosity-selected samples is reproduced very well. At z~3-5, our predictions also match the observed shape and amplitude of the angular two-point correlation function of Lyman-break galaxies (LBGs) on both large and small scales, including the small-scale upturn.Comment: 16 pages 11 figures, ApJ in pres

Structure Formation Inside Triaxial Dark Matter Halos: Galactic Disks, Bulges and Bars

We investigate the formation and evolution of galactic disks immersed in assembling live DM halos. Disk/halo components have been evolved from the cosmological initial conditions and represent the collapse of an isolated density perturbation. The baryons include gas (which participates in star formation [SF]) and stars. The feedback from the stellar energy release onto the ISM has been implemented. We find that (1) The growing triaxial halo figure tumbling is insignificant and the angular momentum (J) is channeled into the internal circulation; (2) Density response of the disk is out of phase with the DM, thus diluting the inner halo flatness and washing out its prolateness; (3) The total J is neathly conserved, even in models accounting for feedback; (4) The specific J for the DM is nearly constant, while that for baryons is decreasing; (5) Early stage of disk formation resembles the cat's cradle -- a small amorphous disk fueled via radial string patterns; (6) The initially puffed up gas component in the disk thins when the SF rate drops below ~5 Mo/yr; (7) About 40%-60% of the baryons remain outside the SF region; (8) Rotation curves appear to be flat and account for the observed disk/halo contributions; (9) A range of bulge-dominated to bulgeless disks was obtained; Lower density threshold for SF leads to a smaller, thicker disk; Gravitational softening in the gas has a substantial effect on various aspects of galaxy evolution and mimics a number of intrinsic processes within the ISM; (10) The models are characterized by an extensive bar-forming activity; (11) Nuclear bars, dynamically coupled and decoupled form in response to the gas inflow along the primary bars.Comment: 18 pages, 16 figures, accepted by the Astrophysical Journal. Minor revisions. The high-resolution figures can be found at http://www.pa.uky.edu/~shlosman/research/galdyn/figs07a