1.5.3: Name -- Nuclear Dense Gas in Active Galaxies
       Authors: E. Schinnerer, C. Carilli

2. Science goal: The polar molecule HCN is commonly used as a tracer
   of dense molecular gas. The correlation between the HCN luminosity
   and the far-infrared (FIR) luminosity is found to be tighter than the
   well-known correlation between the CO luminosity and the FIR
   luminosity, and more importantly, the HCN-FIR correlation remains
   linear to high FIR luminosity, unlike the CO-FIR correlation, which
   'saturates' at high FIR luminosity. Since HCN is a dense
   gas tracer, likely associated with active star forming regions,
   the linear HCN-FIR correlation suggests that the FIR
   luminosity originates from star formation rather than AGN activity
   in IR luminous galaxies. However, the HCN to CO intensity ratio varies
   substantially among luminous galaxies, which may indicate
   variation of the star formation efficiency (= star formation
   rate/total molecular gas mass) as a function of IR luminosity.
   The excitation conditions for HCN can be met either in star
   forming regions, or in gas close to the nuclei of galaxies, where AGN
   may heat  the gas and dust. Therefore, we propose to image HCN in a
   representative sample of starburst and AGN galaxies to identify the
   excitation source of HCN. High angular resolution is essential to
   discriminate between possible nuclear star forming regions and the
   AGN itself. In addition the high angular resolution of 0.1'' will
   allow detailed comparison to high resolution ground-based adaptive
   optics MIR data to test the HCN-FIR correlation on scales of a few
   10 pc.

3. Number of sources: 10

4. Coordinates:

  4.1. active galaxies at ~20-80 Mpc distance:
       e.g. N1068,N2273,Arp220,N6240,N3256,N7469, ...

  4.2. Moving target: no

  4.3. Time critical: no

5.  Spatial scales:

  5.1. Angular resolution: 0.1"

  5.2. Range of spatial scales/FOV: 0.1" to 15"

  5.3. Single dish: no

  5.4. ACA: no

  5.5. Subarrays: no

6. Frequencies:

  6.1. Receiver band: Band 6 -- 265 GHz  in Configuration BC

  6.2. Lines and Frequencies: HCN(3-2) @ 266 GHz

  6.3. Spectral Resolution (km/s): 20 km/s

  6.4. Bandwidth or spectral coverage: ~ 1200 km/s

7. Continuum flux density:

  7.1. Typical value:

  7.2. Continuum peak value: < 0.5 mJy/beam at 230 GHz

  7.3. Required continuum rms:

  7.4. Dynamic range in image:

8. Line intensity:

  8.1. Typical value: ~ 0.9 mJy/beam at 266 GHz

  8.2. Required rms per channel: 0.15 mJy/beam

  8.3. Spectral dynamic range: >5

9. Polarization: no

10. Integration time per setting: including calibration
    1 track (+/- 4hr) at 266  x 10 sources x 2 configurations

11. Total integration time for program: 160 hr

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Review Jean Turner: HCN is an interesting molecule for starbursts. One
issue is the one that Phil Solomon has pointed out, that HCN is better
correlated with Lir than CO. This is worth pursuing with high spatial
resolution.  HCN could well trace the star formation much better than
CO (and presumably these galaxies would all already have comparable CO
maps). However, it seems to me premature to do 10 sources. HCN could
be confusing.  Especially in exotic objects, with a lot of mid-IR
emission. With the high spatial resolution one can isolate the AGN to
some degree and perhaps simplify the problem to learn interesting
things about dense gas and HCN in these two different categories of
systems. This project will be done by ALMA. I think the dust continuum
will be exceedingly useful in helping out with where the gas is (this
would potentially require band 3 to weed out the free-free
contribution)

Scope: I think this is an awful lot of time to devote to HCN. At
the moment it is unclear how much one might learn, with potential
excitation issues, particularly in AGN, but anywhere there is strong
mid-IR emission. Ewine will know better about the mid-IR pumping of
HCN. If there are SSCs forming, there is VERY strong and localized mid-IR.
Ditto AGN. Two galaxies carefully chosen galaxies would be sufficient
to reveal unusual HCN properties.

Technical: High resolution is needed to separate AGN; however, too
high is not good for molecules. They propose 0.1", this is a
reasonable compromise.

Integration time is fine per galaxy. Total time for project should
be cut, to possibly 2 sources instead of 10, for a total of 32 hrs.

Comment Ewine: Both collisions and mid-IR pumping can indeed affect the HCN
excitation in these hot cores. Two sources seems too few to me to test any
relation, so I propose to cut to 6 sources = 96 hrs