The Influence of Galactic Outflows on the Formation of Nearby Dwarf Galaxies.

We show that the gas in growing density perturbations is vulnerable to the influence of winds outflowing from nearby collapsed galaxies that have already formed stars. This suggests that the formation of nearby galaxies with masses less, similar10(9) M( middle dot in circle) is likely to be suppressed, irrespective of the details of galaxy formation. An impinging wind may shock-heat the gas of a nearby perturbation to above the virial temperature, thereby mechanically evaporating the gas, or the baryons may be stripped from the perturbation entirely if they are accelerated to above the escape velocity. We show that baryonic stripping is the most effective of these two processes, because shock-heated clouds that are too large to be stripped are able to radiatively cool within a sound crossing time, limiting evaporation. The intergalactic medium temperatures and star formation rates required for outflows to have a significant influence on the formation of low-mass galaxies are consistent with current observations, but may soon be examined directly via associated distortions in the cosmic microwave background and with near-infrared observations from the Next Generation Space Telescope, which may detect the supernovae from early-forming stars.

A Spectroscopic Redshift for the Cl 0024+16 Multiple Arc System: Implications for the Central Mass Distribution.

We present a spectroscopic redshift of z=1.675 for the well-known multiply lensed system of arcs seen in the z=0.39 cluster Cl 0024+16. In contrast to earlier work, we find that the lensed images are accurately reproduced by a projected mass distribution which traces the locations of the brightest cluster elliptical galaxies, suggesting that the most significant minima of the cluster potential are not dynamically erased. The averaged mass profile is shallow and consistent with predictions of recent numerical simulations. The source redshift enables us to determine an enclosed cluster mass of M(<100 kpc h-1&parr0;=1.11+/-0.03x1014 h-1 M middle dot in circle (Omega=1) and a mass-to-light ratio of M&solm0;LB(<100 kpc h-1&parr0;=320+/-30 h &parl0;M&solm0;LB&parr0; middle dot in circle (virtually independent of curvature), after correction for passive stellar evolution. The arc spectrum contains many ionized absorption lines and closely resembles that of the local Wolf-Rayet galaxy NGC 4217. Our lens model predicts a high magnification ( approximately 20) for each image and identifies a new pair of multiple images of a galaxy at a predicted redshift of z=1.3.

Detecting the Gravitational Redshift of Cluster Gas.

We examine the gravitational redshift of radiation emitted from within the potential of a cluster. Spectral lines from the intracluster medium (ICM) are redshifted in proportion to the emission-weighted mean potential along the line of sight, amounting to approximately 50 km s-1 at a radius of 100 kpc h-1, for a cluster dispersion of 1200 km s-1. We show that the relative redshifts of different ionization states of metals in the ICM provide a unique probe of the three-dimensional matter distribution. An examination of the reported peculiar velocities of cD galaxies in well-studied Abell clusters reveals that they are typically redshifted by an average of approximately 200 km s-1. This can be achieved by gravity with the addition of a steep central potential associated with the cD galaxy. Note that, in general, gravitational redshifts cause a small overestimate of the recessional velocities of clusters by an average of approximately 20 km s-1.

Young Red Spheroidal Galaxies in the Hubble Deep Fields: Evidence for a Truncated Initial Mass Function at approximately 2 M middle dot in circle and a Constant Space Density to z approximately 2.

The optical-IR images of the northern and southern Hubble Deep Fields are used to measure the spectral and density evolution of early-type galaxies. The mean spectral energy distribution is found to evolve passively toward a mid-F star-dominated spectrum by z approximately 2, becoming more sharply peaked around the 4000 Å break. We demonstrate with realistic simulations that hotter elliptical galaxies would be readily visible if evolution progressed blueward and brightward at z>2, following a standard initial mass function (IMF). The color distributions are best fitted by a "red" IMF, deficient above approximately 2 M middle dot in circle and with a spread of formation in the range 1.51 results from a selection bias against distant red galaxies in the optical, where the flux is too weak for morphological classification, but is remedied with relatively modest IR exposures that reveal a roughly constant space density to z approximately 2, with 32 and 16 elliptical galaxies detected above and below z=1, respectively. We point out that the lack of high-mass star formation inferred here and the requirement of metals implicates cooling flows of preenriched gas in the creation of the stellar content of spheroidal galaxies. Deep-field X-ray images will be very helpful in examining this possibility.

Photometry and spectroscopy of the GRB 970508 optical counterpart

An optical transient within the error box of the gamma ray burst GRB 970508 was imaged 4 hours after the event. It displayed a strong ultraviolet excess, and reached maximum brightness 2 days later. The optical spectra did not show any emission lines, and no variations on time scales of minutes were observed for 1 hour during the decline phase. According to the fireball and afterglow models, the intensity should rise monotonically before the observed optical maximum, but the data indicate that another physical mechanism may be responsible for the constant phase seen during the first hours after the burst.

Redshift survey with multiple pencil beams at the galactic poles.

Observations of the large-scale structure of the universe suggest inhomogeneities on scales between 100h-1 and 150h-1 Mpc (where h approximately 0.5-1 is the Hubble constant in units of 100 km.s-1.Mpc-1; 1 pc = 3.09 x 10(16) m). A deep redshift survey with a "pencil-beam" geometry of galaxies at the galactic poles indicated strong clustering, with a provocative regularity at 128h-1 Mpc [Broadhurst, T. J., Ellis, R. S., Koo, D. C. & Szalay, A. S. (1990) Nature (London) 343, 726-728]. Using newly acquired data, we demonstrate how multiple deep probes overcome most of the statistical problems associated with single pencil beams. Our results from cross correlations of multiple pencil beams, containing over 1200 galaxies, indicate that the strong peak in the power spectrum results from structures of large transverse size, in agreement with our original conjecture. We also discuss the sensitivity of pencil-beam surveys to the topology of large-scale structures and compare them with sparsely sampled wide-angle local surveys.