1. Tony Wong CSIRO Australia Telescope & University of New South Wales
Radio continuum, CO, and thermal infrared emission in nearby star-forming galaxies
Tony Wong
CSIRO Australia Telescope & University of New South Wales
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
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. Why It’s Surprising Courtesy Ron Ekers High Correlation!
Molecular clouds
FIR
Hot
stars
Heating
Warm
Dust
Stars
form
Grain props UV SN
SNR
ISM
Shocks
Cosmic Ray
Acceleration
Synchrotron
Radio
High Correlation!
What’s most remarkable about the FIR-radio correlation is how it couples a supposedly thermal process (dust emission) with a supposedly non-thermal process (synchrotron emission). In the conventional model, both the FIR and radio emission trace massive star formation, albeit on somewhat different timescales. Despite the large number of steps required to go from the formation of a massive star to the final production of FIR or radio photons, the correlation remains strong.
Magnetic
Field
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 for B-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
The difficulty in obtaining high-resolution FIR images of galaxies has led us to examine the correlation between radio continuum and CO emission, since CO traces the molecular gas from which stars form and is known to be correlated with FIR emission. In NGC 6946, for example, the CO/radio correlation is quite impressive down to scales of 170 pc. This raises the question of whether the correlation is really due to star formation or to the properties of the dense gas from which stars form.
Murgia et al. (2005)

16. Spiral Galaxies NGC 4736 NGC 7331
For nearby spiral galaxies at distances of several Mpc, far-IR images are generally not available at sub-kpc resolution. However, the MIPS instrument on Spitzer is capable of 6” imaging at 24 microns, which allows us to look at correlations between mid-infrared emission, which is also expected to trave massive SF, and CO & RC.
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.
This is only apparent at resolutions that distinguish between the locations of past and future SF, I.e. ~1 kpc.

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3P0 transition (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.
If nonthermal radio is coincident with neither past nor future star formation…
Is the breakdown in the mid-IR/radio correlation on intermediate scales due to the neglect of an extended far-IR component?
Do old stars contribute significantly to the far-IR? Since most theories assume the correlation derives ultimately from young stars, this remains an important issue to resolve.