arXiv subjects

Leo Blitz

Publications and source records attributed to Leo Blitz.

At least 109 records · Page 6Linked to original sources

arXiv2004

The Initial Mass Function of GMCs

The properties of GMCs in several Local Group galaxies are quantified and compared. It is found that the mass spectrum of GMCs varies from galaxy to galaxy. The variations are significant and do not appear to be the result of systematic uncertainties. Nevertheless, it appears that all of the GMCs follow the same size-linewidth and mass-linewidth relations with little scatter. The power law indices of these relations imply that the GMCs are self-gravitating, and that the mean surface density of Local Group GMCs is approximately constant. This, in turn, implies that the mean internal pressure of GMCs is also constant. If the IMF of stars is determined by a Jeans instability, this constant internal pressure suggests that the distribution of stellar masses does not vary significantly in galactic disks when averaged over suitably large areas. Thus, although the distribution of GMC masses produced by various Local Group galaxies is quite variable, the large-scale properties of the GMCs is not.

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arXiv2004

GMC Formation in M33: the Role of Pressure

We present a complete map of the Giant Molecular Clouds in M33 and show that they lie exclusively on filaments of HI. The GMCs are localized on the extended HI filaments, and HI filaments extend well beyond the radius where the GMCs are found, suggesting that the H_2 forms from the HI and not vice-versa. We propose that the fraction of neutral gas that is molecular at a particular radius is determined primarily by the ambient hydrostatic pressure at that radius. This leads to a prediction that the transition radius, the radius where the mean surface density of HI and H_2 are equal, occurs at a constant value of the stellar surface density. We show that for a sample of 26 galaxies, the stellar surface density at the transition radius is constant to 40%.

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arXiv2004

The Role of Pressure in GMC Formation

We examine the hypothesis that hydrostatic pressure alone determines the ratio of atomic to molecular gas averaged over a particular radius in disk galaxies. The hypothesis implies that the transition radius, the location where the ratio is unity, should always occur at the same value of stellar surface density in all galaxies. We examine data for 28 galaxies and find that the stellar surface density at the transition radius is indeed constant to 40% at a value of 120 M_sun/pc^2. If the hypothesis can be confirmed at all radii within a large range of galaxy types and metallicities, combining it with the observed constancy of the star formation rate with H_2 surface density may enable a physically motivated star formation prescription with wide applicability.

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arXiv2004

A Search for Kinematic Evidence of Radial Gas Flows in Spiral Galaxies

CO and HI velocity fields of seven nearby spiral galaxies, derived from radio-interferometric observations, are decomposed into Fourier components whose radial variation is used to search for evidence of radial gas flows. Additional information provided by optical or near-infrared isophotes is also considered, including the relationship between the morphological and kinematic position angles. To assist in interpreting the data, we present detailed modeling that demonstrates the effects of bar streaming, inflow, and a warp on the observed Fourier components. We find in all of the galaxies evidence for either elliptical streaming or a warped disk over some range in radius, with deviations from pure circular rotation at the level of ~20-60 km/s. Evidence for kinematic warps is observed in several cases well inside R_{25}. No unambiguous evidence for radial inflows is seen in any of the seven galaxies, and we are able to place an upper limit of ~5-10 km/s (3-5% of the circular speed) on the magnitude of any radial inflow in the inner regions of NGC 4414, 5033 and 5055. We conclude that the inherent non-axisymmetry of spiral galaxies is the greatest limitation to the direct detection of radial inflows.

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arXiv2003

Dark Matter in Dwarf Galaxies: High Resolution Observations

We present observations and analysis of rotation curves and dark matter halo density profiles in the central regions of four nearby dwarf galaxies. This observing program has been designed to overcome some of the limitations of other rotation curve studies that rely mostly on longslit spectra. We find that these objects exhibit the full range of central density profiles between constant density and NFW halos. This result suggests that there is a distribution of central density slopes rather than a unique halo density profile.

