1. Molecular Line Studies and Chemistry in Interacting and Starburst Galaxies Susanne Aalto Department of Radio and Space Science With Onsala Space Observatory Chalmers University of Technology

2. Collaborators M. Spaans, JP Perez Beaupuits (Kapteyn) F. Van der Tak (SRON) D. Wilner, S. Martin (CfA, Harvard, USA) J. Martin-Pintado (CSIC, Spain) M. Wiedner ( Cologne, Germany) F. Costagliola, E. Olsson, R. Monje, J. Black (OSO, Sweden) R. Beswick, (Jodrell Bank, UK) K. Sakamoto (ASIAA, Taiwan) J. Gallagher (Michigan, USA) E. Manthey (ASTRON)

3. Outline Why study molecular gas in galaxies? Tracing the gas Molecular lines as diagnostic tools CO and 13 CO – ISM large scale structure, impact of dynamics and temperature Dense gas and chemistry in galaxy centres: HCN and HNC HCO + CN HC 3 N H 3 O +

4. Why study the molecular gas? Serves as fuel for both starburst and AGN activity. Significant mass in galaxy nuclei ”Extinction-free” tracer Interesting dynamics Multitude of spectroscopic tools to determine physical conditions and chemistry. H 2 is a ”silent” molecule – needs tracer species NGC 1365

5. Molecular gas in galaxies - fuel for starbursts and AGNs CO as standard tracer of H 2 distribution, dynamics and mass CO luminosity to H 2 mass conversion factor HCN as tracer of high density (n>10 4 cm -3 ) gas HCN-FIR correlation (Solomon et al -92). (New plot by Gao and Solomon 2004)

6. Molecular gas distribution in interacting galaxies CO morphology: Signature of interaction type and age – as well as evolutionary stage of the central activity. Interaction NGC5218/NGC5216 NGC4194 – The Medusa merger

7. Molecular line ratios A.. 12 CO / 13 CO line ratio as a tracer of ISM- structure, temperature and dynamics. B.…and the dense gas: 1. HCO + /HCN 2. HNC/HCN 3. CN/HCN 4. HC 3 N Note: Even with exisiting telescope arrays, we are looking at ensembles of clouds -> Average properties of the molecular gas within the beam – but ALMA will change all of this. Issues of radiative transfer and optical depth

8. A. CO/ 13 CO line ratio - Global CO/ 13 CO J=1-0 line ratio increases with increasing 60/100  m flux ratio (e.g. Young and Sanders 1986, Aalto et al 1991, 1995) Elevated CO/ 13 CO J=1-0 line ratio caused by moderate optical depths : 1. high kinetic temperatures, or 2. presence of diffuse, unbound gas. Additional abundance effects in outskirts of galaxies, low metallicity gas. selective dissociation in PDRs (Photon Dominated Regions) in galaxy nuclei. Luminous mergers Arp220 Serves as tracer of large scale ISM structure and impact by dynamics and starformation

9. Large scale ISM property gradients a) Temperature gradients Temperature gradient in the molecular gas of the merger Arp299 Faint 13 CO 1-0 in the nuclei of IC 694 and NGC 3690, but bright 13 CO 2-1 emission. High 13 CO 2-1/1-0 line ratio expected when temperatures and densities are high 13 CO 1-0 13 CO 2-1 log n 13 CO 2-1/1-0

10. Large scale ISM property gradients b) Diffuse molecular gas Diffuse molecular gas in dust-lane of the medusa merger. Large-scale shift in CO and 13 CO 1-0 peaks. CO emission is tracing dust lane and nuclear starburst region 13 CO is not associated with dust lane but with the western side of the starburst. 13 CO peaks are one kpc away from CO peak.

11. Diffuse molecular gas 13 CO 1-0 peaks downstream from CO 1-0 Tosaki et al. CO 1-0 13 CO 1-0

12. 12 CO Gray scale: 12 CO Contours: 13 CO Hüttemeister, Aalto, Das & Wall, 2000 OVRO Diffuse, unbound gas close to bar shock GMCs/possible star formation downstream (offset to leading edge) within bar: 4 - 40 central 1 kpc: 10 - 30 Strong variation in R 10 SBc Starburst/ LINER NGC 7479

13. To conclude Investigate with different molecular tracers: - Transition to center - SFR and SFE - Phases and correlations at high resolution... The phases of molecular gas in bars (and starbursts): Traced by studies of molecular line ratios Evolutionary differences even in small sample Evidence for (at least) two-phase medium Diffuse gas Star-forming clouds Down - stream

14. B. Chemistry as a diagnostic tool Assist in identifying dust enshrouded nuclear power sources: AGN or starburst – XDR or PDR chemistry? Tracer of starburst evolution. Tracer of type of starburst? Are all starbursts alike – or do their properties vary with environment? Starburst-AGN connection. ISM chemistry tracers are particularly important for the deeply obscured activity zones of luminous and ultraluminous galaxies. Kohno et al.

