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Tom Mazanec
2012-Jan-24, 01:15 PM
I have seen estimates that there are several supernovae in the MW every century, and also that M31 is about twice the size of the MW. We would miss most of the MW supernovae because we are in the plane of the galaxy, and dust would obscure them. But we should see most of M31's supernovae. AFAIK, only SN1885 was observed in M31. Are the estimates off, or is there something else going on?

antoniseb
2012-Jan-24, 03:21 PM
... Are the estimates off, or is there something else going on?

This is an interesting question. There was one SN remnant found in the Milky Way that is only a couple hundred years old, and it is embedded in a thick cloud. The same might be true for M31 ... i.e. that there have been none since we've gained the ability to observe in far infrared. I suspect that another part of the answer is that the average of one per galaxy per century might average in galaxies with significant star-creation activity, which the Milky Way and M31 may not be having. I do not actually know where the "one per century" rule comes from, but it is an old rule.

jfribrg
2012-Jan-24, 03:33 PM
I would expect that most of M31 is also obscured by dust and the galactic bulge and that for us to see a supernova it would have to be on our side of the galactic bulge and relatively near the edge of the galaxy. My guess ( and it's only a guess) is that 40% of the volume of M31 is in our "detectable region".

Romanus
2012-Jan-25, 10:46 AM
^
Interesting point; e.g., M 82 is supposed to have a much higher SN rate than any galaxy in the Local Group, yet none have been recorded, apparently due to obscuration.

For my part, I think the next SN in the Local Group will once more be in either the Magellanic Clouds, or more likely M 33, which is larger than either (that is, has a larger "sample size"), and a very high star formation rate.

Tom Mazanec
2012-Jan-25, 11:43 AM
10.2 in http://www.talkorigins.org/faqs/supernova/#BM102 gives a number of estimates for the Milky Way, most of them two or three per century.
http://adsbit.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1994ApJS...92..487T gives 0.6/century for the Magellinic Clouds, and 1.8/century for M31+M33.
And M33 is face on, so we should see all of the SNe in it.

StupendousMan
2012-Jan-25, 01:01 PM
A number of variable radio sources have been detected in M82, which may be obscured supernova. At least one of these sources has been so well observed that it was given an official designation as a supernova: SN 2008iz. See, among others,

http://arxiv.org/abs/0909.5535
http://arxiv.org/abs/1011.5316

SagittariusAStar
2012-Jan-25, 04:38 PM
I have seen estimates that there are several supernovae in the MW every century, and also that M31 is about twice the size of the MW. We would miss most of the MW supernovae because we are in the plane of the galaxy, and dust would obscure them. But we should see most of M31's supernovae. AFAIK, only SN1885 was observed in M31. Are the estimates off, or is there something else going on?

Something else is going on.

Although larger than the Milky Way, Andromeda has a lower star formation rate and thus a lower supernova rate.

From page 38 of Sidney van den Bergh's book The Galaxies of the Local Group: "The fact that the number of supernova remnants discovered in this galaxy is relatively small suggests that the low star formation rate in M31 has resulted in a low supernova rate."