The gas and dust properties of NGC 891 with Herschel

Transcription

1 The gas and dust properties of NGC 891 with Herschel Tom Hughes Herschel-PACS [OIII] 88 μm

2 Herschel photometry To trace the FIR/submm SED: PACS 70, 100, 160 μm SPIRE 250, 350, 500 μm SCUBA 850 μm Ancillary data: IRAC 3.6 and MIPS 24 μm HI 21 cm (Oosterloo+ 2007) JCMT/HARP-B CO (J=3-2)

3 Notes on FIR morphology Modelling vertical profiles with double exponential disc find scale heights of kpc with increasing wavelength c.f. Xilouris+ 1998, kpc from B - K band images Radial profiles main morphological features in FIR bands correlate better with MIPS 24 μm than IRAC 3.6 μm profile, hinting at dust heating source.

4 Source of dust heating Compare FIR colours to 3.6 and 24 μm: μm emission correlates with 24 μm emission, suggests heating from SF regions >350 μm ambiguous, possible heating from combination of total stellar pop. + SF regions Dust heating in NGC 891 dominated by SF regions may explain radial scale lengths akin to young stars (B band) rather than the old stellar disc (K band) required in models of Bianchi & Xilouris (2011)

5 Integrated SED fitting Fit μm with single-component modified black body: Power-law dust emissivity with к ѵ = m 2 kg -1 at 350 μm, with β as both a fixed (=1.8) and free parameter. Errors determined from bootstrap technique. Best-fit global dust properties: M d 8.5 x 10 7 M, T 23 K, and β 1.8 (c.f. Dupac et al. 2003)

6 Resolved SED fitting Dust smoothly distributed Two regions of warmer dust, coincident with observed asymmetry in northeastern side of disc Mean χ 2 ~ 1 remains likely we are over estimating the actual errors

7 Gas-to-dust ratios Low ratio in center, peaking towards disc edge Expected for the inner regions of the disc Majority of disc has G/D ratios that correlate with dust temperature. Centre has lower G/D, but still warmer dust?

8 Gas-to-dust ratio Correlation in H 2 -to-dust ratio with dust temperature, most likely a consequence of the Kennicutt-Schmidt law. Central pixels could be predominantly heated from total stellar population in bulge. Remains possibility we are simply underestimating the H 2 surface density with incorrect XCO factor.

9 Gas diagnostics Fine-structure cooling lines: collisionally-excited atoms de-excite through forbidden transitions, emitting photons in the far-infrared e.g. [C II] 158 μm ( [O I] 63 + [C II] 158) / F TIR - probe of photoelectric heating efficiency of FUV radiation field [C II] 158 / [O I] 63 - sensitive to the gas density [O I] 145 / [O I] 63 - probe temperatures T gas approx. 300 K Physical gas properties derived via comparison with PDR models, e.g. Tielens & Hollenbach (1985) gas characterised by FUV rad. strength G o incident on slab of hydrogen density n Lines previously observed with e.g. KAO, ISO.

10 Herschel spectroscopy PACS: 25 x 9.4 spaxals in a 5x5 grid, strips + maps, res approx [O III] 88 μm, [O I] 145 & 63 μm, [C II] 158 μm, [N II] 122 μm SPIRE FTS: Two bolometer arrays, sparse and intermediate raster maps, res approx [NII] 205 μm Ancillary data: 24, μm for deriving TIR map (Galametz+ 2013)

11 Radial bins at 12 resolution

12 Gas heating and cooling Lower [C II]/TIR in centre higher fraction of FUV photons absorbed by dust instead of gas Heating efficiency variation gas heating more efficient in disc, consistent with observations of M51 [C II] primary gas coolant possibly under estimating [O I] 63 due to optical depth effects

13 PDR modelling Use ([O I]63+[C II] 158)/F TIR versus [C II] 158 / [O I] 63 diagnostic diagram PDR models of Kaufman+ (1999, 2006), available on web PDR Toolbox Pixel-by-pixel line fitting produces maps of G o and n (and χ 2 )

14 Radial bins at 12 resolution

15 PDR modelling - Derive: 1.7 < log G < 3 1 < log (n/cm 3 ) < < T gas < 500 K - Consistent with: integrated ISO observations of NGC 891 (Stacey+ 2010) and other normal star-forming galaxies (Malhotra ). Herschel observations of resolved galaxies, particularly M51 s spiral arms (Parkin+ 2013).

16 Summary First time mapping the dust properties at sub-kpc resolution Find strong correlations between gas-to-dust ratio and dust temperature Majority of the disc has similar gas properties as spiral arm and inter-arm regions of M51 Estimate a stronger on average FUV radiation field in the NE region but still far from detail reached in optical-nir bands! Hughes et al. 2014, A&A, 565, 4H / Hughes et al. in prep.