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arXiv2003

Molecular Gas in Nearby Dwarf Galaxies: Single Dish and Interferometric Results

We present results from an extensive survey of molecular gas in nearby dwarf galaxies. This survey consists of a single-dish search for CO emission at the UASO Kitt Peak 12m telescope and interferometric follow-up using the Berkeley-Illinois-Maryland Array. We have detected 41 new sources, approximately doubling the number of detections of such objects in the literature. In this paper we discuss two results from the survey that lead us to agree with recent suggestions that, above a metallicity of 12+log[O/H]~8.0, there appears to be little variation in the CO-to-H2 conversion factor for star-forming molecular gas. We find two major pieces of evidence for this result: 1) the virial mass to molecular mass ratio found from the interferometer observations in a variety of sources shows no clear trend with metallicity; and 2) dwarf galaxies appear to exhibit a CO-to-radio continuum relationship that is similar to that observed in larger, presumably metal-rich, galaxies leaving little room for a large systematic variation of the CO-to-H2 conversion factor with metallicity in the star-forming gas.

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arXiv2003

Giant Molecular Clouds in M33 -- II. High Resolution Observations

We present 12CO(1->0) observations of 45 giant molecular clouds in M33 made with the BIMA array. The observations have a linear resolution of 20 pc, sufficient to measure the sizes of most GMCs in the sample. We place upper limits on the specific angular momentum of the GMCs and find the observed values to be nearly an order of magnitude below the values predicted from simple formation mechanisms. The velocity gradients across neighboring, high-mass GMCs appear preferentially aligned on scales less than 500 pc. If the clouds are rotating, 40% are counter-rotating with respect to the galaxy. GMCs require a braking mechanism if they form from the large scale radial accumulation of gas. These observations suggest that molecular clouds form locally out of atomic gas with significant braking by magnetic fields to dissipate the angular momentum imparted by galactic shear. The observed GMCs share basic properties with those found in the Galaxy such as similar masses, sizes, and linewidths as well as a constant surface density of 120 M_sun pc^{-2}. The size-linewidth relationship follows Delta V ~ r^{0.45\pm 0.02}, consistent with that found in the Galaxy. The cloud virial masses imply that the CO-to-H_2 conversion factor has a value of 2 X 10^20 H2 cm^-2/(K km/s) and does not change significantly over the disk of M33 despite a change of 0.8 dex in the metallicity.

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arXiv2003

Dark Matter in Dwarf Galaxies: The First Dark Galaxy?

We present new HI observations of the high-velocity cloud (HVC) that we resolved near the Local Group dwarf galaxy LGS 3. The cloud is rotating, with an implied mass that makes it dark matter-dominated no matter what its distance from the Milky Way is. Our new, high-sensitivity Arecibo observations demonstrate that the faint HI features that we previously described as tidal tails are indeed real and do connect to the main body of the HVC. Thus, these observations are consistent with our original hypothesis of a tidal interaction between the HVC and LGS 3. We suggest that the HVC may be one of the missing dark matter satellites in the Local Group that are seen in Cold Dark Matter numerical simulations but have not yet been identified observationally.

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arXiv2003

Dark Matter in Dwarf Galaxies: Latest Density Profile Results

We present high-resolution two-dimensional velocity fields in Halpha and CO of the nearby dwarf galaxy NGC 2976. Our observations were made at both higher spatial resolution (~75 pc) and higher velocity resolution (13 km/s in Halpha and 2 km/s in CO) than most previous studies. We show that NGC 2976 has a very shallow dark matter density profile, with rho(r) lying between rho ~ r^-0.3 and rho ~ r^0. We carefully test the effects of systematic uncertainties on our results, and demonstrate that well-resolved, two-dimensional velocity data can eliminate many of the systematic problems that beset longslit observations. We also present a preliminary analysis of the velocity field of NGC 5963, which appears to have a nearly NFW density profile.

