The edge-on, nearby spiral galaxy NGC 5907 has long been used as the prototype of a "non-interacting" warped galaxy. We report here the discovery of two interactions with companion dwarf galaxies that substantially change this picture. First, a faint ring structure is discovered around this galaxy that is likely due to the tidal disruption of a companion dwarf spheroidal galaxy. The ring is elliptical in shape with the center of NGC 5907 close to one of the ring's foci. This suggests the ring material is in orbit around NGC 5907. No gaseous component to the ring has been detected either with deep Hα images or in Very Large Array (VLA) HI 21-cm line maps. The visible material in the ring has an integrated luminosity ≤ 10 8 L ⊙ and its brightest part has a color R-I ∼ 0.9. All of these properties are consistent with the ring being a tidally-disrupted dwarf spheroidal galaxy. Second, we find that NGC 5907 has a dwarf companion galaxy, PGC 54419, projected to be only 36.9 kpc from the center of NGC 5907, close in radial velocity (∆V = 45 km s −1 ) to the giant spiral galaxy. This dwarf is seen at the tip of the HI warp and in the direction of the warp. Hence, NGC 5907 can no longer be considered "non-interacting," but is obviously interacting with its dwarf companions much as the Milky Way interacts with its dwarf galaxies. These results, coupled with the finding by others that dwarf galaxies tend to be found around giant galaxies, suggest that tidal interaction with companions, even if containing a mere 1% of the mass of the parent galaxy, might be sufficient to excite the warps found in the disks of many large spiral galaxies.

We present the results of 3 yr of ground-based observations of the Seyfert 1 galaxy NGC 5548, which, combined with previously reported data, yield optical continuum and broad-line Hb light curves for a total of 8 yr. The light curves consist of over 800 points, with a typical spacing of a few days between observations. During this 8 yr period, the nuclear continuum has varied by more than a factor of 7, and the Hb emission line has varied by a factor of nearly 6. The Hb emission line responds to continuum variations with a time delay or lag of D10È20 days, the precise value varying somewhat from year to year. We Ðnd some indications that the lag varies with continuum Ñux in the sense that the lag is larger when the source is brighter.

We perform photometric measurements on a large HST snapshot imaging survey sample of 97 ultraluminous infrared galaxies (ULIRGs). We select putative nuclei from bright clumps in all the sample targets, mainly based on a quantitative criterion of I-band luminosity as well as the global and local morphological information. All the sources are then classiÐed into three categories with multiple, double, and single nucleus/nuclei. The resultant fractions of multiple, double, and single nucleus/nuclei ULIRGs are 18%, 39%, and 43%, respectively. This supports the multiple merger scenario as a possible origin of ULIRGs, in addition to the commonly accepted pair merger model. Further statistical studies indicate that the AGN fraction increases from multiple (36%) to double (65%) and then to single (80%) nucleus/ nuclei ULIRGs. For the single nucleus category, there is a high-luminosity tail in the luminosity distribution, which corresponds to a Seyfert 1/QSO excess. This indicates that active galactic nuclei tend to appear at Ðnal merging stage. For multiple/double nuclei galaxies, we also Ðnd a high fraction of very close nucleus pairs (e.g., 3/4 for those separated by less than 5 kpc). This strengthens the conclusion that systems at late merging phase preferentially host ULIRGs.

We present a deep, 42.79 hr image of the nearby, edge-on galaxy NGC 4565 in the Beijing-Arizona-Taipei-Connecticut (BATC) 6660Å band using the large-format CCD system on the 0.6m Schmidt telescope at the Xinglong Station of the National Astronomical Observatories of China (NAOC). Following the procedures previously developed by our team for the analysis of deep images of galaxies (Zheng et al.), we obtain a final image that is calibrated to an accuracy of 0.02 mag in zero point, and for which we can measure galaxy surface brightness to an accuracy of 0.25 mag at a surface brightness at 27.5 mag arcsec −2 at 6660Å, corresponding to a distance of 22 kpc from the center of the disk. The integrated magnitude of NGC 4565 in our filter is m 6660 = 8.99 (= R magnitude of 9.1) to a surface brightness of 28 mag arcsec −2 . We analyze the faint outer parts of this galaxy using a two-dimensional model comprised of three components: an exponential thin disk, an exponential thick disk, and a power-law halo. Combined with a need to provide a cut-off radius for the disk, a total of 12 parameters are included in our model.We determine the best values of our model parameters via 10,000 random initial values, 3,700 of which converge to final values. We then plot the χ 2 for each converged fit versus parameter value for each of the 12 parameters. The thin disk and thick disk parameters we determine here are consistent with those of previous studies of this galaxy. However, our very deep image permits a better determination of the power law fit to the halo, constraining this power law to be between r −3.2 and r −4.0 , with a best fit value of r −3.88 . We find the axis ratio of the halo to be 0.44 and its core radius to be 14.4 kpc (for an adopted distance of 14.5 Mpc). We also agree with others that the bulge of NGC 4565 is fit well by an exponential luminosity distribution with scale height similar to that found for the thin disk.