VLA Expansion Project Science Working Group:
The Obscured Universe
(4 December 1998)

This is the pre-meeting document prepared by Jim Ulvestad and Joan Wrobel, with additions/emendations by M. Rupen to reflect the discussion -- these are embedded between square brackets, [like so].

Chaired by Joan Wrobel and Jim Ulvestad.
Also present:
  Peter Barnes, Chris Carilli, Chris Fassnacht, Jim Herrnstein, Jack Gallimore,
  Miller Goss, John Hibbard, Ken Kellerman, Leonid Kogan, Alison Peck,
  Rick Perley, Alan Roy, Michael Rupen, Greg Taylor, Marc Verheijen,
  Craig Walker.

Outline for obscured universe session for VLA upgrade, 4 Dec. 1998

(1) Nearby starbursts (jsu)
    - go much deeper in luminosity function for M82 & NGC 253 [at 3.6cm]
       - detect about 30-40 sources, with about 10-15 down
         to CasA strength  [CasA is 1 mJy in N253, 2.5--3 Mpc]
       - could go 15 times deeper [made it up -- NEED A REAL NUMBER HERE],
         Which would be confusion-limited unless you have A+ array.  This may 
         provide hundreds of new sources, and give a good luminosity function
       - 1 pc is about 0.08 arcsec, and most sources seem to be
         about 1 pc in size.  A+ would give 0.04 arcsec resolution,
         so it should be possible to image the SNRs & HII regions
       - Radio sources do NOT generally coincide with X-ray point
         sources or HST star clusters, because the radio sources
         are obscured at short wavelengths
       - measure expansion rates
       - HI absorption as for M82 with MERLIN 
      [- turnover due to f-f abs.]
    - get sensitivity and resolution comparable to CasA at
        considerably greater distance
       - "The Antennae" at 21 Mpc reveal about 100-200 sources above
         30-50 microJy and resolution near 1 arcsec.  These are
         complexes, rather than individual SNRs, with the faintest
         sources being about 3-5 times CasA and 10-20 pc in size.
       - Many other galaxies at similar distances have starbursts
         and compact radio sources (e.g., NGC 4736)
       - Ultra-sensitive array would detect sources fainter than 
         CasA, and A+ would give resolution of 1-3 pc
       - comparison with UV, etc.

   [- high z ULIRGs: cf. Arp 220 (5200km/s) figure --- 8.4 GHz in A+]

(2) Absorption in Seyferts (jsu)
    - how common is it, how big are the absorbing clouds, how do they
        relate to the "torus?"
       - Mrk 231 VLBI shows free-free absorption on 3-5 pc scales
       - MERLIN NGC 4151 shows free-free absorption on 50 pc scales
         (Pedlar et al, MNRAS, 300, 1071--just out)
      [- turn-over freq. is about 2 GHz -- may not be f-f?]
       - HI absorption and millihalos on scales of a few hundred pc
         (3C84 = NGC 1275, Mrk 231, Mrk 1?)
       - Crucial spatial scale looks like A+ and out to inner VLBA
         stations, with complete spectral coverage being critical
      [- NLRs
         * supposed to have tau>1 at 18cm -- what if filling factor if
           they're not seen?
         * A+/4cm, A/2cm match HST resolution (10s of mas)
         * What are the Faraday depths of the optical emitting
      [ - "gas in extended accretion disks"]

(3) Polarimetry of weak AGN (jmw)
    - Seyferts: limits are only a few percent on many galaxies, could
        push the limits much lower or detect lower polarizations.
        Another way to learn about the ionized gas.
    - RQQs: not much is known
    - UGCs: polarimetry on background sources through the galaxies
       - UGC galaxies are all over 1 arcminute in size, so you can
         measure the polarization of a number of background sources
         and map the distribution of rotation measure at various
         points in the few hundred (?) objects bigger than about
         3 arcminutes (smaller ones don't have the background

(4) High column density QSO absorption line systems (clc)
    - see qsoabs4dec98.ps, Carilli's
      notes on the subject

(5) Obscured quasars and type-2 [(narrow-line)] AGNs
    - NGC 4258's with highly obscured optical nuclei 
       - Detection of the compact radio sources, water masers
         (isn't the GBT better for this?)
    - Quasar 2's, anything useful to do?
    - Very distant red quasars
(6) Recombination lines 
    - Needs sensitivity, does it need A+?

(7) Gravitational lenses? 
   [- more objects --> measure lambda
    - pol'n]

(8) What else?
  [- AGNs vs. star formation: is that an obscured starburst or a dusty AGN?]
  [- dust heating mechanisms]

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