[en] A technique is presented for calculating the temperature distribution of interstellar dust grains in the presence of a radiation field or collisional heating by a hot gas, or both. The distribution functions are computed for grain sizes ranging from 0.02 micron to 2.5 A for graphite and silicate grains. Five different radiation fields are considered: one and three times the average interstellar radiation in the solar neighborhood, the radiation field 0.3 pc from a B3V star appropriate for visual reflection nebulae, and the radiation fields in interstellar clouds at optical depths corresponding to visual extinctions of A(V) = 0.25 mag and A(V) = 0.50 mag. For the interstellar radiation field in the solar neighborhood, a minimum grain size N(crit) of roughly 23 for graphite grains and N(crit) of roughly 37 for silicate grains, where N(crit) is the number of atoms in a grain for which the lifetime against sublimation is 10 to the 13th s. 27 refs

[en] The cold interstellar and intergalactic medium is in the small group of galaxies whose brightest member is the elliptical galaxy NGC5018. Researchers' attention was first drawn to this galaxy as possibly containing cold interstellar gas by the detection by the Infrared Astronomy Satellite (IRAS) of emission at lambda 60 microns and lambda 100 microns at an intensity of about 1 Jy (Knapp et al. 1989), which is relatively strong for an elliptical (Jura et al. 1987). These data showed that the temperature of the infrared emission is less than 30K and that its likely source is therefore interstellar dust. A preliminary search for neutral hydrogen (HI) emission from this galaxy using the Very Large Array (VLA) showed that there appears to be HI flowing between NGC5018 and the nearby Sc galaxy NGC5022 (Kim et al. 1988). Since NGC5018 has a well-developed system of optical shells (cf. Malin and Carter 1983; Schweizer 1987) this observation suggests that NGC5018 may be in the process of forming its shell system by the merger of a cold stellar system with the elliptical, as suggested by Quinn (1984). Researchers describe follow-up HI observations of improved sensitivity and spatial resolution, and confirm that HI is flowing between NCG5022 and NGC5018, and around NGC5018. The data show, however, that the HI bridge actually connects NGC5022 and another spiral in the group, MCG03-34-013, both spatially and in radial velocity, and that in doing so it flows through and around NGC5018, which lies between the spiral galaxies. This is shown by the total HI map, with the optical positions of the above three galaxies labelled

[en] The data from the IRAS satellite were coadded for about 1150 early-type galaxies with apparent magnitudes brighter than 14 mag. The sample selection, averaging procedure, and errors are discussed. Results are reported of galaxies of large angular size compared with the IRAS beamwidth and for a sample of dwarf ellipticals in the Virgo cluster. At 60 and 100 microns, about 45 percent of the ellipticals and 68 percent of the S0 galaxies are detected, showing that a large fraction of these galaxies contain cool interstellar matter. 46 refs

[en] Far-IR morphology-correlated structure has been noted in the visible range in the present multicolor images of isolated, medium-high latitude Galactic 'cirrus' clouds; the scale of the structure ranges from 30 arcsec to more than 10 arcmin. The novel imaging technique used a mosaic of contiguous CCD frames. The red B(J)-R color of the 'cirrus' is not due to H-alpha emission. Visible/100-micron brightness ratios are used to investigate the nature of a faint, extended optical component seen in a deep CCD survey of high-redshift galaxies which is known to be unrelated to galactic dust. 49 refs

[en] A definitive upper limit for the redshift of the population of faint blue galaxies found in deep imaging surveys is obtained. The U-B(j), and particularly the B(j)-R, colors of these objects show a blueing trend toward fainter magnitudes. A typical galaxy at R = 26 has colors that are only slightly redder than a flat spectrum. For any reasonable Lyman limit break, this constrains 93 percent or more of the galaxies to under z about 3, beyond which the break gets redshifted through the U band. The galaxies appear to be undergoing relatively recent evolution with rest frame spectra that are approximately flat down to the Lyman limit. 23 refs

