arXiv subjects
Publications and source records attributed to J. M. Stil.
This paper is the first in a series presenting a sample of 29 late-type dwarf galaxies, observed with the Westerbork Synthesis Radio Telescope (WSRT) in the 21-cm line of neutral atomic hydrogen (HI). The sample itself, the HI content of and the HI distribution in the sample galaxies are briefly discussed. Four sample galaxies were also detected in the continuum.
This paper is the second in a series presenting a sample of 29 late-type dwarf galaxies observed with the Westerbork Synthesis Radio Telescope in the 21-cm line of neutral hydrogen (HI). Here we present rotation curves, maps of the velocity field and maps of the velocity dispersion across the sample galaxies.
We present new data of the dwarf galaxy NGC 1569 at 450 $μ$m, 850 $μ$m and 1200$μ$m taken with SCUBA at the JCMT and the bolometer array at the IRAM 30m telescope. After including data from IRAS at 12, 25, 60 and 100 $μ$m, we have successfully fitted the dust grain population model of Désert et al. (1990) to the observed midinfrared-to-millimeter spectrum. The fit requires a combination of both large and very small grains exposed to a strong radiation field as well as an enhancement of the number of very small grains relative to the number of large grains. We interpret this as the consequence of large grain destruction due to shocks in the turbulent interstellar medium of NGC 1569. The contribution of polyaromatic hydrocarbons (PAH's) is found to be negligible. Comparison of the dust emission maps with an HI map of similar resolution shows that both dust and molecular gas distributions peak close to the radio continuum maximum and at a minimum in the HI distribution. From a comparison of these three maps and assuming that the gas-to-dust mass ratio is the same everywhere, we estimate the ratio of molecular hydrogen column density to integrated CO intensity to be about 25 -- 30 times the local Galactic value. The gas-to-dust ratio is 1500 -- 2900, about an order of magnitude higher than in the Solar Neighbourhood.
The properties of the Galactic HI shell GSH 138-01-94 are derived from data of the Canadian Galactic Plane Survey. The basic parameters of GSH 138-01-94 were determined by fitting the expansion of a thin shell to the expansion velocity field on the sky. The kinematic distance is 16.6 kpc for v_LSR=-94.2 $\pm$ 0.5 km/s. The radius is 180 $\pm$ 10 pc, the expansion velocity v_exp = 11.8 $\pm$ 0.9 km/s, and the mass 2 * 10^5 M_sun. No radio continuum counterpart of the shell was detected at 21 cm or at 74 cm. Absorption of a background continuum source constrains the spin temperature of \HI in the shell to T_s=230^{367}_{173} K. The expansion age of GSH 138-01-94 is 4.3 Myr. These observables are in excellent agreement with predictions from hydrodynamic models for a supernova remnant in a low-density low-metallicity environment such as the outer Galaxy. GSH 138-01-94 is then the largest and the oldest supernova remnant known. It provides direct evidence for the release of mechanical energy in the interstellar medium by stars in the outer galaxy. It is argued that such old supernova remnants be found in low-density, low-metallicity environments such as the outer Galaxy, dwarf galaxies and low surface brightness galaxies.
Deep SCUBA observations of NGC 1275 at 450 micron and 850 micron along with the application of deconvolution algorithms have permitted us to separate the strong core emission in this galaxy from the fainter extended emission around it. The core has a steep spectral index and is likely due primarily to the AGN. The faint emission has a positive spectral index and is clearly due to extended dust in a patchy distribution out to a radius of $\sim$ 20 kpc from the nucleus. These observations have now revealed that a large quantity of dust, $\sim$ 6 $\times$ 10$^7$ $M_\odot$, 2 orders of magnitude larger than that inferred from previous optical absorption measurements, exists in this galaxy. We estimate the temperature of this dust to be $\sim$ 20 K (using an emissivity index of $β$ = 1.3) and the gas/dust ratio to be 360. These values are typical of spiral galaxies. The dust emission correlates spatially with the hot X-ray emitting gas which may be due to collisional heating of broadly distributed dust by electrons. Since the destruction timescale is short, the dust cannot be replenished by stellar mass loss and must be externally supplied, either via the infalling galaxy or the cooling flow itself.
We have studied the relation between HI linewidth and optical luminosity in two samples of dwarf galaxies, with special attention to the accuracy of the inclination correction. The first sample consists of rotationally supported galaxies, and has DDO 154 as a prototype. They have HI linewidths significantly larger than predicted by an extrapolation of the Tully-Fisher relation. The deviation from the extrapolated Tully-Fisher relation correlates with the distance-independent mass-to-luminosity ratio M_HI/L_B. The baryonic correction for these gas rich galaxies is too small to explain the deviation from the Tully-Fisher relation, unless a small mass-to-light ratio (M/L_B)_* <= 0.25 for the stellar population is assumed. The second sample contains dwarf galaxies with comparable rotational and random velocities. This sample, having Leo A as a prototype, is consistent with an extrapolation of the classical Tully-Fisher relation to lower luminosities. These dwarf galaxies are not particularly rich in gas, but their mass is dominated by baryonic matter within their HI radius. We suggest that any evolution of the deviating galaxies towards the Tully-Fisher relation will combine a decrease in linewidth by radial transport of HI with an increase in luminosity due to a larger accumulated stellar mass.
High-sensitivity maps of the large-scale structure of atomic hydrogen in the starburst dwarf galaxy NGC 1569 show evidence for an HI cloud with a mass of 7*10**6 M_sun, at a projected distance of 5 kpc from the parent galaxy. This cloud may be a condensation in a low-column-density HI halo or a companion galaxy/HI-cloud. NGC 1569 and its companion are connected by a low surface brightness HI bridge. At the edge of NGC1569, the HI bridge coincides with H_alpha arcs, also detected in soft X-rays.