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

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

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

Kinetics of CH₂OO reactions with SO₂, NO₂, NO, H₂O and CH₃CHO as a function of pressure

Kinetics of CH₂OO Criegee intermediate reactions with SO₂, NO₂, NO, H₂O and CH₃CHO and CH₂I radical reactions with NO₂ are reported as a function of pressure at 295 K. Measurements were made under pseudo-first-order conditions using flash photolysis of CH₂I₂–O₂–N₂ gas mixtures in the presence of excess co-reagent combined with monitoring of HCHO reaction products by laser-induced fluorescence (LIF) spectroscopy and, for the reaction with SO₂, direct detection of CH₂OO by photoionisation mass spectrometry (PIMS). Rate coefficients for CH₂OO + SO₂ and CH₂OO + NO₂ are independent of pressure in the ranges studied and are (3.42 ± 0.42) × 10‾¹¹ cm³ s‾¹ (measured between 1.5 and 450 Torr) and (1.5 ± 0.5) × 10‾¹² cm³ s‾¹ (measured between 25 and 300 Torr), respectively. The rate coefficient for CH₂OO + CH₃CHO is pressure dependent, with the yield of HCHO decreasing with increasing pressure. Upper limits of 2 × 10−13 cm³ s‾¹ and 9 × 10−17 cm³ s‾¹ are placed on the rate coefficients for CH₂OO + NO and CH₂OO + H₂O, respectively. The upper limit for the rate coefficient for CH₂OO + H₂O is significantly lower than has been reported previously, with consequences for modelling of atmospheric impacts of CH₂OO chemistry

Is There a Fundamental Line for Disk Galaxies?

We show that there are strong local correlations between metallicity, surface brightness, and dynamical mass-to-light ratio within M33, analogous to the fundamental line of dwarf galaxies identified by Prada & Burkert (2002). Using near-infrared imaging from 2MASS, the published rotation curve of M33, and literature measurements of the metallicities of HII regions and supergiant stars, we demonstrate that these correlations hold for points at radial distances between 140 pc and 6.2 kpc from the center of the galaxy. At a given metallicity or surface brightness, M33 has a mass-to-light ratio approximately four times as large as the Local Group dwarf galaxies; other than this constant offset, we see broad agreement between the M33 and dwarf galaxy data. We use analytical arguments to show that at least two of the three fundamental line correlations are basic properties of disk galaxies that can be derived from very general assumptions. We investigate the effect of supernova feedback on the fundamental line with numerical models and conclude that while feedback clearly controls the scatter in the fundamental line, it is not needed to create the fundamental line itself, in agreement with our analytical calculations. We also compare the M33 data with measurements of a simulated disk galaxy, finding that the simulation reproduces the trends in the data correctly and matches the fundamental line, although the metallicity of the simulated galaxy is too high, and the surface brightness is lower than that of M33.Comment: 14 pages, 14 figures (5 in color). Accepted for publication in Ap

An instrument to measure fast gas phase radical kinetics at hight temperatures and pressures

Fast radical reactions are central to the chemistry of planetary atmospheres and combustion systems. Laser-induced fluorescence is a highly sensitive and selective technique that can be used to monitor a number of radical species in kinetics experiments, but is typically limited to low pressure systems owing to quenching of fluorescent states at higher pressures. The design and characterisation of an instrument is reported using laser-induced fluorescence detection to monitor fast radical kinetics (up to 25,000 s-1) at high temperatures and pressures by sampling from a high pressure reaction region to a low pressure detection region. Kinetics have been characterised at temperatures reaching 740 K and pressures up to 2 atm, with expected maximum operational conditions of up to ~ 900 K and ~ 5 atm. The distance between the point of sampling from the high pressure region and the point of probing within the low pressure region is critical to the measurement of fast kinetics. The instrumentation described in this work can be applied to the measurement of kinetics relevant to atmospheric and combustion chemistry

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-Resolution Measurements of the Dark Matter Halo of NGC 2976: Evidence for a Shallow Density Profile

