Up: Minutes of the 1st
Minutes of the talk on the:
HI Mass Function
Steve Schneider (U. Mass.)
I note that "Z" plays an important role in what the ALFA surveys will look for:
|HI rich objects || few metals || low Z |
|nearby low mass || low redshift || low z|
|Arecibo || low zenith angle || Low Z|
Arecibo Blind Surveys
Several surveys have been undertaken in the recent past at Arecibo:
- AHISS (Arecibo HI Sky Survey): (Sorar 1994).
- Scanned 19h at fixed declinations (rms of 0.9 mJy)
- 1 object with MH < 108 solar masses.
- Slice Survey: Spitzak & Schneider.
- Scanned 60 sq deg area in Pisces-Perseus region at rms 1.7 mJy
- 3 objects with MH < 108 solar masses.
- ADBS (Arecibo Dual Beam Survey): Rosenberg & Schneider 2002.
- Scanned 450h in drift scans.
- A few sources with unobvious optical counterparts.
HI Selected Galaxies:
For the ADBS, we used the J band luminosities to try to understand the underlying population
detected by the survey. At any given HI mass, there are a wide range of stellar luminosities (J band)
- There is lots of gas relative to the stellar mass in the detected objects.
Deriving the HIMF:
The HIMF derived from the ADBS is quite different from that derived from the AHISS
by Zwaan et al. Among other things, we used different methods. So the questions are:
How tricky is it to actually derive HIMF? Coverage wins for volume, but if you do a shallow
survey you are sampling only the nearby volume at low masses. The environment locally may be atypical.
In particular, you have to worry about the Eddington bias.
- Are the difference real?
- Could they be due to environment? We find a hint that the HIMF is different in Virgo.
- 5-sigma sounds good, but:
- Sources scatter in and out (flux bias).
- Baselining & interference excision introduces bias.
- Completeness tests fail at this level.
- So need artificial test sources to find completeness.
- What if you integrate only long enough to see half as far?
- You can cover twice as much volume in same time.
so you get more volume by going shallow over area.
- But what is the tradeoff?
- Would like to learn about HI flux versus redshift.
- For a given flux limit, you are sampling very small volume for low mass objects.
- Need to ask: how far out do you have to go in order to cover enough volume?
- Total volume within survey in which source could have been detected.
- Need to account for beam shape, frequency dependence, sensitivity variations, line widths
- Must add up the volumes properly.
- LSS in the ADBS:
- Can see difference between galaxies within
of Virgo vs outside Virgo.
- Very different galaxy distribution because of LSS;
this needs to be taken into account
- Eddington bias:
- Introduced through worrying about Virgo.
- Sources get shifted to lower cz and then their distances are misinterpreted.
- Not a significant result where the velocity dispersion ~ 300 km/s,
but can be a problem in cluster regions where the dispersion is
more like 1000 km/s.
- 5 sigma limits turn out not to be enough. You have to be cautious doing a simple cutoff,
thinking you are complete to that limit, because sources get scattered in and out.
You probably need to cut at a limit of more like 10 sigma to be sure.
- You need to spend time worrying about testing completeness.
- Inject fake sources and test if they are found. .
- Methods for quality control of source extraction methods.
- It is surprising how strong a source can be and sometimes be missed.
Don't you have a bias at low MHI so that you don't detect many?|
This is an HI selected sample.|
The same effect is seen in optical surveys because of the small volume.|
Up: Minutes of the 1st
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Last modified: Sat Apr 19 14:15:36 EDT 2003