[en] Two ellipticals, NGC 2974 and NGC 5018, selected because of their relatively strong infrared fluxes, were detected in H I in VLA 21 cm observations. NGC 2974 has a regular distribution of gas. The H I appears to be in a rotating disk with the rotation axis well aligned with the optical minor axis of the galaxy. In contrast, NGC 5018 contains little hydrogen, which may be flowing into it from the nearby spiral NGC 5022. The H I distribution of NGC 5022 is distorted toward NGC 5018. Three other H I features in the field of NGC 5018/5022 are probably gas-rich dwarfs which may have played some part in the acquisition of gas by the elliptical galaxy. The radio continuum map at 21 cm shows two sources near NGC 5018, one at its center and another about one arcmin away. 41 references

[en] Results are reported from a 21 cm survey of fields in the Perseus-Pisces supercluster and a foreground void, which was designed to find gas-rich dwarf galaxies or optically faint H I clouds with masses greater than 10 to the 8th solar masses. Sixteen objects have been detected in the supercluster, nine of them previously uncataloged. No objects were found in the void fields, an indication that H I dwarf galaxies follow the spatial structure defined by bright galaxies, although the results do not rule out a moderate 'bias' between dwarfs and bright galaxies. The number of galaxies detected is smaller than expected from a simple extrapolation of the optical luminosity function. These data leave no room for a substantial population of H I dwarfs or gas-rich, low surface brightness giants that could have been missed by conventional, optical galaxy catalogs. 24 refs

[en] Based on long baseline (5-7 years) multi-epoch HST/ACS photometry, used previously to measure the proper motion of M31, we present the proper motions (PMs) of 13 main-sequence Milky Way halo stars. The sample lies at an average distance of r ≅ 24 kpc from the Galactic center, with a root-mean-square spread of 6 kpc. At this distance, the median PM accuracy is 5 km s^–1. We devise a maximum likelihood routine to determine the tangential velocity ellipsoid of the stellar halo. The velocity second moments in the directions of the Galactic (l, b) system are < v²_l>^1/2= 123⁺²⁹_-23 km s^–1, and < v²_b>^1/2= 83⁺²⁴_-16 km s^–1. We combine these results with the known line-of-sight second moment, < v²_los>^1/2= 105±5 km s^–1, at this (r) to study the velocity anisotropy of the halo. We find approximate isotropy between the radial and tangential velocity distributions, with anisotropy parameter β= 0.0^+0.2_-0.4. Our results suggest that the stellar halo velocity anisotropy out to r ∼ 30 kpc is less radially biased than solar neighborhood measurements. This is opposite to what is expected from violent relaxation, and may indicate the presence of a shell-type structure at r ∼ 24 kpc. With additional multi-epoch HST data, the method presented here has the ability to measure the transverse kinematics of the halo for more stars, and to larger distances. This can yield new improved constraints on the stellar halo formation mechanism, and the mass of the Milky Way.

[en] We model the dynamics of dwarf early-type galaxies in the Virgo cluster when subject to a variety of environmental processes. We focus on how these processes imprint trends in the dynamical state (rotational versus pressure support as measured by the λ_Re/2^∗ statistic) with projected distance from the cluster center, and compare these results to observational estimates. We find a large scatter in the gradient of λ_Re/2^∗ with projected radius. A statistical analysis shows that models with no environmental effects produce gradients as steep as those observed in none of the 100 cluster realizations we consider, while in a model incorporating tidal stirring by the cluster potential 34% of realizations produce gradients as steep as that observed. Our results suggest that tidal stirring may be the cause of the observed radial dependence of dwarf early-type dynamics in galaxy clusters

[en] We exploit the large number of archival Hubble Space Telescope images of 47 Tuc to examine its subgiant branch (SGB) and main sequence (MS) for signs of multiple populations. In the cluster core, we find that the cluster's SGB exhibits a clear spread in luminosity, with at least two distinct components: a brighter one with a spread that is real but not bimodal, and a second one about 0.05 mag fainter, containing about 10% of the stars. In a less crowded field 6' from the center, we find that the MS is broadened much more than can be accounted for by photometric errors, and that this broadening increases at fainter magnitudes.