15. HCO+ and HCN in XDR models Large X[HCN]/X[HCO+] ratio is expected in some XDR models - e.g. Maloney et al (1996). Selective destruction of HCO+ combined with formation of HCN. However, recent modelling by Meijerink and Spaans, Meijerink et al (2005, 2006, 2007) predict large HCO+ abundances in XDRs Elevated HCN/HCO+ abundance ratios also in young, pre-supernova, starformation.  Line ratio serves as an indication of evolution X[HCO+] and X[HCN] in XDR (from Lepp and Dalgarno (1996) – plotted in the same figure

16. 2. HNC in luminous galaxies In Milky way GMCs: X[HNC] is decreasing with increasing temperature (e.g. Schilke et al 92) In cold dark clouds: X[HNC]>X[HCN] In hot cores: X[HNC]<<X[HCN ] … but this behaviour appears NOT to be generally reproduced in external galaxies. Survey results indicate bright HNC 1-0 emission in many warm starbursts and AGNs Nearby galaxies: Hüttemeister et al. (1995) ULIRGs and LIRGs: Aalto et al. (2002, 2007), Baan et al (2007) Galaxies with similar CO/HCN 1-0 line ratio often have very different HCN/HNC 1-0 ratios: 1 to >6

17. What is causing bright HNC line emission in warm environments? Abundance: Ion-neutral chemistry governs the HCN/HNC abundance ratio – which is independent of temperature X[HNC]=X[HCN in PDRs X[HNC]>X[HCN] in warm, dense (n>10 5 cm -3 ) XDRs (Meijerink and Spaans 2005). Optical depth and cloud size Excitation: mid-IR pumping of HNC via bending mode occurs at 21.5  m at 669 K –pumping starts to become effective at T B (IR) = 50 K

18. HNC 3-2 in Arp 220 – SMA high resolution study Recent SMA result by Aalto, Wilner, Wiedner, Spaans, Black (2008) Preliminary results: HNC 3-2 emission primarily associated with western nucleus. Peak T B in 0.”5x0.”3 beam is 36 K: CO 2-1/HNC 3-2 line intensity ratio of < 2 in inner 0.”5. About 50% of emission is extended on scales of 0.”7.

19. HNC 3-2 in Arp 220 Narrow, luminous feature on western nucleus. Occuring where CO 2- 1 has deep absorption through.

20. HNC, a new Astronomical maser?

21. Extended HNC emission Tapered (low resolution) map showing north-south, bipolar emission Coincident with OH megamaser emission towards western nucleus. Outflow? Excitation of HNC? Chemistry?

22. 3. CN in external galaxies CN is both a PDR and an XDR tracer (Krolik and Kallman 1983; Lepp and Dalgarno; Sternberg; Meijerink and Spaans 2005 Survey of 15 luminous galaxies show CN 1-0 to be somewhat fainter than we expected for a PDR tracer. Slight tendency for CN luminosity to decrease with galaxy luminosity – but must be confirmed with larger sample. (Aalto et al 2002) IC 694 nucleus: HCN/CN = 1 Overlap region: HCN/CN = 1 NGC 3690 nucleus: HCN/CN > 5 OVRO CN 1-0 (Aalto et al 2005) CN 1-0 in the Arp299 merger

23. 4. HC 3 N in LIRGs Surveys of LIRGs have revealed a handful of galaxies with luminous HC 3 N 10-9 emission. Tracer of warm, dense, shielded gas. Quickly destroyed by UV photons and by reactions with C + ”Hot core molecule” – i.e. young star formation or very dusty, embedded AGNs? HC 3 N luminous in LIRGs with deep IR silicate absorption (Costagliola et al 2008) Correlation with IR excitation temperature (as derived by Lahuis et al 2007). ”Extended” hot core phase?

24. Examples: A. NGC4418 – Dusty LIRG. Dominated by compact nuclear emission. Nascent starburst or AGN? B. Arp220 – Dusty ULIRG. Two luminous merging nuclei. Starburst and/or AGN?

