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Molecular Line Studies and Chemistry in Interacting and Starburst Galaxies

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. M. Spaans, JP Perez Beaupuits (Kapteyn) F. Van der Tak (SRON)

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Molecular Line Studies and Chemistry in Interacting and Starburst Galaxies

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  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. 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) Collaborators

  3. Outline • Why study molecular gas in galaxies? • Tracing the gas • Molecular lines as diagnostic tools • CO and 13CO – ISM large scale structure, impact of dynamics and temperature • Dense gas and chemistry in galaxy centres: • HCN and HNC • HCO+ • CN • HC3N • H3O+

  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. • H2 is a ”silent” molecule –needs tracer species NGC 1365

  5. Molecular gas in galaxies- fuel for starbursts and AGNs • CO as standard tracer of H2distribution, dynamics and mass • CO luminosity to H2 mass conversion factor • HCN as tracer of high density (n>104 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. A.. 12CO/ 13CO line ratio as a tracer of ISM-structure, temperature and dynamics. B.…and the dense gas: HCO+/HCN HNC/HCN CN/HCN HC3N 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 Molecular line ratios

  8. Global CO/13CO J=1-0 line ratio increases with increasing 60/100 mm flux ratio (e.g. Young and Sanders 1986, Aalto et al 1991, 1995) Elevated CO/13CO J=1-0 line ratio caused by moderate optical depths : high kinetic temperatures, or 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. A. CO/13CO line ratio - Serves as tracer of large scale ISM structure and impact by dynamics and starformation Luminous mergers Arp220

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

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

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

  12. SBc Starburst/ LINER OVRO Gray scale: 12CO Contours: 13CO 12CO Strong variation in R10 central 1 kpc: 10 - 30 within bar: 4 - 40 Diffuse, unbound gas close to bar shock GMCs/possible star formation downstream (offset to leading edge) Hüttemeister , Aalto, Das & Wall, 2000 NGC 7479

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

  14. B. Chemistry as a diagnostic tool ISM chemistry tracers are particularly important for the deeply obscured activity zones of luminous and ultraluminous galaxies. • Assist in identifying dust enshrouded nuclear power sources: AGN or starburst – XDR or PDR chemistry? • Tracer of starburst evolution. • Tracer of typeof starburst? Are all starbursts alike – or do their properties vary with environment? • Starburst-AGN connection. 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. 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] …butthis 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 2. HNC in luminous galaxies

  17. 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>105 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 mm at 669 K –pumping starts to become effective at TB(IR) = 50 K What is causing bright HNC line emission in warm environments?

  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 TB 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) CN 1-0 in the Arp299 merger OVRO CN 1-0 (Aalto et al 2005) IC 694 nucleus: HCN/CN = 1 Overlap region: HCN/CN = 1 NGC 3690 nucleus: HCN/CN > 5

  23. 4. HC3N in LIRGs • Surveys of LIRGs have revealed a handful of galaxies with luminous HC3N 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? • HC3N 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 TB(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 H2 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 HC3N 10-9,16-15,25-24 detected. Ortho- H2CO, CN, HCN, HCO+, OCS (tentative). HNCO not detected. All species - apart from HNC and HC3N - are subthermally excited and can be fitted to densities 5x104 – 105 cm-3 (Aalto, Monje, Martin 2007). HC3N 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 CN 1-0 CN1-0 CN2-1 CN2-1

  28. NGC4418 – buried AGN or young starburst. How can we tell? • Bright HC3N 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 HC3N can survive. Bright HC3N emission is inconsistent with a large scale XDR component. NGC4418 From Lisenfeld et al 1996

  29. 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. Conclusions – HCO+

  30. 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 Conclusions - HNC

  31. CN not as bright as expected in ULIRGs – where are the PDRs in the super-starbursts? Or the XDRs around the AGNs? HC3N lines bright in NGC4418, Arp220, UGC5101 - dusty, luminous galaxies. Up to 50% of HCN 1-0 luminosity. Young starbursts? Conclusions - CN

  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+ NGC 5033 (Kohno 2005) • 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?

  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.

  35. R21 12CO (2-1)/(1-0) 15 20 R10 0.6 Characteristic gas densities and kinetic temperatures Example: low density, fairly high Tkin solution 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

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

  37. Compact (35 x 20 pc), hot (TB = 90 K at 1.3 mm, TD =170 K), massive,nuclear dust disk. Black body luminosity of nuclear dust source: 1012 Lo – required emission surface brightness: 5 x 1014 Lo 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 = 109 Mo Alternative (Sakamoto et al 2008): buried young starburst equivalent to >100 SSCs B. Arp220 - Black hole in the western nucleus? (Downes and Eckart 2007)

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