We present a catalogue and images of visually detected features, such as asymmetries, extensions, warps, shells, tidal tails, polar rings, and obvious signs of mergers or interactions, in the faint outer regions (at and outside of R_25) of nearby gal
axies. This catalogue can be used in future quantitative studies that examine galaxy evolution due to internal and external factors. We are able to reliably detect outer region features down to a brightness level of 0.03 MJy/sr per pixel at 3.6 microns in the Spitzer Survey of Stellar Structure in Galaxies (S4G). We also tabulate companion galaxies. We find asymmetries in the outer isophotes in 22+/-1 per cent of the sample. The asymmetry fraction does not correlate with galaxy classification as an interacting galaxy or merger remnant, or with the presence of companions. We also compare the detected features to similar features in galaxies taken from cosmological zoom re-simulations. The simulated images have a higher fraction (33 per cent) of outer disc asymmetries, which may be due to selection effects and an uncertain star formation threshold in the models. The asymmetries may have either an internal (e.g., lopsidedness due to dark halo asymmetry) or external origin.
We obtained Spitzer/IRAC 3.6-8 micron images of the nearby spiral galaxy NGC 4258 to study possible interactions between dust and the radio jet. In our analysis we also included high-resolution radio continuum, H-alpha, CO, and X-ray data. Our data r
eveal that the 8 micron emission, believed to originate largely from PAH molecules and hot dust, is an excellent tracer of the normal spiral structure in NGC 4258, and hence it originates from the galactic plane. We investigated the possibility of dust destruction by the radio jet by calculating correlation coefficients between the 8 micron and radio continuum emissions along the jet in two independent ways, namely (i) from wavelet-transformed maps of the original images at different spatial scales, and (ii) from one-dimensional intensity cuts perpendicular to the projected path of the radio jet on the sky. No definitive sign of a correlation (or anticorrelation) was detected on relevant spatial scales with either approach, implying that any dust destruction must take place at spatial scales that are not resolved by our observations.
We have observed warm molecular hydrogen in two nearby edge-on disk galaxies, NGC 4565 and NGC 5907, using the Spitzer high-resolution infrared spectrograph. The 0-0 S(0) 28.2 micron and 0-0 S(1) 17.0 micron pure rotational lines were detected out to
10 kpc from the center of each galaxy on both sides of the major axis, and in NGC 4565 the S(0) line was detected at r = 15 kpc on one side. This location lies beyond a steep drop in the radio continuum emission from cosmic rays in the disk. Despite indications that star formation activity decreases with radius, the H2 excitation temperature and the ratio of the H2 line and the far-IR luminosity surface densities, Sigma_L(H2}/Sigma_L(TIR}, change very little as a function of radius, even into the diffuse outer region of the disk of NGC 4565. This suggests that the source of excitation of the H2 operates over a large range of radii, and is broadly independent of the strength and relative location of UV emission from young stars. Although excitation in photodissociation regions is the most common explanation for the widespread H2 emission, cosmic ray heating or shocks cannot be ruled out. The inferred mass surface densities of warm molecular hydrogen in both edge-on galaxies differ substantially, being 4(-60) M_solar/pc^2 and 3(-50) M_solar/pc^2 at r = 10 kpc for NGC 4565 and NGC 5907, respectively. The higher values represent very unlikely point-source upper limits. The point source case is not supported by the observed emission distribution in the spectral slits. These mass surface densities cannot support the observed rotation velocities in excess of 200 km/s. Therefore, warm molecular hydrogen cannot account for dark matter in these disk galaxies, contrary to what was implied by a previous ISO study of the nearby edge-on galaxy NGC 891.
The barred galaxy NGC 7479 hosts a remarkable jet-like radio continuum feature: bright, 12-kpc long in projection, and hosting an aligned magnetic field. The degree of polarization is 6%-8% along the jet, and remarkably constant, which is consistent
with helical field models. The radio brightness of the jet suggests strong interaction with the ISM and hence a location near the disk plane. We observed NGC 7479 at four wavelengths with the VLA and Effelsberg radio telescopes. The equipartition strength is 35-40 micro-G for the total and >10 micro-G for the ordered magnetic field in the jet. The jet acts as a bright, polarized background. Faraday rotation between 3.5 and 6 cm and depolarization between 6 and 22 cm can be explained by magneto-ionic gas in front of the jet, with thermal electron densities of ~0.06 cm**(-3) in the bar and ~0.03 cm**(-3) outside the bar. The regular magnetic field along the bar points toward the nucleus on both sides. The regular field in the disk reveals multiple reversals, probably consisting of field loops stretched by a shearing gas flow in the bar. The projection of the jet bending in the sky plane is in the sense opposite to that of the underlying stellar and gaseous spiral structure. The bending in 3-D is most easily explained as a precessing jet, with an age less than 10**6 years. Our observations are consistent with very recent triggering, possibly by a minor merger. NGC 7479 provides a unique opportunity to study interaction-triggered 15-kpc scale radio jets within a spiral galaxy.
