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Neutral hydrogen gas in interacting galaxies: the NGC 6221/6215 galaxy group
Neutral hydrogen observations of the spiral galaxies NGC 6221 and 6215with the Australia Telescope Compact Array (ATCA) reveal a wide,two-stranded bridge of at least 3 × 108Msolar which can be traced between the two galaxies over aprojected distance of 100 kpc. The velocity gradient of the HI bridgeprovides a rough estimate for the time since the encounter of 500 Myr.For NGC 6221, the brightest and most massive galaxy of the group, wederive a dynamical mass of Mtot= 8 × 1010Msolar, while its companion NGC 6215 has a mass of onlyMtot~ 2 × 109 Msolar. Further, wefind three low-surface-brightness dwarf galaxies (Dwarfs 1, 2 and 3) inthe neighbourhood of NGC 6221/15 with HI masses of 3.3, 0.6 and 0.3× 108 Msolar, respectively. The smallest,previously uncatalogued galaxy, Dwarf 3, lies between NGC 6221 and 6215,and may have formed out of bridge material.The brightest part of the HI bridge lies roughly halfway between theinteracting galaxies, indicating that bridge gas close to NGC 6221 and6215 may have fallen back to the galaxies. The asymmetric extensions tothe HI envelope of NGC 6221 are likely to be reaccreted gas, stillsettling in. Also, the peculiar velocity field of NGC 6215 may beexplained by accreted bridge material settling into a plane offset fromthe old disc.
| The Obscuring Starburst of NGC 6221 and Implications for the Hard X-Ray Background
We present NGC 6221 as a case study of ``X-ray-loud compositegalaxies,'' which appear similar to starbursts at optical wavelengthsand resemble traditional active galactic nuclei in X-rays. The netoptical spectrum of NGC 6221 is properly characterized as a starburstgalaxy, but in X-rays, NGC 6221 is similar to Seyfert 1 galaxies,exhibiting a power-law continuum spectrum, a broad Fe Kα line, andcontinuum variability on timescales of days and years. High-resolutionimages reveal that the detected active nucleus is relatively weak, notonly at optical, but also at near-infrared wavelengths. An obscuringstarburst, in which the interstellar gas and dust associated with thestarburst conceal the active nucleus, accounts for these peculiarfeatures. We demonstrate quantitatively that obscuration by columndensity NH=1022 cm-2 combined withrelatively weak intrinsic nuclear activity can produce an opticalspectrum that is characteristic of the surrounding starburst alone.While optical surveys would not identify the active nuclei that makethese galaxies significant X-ray sources, such galaxies may, in fact, beimportant contributors to the X-ray background.
| A catalogue of galaxies behind the southern Milky Way. II. The Crux and Great Attractor regions (l~ 289o to 338o)
In this second paper of the catalogue series of galaxies behind thesouthern Milky Way, we report on the deep optical galaxy search in theCrux region (289o <= l <= 318o and-10o <= b <= 10o) and the Great Attractorregion (316o <= l <= 338o and-10o <= b <= 10o}). The galaxy cataloguesare presented, a brief description of the galaxy search given, as wellas a discussion on the distribution and characteristics of the uncoveredgalaxies. A total of 8182 galaxies with major diameters D >~ 0.2arcmin were identified in this ~ 850 square degree area: 3759 galaxiesin the Crux region and 4423 galaxies in the Great Attractor region. Ofthe 8182 galaxies, 229 (2.8%) were catalogued before in the optical (3in radio) and 251 galaxies have a reliable (159), or likely (92)cross-identification in the IRAS Point Source Catalogue (3.1%). A numberof prominent overdensities and filaments of galaxies are identified.They are not correlated with the Galactic foreground extinction andhence indicative of extragalactic large-scale structures. Redshiftsobtained at the South African Astronomical Observatory (SAAO) for 518 ofthe newly catalogued galaxies in the Crux and Great Attractor regions(Fairall et al. \cite{Fairall98}; Woudt et al. \cite{Woudt99}) confirmdistinct voids and clusters in the area here surveyed. With this opticalgalaxy search, we have reduced the width of the optical ``Zone ofAvoidance'' for galaxies with extinction-corrected diameters larger than1.3 arcmin from extinction levels AB >= 1.0m toAB >= 3.0m: the remaining optical Zone of Avoidance is nowlimited by | b | <~ 3o (see Fig. \ref{cruxf1new}). The twooptical catalogues and their respective listings of IRAScross-identifications are available in electronic format at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/441
