HI, HII, and R-Band Observations of a Galactic Merger Sequence

Abstract

We present high-quality aperture synthesis observations of the neutral hydrogen distribution in a sample of five galactic systems believed to be involved in progressive stages of merging: Arp 295, NGC 4676, NGC 520, NGC 3921, and NGC 7252. This is the first time that the atomic hydrogen in such a broad range of disk mergers has been imaged. These data are supplemented by wide-field images taken through a narrow band Hα filter, and by deep (µ R ∼ > 26.5 mag arcsec −2 ) R-band surface photometry.We identify several trends along the merging sequence. In the early stages, large amounts of H i still exist within the galactic disks and star formation is widespread. The ionized gas emission often takes on the appearance of plumes and arcs emanating from the nuclear regions, which are presumably the sites of interaction-induced starbursts. In the final stages there is little if any H i within the remnant bodies, and tidal material is seen moving inward. We conclude that as the merger rearranges the light profiles of the progenitor disk galaxies into r 1/4 profiles, it leads to an efficient conversion of the atomic gas into other forms within the main bodies of the merger remnants. This suggests that these remnants will evolve into elliptical galaxies in their atomic gas contents as well as their photometric properties. However the observations of NGC 520 reveal an extensive rotating gaseous disk, suggesting that perhaps some mergers will not destroy the atomic gas disks of the progenitors.Although the observed tidal features contribute only of order 10% of a system's optical luminosity, they contain the majority of the neutral hydrogen. The morphological similarity between the gaseous and stellar tails and the smooth gas kinematics confirm that gravity plays the dominant role in producing them. There are, however, some striking differences between the faint gas and light distributions. H i mapping often reveals gaseous extension not at all apparent optically, and tidal features of different optical morphologies have different gas characteristics: the edge-brightened tails are gas-rich, while the featureless plumes and halos are gas-poor. Some of these features may be explained by the different velocity dispersions of the gas and stars, some by different gas contents in the progenitors, and some remain unexplained. Overall, large quantities of both gas and starlight (M HI > 10 9 h −2 M ⊙ , L R > 10 9 h −2 L ⊙ ) are seen at large radii (r >50 h −1 kpc). Since this material evolves on very long time scales, it may leave observable signatures for many Gyr.

B2 0648+27: A radio galaxy in a major merger

“…Hibbard & van Gorkom 1996;Wilson 1996;Mihos & Hernquist 1994;Morganti et al 1997) This is presented in Fig. 6.…”

“…Perhaps either kind of merger plays an important role, as long as the initial conditions are such that the gas will be able to reach the very inner regions (on pc-like scales), see e.g. Hibbard (1995), Wilson (1996). Another uncertain aspect is at which stage of the merger the onset of the radio activity takes place.…”

“…Part of the HI is found in two long tidal tails. NGC 7252 (2 nd from left) represents a somewhat later stage where the central remnant has already more or less taken the shape of an elliptical galaxy (Hibbard et al 1996). The HI is mainly in large tails at large radii, while the gas in the centre, where much star formation is still occurring, is mainly molecular.…”

Star and Cluster Formation in Extreme Environments

“…In both of these interacting systems we find several dozen YMCs (or, alternatively, compact star-forming regions), which overlap spatially with regions of active star formation in the galaxies' tidal tails and spiral arms (from a comparison with Hα observations that trace active star formation; Hibbard & van Gorkom 1996). The tidal tail of the Tadpole system is dominated by starforming regions, which contribute ∼ 70% of the total flux in the HST F814W filter (decreasing to ∼ 40% in the F439W filter).…”

Tracing interactions in HCGs through the H I

“…Since the atomic hydrogen is a sensitive tracer of tidal interaction (e.g. Hibbard & Van Gorkom 1996, Hibbard 1999, we have performed VLA mapping of a large number of HCGs which added to the previously published ones (Williams & Van Gorkom 1988-WVG88, Williams, Mac Mahon and Van Gorkom 1991-WMVG 91, Williams & Van Gorkom 1995-WVG95, Williams et al 1997) and lead to a total sample of 16 groups. In two cases (HCG 31 and HCG 79) we have obtained higher spatial and spectral resolution data which have helped to improve our understanding of these systems.…”