Archive for January 7th, 2012

January 7, 2012

A hyperactive young galaxy

Active galaxies contain a supermassive black hole (SMBH) that causes vigorous radiation of electromagnetic energy as a result of rapid accretion of gas and dust. While almost all galaxies except dwarfs contain an SMBH in the center, active galaxies are rare – fewer that 1% of galaxies in the present universe. A very few active galaxies contain two active SMBHs. Even fewer have three. Before the latest discovery, only two examples had ever been documented in the literature.

The most recent example was so obscure it doesn’t even have a name. This example is unusual in other respects as well. It is quite distant, having a redshift of z=1.35. That means its light has taken 8.92 billion years to reach us. We see it as it looked about 4.75 billion years after the big bang. Most galaxies at that time were fairly mature. Not this one. It seems to be quite young and irregular in form, with 4 separate components. Three of those appear to contain active SMBHs.

Some of the details about this galaxy are not known very precisely. It’s (barely) possible that one or more of the apparent active SMBHs are actually bulges hosting very active star formation. But the spectroscopic evidence is heavily against that.

The three SMBHs are not especially large as such things go. The masses could be as much as 3.1×106 M, 1.0×107 M, and 1.2×107 M. However, these are upper bounds, and the actual masses could be only 20% as large. For comparison, the Milky Way’s SMBH is 4.2×106 M, but many SMBHs are in the 109 to 1010 M range.

There are several intriguing questions about this object. The first: how did it form? Is it a 3-way merger of smaller galaxies of similar size, each of which had its own SMBH already? In the modern universe, such mergers are very rare. The evidence is that they were also rare in the era of 5 billion years after the big bang. It’s a lot more likely that this galaxy is still rather young and in an early stage of formation.

If that is the case, then there are very interesting questions about how the object can have three active nuclei.

read more »