| Extragalactic large-scale structures behind the southern Milky Way. III. Redshifts obtained at the SAAO in the Great Attractor region
In the third of a series of papers on large-scale structures behind thesouthern Milky Way, we report here on redshifts obtained at the SouthAfrican Astronomical Observatory (SAAO) in the Great Attractor region(318deg <~ l <~ 340deg , |b| <= 10deg , Woudt 1998). Thisregion encompasses the peak in the reconstructed mass density field,associated with the Great Attractor (Kolatt et al. 1995, Dekel et al.1998) and covers the crossing of the Supergalactic Plane with theGalactic Plane. Our deep optical galaxy search in the Zone of Avoidance(ZOA) in this region (Woudt 1998) has resulted in the detection of 4423galaxies with observed diameters larger than 0.2 arcmin. We haveobtained reliable redshifts for 309 galaxies of the 4423 galaxies withthe ``Unit'' spectrograph (first with a Reticon, then with a CCDdetector) at the 1.9-m telescope of the SAAO. An additional 13 tentativeredshifts are presented. Before our survey, 127 galaxies had apreviously recorded redshift (NED and SRC96). Given a small overlap withthe literature (44 galaxies), we present here redshifts for 265 galaxiesthat had no previous recorded velocity. In addition, we present centralvelocity dispersion (sigma_o ) measurements for 34 galaxies in ACO 3627.It is known that the Great Attractor (GA) region is overdense ingalaxies at a redshift-distance of v ~ 5000 {km s-1 }(Fairall 1988, Dressler 1991, Visvanathan & Yamada 1996, di Nella etal. 1997). We realise here, however, that the Great Attractor region isdominated by ACO 3627 (hereafter referred to as the Norma cluster), ahighly obscured, nearby and massive cluster of galaxies close to theplane of the Milky Way (l, b, v) = (325.3deg , -7.2deg , 4844 {kms-1 }) (Kraan-Korteweg et al. 1996, Woudt 1998). Previousredshift surveys in the GA region have failed to gauge the significanceof the Norma cluster, primarily due to the diminishing effects of theGalactic foreground extinction on the partially obscured galaxies. Inthe absence of the obscuring effects of the Milky Way, the Norma clusterwould have appeared as prominent as the well-known Coma cluster, butnearer in redshift-space. This cluster most likely marks the bottom ofthe potential well of the Great Attractor (Woudt 1998). All the tablesare only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| General study of group membership. II - Determination of nearby groups
We present a whole sky catalog of nearby groups of galaxies taken fromthe Lyon-Meudon Extragalactic Database. From the 78,000 objects in thedatabase, we extracted a sample of 6392 galaxies, complete up to thelimiting apparent magnitude B0 = 14.0. Moreover, in order to considersolely the galaxies of the local universe, all the selected galaxieshave a known recession velocity smaller than 5500 km/s. Two methods wereused in group construction: a Huchra-Geller (1982) derived percolationmethod and a Tully (1980) derived hierarchical method. Each method gaveus one catalog. These were then compared and synthesized to obtain asingle catalog containing the most reliable groups. There are 485 groupsof a least three members in the final catalog.
| ESO 138 G1 - A high excitation Seyfert 2 nucleus in a low luminosity early-type galaxy
The spectral properties of the active nucleus ESO 138 G1 are analyzed.The continuous energy distribution is monitored over the wavelengthrange 3400-9700. A detailed analysis of the emission lines reveals thatthere exists in this object a stratification of the emitting gas cloudsaccording to their density and dominant ionization stage. The width ofthe lines is observed to correlate with their critical density. The N/Oabundance ratio in the gas is about three times larger than the solarvalue. Compared to the other two active nuclei which are known so far toexhibit such a high excitation line spectrum, IIIZw77 and Tol 0109-383,ESO 138 G1 builds up a sequence of spectral properties which can beunderstood if the mean electron density and the excitation of the gasclouds decrease from IIIZw77 to ESO 138 G1.
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