We have obtained two-dimensional velocity fields of the dwarf spiral galaxy NGC 2976 in Halpha and CO. The high spatial (~75 pc) and spectral (13 km/s and 2 km/s, respectively) resolution of these observations, along with our multicolor optical and near-infrared imaging, allow us to measure the shape of the density profile of the dark matter halo with good precision. We find that the total (baryonic plus dark matter) mass distribution of NGC 2976 follows a rho_tot ~ r^(-0.27 +/- 0.09) power law out to a radius of 1.8 kpc, assuming that the observed radial motions provide no support. The density profile attributed to the dark halo is even shallower, consistent with a nearly constant density of dark matter over the entire observed region. A maximal disk fit yields an upper limit to the K-band stellar mass-to-light ratio (M*/L_K) of 0.09^{+0.15}_{-0.08} M_sun/L_sun,K (including systematic uncertainties), with the caveat that for M*/L_K > 0.19 M_sun/L_sun,K the dark matter density increases with radius, which is unphysical. Assuming 0.10 M_sun/L_sun,K < M*/L_K < 0.19 M_sun/L_sun,K, the dark matter density profile lies between rho_dm ~ r^-0.17 and rho_dm ~ r^-0.01. Therefore, independent of any assumptions about the stellar disk or the functional form of the density profile, NGC 2976 does not contain a cuspy dark matter halo. We also investigate some of the systematic effects that can hamper rotation curve studies, and show that 1) longslit rotation curves are far more vulnerable to systematic errors than two-dimensional velocity fields, 2) NGC 2976 contains large radial motions at small radii, and 3) the Halpha and CO velocity fields of NGC 2976 agree within their uncertainties. [slightly abridged]Comment: 30 pages, 4 tables, 13 figures (7 in color; Figures 1 and 3 are low-resolution to save space). Accepted for publication in ApJ. Version with full-resolution figures available at http://astro.berkeley.edu/~bolatto/ngc2976rotation.ps (46 MB

Hyperstrong Radio-Wave Scattering in the Galactic Center. II. A Likelihood Analysis of Free Electrons in the Galactic Center

The scattering diameters of Sgr A* and several nearby OH masers (~ 1" at 1 GHz) indicate that a region of enhanced scattering is along the line of sight to the Galactic center. We combine radio-wave scattering data and free-free emission and absorption measurements in a likelihood analysis that constrains the following parameters of the GC scattering region: The GC-scattering region separation, d; the angular extent of the region, \psi_l; the outer scale on which density fluctuations occur, l_0; and the gas temperature, T. The maximum likelihood estimates of these parameters are d = 133_{-80}^{+200} pc, 0.5 degrees <= \psi_l <~ 1 degrees, and (l_0/1 pc)^{2/3}T^{-1/2} = 10^{-7 +/- 0.8}. As host media for the scattering, we consider the photoionized surface layers of molecular clouds and the interfaces between molecular clouds and the 10^7 K ambient gas. We are unable to make an unambiguous determination, but we favor an interface model in which the scattering medium is hot (T ~ 10^6 K) and dense (n_e ~ 10 cm^{-3}). The GC scattering region produces a 1 GHz scattering diameter for an extragalactic source of 90", if the region is a single screen, or 180", if the region wraps around the GC, as appears probable. We modify the Taylor-Cordes model for the Galactic distribution of free electrons in order to include an explicit GC component. Pulsars seen through this region will have a dispersion measure of approximately 2000 pc cm^{-3}, of which 75% arises from the GC component. We stress the uniqueness of the GC scattering region, probably resulting from the high-pressure environment in the GC.Comment: 39 pages with 9 PostScript figures; LaTeX2e with AASTeX macro aaspp4, to be published in Ap

Constraints on the galactic distribution of cosmic rays from the COS-B gamma-ray data

The diffuse component of the galactic high energy gamma rays results mainly from the interaction of CR nuclei and electrons with the nuclei of the interstellar gas. An additional contribution is obtained from the interaction of CR electrons with the interstellar photons through the inverse-Compton (IC) process. Gamma ray astronomy therefore offers an excellent means to study the distribution of CR particles throughout the Galaxy, but it is essential to know the distribution of the target interstellar gas particles, the major constituents being atomic and molecular hydrogen. Large scale millimeter wave surveys of the CO molecule covering more than half of the Milky Way, obtained with the Columbia 1.2 m telescopes, are currently available and are used to trace the H2; the COS-B observations have sufficient resolution and sensitivity to constrain the relation between the integrated CO line intensity and the molecular hydrogen column density