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arXiv2003

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]

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arXiv2003

The BIMA Survey of Nearby Galaxies (BIMA SONG). II. The CO Data

The BIMA Survey of Nearby Galaxies is a systematic imaging study of the 3 mm CO J = 1--0 molecular emission within the centers and disks of 44 nearby spiral galaxies. The typical spatial resolution of the survey is 6", or 360 pc at the average distance (12 Mpc) of the sample, over a field of view of 10kpc. The velocity resolution of the CO observations is 4 km/s. The sample was not chosen based on CO or infrared brightness; instead, all spirals were included that met the selection criteria of vsun <= 2000 km/s, dec >= -20deg, inc <= 70deg, D25 < 70', and BT < 11.0. The detection rate was 41/44 sources or 93%. Fully-sampled single-dish CO data were incorporated into the maps for 24 galaxies; these single-dish data comprise the most extensive collection of fully-sampled, two-dimensional single-dish CO maps of external galaxies to date. We also tabulate direct measurements of the global CO flux densities for these 24 sources. We demonstrate that the measured ratios of flux density recovered are a function of the signal-to-noise of the interferometric data. We examine the degree of central peakedness of the molecular surface density distributions and show that the distributions exhibit their brightest CO emission within the central 6" in only 20/44 or 45% of the sample. We show that all three Local Group spiral galaxies have CO morphologies that are represented in SONG, though the Milky Way CO luminosity is somewhat below the SONG average, and M31 and M33 are well below average. This survey provides a unique public database of integrated intensity maps, channel maps, spectra, and velocity fields of molecular emission in nearby galaxies.

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arXiv2002

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.

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arXiv2002

The Absence of Stars in Compact High-Velocity Clouds

We present the results of our search for faint Local Group dwarf galaxies in compact high-velocity clouds (HVCs). We used digitized Palomar Observatory Sky Survey (POSS) data to examine 1 square degree of sky around each of ~250 northern hemisphere HVCs. The POSS images were processed to remove foreground stars and large-scale backgrounds, smoothed to enhance arcminute-sized low surface brightness features, and then compared to the original plates. Using this technique, we located 60 candidate dwarf galaxies in the ~250 square degrees that we surveyed. Followup observations of these candidates have revealed several distant clusters of galaxies and a number of possible Galactic cirrus clouds, but no Local Group dwarfs. It appears that many of the low surface brightness features in the sky survey data are plate flaws. The second-generation red POSS plates are sensitive down to surface brightness levels of 25-26 magnitudes per square arcsecond, and our followup images reached 10-sigma limiting magnitudes of R = 21-23 for point sources. Given these limits, all known Local Group galaxies except four of the very diffuse, extended dwarf spheroidals located within 100 kpc of the Milky Way would have been detected had they been in our survey. Therefore, we can rule out the possibility that these HVCs are associated with normal but faint dwarf galaxies. If compact HVCs do contain stars, they must have surface brightnesses > 1 magnitude per square arcsecond fainter than most known Local Group galaxies.

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arXiv2001

The Relationship Between Gas Content and Star Formation in Molecule-Rich Spiral Galaxies

We investigate the relationship between HI, H_2, and the star formation rate (SFR) using azimuthally averaged data for seven CO-bright spiral galaxies. Contrary to some earlier studies based on global fluxes, we find that the SFR surface density exhibits a much stronger correlation with the H_2 than with the HI, as the HI surface density saturates at a value of $\sim$10 Msol pc^{-2} or even declines at large SFR surface densities. Hence the good correlation between the SFR surface density and the total (HI+H_2) gas surface density is driven by the molecular component in these galaxies. We find no clear evidence for a link between the gravitational instability parameter for the gas disk (Q_g) and the SFR, and suggest that Q_g be considered a measure of the gas fraction. This implies that for a state of marginal gravitational stability to exist in galaxies with low gas fractions, it must be due to instability of the stellar disk. In regions where we have both HI and CO measurements, the ratio of HI to H_2 surface density scales with radius R as roughly R^{1.5}, and we suggest that the balance between HI and H_2 is determined primarily by the midplane interstellar pressure. These results favor a "law" of star formation in quiescent disks in which the ambient pressure and metallicity control the formation of molecular clouds from HI, with star formation then occurring at a roughly constant rate per unit H_2 mass. (abstract abridged)

