Radio continuum, CO, and thermal infrared emission in nearby star-forming galaxies

1. Radio continuum, CO, and thermal infrared emission in nearby star-forming galaxies With A. Hughes2, R. Ekers1, R. Paladino3, M. Murgia3,4, L. Blitz5, T. Helfer5, L. Moscadelli3, L. Gregorini4, L. Staveley-Smith1, M. Filipovic6, Y. Sofue7, N. Mizuno7 Tony Wong CSIRO Australia Telescope & University of New South Wales 1ATNF, 2Swinburne U., 3INAF-Cagliari, 4CNR Italy, 5UC Berkeley, 6U. Western Sydney, 7Nagoya U. SMA meeting, 13 Jun 2005

2. The Radio/FIR Correlation • Amazingly tight correlation (factor of 2 scatter over >4 orders of magnitude) • Correlation persists even when normalizing by mass (Xu et al. 1994). • Correlation of radio with FIR better than with other SF tracers (e.g. UV, H). • Has been used as redshift indicator (Carilli & Yun 1999) and to identify high-redshift submm galaxies. Yun, Reddy, & Condon 2001 LMC

3. Molecular clouds Hot stars Heating Stars form Warm Dust SN FIR Radio SNR Synchrotron ISM Shocks Cosmic Ray Acceleration Magnetic Field Why It’s Surprising Grain props UV High Correlation! Courtesy Ron Ekers

4. LMC images To examine correlations as a function of size scale, we compared several images, all at ~1’-2’ resolution: • IRAS 60µm image processed via HIRES algorithm (courtesy J. Surace, IPAC) • ATCA + Parkes 1.4 GHz continuum image (courtesy M. Wolleben & L. Staveley-Smith) • ATCA + Parkes 21cm HI image (Kim et al. 2003) • NANTEN CO image (courtesy Fukui et al., Nagoya U.) • SHASSA H image (Gaustad et al. 2001)

5. 1.4 GHz Continuum and HI

6. FIR and CO

7. Wavelet filtered images

8. The Pet Hat Wavelet The “Pet Hat” wavelet is essentially an annulus in the Fourier plane that picks out structure on a certain scale. 5 kl 20” Frick et al. (2001)

9. An Example 0º 30º

10. LMC Subregion Each image 2° square

11. LMC Subregion Each image 2° square

12. Correlations Compared • Best correlations among 60 µm, H, and 1.4 GHz. • Correlations with gas tracers (CO, HI) degrade below ~200 pc (12.5 arcmin).

13. Correlations Compared • Best correlations among 60 µm, H, and 1.4 GHz. • Results even more pronounced for galaxy as a whole. • Correlations with gas tracers degrade below ~1 kpc.

14. LMC Results We have examined the correlations between FIR, 1.4 GHz radio, CO, H, and HI emission in the LMC from scales of 2’ (40 pc) to 4° (4 kpc). • Best correlations are between the “star formation tracers” FIR, 1.4 GHz, and H, beginning to break down only scales of <0.1 kpc. • Correlation of FIR and 1.4 GHz with cold gas tracers (e.g., HI) relatively poor on sub-kpc scales. Problem forB-gas coupling? • Caveat is larger fraction of thermal emission in the LMC (~50% at 1.4 GHz, Haynes et al. 1991).

15. The CO/RC Correlation CO/RC ratio map RC vs. CO fluxes Murgia et al. (2005)

16. Spiral Galaxies NGC 4736 NGC 7331

17. RC vs. mid-IR Although correlation between 24 µm and 1.4 GHz emission is good on a pixel-by-pixel basis, a drop in scale-dependent correlation is often seen on scales of < 2 kpc.

18. RC vs. CO The correlation between CO and 1.4 GHz emission is better in some galaxies and worse in others, but also shows breakdown in the 1-2 kpc range.

19. mid-IR vs. CO The best correlation is probably between 24 µm and CO emission, although still breaking down on sub-kpc scales.

20. Caveats • 24 µm emission dominated by dust heated by SF; may exclude diffuse emission from cooler dust that contributes to overall FIR flux. • Sensitivity of RC images to bright point sources (e.g. nuclear or background AGN), which contribute power on all scales. • Possibility of missing interferometer spacings (e.g. combining VLA B- and D-arrays) • Signal-to-noise differs between images, producing an artificial decorrelation in wavelet analysis.

21. Conclusions 1.4 GHz radio emission in LMC: mostly thermal? • Similarity of FIR, RC, H suggest they all trace recent SF. • Overall synchrotron weak (<~50%) and may be dominated by compact SNRs (Klein et al. 1989). • Lack of synchrotron (compared to massive gals) could be due to greater ease of cosmic ray escape (e.g. Chi & Wolfendale 1990). In nearby spirals, correlations of radio continuum with mid-IR and CO break down at 1-2 kpc scales. • Non-thermal emission neither correlating with recent star formation nor tightly coupled to molecular clouds. • Need better radio images & thermal/non-thermal separation.

22. Role of Sub-mm Observations Does non-thermal RC trace the interface between strong CR flux and strong B field? • Can compare with classic “PDR” tracers such as CI and CII. • CI: 3P13P0transition (492 GHz):ncr~103 cm-3, ∆E~24 K, ~0.1-1. • In well-shielded regions, CI abundance may be enhanced due to cosmic ray heating (e.g. Flower et al. 1994). Importance of cold (T < 20 K) dust • Coldest dust dominates at sub-mm wavelengths and correlates with HI+H2: how well does it correlate with radio continuum? • Examine low Lfir/Lopt regions at >100µm to assess importance of dust heating by old stars & its effect on correlation.