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

NGC 253 – Probing a Nuclear Starburst with High-Resolution PAH Spectra. Steve Croft. Paul Festler. Benny Trakhtenbrot. Mikołaj Kopernik. David Raban. Mid-IR Starburst Spectra. Spitzer IRS spectrum (aph/0406172) of NGC 253 (our target – we will also compare to the IRS results)

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

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  1. NGC 253 – Probing a Nuclear Starburst with High-Resolution PAH Spectra Steve Croft Paul Festler Benny Trakhtenbrot Mikołaj Kopernik David Raban

  2. 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), Δλ/λ = 600, 10 – 20 µm IRS spectrum (aph/0701124) of NGC 1097 (a similar starburst galaxy)

  3. 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 <50 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

  4. 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 Δλ/λ = 230. • We don’t 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 don’t resolve out the structures) so we choose U2 – U3. • MIDI will give Δλ/λ = 230 from 8 – 13 µm

  5. 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

  6. 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.

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