An Investigation of the Molecular-FIR-Radio correlation at small scales in the Galaxy

1. An Investigation of the Molecular-FIR-Radio correlation at small scales in the Galaxy Mónica Ivette Rodríguez Dr. Laurent Loinard (UNAM - México) Dr. Tommy Wiklind (STScI - USA)

2. Introduction The main goal of studying spiral galaxies is to understand how stars form and how the star formation is related to dynamical and physical conditions in the interstellar medium through several different diagnostics. Examples : • Ionizing continuum radiation (UV) • Balmer lines • near-infrared, mid-infrared • Dust emission (far-infrared) • Molecular emission (CO) • Radio continuum

3. Introduction Several diagnostics are correlated FIR-RC correlation FIR-CO correlation RC-CO correlation Condon1992 Tutui et al. (2002) Paladino et al. (2006)

4. Introduction These correlations hold when viewing galaxies on global scales, however the emission mechanisms and the processes driving the emission are different. The physical bases for understanding the molecular-FIR-RC correlation is not well understood, and several effects can modify the basic correlation such as density waves, etc. In my PhD. Project I will study these correlations, most notably, the far-infrared and radio continuum correlation on scales corresponding to the size of small molecular clouds. I will also study the possibility that the correlation between CO and far-infrared luminosities is caused by strong selection effects: molecular CO emission is only detected in warm regions. An alternative is searching for very cold molecular gas using the anomalous 6-cm line of formaldehyde (H2CO).

5. Molecular Gas The molecular gas is mainly traced by 12CO emission Since it is an optically thick line, the line does no give any information about density Absorption lines can be used as tracer of cold molecular gas since the excitation characteristics looks different But the absence of radio continuum sources limits the use of absorption lines The 6-cm H2CO line is an absorption line against the Cosmic Microwave Background, seems to offers an alternative …

6. Formaldehyde (H2CO) Several transitions 2-mm, 2-cm & 6-cm At 6-cm (4829.660 Mhz) : Was discovered in 1969 (Palmer et al. 1969) Low excitation energy ( ~ 1.7 K) It is an absorption line against the Cosmic Microwave Background (CMB) Energy-level diagram (Townes & Cheung 1969)

7. Formaldehyde (H2CO) Townes & Cheung 1969 used a classical calculation for collisional excitation The collisional pumping mechanism is more effective at high collision rates (Evans et al. 1975), however they showed that the mechanism would still be effective at low temperatures More precise calculations in Garrison et al. (1975) suggest a smaller effect at very low kinetic temperatures This leaves open the possibility that high-density, cold molecular gas may be detected using H2CO Structure of the H2CO molecule (Townes & Cheung 1969)

8. Introduction to the target sources Galactic Anticenter : The Galactic non-thermal background is faint in this direction The velocity gradient is small in this direction, enhancing the probability of detection

9. Introduction to the target sources L1204/S140 Region : Photodisociated region caused by a very close nearby B0V star Close star forming region

10. Observations The observations were obtained during three sessions (January 2004, September - October 2004, May 2005) with the 25.6m telescope of the Onsala Space Observatory (OSO) At 6 cm, the angular resolution of the 25 m is 10’.

11. Results Galactic Anticenter : 143 positions H2CO absorption at 10 % No H2CO emission

12. Results l = 182o, b = 0o H2CO 12CO 63 positions

13. Results l = 190o, b = 0o 101 positions H2CO 12CO

14. Results Proving the nature of the H2CO absorption toward the Anticenter Grey scale : H2CO absorption Contours : 21-cm radio continuum

15. Results 1) L1204/S140 Region Photodissociated Region 2) First panel : -12 km/sec < v < -5 km/sec Second panel : -10 km/sec < v < -5 km/sec Third panel : -12 km/sec < v < -10 km/sec 3)

16. Results L1204/S140 Region : l = 107ob = 5.3o 72 positions H2CO peak is at 10’ offset of 12 CO peak H2CO 12CO

17. Relation of the H2CO CMB absorption to CO(1-0) emission Galactic Anticenter : Intensity ratio Points : both tracer Open circles : CO I(H2CO) K km/sec I(12CO) K km/sec

18. Relation of the H2CO CMB absorption to CO(1-0) emission L1204/S140 Region : Intensity ratio Points : both tracer Open circles : CO I(H2CO) K km/sec I(12CO) K km/sec

19. Conclusions The excitation characteristics of both lines are similar H2CO and 12CO lines trace warm, dense molecular gas The H2CO absorption line is not a viable tracer of cold molecular gas The question that clouds of cold and dense molecular gas may exist remains open

20. Publications The results of the H2CO observations toward the Galactic Anticenter were presented in the article : “Anomalous H2CO Absorption Toward the Galactic Anticenter : A Blind Search for Dense Molecular Gas “(Rodriguez el al., 2006 Astro-ph/0607616) (submitted and accepted to ApJ) The results of the H2CO observations toward the Galactic Cloud L1204 will presented in the article : “Anomalous H2CO Absorption in the L1204/S140 Region and a Comparison with CO(1-0) emission” (to be submitted to ApJ)

21. Future Work The work plan for the up coming year, will be focused in the behavior of far-infrared and continuum correlation, on scales corresponding to the size of the small molecular cloud. Following the calorimeter theory (Voelk 1989) such correlation is not expected at local scales Hoernes, Berhuijsen & Xu (1998) showed that it still holds at scales of about 1 kpc in M31 Murphy et al. 2005 combined new Spitzer data with archival radio observations of M51 conclude that this correlation still holds at 750 pc Then the scale of infrared-radio remains unknown … Such correlation is indeed needed to explain the overall radio-infrared correspondence We proposed to study it at much smaller scales … Milky Way

22. Example Comparison between the IRAS 100 m image and the 408 MHz radio image of the Galactic region around (135,+2)

23. Work plan • Get 1.4 GHz and 408 MHz and IRAS images of the Dominion Radio Astrophysical Observatory (DRAO) (Taylor et al. 2003) • Identify several prototypical Galactic regions, we proposed 20-25 examples Infrared Radio continuum 408 MHz 60  100  1420 MHz

24. 408 MHz 60  10’ 100  1420 MHz

25. 408 MHz 60  10’ 100  1420 MHz

26. Work plan • Clean the bright point sources • Get the index spectral map for every region • Obtain the pure non-thermal images combining the two wave lengths • Compare them quantitatively with the far-infrared data from IRAS • Compare our local correlation FIR-RC with the global correlation

27. Goals Hoernes, Berhuijsen & Xu (1998) proposed that if the FIR-RC still holds on small scales there should be a strong coupling between the interstellar gas located in the clouds traced by IRAS and the magnetic field If we confirm the existence of a tight radio/infrared correlation at parsec scales, we shall attempt to explain it using similar models, and be able to put stronger constraints on the theoretical models If the results are in agreement with our expectations, we will consider extending our studies over the entire Galactic disk.