Molecular Line Studies and Chemistry in Interacting and

Starburst Galaxies

Susanne Aalto

Department of Radio and Space ScienceWith Onsala Space Observatory

Chalmers University of Technology

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)

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+

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

Molecular gas in galaxies- fuel for starbursts and AGNs CO as standard

tracer of H2 distribution, 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)

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

Molecular line ratios A.. 12CO/ 13CO line ratio

as a tracer of ISM-structure, temperature and dynamics.

B.…and the dense gas:1. HCO+/HCN2. HNC/HCN 3. CN/HCN4. 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

A. CO/13CO line ratio - Global CO/13CO 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/13CO 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

mergersArp220

Serves as tracer of large scale ISM structure and impact by dynamics and starformation

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 1-0 13CO 2-1

log n

13CO 2-1/1-0

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.

Diffuse molecular gas

13CO 1-0 peaks downstream from CO 1-0

Tosaki et al.

CO 1-0 13CO 1-0

12COGray scale: 12COContours: 13CO

Hüttemeister , Aalto, Das& Wall, 2000

OVRO

Diffuse, unbound gasclose to bar shock

GMCs/possible starformation downstream(offset to leading edge)

within bar: 4 - 40

central 1 kpc:10 - 30

Strong variationin R10

SBc Starburst/LINER

NGC 7479

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 mediumDiffuse gas

Star-forming clouds

Down-stream

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 ISM chemistry tracers are particularly important for the deeply obscured activity

zones of luminous and ultraluminous galaxieszones of luminous and ultraluminous galaxies..

Kohno et al.

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 andDalgarno (1996) – plotted in the same figure

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

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>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 m at 669 K –pumping starts to become effective at TB(IR) = 50 K

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.

HNC 3-2 in Arp 220 Narrow, luminous

feature on western nucleus.

Occuring where CO 2-1 has deep absorption through.

HNC, a newAstronomical maser?

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?

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

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?

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?

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)

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 befitted to densities 5x104 – 105 cm-3 (Aalto, Monje, Martin 2007).

HC3N is vibrationally excited – governed by IR-field not collisions

HCN, HNC, HCO+,CN Overluminous HNC 3

radiative excitation?

Luminous high-J HCO+ HCO+-rich core?

CN relatively brightCN1-0

CN2-1

CN 1-0

CN 2-1

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

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.

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

Conclusions - CN 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?

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?

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)

…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.

Line ratio analysis – radiative transfer

Non-LTE radiativetransfer model (LVG)

As many transitionsas possible to breakdegeneracies.

Intersections ofmeasured ratios +... assumptions ...e.g. collisional excitation

Characteristic gasdensities and kinetic temperatures

R21

R1

0

12CO(2-1)/(1-0)1520

0.6

Example: low density, fairly high Tkin solution

HCN/HCO+ line ratiosHCN/CO vs. HCN/HCO+

(Kohno et al 2001)

B. Arp220 - Black hole in the western nucleus? (Downes and Eckart 2007)

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