Friday Galaxy Blogging : Active Galaxy Mk 509

Are you ready for this? This week's Friday Galaxy is Mk509:

DSS2 image from Skyview

Admittedly, the image of this galaxy does not rank very high on the "wow, what a cool and pretty looking galaxy" scale. However, this is an interesting galaxy because of what's going on at the nucleus. Like all large galaxies, there is a supermassive black hole at the core of this galaxy. The black hole in this galaxy is being fed, giving rise to what we call an Active Galactic Nucleus (AGN). If you drop gas down into an accretion disk near a supermassive black hole, a tremendous amount of gravitational potential energy is released. That energy goes into heating up the accretion disk, and the radiation from the accretion disk energies all sorts of other fun behavior in the galaxy.

How do we know? From the spectrum. We see strong emission lines that come from gas that has been ionized by high energy radiation. We also see high-ionization lines. A couple of weeks ago, my graduate student Katie Chynoweth obtained a spectrum of this galaxy in Chile at my request. Actually, I was hoping to get Mk 590, but in a fit of dyslexia, I accidentally gave her the wrong galaxy name... oops. Here's a part of the spectrum:


The Hβ line comes from Hydrogen gas, whereas the two "[OIII]" lines come from doubly-ionized Oxygen (that is, Oxygen that has had two of its electrons stripped off of it from the high-energy radiation). To put this in context, let's compare the spectrum to that of another active galactic nucleus, NGC 7130:


There are a number of obvious differences. First, you may have noticed that the lines are showing up at different wavelengths. That's just because the galaxies are at different redshifts. NGC 7130 is at a redshift of z=0.016, putting it a "mere" 220 million light years away. Mk 509 is at a redshift of z=0.034, and is 460 million light-years away.

The second obvious difference is in the Hydrogen-β line. In NGC 7130, the Hβ line is no wider than the [OIII] lines; slightly narrower, in fact. However, in Mk 509, the Hβ line is very wide. The width of the lines come from motions of gas. Compared to the overall redshift of the galaxy, some gas is moving towards you, some is moving away, and what we see is a mishmash of all of that gas. The width of the Hβ line in Mk 509 tells us that we have gas moving back and forth with velocities of several thousand kilometers per second. This is gas that is right down near the center of the AGN, within a few light-weeks of the supermassive black hole itself. NGC 7130 almost certainly also has clouds like this, but we don't see the broad line because there is obscuration. The orientation of the structures near the nucleus is such that the so-called "broad line region" is blocked to our view.

More like this

So let me try to understand. The HBeta (Balmer line?) is primarily present in the rapidly spinning accretion disk. The OIII in the slower moving emmision nebulae. I woulda thought that the OIII is a higher ionization state, i.e. higher temps energies etc. So is the accretion disk cooler? Or is the pressure in the disk much much higher than the emmision nebulae -which a presume are ionized by UV from nearby stars?

(Yeah I was a wanna-be AstroPhyscist- who ended up doing other stuff).

Actually, the H-beta is present in clouds orbiting about the center of the nucleus, rather than in the accretion disk itself.

Re: the OIII, there is doubly ionized oxygen in those clouds. It's just that when the atoms sit there in the upper state of the transition, the upper state is long-lived enough that the atmos are likely to collide with an electron (thereby knocking the atoms out of the upper stated) long before they get a chance to radiate.

Out where the narrow lines are coming from, the densities are lower. The mean time between collisions is longer than the average lifetime of the tranisition.


I had a lot of fun looking at galaxies like this for my undergraduate project (which developed into a summer project), trying to use the response of the broad lines to the continuum changes to infer properties of the broad-line region ("reverberation mapping"). Didn't quite get anything much out of it, but who knows, perhaps I'll get another shot at it sometime :-)

By Philip Rodrigues (not verified) on 30 Apr 2007 #permalink