HI in starburst galaxies

Bärbel Koribalski (ATNF)

(1996), in ``Starburst Activity in Galaxies'', Tonantzintla, eds. J. Franco, R. Terlevich, & G. Tenorio-Tagle, Revista Mexicana de Astronomia y Astrofisica Serie de Conferencias, Vol. 6, p. 55

Resumen. Cantidades grandes de gas fri'o son un ingrediente necesario para la actividad de `starburst' en la regio'n nuclear de galaxias. Mientras sufficiente gas fluye hacia la regio'n nuclear la actividad de `starburst' continua. Esa actividad ha sido muy probablemente provocada y aumentada por el transporte de masa hacia el centro iniciado por una barra. La corriente de gas a lo largo de la barra es bien visible en el campo de velocidad de HI. Interaccio'nes entre galaxias tambien tienen una gran importancia porque pueden provocar la formatio'n de una barra o en caso de interaccio'nes fuertes o mergers directamente mover gas hacia el centro. Barras y indicios de interaccio'nes con galaxias cercanas se observa frequentamente en galaxias `starburst' (por exemplo en el Grus-Quartet). Para entender la evolutio'n de las galaxias `starburst' necesitamos estudiar suya dina'mica del gas y tambien su medio ambiente.

Abstract. Large amounts of cold gas are a necessary ingredient for nuclear starburst activity to happen in galaxies. As long as sufficient gas is supplied to the nuclear region the starburst activity continues. One possible mechanism to trigger and fuel the nuclear activity of spiral galaxies is through the transport of gas from the disk into the center as induced by the potential of a bar. Such bar-induced streaming motions are visible in the HI velocity field. Galaxy interactions also play an important role as they can trigger the formation of bars or in case of strong interactions or mergers directly drive gas into the center. Bars as well as signs of interactions with a companion are often observed in starburst galaxies (see e.g. the Grus-Quartet). Thus, to understand the evolution of these most exciting galaxies we need to study their gas dynamics and their nearby environment.

Keywords: GALAXIES: GAS DYNAMICS - INDIVIDUAL: GRUS-QUARTET, NGC 253, CIRCINUS, NGC6221-GROUP --- LINE: 21-CM

Introduction
Why is the study of neutral atomic hydrogen gas (HI) in starburst galaxies important ?

• In general, the HI emission line is still the best tracer for the overall gas distribution and kinematics in galaxies. The HI envelope of spiral galaxies usually extends much further out than the optical emission, thus providing a unique means of determining the mass distribution in their outer regions.
• In interacting galaxies the HI line is the best tracer for tidal tails, bridges between galaxies, and low-surface brightness companions.
• In barred galaxies (type SB and SX) we can, e.g., use high-resolution HI velocity fields to study the gas flow in and around the bar. The galaxy velocity field directly reflects the galaxy potential.
• And in starburst galaxies, which are often found to be barred and interacting with neighbouring galaxies, we can study both phenomena by using the HI emission line. We may thus find out which mechanisms triggered and sustained the starburst activity. In addition, we can use the HI absorption against the bright and often extended nuclear continuum source to study the nuclear gas kinematics and therefore the central mass concentration. The latter technique requires the largest configurations of the existing synthesis telescopes and VLBI for distant ultraluminous galaxies.

Observations and Results

The Grus-Quartet. The Grus-Quartet consists of four large spiral galaxies: NGC~7590, NGC~7599, NGC~7582, NGC~7552. Two of the galaxies, NGC~7582 and NGC~7552, are well-known starburst galaxies. Both of them are classified as barred. HI observations with various configurations of the Australia Telescope Compact Array (ATCA) reveal the following

• clear signs of tidal interactions, e.g. a bridge between NGC~7590 and NGC~7582 as well as an extension from NGC~7582 toward NGC~7552,
• an S-shaped velocity field in the nearly face-on galaxy NGC~7552 showing the bar, and
• fast-rotating nuclear rings in both NGC~7582 and NGC~7552 as indicated by high-resolution HI absorption measurements. A nuclear disk (diameter about 1 kpc) of HI regions and outflow of ionized gas has been suggested by Morris et al. (1985).

The starburst galaxy NGC~253. One of the closest and most active galaxies in the southern sky is the well-known starburst galaxy NGC~253, a member of the Sculptor group. ATCA HI observations by Koribalski, Whiteoak, & Houghton (1995) show the following

• a gas envelope only slightly larger than the optical B_25 emission,
• a rather regular velocity field, and
• a fast-rotating nuclear ring. There are also indications of a nuclear outflow.

The Circinus Galaxy. Maybe less known but at least as exciting as NGC~253 is the Circinus galaxy. It is a nearby spiral galaxy only 4 degrees below the Galactic plane. The nuclear activity is caused by star formation and a prominent Seyfert 2 nucleus. Giant radio lobes (perhaps associated with outflow) are emerging from the nuclear region (Elmouttie et al. 1995) very similar to those in NGC 3079. Freeman et al. (1977) measured a half-width of at least 32 arcmin x 15 arcmin for the HI extent of Circinus, much larger than the optical diameter (about 10 arcmin) of the galaxy. Jones et al. (1996), and Koribalski & Whiteoak (1996) have recently mapped Circinus with several configurations of the ATCA. The measured HI distribution shows

• several spiral arms extending over an area of at least 30 arcmin,
• an elongated central disk (length about 5 arcmin) plus a peculiar velocity field indicating a bar, and
• a central `hole' caused by HI absorption against the bright central continuum source revealing a fast-rotating nuclear ring.

The NGC~6221-Group. This group consists of two big spiral galaxies, NGC~6215 and NGC~6221, and several newly detected low-surface brightness galaxies. The most prominent member of the group is the barred starburst galaxy NGC~6221. ATCA observations revealed an HI bridge between NGC~6221 and NGC~6215. In addition, NGC~6221 shows several peculiar HI extensions, some of them with rather high velocity dispersion and a rather distorted HI velocity field.

SUMMARY
Although HI observations cannot shed much light on the starburst activity in galaxies itself, they can provide an enourmous amount of information regarding the mechanisms, e.g. the transport of gas from the disk into the nuclear region, which trigger and sustain the starburst. HI observations contribute strongly to the understanding of the overall evolution of starburst galaxies. In particular, high-resolution HI absorption measurements reveal a large amount of cold neutral atomic gas in the nuclear region of starburst galaxies, mixed within molecular gas and HI regions, and rotating at rather high speed around the central mass concentration. A specific aim of this study is to understand the behaviour of the rotating curve in the nuclear region of galaxies.

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