Topology of COBE Microwave Background Fluctuations

We have measured the topology (genus) of the fluctuations in the cosmic microwave background seen in the recently completed (four-year) data set produced by the COBE satellite. We find that the genus is consistent with that expected from a random-phase Gaussian distribution, as might be produced naturally in inflationary models.Comment: 2 pages, one Post-Script figure, MNRAS LaTeX Style (mn.sty), submitted to MNRA

Hourly Variability in Q0957+561

We have continued our effort to re-reduce archival Q0957+561 brightness monitoring data and present results for 1629 R-band images using the methods for galaxy subtraction and seeing correction reported previously. The new dataset comes from 4 observing runs, several nights apiece, with sampling of typically 5 minutes, which allows the first measurement of the structure function for variations in the R-band from timescales of hours to years. Comparison of our reductions to previous reductions of the same data, and to r-band photometry produced at Apache Point Observatory shows good overall agreement. Two of the data runs, separated by 417 days, permit a sharpened value for the time delay of 417.4 days, valid only if the time delay is close to the now-fashionable 417-day value; our data do not constrain a delay if it is more than three days from this 417-day estimate. Our present results show no unambiguous signature of the daily microlensing, though a suggestive feature is found in the data. Both time delay measurement and microlensing searches suffer from from the lack of sampling at half-day offsets, inevitable at a single observatory, hence the need for round-the-clock monitoring with participation by multiple observatories.Comment: AASTeX 4.0 preprint style, 21 pages, 8 EPS figure

Dynamics of ``Small Galaxies'' in the Hubble Deep Field

We have previously found in the Hubble Deep Field a significant angular correlation of faint, high color-redshift objects on scales below one arcsecond, or several kiloparsecs in metric size. We examine the correlation and nearest neighbor statistics to conclude that 38% of these objects in the HDF have a companion within one arcsecond, three times the number expected in a random distribution with the same number of objects. We examine three dynamical scenarios for these object multiplets: 1) the objects are star-forming regions within normal galaxies, whose disks have been relatively dimmed by K-correction and surface brightness dimming; 2) they are fragments merging into large galaxies; 3) they are satellites accreting onto normal L_* galaxies. We find that hypothesis 1 is most tenable. First, large galaxies in the process of a merger formation would have accumulated too much mass in their centers (5e12 M_sun inside 2 kpc) to correspond to present day objects. Second, accretion by dynamical friction occurs with a predictable density vs. radius slope, not seen among the faint HDF objects. Since the dynamical friction time is roughly (1 Gyr), a steady-state should have been reached by redshift z < 5. Star-forming regions within galaxies clearly present no dynamical problems. Since large spirals would still appear as such in the HDF, we favor a scenario in which the faint compact sources in the HDF are giant starforming regions within small normal galaxies, such as Magellanic irregulars. Finally we checked that reduction in mass-to-light from induced star-formation cannot alone explain the luminosity overdensity.Comment: AASTeX 4.0 (preprint), 4 PostScript figure

A Rapid Microlensing Event in the Q0957+561 A,B Gravitational Lens System

We re-analyze brightness data sampled intensively over 5 nights at two epochs separated by the quasar lens time delay, to examine the nature of the observed microlensing. We find strong evidence for a microlensing event with an amplitude of 1% and a time scale of twelve hours. The existence of such rapid microlensing, albeit at low amplitude, imposes constraints on the nature of the quasar and of the baryonic dark matter