A new version of this page exists for CASA 3.3: N891 simdata (CASA 3.3).
Nearby edge-on spiral
Roughly modeled after NGC891
- mostly correct, but probably not very thoroughly explained. Updated for CASA 3.1 (simdata2->simdata)
- I binned the cube to coarser velocity resolution in order to speed the simulation. the fits file is grs-12kms.fits
- units: K - first convert to flux surface brightness
Jy/Sr = 2x1023 k T / λ2, = 4x108T at 110GHz.
now we need to decide if this model data will work at the desired pixel scale
- the GRS resolution of 40" at ~10kpc is 0.04" at 10Mpc, so we should be able to do a simulation of observing at ~0.1-0.2". The resolution plot () indicates that for ALMA at 100GHz, configuration 20 is appropriate.
- if we intend to set cell=0.04arcsec in simdata, then the cube needs to be multiplied by
4x108 * (.04/206265)2 = 1.4x10-5 to obtain Jy/pixel. The cube peaks at ~20K, so we can perform the simulation with inbright=3e-4, which should yield a peak of ~1mJy/bm.
will we be dominated by the noise in the input model?
- input noise ~150mK or S/N~20, so at our scaled intensity, ~0.05 mJy/bm. The exposure time calculator says that ALMA will achieve 2.5mJy/bm in 2 hours for the input 212m/s channel width (0.075MHz), so the noise in the input model should not affect our results.
- We do have a sensitivity issue though - if we decrease the spectral resolution by a factor of 6 (bin the input channels in some other program - simdata will know how to do that in the future but not yet), and plan for 3 8-hr tracks, then the sensitivity calculator suggests that we'll get <0.25mJy rms, or S/N>10 per beam. Rather than simulate 3 days of observing, I'll increase inbright by sqrt(3) and simulate one 8 hour track.
- the ALMA 12m primary beam is 50" so we'd space a mosaic by 25", but the model cube has 326x357 pixels, or 13 arcsec with our small pixels. That's a lot smaller than the primary beam, so it doesn't matter much what output image size we ask for.
- there are 659 channels in the input cube, but as noted above we want to bin those to 109 channels of 1.2 km/s each.
here are the simdata inputs :
taskname = "simdata" project = "n891d"
modifymodel = T
skymodel = "grs-12kms.fits"
incenter = "110.1777GHz"
inwidth = "-.468MHz"
inbright = "1.4e-4"
indirection = 'J2000 7h00m34 -23d03m00'
incell = "0.2arcsec"
setpointings = T
integration = "300s"
pointingspacing = "25arcsec"
mapsize = '60arcsec'
graphics = "both"
verbose = True
overwrite = True
antennalist = "alma;0.5arcsec"
totaltime = "3600s"