HI Imaging of LGS 3 and an Apparently Interacting High-Velocity Cloud

We present a 93' by 93' map of the area near the Local Group dwarf galaxy LGS 3, centered on an HI cloud 30' away from the galaxy. Previous authors associated this cloud with LGS 3 but relied on observations made with a 36' beam. Our high-resolution (3.4'), wide-field Arecibo observations of the region reveal that the HI cloud is distinct from the galaxy and suggest an interaction between the two. We point out faint emission features in the map that may be gas that has been tidally removed from the HI cloud by LGS 3. We also derive the rotation curve of the cloud and find that it is in solid-body rotation out to a radius of 10', beyond which the rotation velocity begins to decline. Assuming a spherical geometry for the cloud, the implied mass is 2.8 x 10^7 (d/Mpc) M_{Sun}, where d is the distance in Mpc. The observed HI mass is 5.5 x 10^6 (d/Mpc)^2 M_{Sun}, implying that the cloud is dark-matter dominated unless its distance is at least 1.9 Mpc. We propose that the cloud is a high-velocity cloud that is undergoing a tidal interaction with LGS 3 and therefore is located roughly 700 kpc away from the Milky Way. The cloud then contains a total mass of ~2.0 x 10^7 M_{Sun}, 82% of which consists of dark matter.Comment: 5 pages, 2 color figures. Accepted for publication in ApJ Letter

OH yields from the CH3CO+O-2 reaction using an internal standard

Laser flash photolysis of CH3C(O)OH at 248 nm was used to create equal zero time yields of CH3CO and OH. The absolute OH yield from the CH3CO + O2 (+M) reaction was determined by following the OH temporal profile using the zero time OH concentration as an internal standard. The OH yield from CH3CO + O2 (+M) was observed to decrease with increasing pressure with an extrapolated zero pressure yield close to unity (1.1 ± 0.2, quoted uncertainties correspond to 95% confidence limits). The results are in quantitative agreement with those obtained from 248 nm acetone photolysis in the presence of O2

Tightly Correlated HI and FUV Emission in the Outskirts of M83

We compare sensitive HI data from The HI Nearby Galaxy Survey (THINGS) and deep far UV (FUV) data from GALEX in the outer disk of M83. The FUV and HI maps show a stunning spatial correlation out to almost 4 optical radii (r25), roughly the extent of our maps. This underscores that HI traces the gas reservoir for outer disk star formation and it implies that massive (at least low level) star formation proceeds almost everywhere HI is observed. Whereas the average FUV intensity decreases steadily with increasing radius before leveling off at ~1.7 r25, the decline in HI surface density is more subtle. Low HI columns (<2 M_solar/pc^2) contribute most of the mass in the outer disk, which is not the case within r25. The time for star formation to consume the available HI, inferred from the ratio of HI to FUV intensity, rises with increasing radius before leveling off at ~100 Gyr, i.e., many Hubble times, near ~1.7 r25. Assuming the relatively short H2 depletion times observed in the inner parts of galaxies hold in outer disks, the conversion of HI into bound, molecular clouds seems to limit star formation in outer galaxy disks. The long consumption times suggest that most of the extended HI observed in M83 will not be consumed by in situ star formation. However, even these low star formation rates are enough to expect moderate chemical enrichment in a closed outer disk.Comment: Accepted for Publication in ApJ

Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

The kinetics of reactions of the OH radical with ethene, ethyne (acetylene), propyne (methyl acetylene) and t-butyl-hydroperoxide were studied at temperatures of 69 and 86 K using laser flash-photolysis combined with laser-induced fluorescence spectroscopy. A new pulsed Laval nozzle apparatus is used to provide the low-temperature thermalised environment at a single density of similar to 4 x 10(16) molecule cm(-3) in N-2. The density and temperature within the flow are determined using measurements of impact pressure and rotational populations from laser-induced fluorescence spectroscopy of NO and OH. For ethene, rate coefficients were determined to be k(2) = (3.22 +/- 0.46) x 10(-11) and (2.12 +/- 0.12) x 10(-11) cm(3) molecule(-1) s(-1) at T = 69 and 86 K, respectively, in good agreement with a master-equation calculation utilising an ab initio surface recently calculated for this reaction by Cleary et al. (P. A. Cleary, M. T. Baeza Romero, M. A. Blitz, D. E. Heard, M. J. Pilling, P. W. Seakins and L. Wang, Phys. Chem. Chem. Phys., 2006, 8, 5633-5642) For ethyne, no previous data exist below 210 K and a single measurement at 69 K was only able to provide an approximate upper limit for the rate coefficient of k(3) < 1 x 10(-12) cm(3) molecule(-1) s (-1), consistent with the presence of a small activation barrier of similar to 5 kJ mol (-1) between the reagents and the OH-C2H2 adduct. For propyne, there are no previous measurements below 253 K, and rate coefficients of k(4) = (5.08 +/- 0.65), (5.02 +/- 1.11) and (3.11 +/- 0.09) x 10(-12) cm(3) molecule(-1) s(-1) were obtained at T = 69, 86 and 299 K, indicating a much weaker temperature dependence than for ethene. The rate coefficient k(1) = (7.8 +/- 2.5) x 10(-11) cm(3) molecule(-1) s (-1) was obtained for the reaction of OH with t-butyl-hydroperoxide at T = 86 K. Studies of the reaction of OH with benzene and toluene yielded complex kinetic profiles of OH which did not allow the extraction of rate coefficients. Uncertainties are quoted at the 95% confidence limit and include systematic errors

The Distances of SNR W41 and overlapping HII regions

New HI images from the VLA Galactic Plane Survey show prominent absorption features associated with the supernovae remnant G23.3-0.3 (SNR W41). We highlight the HI absorption spectra and the $^{13}$CO emission spectra of eight small regions on the face of W41, including four HII regions, three non-thermal emission regions and one unclassified region. The maximum velocity of absorption for W41 is 78$\pm$2 km/s and the CO cloud at radial velocity 95$\pm$5 km/s is behind W41. Because an extended TeV source, a diffuse X-ray enhancement and a large molecular cloud at radial velocity 77$\pm$5 km/s are also projected at the center of W41, these yield the kinematic distance of 3.9 to 4.5 kpc for W41. For HII regions, our analyses reveal that both G23.42-0.21 and G23.07+0.25 are at the far kinematic distances ($\sim$9.9 kpc and $\sim$ 10.6 kpc respectively) of their recombination-line velocities (103$\pm$0.5 km/s and 89.6$\pm$2.1 km/s respectively), G23.07-0.37 is at the near kinematic distance (4.4$\pm$0.3 kpc) of its recombination-line velocity (82.7$\pm$2.0 km/s), and G23.27-0.27 is probably at the near kinematic distance (4.1$\pm$0.3 kpc) of its recombination-line velocity (76.1$\pm$0.6 km/s).Comment: 11 pages, 3 figs., 2 tables, accepted by A

Gas Rich Dwarf Spheroidals

We present evidence that nearly half of the dwarf spheroidal galaxies (dSph and dSph/dIrr) in the Local Group are associated with large reservoirs of atomic gas, in some cases larger than the stellar mass. The gas is sometimes found at large distance (~10 kpc) from the center of a galaxy and is not necessarily centered on it. Similarly large quantities of ionized gas could be hidden in these systems as well. The properties of some of the gas reservoirs are similar to the median properties of the High-Velocity Clouds (HVCs); two of the HI reservoirs are catalogued HVCs. The association of the HI with the dwarf spheroidals might thus provide a link between the HVCs and stars. We show that the HI content of the Local Group dSphs and dIrrs exhibits a sharp decline if the galaxy is within 250 kpc of either the Milky Way or M31. This can be explained if both galaxies have a sufficiently massive x-ray emitting halo that produces ram-pressure stripping if a dwarf ventures too close to either giant spiral. We also investigate tidal stripping of the dwarf galaxies and find that although it may play a role, it cannot explain the apparent total absence of neutral gas in most dSph galaxies at distances less than 250 kpc. For the derived mean density of the hot gas, n_0 = 2.5e-5 cm^-2, ram-pressure stripping is found to be more than an order of magnitude more effective in removing the gas from the dSph galaxies. The hot halo, with an inferred mass of 1e10 solar masses, may represent a reservoir of ~1000 destroyed dwarf systems, either HVCs or true dwarf galaxies similar to those we observe now.Comment: AASTex preprint style, 27 pages including 12 figures. Submitted to ApJ. See also http://astro.berkeley.edu/~robisha

