J. Donley, B. Koribalski (ATNF); R. Kraan-Korteweg (University of Guanajuota, Mexico); A. Schroeder (University of Leicester, UK); L. Staveley-Smith (ATNF) and the ZOA team

How massive can a spiral galaxy become? Current theoretical models predict that spiral galaxies like the Milky Way form from the merger of smaller galaxies at times when the Universe was between 25% and 50% of its current age. This merger process is thought to create a population of present-day spiral galaxies whose number densities decline exponentially at masses higher than a characteristic value, M*. Observational evidence (e.g. from HIPASS, see page 32) support this theory and set M* to a neutral hydrogen (HI) mass of approximately 6x10⁹ solar masses. The density of galaxies much more massive than this characteristic value is poorly constrained, however, as these super-massive galaxies are very rare.

Through a large survey that utilised the Parkes multibeam receiver, we have discovered a super-massive spiral galaxy at a distance of 140 Megaparsecs. Data from the Parkes telescope suggest that the galaxy HIZOA J0836-43 has an HI mass of 7x10¹⁰ solar masses and a total dynamical mass of 1.5x10¹² solar masses. This HI mass is 20 times that of our own Milky Way Galaxy and the total mass is at least five times the Milky Way's mass! This places HIZOA J0846-43 among the most massive spiral galaxies known, with only a handful of galaxies whose HI and/or total dynamical masses are comparable. Some such galaxies include Malin I, ISOHDFS 27, UGC 1752, UGC 12591, and NGC 1961.

Follow-up high-resolution observations with the Compact Array confirmed that HIZOA J0836-43 is in fact a single system, with a disk radius of 66 kiloparsecs and an inclination-corrected rotational velocity of 312 kilometres per second. The galaxy's rotation curve, the rotational velocity as a function of radius, is constant at large radii. This suggests that, like normal spiral galaxies, HIZOA J0836-43 has a large dark matter halo extending well beyond the disk radius, a property also predicted by current hierarchical clustering models of galaxy formation. This galaxy is so large and massive that, if formed at the very beginning of the Universe, it would only have had time to complete 10 rotations! In addition to giving us insights into the HI properties of HIZOA J0836-43, our observations at the Compact Array have also provided radio continuum data. From these data, we have determined that J0836-43 is probably forming stars at a rate of about 26 solar masses per year.

We have also observed HIZOA J0836-43 with the Anglo-Australian Telescope (AAT) in the nearinfrared K and H bands. Because HIZOA J0836-43 is located behind the plane of the Milky Way in a region known as the Zone of Avoidance, the optical B-band emission originating from the galaxy is obscured by nearly 12 magnitudes of dust extinction, preventing us from obtaining an optical image of the galaxy. Luckily, the effect on infrared emission is much less severe than that on the optical light. The AAT observations have therefore allowed us to determine both the inclination of the galaxy, 65 degrees, as well as the distribution of its old stellar population. Both a galactic bulge and disk can be inferred from the infrared emission. The bulge has a central surface brightness of 15 magnitudes per square arcsecond and a scale length of 2 kpc. The disk has a scale length of 4 kpc and can be detected out to a radius of 20 kpc, even through 1 magnitude of extinction in the infrared K band.

The galaxy HIZOA J0836-43 is one of the most massive spiral galaxies ever detected. Although a nearby galaxy can be seen in the AAT image, there is no evidence that the two galaxies are interacting. The velocity field and integrated emission of HIZOA J0836-43 show no disturbances, suggesting that it is a normal, albeit very massive, galaxy. The detection of a super-massive galaxy such as HIZOA J0836-43 helps us to answer fundamental questions that arise when formulating theories of galaxy evolution. Large-scale blind surveys with receivers such as the Parkes multibeam receiver provide one of the best ways to detect such hidden giants.

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