View Full Version : Three Dimensional Distribution of Atomic Hydrogen in the Milky Way

2012-Jul-27, 06:30 AM
That's the title of a recent arXiv preprint (http://arxiv.org/abs/1207.6150) (arXiv:1207.6150v1 [astro-ph.GA]), by Maryam Tavakoli:

A new model for three dimensional distribution of atomic hydrogen gas in the Milky Way is derived using the 21cm LAB survey data. The global features of the gas distribution such as spiral arms are reproduced. The Galactic plane warps outside the solar orbit and the thickness of the gas disk flares outward the Galaxy. It is found that the mass of atomic hydrogen gas within a radius of 20 kpc is 4.3*10^9 M_Sun.

It's not one picked up by the regulars who post in Fun Papers In Arxiv (http://cosmoquest.org/forum/showthread.php/123740-Fun-Papers-In-Arxiv?goto=newpost), yet would seem relevant to several recent threads here, on the nature of the mass in the outer parts of spiral galaxies (or at least the spiral galaxy which is our home).

ETA: Almost the very next astro-ph is also interesting, for the same reasons: The H2O southern Galactic Plane Survey(HOPS): NH3 (1,1) and (2,2) catalogues (arXiv:1207.6159v1 [astro-ph.GA] (http://arxiv.org/abs/1207.6159)), by C. R. Purcell et al.:

The H2O Southern Galactic Plane Survey (HOPS) has mapped a 100 degree strip of the Galactic plane (-70deg > l > 30deg, |b| < 0.5deg) using the 22-m Mopra antenna at 12-mm wavelengths. Observations were conducted in on-the-fly mode using the Mopra spectrometer (MOPS), targeting water masers, thermal molecular emission and radio-recombination lines. Foremost among the thermal lines are the 23 GHz transitions of NH3 J,K = (1,1) and (2,2), which trace the densest parts of molecular clouds (n > 10^4 cm^{-3}). In this paper we present the NH3 (1,1) and (2,2) data, which have a resolution of 2 arcmin and cover a velocity range of +/-200 km/s. The median sensitivity of the NH3 data-cubes is sigma_Tmb = 0.20 +/1 0.06 K. For the (1,1) transition this sensitivity equates to a 3.2 kpc distance limit for detecting a 20 K, 400 Msun cloud at the 5-sigma level. Similar clouds of mass 5,000 Msun would be detected as far as the Galactic centre, while 30,000 Msun clouds would be seen across the Galaxy. We have developed an automatic emission finding procedure based on the ATNF DUCHAMP software and have used it to create a new catalogue of 669 dense molecular clouds. The catalogue is 100 percent complete at the 5-sigma detection limit (Tmb = 1.0 K). A preliminary analysis of the ensemble cloud properties suggest that the near kinematic distances are favoured. The cloud positions are consistent with current models of the Galaxy containing a long bar. Combined with other Galactic plane surveys this new molecular-line dataset constitutes a key tool for examining Galactic structure and evolution. Data-cubes, spectra and catalogues are available to the community via the HOPS website.

2012-Jul-27, 12:15 PM
... It's not one picked up by the regulars who post in Fun Papers In Arxiv (http://cosmoquest.org/forum/showthread.php/123740-Fun-Papers-In-Arxiv?goto=newpost), ...

There are certainly a lot of interesting papers we don't highlight. We typically identify between 5 and 10 percent of the papers, where as most of the papers actually have something intriguing about them. Thanks for bringing these to light.

2012-Jul-27, 04:29 PM
There are certainly a lot of interesting papers we don't highlight. We typically identify between 5 and 10 percent of the papers, where as most of the papers actually have something intriguing about them. Thanks for bringing these to light.

Oh, I didn't mean to imply any criticism of what you guys do! {insert relevant smilie here} Far from it; I'm amazed you manage to discipline yourselves to read just a handful (and write reviews of them), week in, week out (I'm more of a binge astro-pher; I pig out for hours, get overwhelmed, then stay away for days, weeks, even months).

These ones particularly caught my attention, given what has been discussed here recently (well, what I've been involved in discussion of anyway). The fact that there are so many drives home the point that in 'hot' topic, it's almost a full-time job just keeping up with the research, let alone having the time to engage in a well-informed discussion. Thank goodness for review papers!

2012-Jul-28, 02:05 AM
...it's almost a full-time job just keeping up with the research...

That's why we don't actually read papers any more. That's what grad students are for!

2012-Jul-30, 11:57 AM
Thank you Neried I will read these with interest. In the "HI paper", I notice they use a MW circular velocity of 220km/s. Yes this is still the industry standard, but several recent studies say this should be nearer 240km/s or even higher. I think using a higher circular velocity would increase their estimate of the total HI (but only in a trifling amount in comparison to the amount of DM required).

I've been wondering about something else: if the efficiency of star formation in molecular clouds is only a few per cent, what happens to the 90%+ of left-over gas?

It seems a bit counter-intuitive to me that it could be blown away (dispersed), then collect together along with fresh gas and start over again. And over and over again, over many cycles, until the gas had been mostly consumed. At least in any reasonable timescale, considering that the MW had a large stellar population many billions of years ago?

2012-Jul-30, 02:14 PM
It is found that the total mass within a radius of 20 kpc is 4.3x109 M sun and only 0.033% of that is due to local gas with peculiar velocities.

A couple questions:

The disk has a radius of about 17 kpc, so the "total mass" interior to 20 kpc is only 4.3x109 M sun while the Galaxy's virial mass is 1012? Do they mean total stellar mass? Or is there 1,000 times more dark matter exterior to 20 kpc?

What does she mean "local gas with peculiar velocities"? Gas that's not in a near-circular orbit? All of the gas is going to have some intrinsic velocity....

2012-Jul-30, 04:55 PM
Cougar -I took it to mean 4.3 E+09 M-sun of HI gas. But I've only had a quick skim so far.

2012-Jul-31, 12:06 PM
I like papers with visual plots of the galaxy like these. I do have some comments though.

Firstly, neither paper is actually a complete mass budget. I don't think that was their intention. On the contrary, only gas clouds big enough to be above the detection limit are included. The HOPS paper includes only gas clouds that have levels of ammonia (NH3) above their detection limit.

In connection with the HOPs paper, I was a bit puzzled by the statement that CO freezes out on dust particles, whereas NH3 does not. Now please don't get the idea I am trying to rubbish the paper on this point, I am sure it is quite reasonable if you know the facts. I am just curious.

The freezing point of ammonia is well above that of CO (CO freezing pt = -205 C, NH3 freezing pt = -78 C). This is at atmospheric pressure of course. But why would NH3 not freeze out on dust in the same conditions that CO would?