25. A. The dusty LIRG NGC 4418 NGC 4418 is a, dusty IR-luminous edge-on Sa galaxy with Seyfert-like mid-IR colours. IR dominated by 80 pc nuclear structure of T B (IR)=85 K (Evans et al). What is driving the IR emission – starburst or AGN activity? FIR-excess, q=3: young starburst? No hard X-rays: starburst? Broad NIR H 2 lines: AGN? HCN/HCO + 1-0 line ratio > 1: AGN? DSS optical NGC4418 NIR image (Evans et al 2003)

26. Rich Chemistry in NGC4418 – buried AGN or nascent starburst? Bright HC 3 N 10-9,16-15,25-24 detected. Ortho- H 2 CO, CN, HCN, HCO +, OCS (tentative). HNCO not detected. All species - apart from HNC and HC 3 N - are subthermally excited and can be fitted to densities 5x10 4 – 10 5 cm -3 (Aalto, Monje, Martin 2007). HC 3 N is vibrationally excited – governed by IR-field not collisions

27. HCN, HNC, HCO+,CN Overluminous HNC 3 radiative excitation? Luminous high-J HCO+ HCO + -rich core? CN relatively bright CN1-0 CN2-1 CN 1-0 CN 2-1

28. NGC4418 – buried AGN or young starburst. How can we tell? Bright HC 3 N emission combined with high HCN abundance can be understood in terms of hot-core chemistry (e.g. Blake 1985) - i.e. young star formation Line ratios can also be understood as deeply buried AGN, In this case, the impact of the AGN should be quite local, where the dust column absorbs nuclear emission so that HC 3 N can survive. Bright HC 3 N emission is inconsistent with a large scale XDR component. NGC4418 From Lisenfeld et al 1996

29. Conclusions – HCO+ What does an elevated HCN/HCO + 1-0 line ratio really mean? XDRs? Existing models give different predictions. HCN/HCO + 3-2 line ratios may give opposite line ratio to 1-0 (e.g. NGC4418) Young starburst? In hot cores we may indeed expect an elevated HCN/HCO + abundance ratio. Something else? If it is not an XDR effect – why are we seeing elevated HCN/HCO + 1-0 line ratios in some Seyfert galaxies? Starburst-AGN connection? We must continue our studies at higher transitions – and other molecules.

30. Conclusions - HNC HNC emission often bright in luminous galaxies – can be explained by ion-neutral chemistry – in PDRs or XDRs. Mid-IR pumping Optical depth effect – scale? Other? Other luminous galaxies have no HNC emission – despite luminous HCN emission. This remains to be understood

31. Conclusions - CN CN not as bright as expected in ULIRGs – where are the PDRs in the super-starbursts? Or the XDRs around the AGNs? HC 3 N lines bright in NGC4418, Arp220, UGC5101 - dusty, luminous galaxies. Up to 50% of HCN 1-0 luminosity. Young starbursts?

32. The Impact of ALMA We are still working on the interpretation of molecular lines towards obscured galactic nuclei. ALMA will help enormously through offering resolution and sensitivity: We can image ULIRGs with GMC-scale resolution. Instead of interpreting individual (or a handful of) lines, will it be possible to develop modelling tools that will deal with whole line-scans? A ”STARBURST99” for the starburst molecular ISM?

33. 1. HCN and HCO+ Kohno et al find HCN/HCO+ 1-0 line ratios greater than unity in several Seyfert nuclei – where also the HCN/CO 1-0 line ratio is high. Gracia-Garpio find elevated HCN/HCO+ 1-0 line ratios in ULIRGs (2006). Is an elevated HCN/HCO+ 1-0 line ratio an AGN indicator? NGC 5033 (Kohno 2005)

34. …ALMA – a new era Resolution will allow GMC-scale studies of the molecular properties of Ultraluminous and Seyfert galaxies. Observe from 115 GHz into the THz regime – new lines – new astronomy? The Atacama Large Millimeter Array -with the ACA (the compact array) to the right.

36. Line ratio analysis – radiative transfer Non-LTE radiative transfer model (LVG) As many transitions as possible to break degeneracies. Intersections of measured ratios +... assumptions... e.g. collisional excitation Characteristic gas densities and kinetic temperatures R 21 R10R10 12 CO (2-1)/(1-0) 15 20 0.6 Example: low density, fairly high T kin solution

37. HCN/HCO + line ratios HCN/CO vs. HCN/HCO + (Kohno et al 2001)

38. B. Arp220 - Black hole in the western nucleus? (Downes and Eckart 2007) Compact (35 x 20 pc), hot (T B = 90 K at 1.3 mm, T D =170 K), massive,nuclear dust disk. Black body luminosity of nuclear dust source: 10 12 L o – required emission surface brightness: 5 x 10 14 L o kpc -2. i.e. 30 times the luminosity of M82 packed into a 1000 times smaller volume. CO appears to be rotating in the potential of a centrally concentrated mass: enclosed mass at 30 pc = 10 9 M o Alternative (Sakamoto et al 2008): buried young starburst equivalent to >100 SSCs