NGC 253 Probing a Nuclear Starburst with HighResolution PAH Spectra

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NGC 253 Probing a Nuclear Starburst with
High-Resolution PAH Spectra

• David Raban

Steve Croft
Paul Festler
Benny Trakhtenbrot
Mikolaj Kopernik
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Mid-IR Starburst Spectra

• Spitzer IRS spectrum (aph/0406172) of NGC 253
• (our target we will also compare to the IRS
  results)
• 11.8'' 5.3'' slit (133 60 pc), ??/? 600, 10
  20 µm

IRS spectrum (aph/0701124) of NGC 1097 (a similar
starburst galaxy)
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Properties of NGC 253

• Starburst galaxy at z 0.000811 (3.4 Mpc)
• 00h47m33.1s, -25d17m18s (J2000)
• Core fluxes
• f8.9 µm 1.1 Jy
• f11.9 µm 2.8 Jy
• (bright enough!)
• There may be an AGN on scales
• lt50 mas (radio observations of
• Ulvestad Antonucci 1997) so there might
• be a chance to see synchrotron in the
• mid-IR Seyfert nature of the object
• is unconfirmed
• How does SFR change
• with distance to the core?

Raban et al. submitted
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Why we want high resolution

• Raban et al. used a 3.6-m telescope to probe
  seeing-limited angular resolutions of 0.5"
  (spatial scales of 8 pc) in N-band imaging.
• Existing IRS spectra cover a 12" x 5" region, so
  yield integrated spectral properties on 200 pc
  spatial scales.
• Individual 8-m UTs will give angular resolution
  of 0.2" (spatial scales of 3 pc).
• VLTI with 40-m baselines will give angular
  resolution of 40 mas (spatial scales of 0.6 pc)
  with ??/? 230.

• We dont want to go much higher in angular
  resolution because we might start to resolve out
  the structures we are interested in.
• We want good sensitivity (so need UTs), but short
  baselines (so we dont resolve out the
  structures) so we choose U2 U3.
• MIDI will give ??/? 230 from 8 13 µm

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Observability and Calibration

• Our target is best placed for observations in
  early October. The delay lines do not limit
  observability (since we are using short
  baselines).
• A 30 minute observation (5 min exposure) will
  provide sufficient S/N
• A suitable calibrator is available

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Path to Science

• Compare IRS and MIDI spectra differences in
  line ratios and strengths will tell us how
  concentrated the SF is w.r.t. the Spitzer IRS and
  VLTI MIDI resolutions.
• If the visibilities are zero, we know the SF is
  more spatially extended than the scales probed by
  MIDI and is being resolved out.
• If the core remains unresolved at the high
  spatial resolutions probed by our MIDI
  observations, we can propose for longer baseline
  observations (not currently requested since first
  we wish to know if the short baselines are
  sufficient). These may also help if there turns
  out to be an AGN.