Galactic Survey science case

lsjouwerman
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Joined: Mon Jul 18, 2011 3:49 pm

Galactic Survey science case

Postby lsjouwerman » Tue Mar 11, 2014 4:26 pm

I guess the main discussion topic, on which target area and observing frequency parameters depend, is the main science case of the Galactic Survey: how do we best serve the science drivers in the Galactic community? That is, given the lare range of possibilities with the VLA, are we converging at a survey mainly detecting star formation (thermal) components or physical processes (synchrotron/polarization)? I guess transient observers will be served by either. To do it both seems not feasible, but maybe I am wrong (hope so!)

I think given the opportunity we should aim for the most tricky one (in observing) as the easier one could be done as a stand-alone later or in smaller chunks. My choice therefore would be high frequency/thermal, also because no such previous radio survey is available whereas there is a wealth of thermal data to compare with in the near and far infrared. Furthermore, there should be no imaging nor confusion problems making the data reduction easier and the results easier to interpret. That is how I see it.

elisabethmills
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Joined: Tue Mar 11, 2014 3:58 pm

Re: Galactic Survey science case

Postby elisabethmills » Tue Mar 11, 2014 4:41 pm

To just go deeper into Lorant's point that the high-frequency thermal survey would be unique, I note that there are currently two Galactic plane surveys in progress: THOR (PI: Henrik Beuther), an L-band survey of the plane in HI, OH, recombination lines and continuum, as well as a C-band survey (PI: Karl Menten), which I believe is also focusing on recombination lines, continuum, methanol masers, and possibly H2CO absorption?

I think it is important to acknowledge that these are in progress (though at present, covering a limited area of the plane), and to discuss whether it makes sense to use VLASS to support a faster, larger-scale implementation of these surveys, or whether it the community is better served by a new survey at a different frequency.

My two cents: Although the SKA is the next big thing, I think that VLASS should maximize the unique capabilities of the VLA, and push to the highest frequencies possible for surveying the plane, which will be most useful for comparison with existing multiwavelength surveys of thermal emission in the plane, and vital for the future of high-frequency surveys and observations (e.g., by identifying the necessary calibrators at these frequencies). I also think that it would be unfortunate not to take advantage of the new capability of the VLA to conduct spectral line surveys afforded by the larger bandwidth and slightly more sensitive receivers, and would strongly advocate that ISM tracers should also drive the choice of frequency.

mathompson
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Joined: Fri Mar 14, 2014 11:49 am

Re: Galactic Survey science case

Postby mathompson » Fri Mar 14, 2014 11:59 am

For me at least the largest improvement is in going to higher frequency. The THOR, CORNISH & Karl Menten's "SuperCORNISH" serve the lower frequency side very well and I don't see the advantage in going deeper at lower frequency and doing these surveys again. Because of the steep extragalactic source counts at low frequency you'll just end up adding in more extragalactic sources.

At high frequency the advantage is in capturing the optically thick thermal emission that falls away steeply at low frequency. Plus you have access to recomb lines that aren't massively pressure broadened. These were the main drivers that led me to propose the MeerGAL survey on MeerKAT, which will be at X-band, once these receivers are commissioned. So I would argue to go for as high frequency as possible. Why not even consider K-band and going deeply enough to detect ammonia? That could then be combined with zero-spacing from GBT and the KFPA.

sbhatnag
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Joined: Wed Oct 05, 2011 3:39 pm

Re: Galactic Survey science case

Postby sbhatnag » Fri Mar 14, 2014 4:55 pm

The case of the survey at low frequency is for wider, not just deeper survey (THOR and other "surveys" often mentioned cover but a small part of the Galaxy -- they are more like a pilot survey than a Galactic Plane Survey). There is a lot more to be said about wider survey than collecting more extra Galactic sources (though even these extra-Galactic source are interesting for background sources in polarization studies). Unless the argument for going to higher frequency is to cover similar part of the Galaxy as is done by these low-frequency pilot survey projects, the scientific case at low and high frequency covering large fraction of the Galaxy are comparable (with higher survey speeds at lower frequencies).

lsjouwerman
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Joined: Mon Jul 18, 2011 3:49 pm

Re: Galactic Survey science case

Postby lsjouwerman » Fri Mar 14, 2014 5:25 pm

Right, and that is why we originally proposed for the entire (visible) GP and as wide as the largest extent done, 5 degrees on either side in b, essentially 4 times larger area than you proposed for in your original L band survey. In the Target Area topic we also consider going to higher latitudes than only 5deg if the science case would justify that.

lsjouwerman
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Joined: Mon Jul 18, 2011 3:49 pm

Re: Galactic Survey science case

Postby lsjouwerman » Fri Mar 14, 2014 5:34 pm

mathompson wrote:So I would argue to go for as high frequency as possible. Why not even consider K-band and going deeply enough to detect ammonia? That could then be combined with zero-spacing from GBT and the KFPA.


Long ago I wanted to do this in Q band (45 GHz), but practically this is not feasible. The same holds for K band. If you want to do it there then you have to limit your scope to small areas, ie short programs. This not only because of the small beam, but also because these bands are less sensitive, you need to include pointing scans, the lack of calibrators, and you are competing for high frequency weather, to name a few. All these considerations make it turn into a survey that - if executed - really restrict other high frequency general observing programs access to the telescope for an extended period of time. Note that there is GBT zero spacing for Ku band already, so no new obs are needed there.

elisabethmills
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Joined: Tue Mar 11, 2014 3:58 pm

Re: Galactic Survey science case

Postby elisabethmills » Fri Mar 14, 2014 5:59 pm

Also note that there are already plans underway/ observations in progress for a GBT-KFPA survey of ammonia in the Galactic plane: RAMPS, or the Radio Ammonia Mid Plane Survey (PI: James Jackson). If there were interest in a focused/targeted K-band VLA follow-up survey component, the existence of RAMPS would be a good pathfinder and motivation, but I know nothing more than that 138 hours were successfully proposed for.

I agree that K band is more optimal for a survey tracing the ISM properties in the plane, given the ammonia lines in that band, but as Lorant states, the survey efficiency drops dramatically, making it a very difficult sell.

Ku (14-18 GHz) band offers 12 GHz methanol masers and absorption of the 14 GHz formaldehyde lines against the continuum, but if you want a (relatively) strong ISM tracer, the next best bets aside from ammonia are HC3N 1-0 at X band, and CH in S-band (the latter frequency not really being ideal for detecting thermal sources in the plane).

Ku-band is for me the clear winner as far as unique continuum science in the plane; if one were motivated by the molecular gas structure and distribution however, I would say this band is less ideal, and there could be a case made for CH (as a molecule which can trace CO-dark gas; but I don't have an idea yet what depth it requires)

sellings
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Joined: Sat Mar 15, 2014 3:37 am

Re: Galactic Survey science case

Postby sellings » Sat Mar 15, 2014 3:57 am

To me the decision between a large area Ku band survey and a smaller area Ka band survey is a comparison between a low-risk incremental Galactic Plane survey and a higher risk, but potentially much more interesting project which will potentially find new populations of sources. So my personal vote would be for the doing a smaller area (say centred around l = 30) at around 22 GHz since there are more lines, the thermal emission is stronger and with the increased sensitivity we could get the first unbiased census of hyper-compact HII regions, water masers and a range of other interesting star formation tracers (e.g. class I methanol masers at 25 GHz which are relatively common, but for which there is little observational data). The ATCA observations around the Galactic centre at 22 GHz by Caswell et al. (2011, MNRAS, 410, 1283) shows that even at modest sensitivities (detection limit 0.5 - 1 Jy in a 2 km/s channel width) there are lots of new water maser sources to be found.

chomiuk
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Joined: Fri Feb 28, 2014 4:16 pm

Re: Galactic Survey science case

Postby chomiuk » Sun Mar 16, 2014 8:39 pm

Hi Everyone,
Glad to see the discussion is moving along here!

Can you all lay out your primary science drivers for going to high frequency?

Myself, coming from an interest in compact objects and interacting binaries, I'd like a Galactic plane survey to be as sensitive to point sources as possible, and have decent (~1'') resolution.

X-ray binaries have flat spectra, so it really doesn't matter too much for them what freq we observe at. But we want sensitivity and resolution.

Both novae and symbiotic stars are thermal, and can show spectral indices from alpha ~ -0.1 to 2. It's not obvious to me what the best frequency is for pursuing these, either, although certainly something higher than 1 GHz is better. But I'm not convinced there would be a big difference between 5 and 15 and 45 GHz for these objects. Again, we mostly want sensitivity and resolution.

Pulsars have really steep spectra (alpha ~ -2ish), but a high-res L band survey of the Galactic center could turn up a really juicy population of pulsars.

In all cases, I'm particularly interested in mapping the bulge (so, l = 1--5 deg or so---not in the CORNISH area) and achieving overlap with X-ray surveys.

Should we be trying to compile a list of all the science cases that we would like to simultaneously cover?
Last edited by chomiuk on Sun Mar 16, 2014 8:53 pm, edited 2 times in total.

chomiuk
Posts: 12
Joined: Fri Feb 28, 2014 4:16 pm

Re: Galactic Survey science case

Postby chomiuk » Sun Mar 16, 2014 8:47 pm

Some details on surveys:

THOR:
L band, 54 sq deg with 20" resolution, lines+continuum. 110 hours granted for pilot.
http://www.mpia-hd.mpg.de/homes/ppvi/posters/1S050.pdf

CORNISH:
C band, 110 sq deg with 1.5" resolution, rms ~ 0.4 mJy/beam
http://cornish.leeds.ac.uk/public/index.php

Anyone else know specs on the other surveys we are discussing? Probably worth getting them from the PIs, if they aren't already public.


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