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

Permanent draft genome sequence of Ensifer sp. strain LCM 4579, a salt-tolerant, nitrogen-fixing bacterium isolated from Senegalese soil

The genus Ensifer (formerly Sinorhizobium) contains many species able to form nitrogen-fixing nodules on plants of the legume family. Here, we report the 6.1-Mb draft genome sequence of Ensifer sp. strain LCM 4579, with a G+C content of 62.4% and 5,613 candidate protein-encoding genes

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

Star Clusters in Virgo and Fornax Dwarf Irregular Galaxies

We present the results of a search for clusters in dwarf irregular galaxies in the Virgo and Fornax Cluster using HST WFPC2 snapshot data. The galaxy sample includes 28 galaxies, 11 of which are confirmed members of the Virgo and Fornax clusters. In the 11 confirmed members, we detect 237 cluster candidates and determine their V magnitudes, V-I colors and core radii. After statistical subtraction of background galaxies and foreground stars, most of the cluster candidates have V-I colors of -0.2 and 1.4, V magnitudes lying between 20 and 25th magnitude and core radii between 0 and 6 pc. Using H-alpha observations, we find that 26% of the blue cluster candidates are most likely HII regions. The rest of the cluster candidates are most likely massive (>10^4 Msol) young and old clusters. A comparison between the red cluster candidates in our sample and the Milky Way globular clusters shows that they have similar luminosity distributions, but that the red cluster candidates typically have larger core radii. Assuming that the red cluster candidates are in fact globular clusters, we derive specific frequencies (S_N) ranging from ~0-9 for the galaxies. Although the values are uncertain, seven of the galaxies appear to have specific frequencies greater than 2. These values are more typical of ellipticals and nucleated dwarf ellipticals than they are of spirals or Local Group dwarf irregulars.Comment: 46 pages, 14 figures, 3 tables, accepted by AJ. Higher quality PS version of entire paper available at http://www.astro.washington.edu/seth/dirr_gcs.htm

Angular momentum distribution of hot gas and implications for disk galaxy formation

We study the angular momentum profiles both for dark matter and for gas within virialized halos, using a statistical sample of halos drawn from cosmological hydrodynamics simulations. Three simulations have been analyzed, one is the ``non-radiative'' simulation, and the other two have radiative cooling. We find that the gas component on average has a larger spin and contains a smaller fraction of mass with negative angular momentum than its dark matter counterpart in the non-radiative model. As to the cooling models, the gas component shares approximately the same spin parameter as its dark matter counterpart, but the hot gas has a higher spin and is more aligned in angular momentum than dark matter, while the opposite holds for the cold gas. After the mass of negative angular momentum is excluded, the angular momentum profile of the hot gas component approximately follows the universal function originally proposed by Bullock et al. for dark matter, though the shape parameter $\mu$ is much larger for hot gas and is comfortably in the range required by observations of disk galaxies. Since disk formation is related to the distribution of hot gas that will cool, our study may explain the fact that the disk component of observed galaxies contains a smaller fraction of low angular momentum material than dark matter in halos.Comment: 30 pages, 12 figures, 4 tables, accepted for publication in Ap

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

Macroscopic Discontinuous Shear Thickening vs Local Shear Jamming in Cornstarch

We study the emergence of discontinuous shear-thickening (DST) in cornstarch, by combining macroscopic rheometry with local Magnetic Resonance Imaging (MRI) measurements. We bring evidence that macroscopic DST is observed only when the flow separates into a low-density flowing and a high-density jammed region. In the shear-thickened steady state, the local rheology in the flowing region, is not DST but, strikingly, is often shear-thinning. Our data thus show that the stress jump measured during DST, in cornstach, does not capture a secondary, high-viscosity branch of the local steady rheology, but results from the existence of a shear jamming limit at volume fractions quite significantly below random close packing.Comment: To be published in PR

Some Global Characteristics of the Galactic Globular Cluster System

The relations between the luminosities $M_{V}$, the metallicities $[Fe/H]$, the Galactocentric radii $R$, and the central concentration indices $c$ of Galactic globular clusters are discussed. It is found that the most luminous clusters rarely have collapsed cores. The reason for this might be that the core collapse time scales for such populous clusters are greater than the age of the Galaxy. Among those clusters, for which the structure has not been modified by core collapse, there is a correlation between central concentration and integrated luminosity, in the sense that the most luminous clusters have the strongest central concentration. The outermost region of the Galaxy with $R>10$ kpc was apparently not able to form metal-rich $([Fe/H]>-1.0)$ globular clusters, whereas such clusters (of which Ter 7 is the prototype) were able to form in some nearby dwarf spheroidal galaxies. It is not yet clear how the popular hypothesis that globular clusters were initially formed with a single power law mass spectrum can be reconciled with the observation that both (1) Galactic globular clusters with $R>80$ kpc, and (2) the globulars associated with the Sagittarius dwarf, appear to have bi-modal luminosity functions.Comment: 15 pages, 1 figur

Yield stress and shear-banding in granular suspensions

We study the emergence of a yield stress in dense suspensions of non-Brownian particles, by combining local velocity and concentration measurements using Magnetic Resonance Imaging with macroscopic rheometric experiments. We show that the competition between gravity and viscous stresses is at the origin of the development of a yield stress in these systems at relatively low volume fractions. Moreover, it is accompanied by a shear banding phenomenon that is the signature of this competition. However, if the system is carefully density matched, no yield stress is encountered until a volume fraction of 62.7 0.3%

A Simple Model for the Absorption of Starlight by Dust in Galaxies

We present a new model to compute the effects of dust on the integrated spectral properties of galaxies, based on an idealized prescription of the main features of the interstellar medium (ISM). The model includes the ionization of HII regions in the interiors of the dense clouds in which stars form and the influence of the finite lifetime of these clouds on the absorption of radiation. We compute the production of emission lines and the absorption of continuum radiation in the HII regions and the subsequent transfer of line and continuum radiation in the surrounding HI regions and the ambient ISM. This enables us to interpret simultaneously all the observations of a homogeneous sample of nearby UV-selected starburst galaxies, including the ratio of far-IR to UV luminosities, the ratio of Halpha to Hbeta luminosities, the Halpha equivalent width, and the UV spectral slope. We show that the finite lifetime of stellar birth clouds is a key ingredient to resolve an apparent discrepancy between the attenuation of line and continuum photons in starburst galaxies. In addition, we find that an effective absorption curve proportional to lambda^-0.7 reproduces the observed relation between the ratio of far-IR to UV luminosities and the UV spectral slope. We interpret this relation most simply as a sequence in the overall dust content of the galaxies. The shallow wavelength dependence of the effective absorption curve is compatible with the steepness of known extinction curves if the dust has a patchy distribution. In particular, we find that a random distribution of discrete clouds with optical depths similar to those in the Milky Way provides a consistent interpretation of all the observations. Our model for absorption can be incorporated easily into any population synthesis model. (abridged)Comment: To appear in the 2000 July 20 issue of the Astrophysical Journal; 19 pages with 13 embedded PS figures (emulateapj5.sty