N891 simdata2

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Nearby edge-on spiral

Roughly modeled after NGC891

This article is under construction. Watch this space!

- 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 (Beamsummary.png) 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"

predict=T totaltime = "3600s"


here's the cube with the simdata's scaling and World Coordinate System: N891.coord.png
and a spectral profile in the box marked in green

Sample results: