Search

Using the Very Large Array, we have measured the proper motions of twenty-eight radio pulsars. On average, the pulsars studied are fainter and more distant than those studied in earlier work, reducing the selection biases inherent in surveys restricted to the Solar neighborhood. The typical measurement precision achieved is a few milliarcseconds per year, corresponding to a few tens of kilometers per second for a pulsar a kiloparsec away. While our results compare well with higher-precision measurements done using very-long baseline interferometry, we find that several earlier proper motion surveys appear to have reported overly optimistic measurement uncertainties, most likely because of a failure to fully account for ionospheric effects. We discuss difficulties inherent in estimating pulsar velocities from proper motions given poorly constrained pulsar distances. Our observations favor a distribution with 20% of pulsars in a low velocity component (sigma_1D = 99 km/s) and 80% in a high velocity component (sigma_1D = 294 km/s). Furthermore, our sample is consistent with a scale height of pulsar birthplaces comparable to the scale height of the massive stars that are their presumed progenitors. No evidence is found in our data for a significant population of young pulsars born far from the plane. We find that estimates of pulsar ages based on kinematics agree well with the canonical spin-down age estimate, but agreement is improved if braking indexes are drawn from a Gaussian distribution centered at n=3 with width 0.8.

We fit the fundamental plane, FP, to over 400 early-type galaxies in 20 nearby clusters (cz ~ 4000–11000 km s-1), using our own photometry and spectroscopy, as well as measurements culled from the literature. We find that the quality of fit, rms(log σ), to the average fundamental plane, FP, varies substantially among these clusters. A statistically significant gap in the distribution of rms(log σ) separates the clusters that fit FP well from those that do not. These two subgroups are of roughly equal size (11 low-rms clusters and nine high-rms clusters). The mean redshifts of the two subsamples are 6700 and 8000 km s-1, respectively, and both samples show a mean peculiar velocity of zero. However, the nine high-rms(log σ) clusters exhibit a scatter about the mean peculiar velocity of 785 ± 200 km s-1, similar in amplitude to large-scale motions reported by other observers. In contrast, the other 11 clusters exhibit significantly lower scatter, equal to 245 ± 55 km s-1. Assuming early-type galaxies in clusters are drawn from a single underlying population, there should be no correlation between the scatter in peculiar velocity and rms(log σ). While the source of the high values of rms(log σ) exhibited in this subset of clusters is not entirely known, there is some indication that it is an intrinsic cluster property, possibly related to the presence of significant substructure. For example, we find that all X-ray–bright clusters in our sample fit the FP well, i.e., they exhibit low values for rms(log σ). The evidence suggests that all-sky surveys that mix together both classes of clusters, as characterized by rms(log σ), may artificially inject signal into the observed peculiar velocity field. We therefore suggest that a prefilter such as the X-ray luminosity of clusters or FP scatter be used to select clusters to derive relative distances via the FP. When such a prefilter is applied to our sample a relatively quiescent Hubble flow is revealed.

Deep, multiband observations of high Galactic latitude fields are an essential tool for studying topics ranging from Galactic structure to extragalactic background radiation. The Hubble Deep Field (HDF-N) observations obtained in 1995 December established a standard for such narrow, deep surveys. The field has been extensively analyzed by a variety of groups and has been widely studied with imaging and spectroscopy over wavelengths ranging from 10-3 to 2 × 105 μm. We describe here a second deep field campaign (HDF-S), this time in the southern hemisphere, undertaken by the Hubble Space Telescope (HST) in 1998 October in a program very similar to the northern Hubble Deep Field. Imaging and spectroscopy of three adjacent fields in the southern continuous viewing zone were obtained simultaneously for 150 orbits, and a mosaic of flanking fields was imaged for 27 additional orbits. Two important features of the HDF-S distinguish it from the HDF-N: the campaign included parallel observations by the three main HST instruments—WFPC2, STIS, and NICMOS—and the HDF-S location was selected to place a bright z = 2.24 quasar in the STIS field of view. The HDF-S observations consist of WFPC2 images in filters close to U, B, V, and I, a deep STIS image of the field surrounding the quasar, spectroscopy of the quasar with STIS from 1150 to 3560 A, and deep imaging of an adjacent field with NICMOS camera 3 at 1.1, 1.6, and 2.2 μm. All of the HDF-S data were fully reduced and made publicly available within 2 months of the observations, and we describe here the selection of the fields and the observing strategy that was employed. Detailed descriptions of the data and the reduction techniques for each field, together with the corresponding source catalogs, appear in separate papers.

NICMOS 2 observations are crucial for constraining distances to most of the existing sample of z > 1 SNe Ia. Unlike the conventional calibration programs, these observations involve long exposure times and low count rates. Reciprocity failure is known to exist in HgCdTe devices and a correction for this effect has already been implemented for high and medium count-rates. However observations at faint count-rates rely on extrapolations. Here instead, we provide a new zeropoint calibration directly applicable to faint sources. This is obtained via inter-calibration of NIC2 F110W/F160W with WFC3 in the low count-rate regime using z ~ 1 elliptical galaxies as tertiary calibrators. These objects have relatively simple near-IR SEDs, uniform colors, and their extended nature gives superior signal-to-noise at the same count rate than would stars. The use of extended objects also allows greater tolerances on PSF profiles. We find ST magnitude zeropoints (after the installation of the NICMOS cooling system, NCS) of 25.296 +- 0.022 for F110W and 25.803 +- 0.023 for F160W, both in agreement with the calibration extrapolated from count-rates 1,000 times larger (25.262 and 25.799). Before the installation of the NCS, we find 24.843 +- 0.025 for F110W and 25.498 +- 0.021 for F160W, also in agreement with the high-count-rate calibration (24.815 and 25.470). We also check the standard bandpasses of WFC3 and NICMOS 2 using a range of stars and galaxies at different colors and find mild tension for WFC3, limiting the accuracy of the zeropoints. To avoid human bias, our cross-calibration was "blinded" in that the fitted zeropoint differences were hidden until the analysis was finalized., Accepted for Publication in the Astronomical Journal. New version contains added reference

We analyze the properties of a sample of galaxies identified in a 21-cm, HI-line survey of selected areas in the Perseus-Pisces supercluster and its foreground void. Twelve fields were observed in the supercluster, five of them (target fields) centered on optically bright galaxies, and the other seven (blank fields) selected to contain no bright galaxies. We detected nine previously uncataloged, gas-rich galaxies, six of them in the target fields. We also detected HI from seven previously cataloged galaxies in these fields. Observations in the void covered the same volume as the twelve supercluster fields at the same HI-mass sensitivity, but no objects were detected. Combining our HI data with optical broad-band and H-alpha imaging, we conclude that the properties of HI-selected galaxies do not differ substantially from those of late-type galaxies found in optical surveys. In particular, the galaxies in our sample do not appear to be unusually faint for their HI mass, or for their circular velocity. The previously cataloged, optically bright galaxies in our survey volume dominate the total HI mass density and cross-section; the uncataloged galaxies contribute only 19% of the mass and 12% of the cross-section. Thus, existing estimates of the density and cross-section of neutral hydrogen, most of which are based on optically-selected galaxy samples, are probably accurate.