VPHAS+ ...The ultimate H-alpha survey? Can amateurs do better?!

[madbadgalaxyman (Robert)]

The latest professional H-alpha survey of the southern Milky Way is called VPHAS+

It utilizes the 2.6 meter VLT Survey Telescope (the VST) together with the OmegaCAM array of CCDs ( http://www.eso.org/sci/facilities/pa...omegacam/inst/ ) , and it will survey much of the southern Milky Way:

http://www.vphasplus.org/

[[VPHAS+ is the successor to the deep H-alpha survey of the Milky Way that was done (still on film!) with the UK Schmidt Telescope:
http://www-wfau.roe.ac.uk/sss/halpha/
]]

Some of the first H-alpha images from the newest survey are probably available through the ESO data archive. They are made through the NB 659 filter, which is similar to an H-alpha filter :

http://archive.eso.org/wdb/wdb/adp/phase3_main/form

Madbadgalaxyman's opinionated commentary :

Despite the use of a gigantic telescope, and perhaps because of the prohibitively large time and money expenditure that would be necessary to undertake the best possible survey of the Milky Way with a 2.6 meter telescope, VPHAS+ is far from being the ultimate H-alpha survey of the band of the Milky Way.....

(1)
Firstly, these Large Telescope H-alpha images are not going to be particularly deep images, though the angular resolution of the images is very good; 1 arcsecond or even better.
Apparently only 120 second exposures !!!....
In my view, this means that the survey won't be able to detect ultra-faint nebular structures; It seems to me that, even made with a 100 inch telescope, such short exposures might not detect faint expanding supershells and faint Supernova Remnants.
Why on Earth use a giant telescope and then make shallow exposures??? (actually, the same criticism could be made about many Hubble Telescope exposures)

So I predict that the deepest H-alpha survey of the Milky Way is going to be done by an amateur astronomer.

(2)
Another deficiency of the VPHAS+ is that the survey fields do not stretch very far above the galactic equator (the apparent plane of our galaxy), so highly unusual and interesting structures high above the galactic plane, such as those detected in Greg Bradley's very deep Milky Way exposures, will not be imaged in this survey.
Many of the least known and most interesting gaseous structures in spiral galaxies are well above the principal plane of a galaxy, e.g. gas being ejected from a galaxy and gas falling into a galaxy from its halo. Yet it is precisely these least understood H-alpha structures that won't be surveyed!
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[Shiraz (Ray)]

Interesting info Robert - thanks for posting.

I don't think that amateurs are anywhere near in this league - we cannot do better than this survey, except to maybe provide a little bit of variable quality data from outside of the VPHAS+ region.

To illustrate the problem for amateurs, a well heeled one may have a 0.5m scope, so will collect about 1/25 times as many photons as the 2.6m scope. Using the biggest affordable high QE CCD, the amateur system will cover about 1/10 as much sky as the 268mpixel VST in each exposure. So, allowing for different sampling, QE and the overlap problem of the VST array, the amateur will need to image for maybe 100 times as long as the VST to cover a chosen patch of sky to the same depth and at lower resolution. VST is going to take about 1000 hours to cover the limited survey region, so the amateur would take maybe 100,000 hours to do the same job. Big survey scopes have huge exposure time advantages and can do things that are totally out of the reach of amateur systems.

I wouldn't be concerned that the Ha exposures are only 2 minutes long - the VST gets to better than mag20 at 10sigma in this time, so the survey will be pretty deep. I guess that the depth that can be obtained must involve a tradeoff between sensitivity on dim targets and total loss of data on brighter objects as the sensor saturates - in this case apparently at about mag14. The resolution will be seeing limited at about 1 arcsec (better than Australian conditions) and all data will be calibrated to 0.05mag and 0.1arcsec, which is much better than amateur standards. Looking forward to seeing results, but of course there will be 11TB of data to wade through.

Maybe there is a good case for an additional low resolution amateur survey of the gaseous regions beyond VPHAS+ or follow-up deep imaging of regions of interest. These activities could be carried out by coordinating and combining the resources of a number of amateurs so that the regions are covered efficiently at a consistent resolution, linearity and depth. Not sure how one would go about coordinating amateurs though, we seem to be a bit of a disparate bunch with widely varying equipment, opinions and egos.

Regards Ray

[madbadgalaxyman (Robert)]

G'day Ray,

Thanks for your numerical argument, which is persuasive.

