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.

Intrigued by the initial report of an extended lumiosity distribution perpendicular to the disk of the edge-on Sc galaxy NGC 5907, we have obtained very deep exposures of this galaxy with a Schmidt telescope, large-format CCD, and intermediate-band filters centered at 6660Å and 8020Å. These two filters, part of a 15-filter set, are custom-designed to avoid the brightest (and most variable) night sky lines. As a result, our images are able to go deeper, with lower sky noise than those taken with broad-band filters at similar effective wavelengths: e.g., 0.6 e − arcsec −2 sec −1 for our observations vs. 7.4 e − arcsec −2 sec −1 for the R-band measures of Morrison et al. In our assessment of both random and systematic errors, we show that the flux level where the errors of observation reach 1 mag arcsec −2 are 29.00 mag arcsec −2 in the 6660Å image (corresponding to 28.7 in R-band) and 27.4 mag arcsec −2 in the 8020Å image (essentially on the I-band system).As detailed in Shang et al., our observations show NGC 5907 has a luminous ring around it that most plausibly is due to the tidal disruption of a dwarf spheroidal galaxy by the much more massive spiral. Here we show that, fainter than 27th R mag arcsec −2 , the surface brightness around NGC 5907 is strongly asymmetric, being mostly brighter on NW (ring) side of the galaxy midplane. This asymmetry rules out a halo for the origin of the faint surface brightness we see.We find this asymmetry is likely an artifact owing to a combination of ring light and residual surface brightness at faint levels from stars that our star-masking procedure cannot completely eliminate. The possible existence of an optical face-on warp in NGC 5907, suggested by our VLA HI observations, is too confused with foreground star contamination to be independently studied. Good agreement with the surface photometry of NGC 5907 by Morrison et al. and other workers lead us to conclude that their data are similarly affected at faint levels by ring light and residual effects from their star masking procedures. Inspection of the images published by Morrison et al. and Sackett et al. confirm this to be the case. Thus, we conclude that NGC 5907 does not have a faint, extended halo.In this paper we present the details of our deep surface photometry of this galaxy, this time paying close attention to the issue of the faint luminosity distribution around this galaxy. Section 2 presents our observations, including details of the data reduction process, which are important for the reader to be able to critically assess the accuracy of our method. In Section 3 we study the faint luminosity distribution around NGC 5907 as it appears in our images, including how the ring, foreground stars, and a possible face-on warp can influence what we see. Our results are compared to those previously published in Section 4, in which we also reassess the likelihood of a halo existing around this galaxy. Section 5 summarizes the main results of this paper. OBSERVATIONS AND DATA REDUCTIONThe phases of data reduction that con...