NGC 404
Published on Nov 3, 2008 at 5:54 pm.
2 Comments.
Filed under galaxies.
I started to learn the sky when I was in graduate school. I used to go out to the observatory on off nights and just have fun. I would pick out a star chart for a constellation and just try to find everything that I could in that constellation. On one of those nights, I picked the constellation Andromeda. Naturally, there was M31, the famous Andromeda Galaxy, and its two satellites, M32 and NGC 205 (M110). There were plenty of other objects there, too. Then, I noticed a galaxy designated NGC 404 right next to the star ? And, Mirach. “Oh, that should be easy to find!” I thought. At the time, I was still rather new to actual telescopic observations, and I had already discovered that many galaxies were hard to see. But, Mirach was a bright star, and it would be easy to find. So, I pointed the telescope at the star, and then offset it a tiny bit to where the galaxy should be. I looked in the eyepiece, and sure enough, there was the galaxy! I’ll admit that it wasn’t the easiest one to see. The star Mirach overpowers it. In fact, it is easiest seen when you position the telescope so that the star is just on the edge, or outside, the field of view.
Over the years, this became a favorite galaxy to have students find. It was about the dimmest galaxy that the introductory astronomy students could reliably find due to its proximity to the bright star Mirach. It was also a good teaching tool in that it was easier to see at lower magnifications than at higher powers. Students always try to put the highest power eyepiece that they have available in the telescope, and that is seldom the best choice.
NGC 404 is a difficult galaxy to study, though. Its proximity to Mirach makes photographing it difficult. Long exposures that are needed to bring out structure in the galaxy overexpose the star, often washing out the image of the nearby galaxy. A good picture of the two objects was featured in a recent Astronomy Picture of the Day. Note that when I say that the galaxy and the star are nearby, I mean that they appear near one another in the sky. Mirach is only about 200 light years away, while NGC 404 is more like 10 million light years distant. The two objects are simply along nearly the same line of sight. Because NGC 404 is so overshadowed by Mirach, it has gotten the nicknames of “Ghost of Mirach Galaxy” and “Mirach’s Ghost Galaxy.” Both names are really unofficial, so there is not actually a standard. If you call it either one, then whoever knows it by one or the other name will know what galaxy you are talking about. At a distance of only 10 million light years, NGC 404 is not all that far away in cosmic terms. It would likely be a very well studied galaxy were it not for the fact that Mirach overpowers it.
For years, NGC 404 has been classified as an Sa or S0 (lenticular) galaxy. That is a disk galaxy with very small, or even non existent, spiral arms. The spiral arms of a disk galaxy are where stars form. So galaxies of this type tend to have very little, if any, current star formation going on in them. Disk galaxies, though, typically owe their form to the way that stars form in them. Our own galaxy is a disk galaxy, but with lots of star formation going on. So, we are likely something like an Sb galaxy. In fact, there seems to be a bar in the center of our galaxy, so the Milky Way is most likely more of an SBb galaxy. I blogged about galaxy classification about two years ago, so you can read there a bit more about what these letters mean. As technology has improved, we have learned a lot more about galaxies. What we have sometimes found is that the bright part of the galaxy that we see most easily can look like one type, while a much dimmer component of the galaxy often surrounding the bright part looks like another type of galaxy. There are numerous examples where this has muddied the whole galaxy classification system. NGC 404, as it turns out, is another example where this sort of thing is going on.
A little over five years ago, the Galaxy Evolution Explorer (GALEX) satellite was launched. GALEX was destined to produce an all sky survey in ultraviolet light. Galaxies with active star formation often are bright in UV light, so such a survey would be a good study of such galaxies. Of course, some galaxies, most notably the ellipticals and the S0 galaxies, don’t have much active star formation, so they shouldn’t show up very prominently in an ultraviolet study. Naturally, that suggests that NGC 404 should not look like much to GALEX. But, as often happens in astronomy, NGC 404 turned out to have a surprise for us. In UV, the galaxy is obviously much larger than it appears in most visual images that I have seen. That part is not unexpected, though, because most visual images are limited by the exposure of Mirach, not the galaxy itself. But, what is unexpected is that NGC 404 appears to have a ring of hot gas in the outer parts of its disk. This hot gas is lit by the light of many young stars. You can see this ring in the image at the top of this post. The ring also coincides with a gas ring found by other researchers using the Very Large Array radio interferometer.
So, how did a galaxy that is not supposed to have much star formation wind up with a ring of star formation? Well, NGC 404 is not the only ring type galaxy known. A more famous example would be the Cartwheel Galaxy. While NGC 404 and the Cartwheel have some fundamental differences, both are believed to have a collision with a smaller body as the primary reason for their ring-like features. Even though the Cartwheel is more than ten times farther away than NGC 404, it has been more extensively studied, since there is nothing else bright nearby to get in the way of studies. But, we are slowly learning more about its nearer cousin. It may be that NGC 404 actually absorbed its companion in an event that we call galactic cannibalism. Even if NGC 404 did not absorb its companion, it may have stolen some of the smaller galaxy’s gas, allowing NGC 404 to once again undergo a spurt of star formation. Or, perhaps, the collision simply caused an expanding shock wave to expand outward through the galaxy’s rarefied interstellar medium to initial its last feeble bit of star formation. Over the next few years, renewed interest in this galaxy may help to answer these questions.
Until then, it will remain one of my favorites to show people.
-Astroprof
Image courtesy NASA, GALEX
Mark Seibert on November 3, 2008 at 8:35 pm: 1
Nice description of NGC 404! I suspect it will indeed get a bit more attention now as it is the closest known lenticular.
Andromeda » Blog Archive » Astroprof’s Page » NGC 404 on November 3, 2008 at 10:53 pm: 2
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