GMRT observations of the group Holmberg 124: Evolution by tidal forces and ram pressure?

Abstract

Abstract. We report new radio continuum and 21 cm HI observations using the Giant Metrewave Radio Telescope (GMRT) of the group Holmberg 124 (Ho 124) comprising four late-type galaxies, namely NGC 2820, Mrk 108, NGC 2814 and NGC 2805. The three galaxies, NGC 2820, Mrk 108 and NGC 2814 which are closely located in the sky plane have clearly undergone tidal interactions as seen from the various morphological tidal signatures and debris. Moreover we note various features in the group members which we believe might be due to ram pressure. In this paper, we describe four interesting results emerging from our observations: a) detection of the tidal radio continuum bridge at 330 MHz connecting the galaxies NGC 2820+Mrk 108 with NGC 2814. The radio bridge was discovered at 1465 MHz by van der Hulst & Hummel (1985, A&A, 150, 17). We find that the bridge has a fairly steep spectrum with a spectral index α (S ∝ ν α ) of −1.8 +0.3 −0.2 which is much steeper than the −0.8 quoted by van der Hulst & Hummel (1985); b) detection of other tidal features like the tilted HI and radio continuum disk of NGC 2814, a HI streamer and a radio continuum tail arising from the south of NGC 2814. We also report the detection of a possible tidal dwarf galaxy in HI; c) sharp truncation in the HI distribution in the south of NGC 2820 and in the HI and radio continuum distribution in the north of NGC 2814. The optical disks in both the cases look undisturbed. As pointed out by Davis et al. (1997, AJ, 114, 613), ram pressure affects different components of the interstellar medium to varying degrees. Simple estimates of pressure in different components of the interstellar medium (radio continuum, Hα and HI) in NGC 2820 indicate that ram pressure will significantly influence HI; d) detection of a large one-sided HI loop to the north of NGC 2820. No radio continuum emission or Hα emission is associated with the HI loop. We discuss various scenarios for the origin of this loop including a central starburst, ram pressure stripping and tidal interaction. We do not support the central starburst scenario since the loop is not detected in ionized gas. Using the upper limit on X-ray luminosity of Ho 124 (Mulchaey et al. 2003, ApJS, 145, 39), we estimate an upper limit on the intragroup medium (IGrM) density of 8.8 × 10 −4 cm −3 . For half this electron density, we estimate the ram pressure force of the IGrM to be comparable to the gravitational pull of the disk of NGC 2820. Since tidal interaction has obviously influenced the group, we suggest that the loop could have formed by ram pressure stripping if tidal effects had reduced the surface density of HI in NGC 2820. From the complex observational picture of Ho 124 and the numerical estimates, we suggest that the evolution of the Ho 124 group may be governed by both tidal forces due to the interaction and the ram pressure due to motion of the member galaxies in the IGrM and that the IGrM densities should not be too low (i.e. ≥4 × 10 −4 ). However this needs to be verified by further observations.

Gas and dark matter in the Sculptor group: NGC 300

“…In contrast, only very few cases of ram‐pressure stripping in galaxy groups have been reported so far (e.g. Kantharia et al 2005; McConnachie et al 2007; Bouchard et al 2007).…”

X-ray spectroscopy of the Virgo Cluster out to the virial radius

“…Kenney, van Gorkom & Vollmer 2004; Chung et al 2007; Kantharia, Rao & Sirothia 2008). There are furthermore several examples which suggest that stripping also operates in lower density, lower velocity dispersion environments of poor clusters and groups of galaxies (Kantharia et al 2005; Levy et al 2007). Recent simulations (e.g.…”

Gas stripping in galaxy groups - the case of the starburst spiral NGC 2276

“…This cannot be unambiguously addressed by this study, as both the observed X‐ray and H i tails of NGC 2276 could be accounted for by viscous stripping alone. An indication that starburst outflows are not a prerequisite for mass loss in groups is the fact that certain group galaxies, such as Holmberg II (Bureau & Carignan 2002) and NGC 2820 (Kantharia et al 2005), also appear to be losing gas because of ram‐pressure or viscous stripping, without showing any signs of starburst activity. A 22‐ks ROSAT study with a limiting point‐source sensitivity of ∼10 37 erg s −1 found no diffuse X‐ray gas in Holmberg II (Kerp, Walter & Brinks 2002), implying that any X‐ray tail of this galaxy is orders of magnitude fainter than 2 × 10 39 erg s −1 found for the NGC 2276 tail.…”

“…This has been interpreted by many authors as ruling out the possibility that stripping via interactions with a hot intracluster medium (ICM) plays a major role in morphological transformations, given that it is expected to be much less effective in poor systems than in the cores of rich clusters. This interpretation remains to be tested by direct observations, however, and recently, claims have been made for X‐ray (Sivakoff, Sarazin & Carlin 2004; see also Machacek et al 2005b) and H i (Bureau & Carignan 2002; Kantharia et al 2005) evidence of ram‐pressure stripping in groups. But, as noted by these authors, ram‐pressure stripping is not the only viable explanation for the gas morphology seen in these systems.…”