Probing the Chemistry of Luminous IR Galaxies

Transcription

1 Probing the Chemistry of Luminous IR Galaxies, Susanne Aalto Onsala Space Observatory, Sweden

2 Talk Outline Luminous IR galaxies Chemistry as a tool Observations in NGC 4418 Conclusions

3 Luminous IR Galaxies Most of the energy radiated in the IR, L IR > L Heavily obscured by dust Deep silicate Mid-IR absorption Most luminous sources (ULIRGS) often mergers or interacting systems Dense (n 10 5 cm 3 ), warm (T > 100 K) molecular gas in nuclear regions a) Arp 244, b) NGC7252, c) I19254 d) Arp 220 (Sanders & Mirabel, 1996)

4 Central Power Source: Starburst or AGN? Seyfert-like optical and NIR spectra Accretion on AGN No Hard X-Ray emission Starburst or deeply buried AGN FIR/Radio correlation, slightly higher q Nascent starburst Energy source obscured by dust Radio observations required Free-free thick! Radio continuum doesn t help M82 Cen-A

5 Possible Scenarios PDR XDR Hot Cores AGN FUV dominated Layered: C + C CO X-Ray dominated Not Layered High Ionization Warm, dense, shielded gas Large molecules EACH LEADING TO A DIFFERENT CHEMISTRY!!

6 Chemical Modeling TRANSITION COEFFICIENTS DENSITY RADIATION FIELD ABUNDANCES INTENSITIES CHEMICAL REACTIONS NETWORK DYNAMICS GEOMETRY

7 Chemical Modeling TRANSITION COEFFICIENTS DENSITY RADIATION FIELD ABUNDANCES INTENSITIES CHEMICAL REACTIONS NETWORK DYNAMICS GEOMETRY

8 The LIRG NGC4418: a Molecular Puzzle Edge-on, Sa-type LIRG L IR L Deep mid-ir silicate absrobtion Warm dust ( 80 k) Rich molecular chemistry! Optical (HST)

9 Observations in NGC 4418:

10 Observations in NGC 4418: Observations between Dec 2007 and Aug 2008 at APEX, IRAM30m and JCMT Detected 41 transitions of 11 molecules Bright HNC and HC3N emission HC3N vibrationally excited!

11 What is causing bright HNC emission? Abundance Radiative Excitation Ion-neutral chemistry governs HNC/HCN ratio: X[HNC]=X[HCN] in PDRs X[HNC]>X[HCN] in dense XDRs (n>10 5 cm 3 ) (Meijerink et al, 2007) Mid-IR pumping of bending mode at 21.5µm. Effective at T B =50 K

12 Bright HC 3 N In gas phase from evaporation of ices on dust grains Quickly destroyed by UV (PDR) and reactions with C + (XDR) Requires dense, warm, shielded gas Hot core molecule: young star formation or dusty, deeply buried AGN??

13 LTE Analysis: Rotational Diagram Log(Nu/gu) Rotational diagram for NGC4418 HCN Tx=10.364[K] N=6.5899e+14[cm - 2] HNC Tx=12.402[K] N=5.2907e+14[cm - 2] CO Tx=9.4714[K] N=1.0788e+18[cm - 2] HCO+ Tx=9.8992[K] N=2.9278e+14[cm - 2] HC3N Tx=63.893[K] N=6.0767e+14[cm - 2] CS Tx=18.591[K] N=1.0517e+15[cm - 2] 13CO Tx=6.5423[K] N=4.2367e+16[cm - 2] Eu [K] HC3N vibrational Excitation Log(Nu/gu) HC3N Tx=63.893[K] N=6.0767e+14[cm - 2] HC3N Tx=499.19[K] N=6.9111e+14[cm - 2] HC3N v 6 Tx=71.912[K] N=1.6652e+18[cm - 2] HC3N v 7 Tx=61.077[K] N=3.3786e+16[cm - 2] Eu [K]

14 LTE Results For most high-density tracers we have T 10 K HC3N has a higher rotational velocity T 60 K HC3N not fitted by a single component model Vibrational temperature of HC3N T vib 500 K Radiative excitation! What does T vib mean? X[HNC]/X[HCN] 1 PDR?

15 CO Radex Fit Freq. [GHz] HNC Radex Fit Freq. [GHz] HCN Radex Fit Freq. [GHz] NLTE Analysis: RADEX 2500 Observed Radex, Chi 2 = Observed Radex, Chi 2 = Flux [K km/s] 1500 Flux [K km/s] nh2=532+/ cm -3 T=93+/ K N=2.4156e+18+/ e+17cm nh2= /-18023cm -3 T=230+/ K N=6.868e+14+/ e+13cm Observed Radex, Chi 2 = Flux [K km/s] nh2=4.9879e+05+/-42037cm -3 T=230+/ K N=5.0966e+14+/ e+13cm

16 RADEX Results CO seems to be coming from low density gas n(h 2 ) 500 cm 3 Cannot fit CO: Probably 2 Components! CO/ 13 CO larger for 1-0 transition coming from diffuse gas? HNC and HCN fitted by the same physical conditions at higher densities and temperatures (solution very unstable) Cannot fit CS and HCO + yet

17 Possible Scenario (Just a Sketch) CO 2 1,3 2 HC3N HCN, HNC CO 1 0

18 Summary and Discussion Observed a very rich molecular chemistry in LIRGS and ULIRGS Not unambiguous interpretation of molecular intensities NGC 4418 probably a young Starburst (deeply buried AGN still possible) Power source for LIRGs is still unclear Necessity of more accurate chemical models (eg. including clumping, dynamics, source size, etc...)

19 What to do PUBLISH!!! Improve RADEX fitting Including source size and HC3N vibrations! Interferometric observations to get a spatially resolved chemistry (in progress) Understand and play with chemical modeling (near future) The Future: APEX THz receiver & Herschel satellite Higher molecular transitions ALMA Spatially resolved chemistry at GMC scale

20 Thank You!

21 Chemical Tracers: HCN/HCO + (I) Kohno et al (2005): HCN/HCO + line ratio>1 in Seyfert nuclei Gracia-Garpio (2006): Elevated HCN/HCO + in ULIRGs Is an elevated HCN/HCO + an AGN indicator?

22 Chemical Tracers: HCN/HCO + (II) Observed high HCN/HCO + in LIRGs and AGNs (Imanishi et al, 2007) Some AGN models show an increased HCN abundance (Lintott & Viti, 2006) Recent XDR models predict low [HCN]/[HCO + ] due to enhanced HCO + production (Meijerink et al, 2007) Imanishi (2007): AGNs (Squares), Starbursts (Circles), LIRGs (Stars)