I personally think that the high galactic latitude regions (>5 degrees from the apparent galactic plane) of our own Galaxy could contain some surprises, with the possibility of outflows of gas similar to those emanating from the disk of NGC 253:
http://www.iceinspace.com.au/forum/s...hlight=H-alpha
and
http://www.iceinspace.com.au/forum/s...hlight=H-alpha
(I believe that you are well aware of these posts of mine, as you commented about these issues regarding NGC 253. But I include links to them here. because others may have missed them)
In particular, I mentioned what at least looks to be a vertical dust chimney above the NGC6231 plus OB Association complex.

Gas inflows into the disks of large spiral galaxies, also seem to be occurring, but they are poorly understood in the literature.
The typical Star Formation Rate of most individual Large Spiral Galaxies (1-2 solar masses per annum) has been quite steady for at least the last 5 billion years (as derived from the method of superposing isochrones onto the H-R diagram of a galaxy and thereby deriving the history of its Star Formation), and this rate of star formation would exhaust the available interstellar gas (= the raw material available for continued star formation) within a galaxy in only one or two billion years, leading to a definite indication that the disks of spiral galaxies are plausibly being resupplied with gas that originates in the intergalactic medium....but current observational evidence for this phenomenon is scarce and tentative.

I also showed evidence in another thread, that gaseous material might plausibly be infalling into the very outermost regions of NGC 5128, and thereby forming a new outer disk structure, because the kinematics of its outer stars were shown to be disky by Freeman and Peng:
http://www.iceinspace.com.au/forum/s...disk+formation

In a similar vein, E.K. McNeil-Moylan and a team including Dr Kenneth C Freeman of ANU (Australia's superstar of optical extragalactic astronomy) argued that a disk is now forming in the outer regions of NGC 1316 , with a physically plausible future for this galaxy as another Sombrero:
http://arxiv.org/abs/1201.6010

.....All of which is to say that the regions far above and far below the plane of our own Galaxy are well worth searching for very-low-density gas, whether neutral or ionized.

Best Regards,
Robert
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A recent publication about this broad research area relating to spiral galaxies, which is somewhat cryptically known, in the trade, as "the disk-halo interaction", is :
http://journals.cambridge.org/action...d=0&issueId=-1

In this respect, I note, with great sadness the premature passing of Jorn Rossa, who showed, early on, that many actively-star-forming spiral galaxies have Ha outflows that can extend vertically for a few kiloparsecs above the disk component.

(This looks like a bloody expensive publication, but one can always try to get preprints from arxiv.org)
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P.S. there were a number of papers strongly indicating that the halo of M31 is much more inhomogeneous and has a wider stellar age spread, compared to the halo of our own Galaxy. So it would seem that there could be more gas and "bits and pieces" of small galaxies remaining in the vicinity of some galaxies than in the vicinity of others.
The galaxy-merger history of the M31 halo is far more obvious than that of the halo of our own galaxy e.g. the M31 halo contains a significant population of intermediate-age stars (6-8 billion years old).
[[ This presentation (an 11MB .pdf file) introduces some of the age dating work done on the M31 halo:
http://www.noao.edu/meetings/m31/fil..._princeton.pdf ]]

So there are going to be differences and surprises when we compare even similar-looking spiral galaxies.....accordingly, it is hard to predict what we might find in a deep continuum-emission or line-emission imaging survey of the regions far above the plane of our own galaxy.
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Image of possible dust and gas breakout from the disk of our own Galaxy:

[madbadgalaxyman (Robert)]

Originally Posted by Shiraz

Not sure how one would go about coordinating amateurs though, we seem to be a bit of a disparate bunch with widely varying equipment, opinions and egos.

That is a very perceptive comment.

However, I suppose that our diversity is our strength, as witnessed by the remarkable diversity of interests that are displayed by our IIS members.
There are plenty of us doing strange and unusual and worthwile astronomical work, which often has to be squeezed into our very limited available spare time.
But you may be right that getting us to cooperate is "like trying to herd cats".

[Shiraz (Ray)]

have you come across this composite map? - it's low resolution, but as far as I can tell, shows the higher latitudes to some depth. regards Ray
http://astrometry.fas.harvard.edu/skymaps/halpha/
http://astrometry.fas.harvard.edu/sk...s/comp_big.jpg

EDIT: and to give an idea of the scope of surveys like VPHAS+, the zoomable image from a sister NIR survey of the centre of the milky way is jaw dropping.
http://www.gigapan.com/gigapans/117375. I guess that this also shows another reason for restricting the length of exposures - the brighter star PSFs cover nearby regions - in a dense star field, the skirts of the brighter stars ultimately obliterate deep data as you expose longer. As an aside, I find that this image provides a way of visualising large numbers, for example, every star in this image could correspond to 80 people if one wanted to to get a feel for the size of earth's human population.