“One exciting possibility is that there might be a second black hole in each system”
Astronomers love a good mystery; equally they have rarely been known to turn their noses up at a silly acronym, and this month we look at both in the form of a strange set of galaxies known as Hot Dust Obscured Galaxies, or Hot DOGs.
These unusual systems were first spotted in data from the WISE infrared telescope. They are certainly bright – a Hot DOG called W2246-0526 is the most infrared luminous galaxy known – but it is their colours that make them distinctive. They shine more brightly in longer wavelengths, suggesting that the light we see comes from dust at a temperature of more than 60K (about –210ºC, positively toasty compared to dust in a normal system).
Glowing dust is an important source of light from most star-forming galaxies, but the presence of so much warm material in these systems needs an explanation. The discoverers suggested that the Hot DOGs must be powered by very luminous active galactic nuclei, voraciously feeding supermassive black holes enshrouded and hence hidden from view by immense clouds of dust.
So far, so straightforward. Hot DOGs are found between redshifts of one and four, corresponding to a time when the largest galaxies around were forming the vast majority of their stars and rapidly accreting material onto the black holes at their centres. The scarcity of Hot DOGs suggests that only the largest of galaxies would have active galactic nuclei powerful enough to suitably light up their dust, but a few of their number have a further secret.
Eight of the Hot DOGs known to date are also shining brightly in the ultraviolet. This is odd; if we were seeing the hot gas that must be feeding the active galactic nuclei directly we’d expect it to be bright in these shorter wavelengths, but we need the centre of activity to be shrouded in dust to make it a Hot DOG in the first place. One exciting possibility is that there might be a second black hole lurking in each of these systems.
To test this idea, the team behind this month’s paper used the Chandra X-ray Observatory to stare at one particular Hot DOG. They found, perhaps sadly, no sign of a second active galactic nucleus; everything seems consistent with one extremely greedy black hole sitting at the centre of the galaxy. So where does the ultraviolet light come from?
They consider the idea that it might be a coincidence, with the galaxy happening to undergo a huge burst of star formation just as its black hole grows most rapidly. The rates involved are prodigious, however, with star formation rates a thousand times that of the Milky Way required. Instead, what seems to be happening is that light from the galaxy’s centre is being reflected by curtains of dust – we see what seems to be an extended volume shining in ultraviolet but this is just a mirage.
If this is right – and further Hubble and Chandra observations are planned – then it means these systems are being observed during the most spectacular period of growth for central black holes yet found. Hot DOG indeed.