A low-mass HI companion of NGC 1569?

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.Comment: 5 pages, 4 figures, 1 tabl

SCUBA Observations of NGC 1275

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 $\beta$ = 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.Comment: 13 pages, 4 figures. Figure 4 is colou

The dust SED in the dwarf galaxy NGC 1569: Indications for an altered dust composition?

We discuss the interpretation of the dust SED from the mid-infrared to the millimeter range of NGC 1569. The model developed by D\'esert et al. (1990) including three dust components (Polyaromatic Hydrocarbons, Very Small Grains and big grains) can explain the data using a realistic interstellar radiation field and adopting an enhanced abundance of VSGs. A simple three-temperature model is also able to reproduce the data but requires a very low dust temperature which is considered to be unlikely in this low-metallicity starburst galaxy. The high abundance of Very Small Grains might be due to large grain destruction in supernova shocks. This possibility is supported by ISO data showing that the emission at 14.3 $\mu$m, tracing VSGs, is enhanced with respect to the emission at 6.7 $\mu$m and 850 $\mu$m in regions of high star formation.Comment: 4 pages, conference proceedings paper, "The Spectral Energy Distribution of Gas-Rich Galaxies: Confronting Models with Data", Heidelberg, 4-8 Oct. 2004, eds. C.C. Popescu & R.J. Tuffs, AIP Conf. Ser., in pres

Star formation history and environment of the dwarf galaxy UGCA 92

We present a quantitative star formation history of the nearby dwarf galaxy UGCA 92. This irregular dwarf is situated in the vicinity of the Local Group of galaxies in a zone of strong Galactic extinction (IC 342 group of galaxies). The galaxy was resolved into stars with HST/ACS including old red giant branch. We have constructed a model of the resolved stellar populations and measured the star formation rate and metallicity as function of time. The main star formation activity period occurred about 8 - 14 Gyr ago. These stars are mostly metal-poor, with a mean metallicity [Fe/H] ~ -1.5 -- -2.0 dex. About 84 per cent of the total stellar mass was formed during this event. There are also indications of recent star formation starting about 1.5 Gyr ago and continuing to the present. The star formation in this event shows moderate enhancement from ~ 200 Myr to 300 Myr ago. It is very likely that the ongoing star formation period has higher metallicity of about -0.6 -- -0.3 dex. UGCA 92 is often considered to be the companion to the starburst galaxy NGC 1569. Comparing our star formation history of UGCA 92 with that of NGC 1569 reveals no causal or temporal connection between recent star formation events in these two galaxies. We suggest that the starburst phenomenon in NGC 1569 is not related to the galaxy's closest dwarf neighbours and does not affect their star formation history.Comment: 11 pages, 9 figures, accepted to MNRA

A Molecular Spiral Arm in the Far Outer Galaxy

We have identified a spiral arm lying beyond the Outer Arm in the first Galactic quadrant ~15 kpc from the Galactic center. After tracing the arm in existing 21 cm surveys, we searched for molecular gas using the CfA 1.2 meter telescope and detected CO at 10 of 220 positions. The detections are distributed along the arm from l = 13 deg, v = -21 km/s to l = 55 deg, v = -84 km/s and coincide with most of the main H I concentrations. One of the detections was fully mapped to reveal a large molecular cloud with a radius of 47 pc and a molecular mass of ~50,000 Mo. At a mean distance of 21 kpc, the molecular gas in this arm is the most distant yet detected in the Milky Way. The new arm appears to be the continuation of the Scutum-Centaurus Arm in the outer Galaxy, as a symmetric counterpart of the nearby Perseus Arm.Comment: 11 pages, 4 figures, 1 Table, ApJ Letters, in pres