The Southern Pinwheel Galaxy M83 viewable in the southern hemisphere has not one but two black holes at its center. Possessed of a bright central nucleus and two extensive spiral arms, M83 gives a “sunken in” appearance even in small amateur telescopes. In larger instruments, this 1/3rd full moon diameter barred beauty begins to show tufts and folds near its core – the kind of mottling that tells astronomers – amateur and professional – that vast clouds of gas and cold dust are collapsing under the influence of gravitation to form massive starburst clusters. The galaxy is 30,000 light-years across.

But now a team of investigators capturing near-visible light are telling us that this relatively light-weight – but beautifully-formed face-on galaxy – is being churned up by the presence of not one but two mass-concentrations (nuclei) which could potentially take the form of supermassive black holes (SMBHs). Perhaps most surprising of all is the fact that neither nuclei lies exactly at the galaxy’s very core.

If you click the little image above you will get a full sized image, which I think is VERY DAZZLING! You might have to force Explorer to allow you to view full screen, buy clicking the little box that comes up at the lower right when you hover the mouse over the image — it’s worth the trouble.

Image credit: ESO

M83, the Southern Pinwheel Galaxy, is one of the finest examples of a grand spiral galaxy visible to amateur astronomers in the Night Sky. Possessed of a bright central nucleus and two extensive spiral arms, M83 gives a “sunken in” appearance even in small amateur telescopes. In larger instruments, this 1/3rd full moon diameter barred beauty begins to show tufts and folds near its core – the kind of mottling that tells astronomers – amateur and professional – that vast clouds of gas and cold dust are collapsing under the influence of gravitation to form massive starburst clusters.

But now a team of investigators capturing near-visible light are telling us that this relatively light-weight – but beautifully-formed face-on galaxy – is being churned up by the presence of not one but two mass-concentrations (nuclei) which could potentially take the form of supermassive black holes (SMBHs). Perhaps most surprising of all is the fact that neither nuclei lies exactly at the galaxy’s very core.

In a paper entitled “Double Nucleus in M83″ published Friday, May 13, 2005, astronomers Damián Mast, Ruben Diaz, and Paz Aguero of Córdoba’s National University and the Astronomical Observatory of Cordoba have confirmed “the presence of a secondary nucleus or mass concentration” first suggested by Niranjan Thatte and a team of investigators in the year 2000.

The paper goes on to say this secondary nucleus “was not detected in the optical HST images due, probably, to strong dust extinction” of visual light. Because of this, the team had to use complex software-based spectrographic analysis techniques – plus highly-resolved archival radio-frequency observations – to reveal something only suspected using the 8 meter Very Large Telescope (VLT) in Chile by Thatte et al five years earlier. As a result, that secondary nucleus is now known to lie 3.9 arc seconds (~200 light years) northwest of M83′s optical center.

According to Damián Mast, “the mass concentrations could be star clusters instead of BHs (black holes).” Damián also says that “there is little non-thermal radiation associated with either mass concentration. The only X-ray sources detected are associated with supernova remnants and, considering the magnitude of the (associated) starburst, there must be a lot of them. So, is there a BH in the nuclear region of M83? Our observations do not allow us to confirm or deny that. We only can tell that there are kinematic fingertips of two mass concentrations.”

Confirming the actual existence of a SMBH is rarely easy. In those few instances where a galaxy orients face-on toward us (and its relativistic jets point right at us), high levels of radiation across a wide range of wavelengths makes detection relatively simple. But where such SMBH’s are quiescent, or oriented elsewhere, the primary method is to determine radial velocities of stars using doppler shifts associated with stellar spectra (Damián’s “kinematic fingertips”). Damián points out that “the spatial resolution we have, only allows us to estimate them inside a radius of 1.5 arc seconds”. This inability to resolve the two nuclei fully makes it difficult to confirm that they are SMBH’s – although they may very well be.

In the instance of M83, the investigating team did a series of observations using the Multifunctional Integral Field Spectrograph (MIFS) attached to the 1.54 meter Bosque Alegre Astrophysical Station in the Sierras Chicas Mountains of Argentina. The data was collected from March to May 2001 using the MIFS deep (red-biased) optical light. This allowed them to evaluate shifts in Hydrogen-alpha and Nitrogen II emission bands as dust and gas near the core swirled about in response to the nuclei. Because of limitations imposed by local atmospheric conditions, the team augmented their data with spectral information collected using the Hubble Space Telescope (HST). This enabled them to create 3D maps (in various light-frequency overlays) of M83′s core region. Damián points out that “M83 has been a subject of our team’s investigations for years. We chose it as a target to show how 3D spectroscopy can be used as a powerful tool to elucidate complex systems such as the one found in M83.”

Among the many interesting features seen using the above approach, the investigating team reports “a bright small red arc (emerging) to the SW” and “a giant star forming arc” – one first reported by astronomers in 1991. Within that arc to the northeast of the nucleus, “several star clusters are present”. These clusters were first noted in 2001. The team also reports that “Going counterclockwise through the arc, a highly obscured region is located with a large number of dust patches and a star forming region emerging from the dust”. All this lies some 250 LYs west of the galaxy’s nucleus.

Having clarified the picture of features in M83′s core region, the team analyzed radial velocities associated with ionized gases. This analysis incorporated data from some 1000 LY’s of M83′s core region. They found that “The radial velocity field has a distortion that indicates the presence of two mass concentrations”. Neither of these concentrations lies right at the optical center of the galaxy. According to the team, heated gases in these locales are moving as fast as 634km/sec. This is substantially slower than similar velocities (1500kms per second) associated with the Milky Way’s SMBH (one that is several times more massive than either of those seen in M83).

Perhaps the most intriguing learning coming out of the team’s investigation is the relationship between the second nucleus and the end of the giant arc of star formation it is associated with. In the words of the paper, this relationship suggests that this nucleus “would be the trigger of the nuclear starburst seen in M83″.

Damián took some time from his busy schedule to describe the astronomical implications of the team’s research, “a double nucleus could mean that two galaxies have fused together, but we must consider ‘major mergers’ and ‘minor mergers’. Major-mergers occur when two nearly equal mass galaxies fuse and the remnant would be a giant elliptical galaxy (potentially the destiny of the fusion of The Milky Way and M31). In the case of a minor-merger event, a dwarf satellite galaxy falls into the host. Due to dynamical friction, ‘the intruder’ falls toward the main nucleus of the galaxy while orbiting it. The global appearance of the host galaxy, if we consider a giant spiral, would not be affected.”

Is the arc of star formation seen by the team implicated in all this? Again according to Damián, “There are different works suggesting that, as an intruding dwarf galaxy passes through dense gas and dust media, it sets off star formation. This leaves behind a visible path of star forming regions.”

But as in most things, answers lead to questions. Damián concludes, “To confirm the minor merger hypothesis in M83, numerical simulation must be done.”

Written by Jeff Barbour