arXiv subjects

Tony Wong

Publications and source records attributed to Tony Wong.

98 records · Page 6Linked to original sources

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

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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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 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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arXiv2000

Observations of a Series of Six Recent Glitches in the Crab Pulsar

From 1995 to 1999, daily monitoring of the radio emission from the Crab pulsar at the Green Bank and Jodrell Bank observatories revealed a series of six sudden rotational spinups or glitches, doubling the number of glitches observed for this pulsar since 1969. With these observations, the range of time intervals between significant Crab glitches has widened considerably, indicating that the occurrence of Crab glitches may be more random than previously thought. The new glitch amplitudes ($Δν/ν$) span an order of magnitude from 2e-9 to 3e-8. Except in one case, which we suggest may represent an ``aftershock'' event, the frequency jumps display an exponential recovery with a timescale of $\sim$3 days for the smaller glitches and $\sim$10 days for the largest (1996) glitch. In the largest event, a portion of the spinup was resolved in time, as was previously reported for the 1989 glitch. A pronounced change in frequency derivative also occurs after each glitch and is correlated with the size of the initial frequency jump, although for some of the smaller glitches this appears to be a temporary effect. We discuss the properties of the ensemble of observed Crab glitches and compare them with the properties of Vela glitches, highlighting those differences which must be explained by evolutionary models.

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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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arXiv2000

Non-circular Gas Kinematics and Star Formation in the Ringed Galaxy NGC 4736

We analyze the gas kinematics and star formation properties of the nearby RSab galaxy NGC 4736 using interferometric and single-dish CO(1-0) data and previously published Halpha and HI data. The CO morphology is dominated by a central molecular bar and tightly wound spiral arms associated with a bright ring of star formation. Strong HI emission is also found in the ring, but HI is absent from the central regions. Comparison of the HI and Halpha distributions suggests that HI in the ring is primarily dissociated H$_2$. Modeling of the CO kinematics reveals gas motion in elliptical orbits around the central bar, and we argue that the ring represents both the OLR of the bar and the ILR of a larger oval distortion. The HI kinematics show evidence for axisymmetric inflow towards the ring and are inconsistent with streaming in aligned elliptical orbits, but the highly supersonic (~40 km/s) inflow velocities required, corresponding to mass inflow rates of ~2 Msol/yr, suggest that more sophisticated models (e.g., gas orbiting in precessed elliptical orbits) should be considered. The radial CO and Halpha profiles are poorly correlated in the vicinity of the nuclear bar, but show a better correlation (in rough agreement with the Schmidt law) at the ring. Even along the ring, however, the azimuthal correspondence between CO and Halpha is poor, suggesting that massive stars form more efficiently at some (perhaps resonant) locations than at others. These results indicate that the star formation rate per unit gas mass exhibits strong spatial variations and is not solely a function of the available gas supply. The localization of star formation to the ring is broadly consistent with gravitational instability theory, although the instability parameter $Q \sim 3$ on average in the ring, only falling below 1 in localized regions.

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arXiv1998

Recent Star Formation in Sextans A

We investigate the relationship between the spatial distributions of stellar populations and of neutral and ionized gas in the Local Group dwarf irregular galaxy Sextans A. This galaxy is currently experiencing a burst of localized star formation, the trigger of which is unknown. We have resolved various populations of stars via deep UBV(RI)_C imaging over an area with diameter \sim 5.'3. We have compared our photometry with theoretical isochrones appropriate for Sextans A, in order to determine the ages of these populations. We have mapped out the history of star formation, most accurately for times \lesssim 100 Myr. We find that star formation in Sextans A is correlated both in time and space, especially for the most recent (\lesssim 12 Myr) times. The youngest stars in the galaxy are forming primarily along the inner edge of the large H I shell. Somewhat older populations, \lesssim 50 Myr, are found inward of the youngest stars. Progressively older star formation, from \sim 50--100 Myr, appears to have some spatially coherent structure and is more centrally concentrated. The oldest stars we can accurately sample appear to have approximately a uniform spatial distribution, which extends beyond a surface brightness of μ_B \simeq 25.9 mag arcsec^{-2} (or, a radius r \simeq 2.'3$). Although other processes are also possible, our data provides support for a mechanism of supernova-driven expansion of the neutral gas, resulting in cold gas pileup and compression along the H I shell and sequential star formation in recent times.

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