Observations of the Ocular Galaxy NGC 2535 and its Starburst Companion NGC 2536

Journal article, 1997

H I, radio continuum, and 12CO J=1-->0 observations at resolutions of 12" to 33" (=2.9-8 kpc), and B, I, J, and K-band images, are presented of the galaxy NGC 2535 and its small starburst companion NGC 2536. NGC 2535 has an ocular (eye-shaped) structure with two long tidal arms indicative of a recent, close, nonmerging encounter. Our observations reveal widespread high velocity dispersions (30 km s-1) in the H I gas and five clouds with masses of ˜10^8 Msun in the tidal arms of NGC 2535. These clouds do not correspond to the most luminous blue knots or to major clumps in the old stellar population. Similar phenomena were found previously in other pairs (IC 2163/NGC 2207 and NGC 5774/5), suggesting that enhanced turbulence and massive gas clouds may be a general feature of interacting spiral galaxies during an early phase of post-encounter evolution. A search for 12CO J= 1-->O emission at 11 positions in NGC 2535 and one position in NGC 2536 yielded two clear detections, one at the center of NGC 2535 and the other on its tidal tail but close to the center of NGC 2535. Most of the massive H I clouds in NGC 2535 are not detected in CO emission. The virial masses of these clouds are significantly greater than their measured masses in gas, presumably because of the presence of old disk stars in the clouds. A form of the virial theorem that includes gas and stars is presented. NGC 2535 shows several unusual features, including an intrinsically oval shape to the disk, that are similar to those in the ocular galaxy IC 2163 studied previously. NGC 2535 also contains structures that are not seen in IC 2163. For example, there is an extended (R =48 kpc) H I envelope and a long, outer, elliptically-shaped H I arc in NGC 2535 that may be a gravitational wake produced by the passage of the companion within or close to the envelope. In the radio continuum, NGC 2535 exhibits a bar-like feature that leads the small stellar bar by 50°. The starburst companion, NGC 2536, lies in a 2 X 10^9 Msun clump of H I gas at the outer end of the tidal bridge from NGC 2535. Most of the gas in this clump is associated with the bridge. The H I emission on the anticompanion side of NGC 2536 has the same line-of-sight velocity and projected position as some of the bridge gas there. This observation is consistent with a previous model by Klaric in which NGC 2536 accretes gas from NGC 2535; we even find an H I feature that may represent bridge gas streaming towards NGC 2536. The failure to detect 12CO emission in NGC 2536 places an upper limit of 6 X H2 on its H2 mass if the standard value of the conversion factor between I_CO and N(H2) applies.