The NGC 6221/15 Galaxy Group

Bärbel Koribalski (ATNF)

Neutral Hydrogen Gas in Interacting Galaxies:
The NGC 6221 / 6215 galaxy group

Bärbel Koribalski & John Dickey

(2004) MNRAS 348, 1255

(File: NGC6221.pdf)

Abstract. Neutral hydrogen observations of the spiral galaxies NGC 6221 and NGC 6215 with the Australia Telescope Compact Array (ATCA) reveal a wide, two-stranded bridge of at least 3 x 10^8 Msun which can be traced between the two galaxies over a projected distance of 100 kpc. The velocity gradient of the HI bridge provides a rough estimate for the time since the encounter of half a billion years. For NGC 6221, the brightest and most massive galaxy of the group, we derive a dynamical mass of Mtot = 8 x 10^10 Msun, while its companion NGC 6215 has a mass of only Mtot ~ 2 x 10^9 Msun. Further, we find three low-surface brightness dwarf galaxies (Dwarf 1, 2, and 3) in the neighbourhood of NGC 6221/15 with HI masses of 3.3, 0.6 and 0.3 x 10^8 Msun, respectively. The smallest, previously uncatalogued galaxy, Dwarf 3, lies between NGC 6221 and NGC 6215, and may have formed out of bridge material.

The brightest part of the HI bridge lies roughly half-way between the interacting galaxies, indicating that bridge gas close to NGC 6221 and NGC 6215 may have fallen back to the galaxies. The asymmetric extensions to the HI envelope of NGC 6221 are likely to be re-accreted gas, still settling in. And the peculiar velocity field of NGC 6215 may be explained by accreted bridge material settling into a plane offset from the old disc.

The color figure below shows the HI emission (blue) in the bridge velocity range overlaid onto an optical image.
The inset shows Dwarf 3.

Selected figures from the paper:

Other Images:

An extract from Koribalski (1996, in the ``The Minnesota Lectures on Extragalactic Neutral Hydrogen'', ASP Conf. Series Vol. 106, ed. Evan Skillman, p. 238).

NGC 6221 is a barred spiral galaxy which is interacting with its brightest neighbour, NGC 6215, and possibly also with two newly discovered low-surface brightness galaxies (see Figure). The galaxy group lies only 10 degrees below the Galactic plane which explains the rather crowded field in the optical. Recent measurements with the ATCA 375-m array revealed an HI bridge between the two major galaxies and several extensions of the gas envelope of NGC 6221 which can be attributed to tidal forces between these galaxies (B. Koribalski et al., in prep.). Detailed studies of the Halpha line emission in NGC 6221 by Pence & Blackman (1984) revealed very large non-circular motions of the ionized gas, possibly as a result of tidal interactions and streaming motions along the bar. The HI distribution of NGC 6221 is quite similar to the optical emission; the central depression is caused by HI absorption which is observed over several hundred km/s. A nuclear ring is the most likely explanation regarding the similarity of this galaxy to others discussed here, but further data analysis is needed to confirm this idea. The HI velocity field looks much more regular than the Halpha field which was reconstructed from several long-slit spectra. The nuclear activity is characterized by star formation and a weak Seyfert 2 nucleus. Not much is known about NGC 6215, which is classified as a non-barred spiral galaxy of type SAS5 (RC3). Its HI emission extends from about 1450 to 1650 km/s.

Figure 1: HI distribution (contours) of NGC 6221 and neighbouring galaxies overlaid onto the optical emission (greyscale) from the Digitized Sky Survey (DSS). The contour levels are 0.15, 0.3, 0.6, 1.2, 2.25, 4.5, and 9 Jy/beam km/s. Please note that no primary beam correction has been applied here. A tidal HI bridge is visible between NGC 6221 and its brightest neighbour, NGC 6215 (v_sys = 1555 km/s). The two low-surface brightness galaxies (BK_1: v_sys = 1645 km/s and BK_2: v_sys = 1510 km/s) are newly discovered members of this interacting group of galaxies. These data have been obtained with the 375-m array of the ATCA (12 h). The angular resolution is about 1.5'.

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