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arXiv2001

The Density Profile of the Dark Matter Halo of NGC 4605

We have obtained ~100 pc resolution CO and ~60 pc resolution Halpha observations of the dwarf spiral galaxy NGC 4605. We use them to derive a high resolution rotation curve and study the central density profile of NGC 4605's dark matter halo. We find that these observations do not agree with the predictions of most high resolution Cold Dark Matter calculations. We investigate two extreme cases: 1) NGC 4605 has a maximal exponential disk, that we model using K-band observations and remove to study the structure of its dark matter halo, and 2) NGC 4605 is dark matter dominated and its disk is dynamically negligible. Because the mass-to-light ratio of the maximal disk is already very low we favor the first solution, which indicates the halo has one component with a density profile rho ~ r^{-0.6} out to R~2.8 kpc. In the second case, the rotation curve requires the presence of two components: a small (~600 pc) core surrounded by a much steeper rho ~ r^{-1.1} halo. Removal of intermediate (submaximal) disks does not ameliorate the discrepancy between the predictions and the observations.

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arXiv2001

The BIMA Survey of Nearby Galaxies (BIMA SONG): I. The Radial Distribution of CO Emission in Spiral Galaxies

We present the first results of the BIMA Survey of Nearby Galaxies (BIMA SONG), an imaging survey of the CO J=(1-0) emission in 44 nearby spiral galaxies at a typical resolution of 6". BIMA SONG differs from previous high-resolution CO surveys in that: (1) CO brightness was not an explicit selection criterion; (2) a larger area (200" diameter for most galaxies) of each galaxy was imaged; and (3) fully-sampled single-dish CO data (55" resolution) were obtained for over half of the sample galaxies, so all of the CO flux is imaged in these galaxies. The radial distribution of stellar light in galactic disks is generally characterized as an exponential. It is, therefore, of interest to investigate whether the molecular gas, which is the star-forming medium, has a similar distribution. Though our low-resolution single-dish radial profiles of CO emission can be described by simple exponentials, this is not true for the emission at our full 6" resolution. The scale lengths of the CO disks are correlated with the scale lengths of the stellar disks with a mean ratio of the scale lengths of about one. We also find that:(1) there is also a weak correlation between the ratio of K-band to CO luminosity and Hubble type; (2) in half of the galaxies presented here, CO emission does not peak at the location of the stellar nucleus; (3) averaged over the inner kiloparsec, the CO emission in one-half of the galaxies exhibits an excess over that expected from an exponential disk which is similar to the excess in stellar light caused by the bulge stars; and (4) this excess CO emission may be due to an increase in the total molecular gas content in the bulge region, or alternatively, to an increase in the CO emissivity caused by the increased pressure of the bulge region.

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arXiv2001

Local Goup HVCs: Status of the Evidence

The evidence for locating the High Velocity Clouds in the Local Group is summarized and evaluated. Recent measurements of the H$α$ surface brightness and metallicity of a number of HVCs appear to be fatal to the Galactic fountain as a significant contributor to the HVC phenomenon, but not to the existence of the fountain itself. Observations of extragalactic analogues to HVCs remain the {\it sine qua non} for deciding whether the Local Group hypothesis is viable, but the constraints based on existing surveys appear to be rather weak. MgII quasar absorption lines restrict how many HVC analogues exist at intermediate redshift, depending on where these lines originate. It is concluded that the evidence remains ambiguous, none of the main hypotheses is fully consistent with all of the data, and the Local Group hypothesis remains a viable explanation for the HVC phenomenon.

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arXiv2000

Molecular gas and star formation in BIMA SONG bars

Using a sample of 7 barred spirals from the BIMA Survey of Nearby Galaxies (SONG), we compare the molecular gas distribution in the bar, to recent massive star formation activity. In all 7 galaxies, Halpha is offset azimuthally from the CO on the downstream side. The maximum offset, at the bar ends, ranges from 170-570 pc, with an average of 320 +/- 120 pc. We discuss whether the observed offsets are consistent with the description of gas flows in bars provided by the two main classes of models: n-body models and hydrodynamic models. This work is supported by NSF grants AST 9981289, AST 9981308.

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