High Energy Gamma--Radiation from the Galactic Center due to Neutralino Annihilation

We study the NGS (Non--dissipative Gravitational Singularity) model, which successfully describes the non--linear stage of evolution of perturbations (see [1], [2] and references therein). This model predicts DM density distribution $\rho(r) \sim r^{-\alpha}$ with $\alpha \simeq 1.8$ which holds from very small distances $r_{\rm min} \simeq 0.01~{\rm pc}$ up to very large distances $r_{\rm max} \simeq 5~{\rm Mpc}$. Assuming the neutralino to be a CDM particle, we calculate the annihilation of neutralinos in the vicinity of the singularity (Galactic Center). If neutralinos are the dominant component of DM in our Galaxy, the produced energy is enough to provide the whole observed activity of the GC. Neutralinos of the most general composition and of mass in the range 20~{\rm GeV} \leq m_\c \leq 1~{\rm TeV} are considered. We find the neutralino compositions which give the relic density needed for the Mixed Dark Matter (MDM) model and we evaluate for these compositions the high--energy ($E_{\gamma} > 100 ~{\rm MeV}$) gamma--ray flux under the constraint that the radio flux is lower than the observational limit. The compositions with the detectable gamma--ray flux which we found are provided by a set of almost pure gaugino states with the neutralino mass between $100$ and $500$ GeV. We demonstrate that a detectable high--energy gamma--ray flux is produced by the neutralino annihilation also in the case when neutralinos provide a small fraction (down to $0.1 \%$) of the DM in our Galaxy. The predicted flux is $F_\gamma \sim 10^{-7}-10^{-8}~{\rm cm}^{-2}~{\rm s}^{-1}$ for E_\gamma \gsim 300~{\rm MeV}Comment: Plain TeX 11 pages 4 figures available on request. Preprint numbers LNGS 94/90 - DFTT 5/9

Super-star clusters versus OB associations

Super Star Clusters (Mecl > 10^5 Msol) are the largest stellar nurseries in our local Universe, containing hundreds of thousands to millions of young stars within a few light years. Many of these systems are found in external galaxies, especially in pairs of interacting galaxies, and in some dwarf galaxies, but relatively few in disk galaxies like our own Milky Way. We show that a possible explanation for this difference is the presence of shear in normal spiral galaxies which impedes the formation of the very large and dense super star clusters but prefers the formation of loose OB associations possibly with a less massive cluster at the center. In contrast, in interacting galaxies and in dwarf galaxies, regions can collapse without having a large-scale sense of rotation. This lack of rotational support allows the giant clouds of gas and stars to concentrate into a single, dense and gravitationally bound system.Comment: uses emulateapj-rtx4.cls, 7 pages, 6 figures, 2 tables, accepted for publication by Ap

High-resolution Velocity Fields of Low-mass Disk Galaxies. I. CO Observations

This paper is the first in a series whose aim is to examine the relative distributions of dark and baryonic matter as a function of star formation history in a representative sample of low-mass disk galaxies. In this paper, we present high-resolution 12 CO(j=1→0) interferometry for a sample of 26 nearby dwarf galaxies that were obtained from the Combined Array for Research in Millimeter-wave Astronomy (CARMA). Among these 26 galaxies, 14 have good CO detections, including 6 galaxies previously detected in single-dish CO measurements and 8 newly detected ones. We find a linear correlation between the CO flux and the mid- and far-IR flux from the WISE and IRAS catalogs. Compared to the far-IR flux, the mid-IR flux may be a better indication of whether a galaxy contains sufficient CO for detection at the level of instrument sensitivity of CARMA. This correlation might prove to be useful in future studies to help choosing other CO targets for observation. The median molecular mass (including helium) of our galaxies is 2.8×10 8 M⊙, which is consistent with past observations for dwarf galaxies. The molecular content is weakly correlated with the dynamical mass, r-band luminosity and size of the galaxies. The median ratios of molecular mass versus dynamical mass and molecular mass versus r-band luminosity are M mol M dyn ≈ 0.035 and M mol L r ≈ 0.078M⊙ L r , ⊙, respectively, which are also consistent with past observations for dwarf galaxies