View Full Version : The S4G results....findin the missing baryons?

2011-Aug-02, 02:34 PM
The Spitzer Survey of Stellar Structure in Galaxies...the S4G...may have found some missing baryons. Galaxy formation, still the subject of much research and controversy, has a new ally in the new data. The authors, a substantial international team, conclude that there are different light/mass ratios for the thin and the thick disks seen in edge-on galaxies, and that their results suggest an increase of 10-50 % in the masses so estimated. As usual, a new technique using state-of-the-art instrumentation leads to new ideas, the Spitzer survey done @ ~ 3.6μm and 4.5μm gives a much deeper observation into thick dust and better mass estimates.


2011-Aug-04, 11:51 AM
Thanks for finding this trinitree88

We found that the stellar mass of the thick disk component ranges from one third to two times that of
the thin disk if [.....] and from two thirds to more than three times if [......]. We thus found
that thick disks are much more massive than generally assumed so far.

I've only skimmed the thing so far. It's another one of those model-dependent papers, however I'll trust it for now.

I think they are only proposing extra mass for thick disks, not the thin disk. In our area of the MW galaxy, the thick disk stars are estimated at 12% of the stellar population. (Barnard's Star is a visitor from the thick disk.)

There are quite a few galaxies surveyed. One is said to be very similar to the MW both in size and morphology (however it's circular velocity at 161km/s appears to be well down on that of the MW, which is 220-250km/s depending on who you believe)

That being the case, the MW should not be an exception to this finding. So can the negative MACHO surveys and MW models be reconciled with a thick disk increasing in stellar mass to this extent?

2011-Aug-04, 02:06 PM
kzb You're welcome. I don't think they'll find Machos, and I'm sure some observors will be able to answer your question within the year, but I recuse myself here a bit. For theorists, it's interesting to ponder for now. pete

2011-Aug-04, 05:23 PM
trinitree88 wrote: I don't think they'll find Machos

But what I am wondering about is if there was something defective about the MACHO search methods. Form this new paper it looks like there should be a substantial stellar population outside the main stellar disk. (Maybe the MACHO searches were designed to find bodies in the halo population only.)

Another interesting thing about the new paper is that the surveys were truncated at certain galactocentric radii, and this radius is within the already-known stellar disk radius. So I wonder what would be found by Spitzer at larger radii ?