Science and Reason: Supermassive black hole in a dwarf galaxy

Supermassive black hole in a type of galaxy where nobody expected to find one? Henize 2-10 is a small, mostly unremarkable compact dwarf galaxy. Its estimated dynamical mass is about 10¹⁰ M_⊙, only a few percent of our galaxy's mass, and its distance from us is about 30 million light years. It is irregular in shape and does not fit in any category of the standard Hubble sequence.

The only respect in which Henize 2-10 has attracted attention – for several decades – before now is an extremely high rate of star formation in comparison to its size. The rate is 10 times that of the Large Magellanic Cloud, a satellite galaxy of the Milky Way that is also irregular in form and has approximately the mass of Henize 2-10.

This research – An actively accreting massive black hole in the dwarf starburst galaxy Henize 2-10 – recently published Nature, now offers good evidence that at the center of Henize 2-10 is an active black hole of substantial but somewhat uncertain mass between 2×10⁵ M_⊙ and 2×10⁷ M_⊙. That's a lot – it could exceed the mass of the Milky Way's black hole, ~4.2× 10⁶ M_⊙.

The evidence presented that Henize 2-10 contains an actively accreting massive black hole is pretty good. It includes detection of radio emissions with a substantial non-thermal component. In other words, much of the radio emissions is due to something besides black body radiation – perhaps synchrotron radiation typical in active black hole jets. There is also a point source of high-energy X-ray emissions coming from the same location as the radio emissions. The evidence that these emissions are due to an active black hole isn't perfect. In particular, long-baseline interferometry shows gaps in the radio source, and the radio spectrum does not have the shape of a typical radio galaxy's. But consideration of other possible explanations indicates that the alternatives are rather improbable.

However, the paper concludes "the massive black hole in Henize 2-10 does not appear to be associated with a bulge, a nuclear star cluster or any other well-defined nucleus. This unusual property may reflect an early phase of black-hole growth and galaxy evolution that has not been previously observed. If so, this implies that primordial seed black holes could have pre-dated their eventual dwellings."

The authors are implying that this black hole could have existed before Henize 2-10 itself. And further, since galaxies in the very early universe (z≥7) have many similarities to Henize 2-10 (as well as certain differences), that many of these very early galaxies could also have formed around pre-existing massive black holes.

These concluding observations should, on the basis of the evidence provided, be regarded as rather speculative. There are substantial logical gaps in the reasoning.

For one thing, Henize 2-10 is pretty unusual based on its high rate of star formation. This implies an unusual and probably chaotic recent history. And so there really isn't much solid reason to think that the central black hole predated the galaxy.

How closely Henize 2-10 resembles very early galaxies is also open to question. The earliest stars, which made up the earliest galaxies, had very low metallicity and therefore tended to be much larger, brighter, and short-lived than stars forming in the present era. The assumption that galaxy evolution would be pretty similar between now and then is hard to make.

Some of the popular media accounts go even further and suggest that "most" galaxies probably formed around pre-existing black holes. Even if that were true for Henize 2-10, all that can legitimately be inferred is the possibility, not the necessity, of that circumstance in most cases.

There have been reports of the existence of supermassive black holes in galaxies without central bulges (not just irregular galaxies) – here, for example. There have even been studies of active black holes in the early universe that may have predated their galaxies, one of which I wrote about in this article: Which came first - the galaxy or the black hole?. There are also cases of fairly normal galaxies, such as M33, that seem to have at most a very small central black hole – see here.

So it's certainly a very real issue whether, at least in some cases, central black holes form before their galaxies, but the present study is just another interesting data point, not the last word on the subject.

Reines, A., Sivakoff, G., Johnson, K., & Brogan, C. (2011). An actively accreting massive black hole in the dwarf starburst galaxy Henize 2-10 Nature, 470 (7332), 66-68 DOI: 10.1038/nature09724

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