ORTHO-TO-PARA RATIOS IN LOCAL (U)LIRGS WITH HIGH FAR-IR … · 2014. 6. 18. · González-Alfonso...

E. González-Alfonso, J. Fischer, E. Sturm, J. Graciá-Carpio, D. Lutz, A. Poglitsch, A. Contursi, H. Feuchtgruber, S. Veilleux, A. Verma, H.W.W. Spoon, N. Christopher, R. Davies. A. Sternberg, R. Genzel, L. Tacconi

(SHINING)

Based on Herschel/PACS observations

ORTHO-TO-PARA RATIOS IN LOCAL (U)LIRGS WITH HIGH FAR-IR RADIATION DENSITIES:

H2O, H

218O, NH

3, H

2O+ and H

3O+ IN NGC 4418 AND ARP 220

(U)LIRGs with high far-IR radiation densities haveFar-IR spectra dominated by absorption molecular lines

How models work...

González-Alfonso et al (2012, A&A, 541, A4, 2013, A&A, 550, A25)Nuclear regions: Tdust>~100 K : mantle-free dust grainsor “undepleted chemistry”

Expected OPRs

Caveats:1) Line opacity effects2) Line excitation3) Several components contributing to the absorption4) Line blending5) Baseline uncertainty

We wonder whether the observed line fluxes are consistent with the high Tspin OPR values or there are evidences for departures.

Expected H2O OPR

H2O: asymmetric rotorRed: detected in both sourcesBlue: detected in NGC 4418Dashed blue: marginalGreen: contaminated

ortho-H2O ground-state is higher in energy than para-H

2O by 34 K, so that OPR=3 at high

temperature and OPR<3 at low temperature.Lis et al. (2010, A&A, 521, L26) have reported OPR=2.35 toward Sgr B2 (expanding ring)

H2O lines Upper spectra:NGC 4418

Lower spectra: Arp 220

LabelsRed: orthoBlue: para

H2O lines Upper spectra:NGC 4418

Lower spectra: Arp 220

LabelsRed: orthoBlue: para

Models for H2O: ortho/para=3 Upper spectra:NGC 4418

Lower spectra: Arp 220

LabelsRed: orthoBlue: para

Models for H2O: ortho/para=3

Overestimating the ortho lines?

LabelsRed: orthoBlue: para

Nuclear regions:

NGC 4418:Tdust~130-150 KN(H

2O)/τ

50~

(2-6)x1018 cm-2

Arp 220: Tdust~90-110 KN(H

2O)/τ

50~

(0.8-6)x1018 cm-2

Very high-lying lines of H2O Upper spectra:NGC 4418

Lower spectra: Arp 220

LabelsRed: orthoBlue: para

Models for H2O: ortho/para=3 Upper spectra:NGC 4418

Lower spectra: Arp 220

LabelsRed: orthoBlue: para

No significant departure from an ortho-to-para H

2O ratio of 3 (high-temperature limit) is found

Red: orthoBlue: para

NGC 4418 (nuclear):Tdust~130-150 KN(H

2O)/τ

50~(2-6)x1018 cm-2

Arp 220 (nuclear): Tdust~90-110 KN(H

2O)/τ

50~(0.8-6)x1018 cm-2

Ps: the fit does not include the submillimeter H2O lines detected in emission, which indicate an extended component more complex than in this model but with still an OPR consistent with the high-T value of 3.

H218O lines Upper spectra:NGC 4418

Lower spectra: Arp 220

LabelsRed: orthoBlue: para

H218O lines: ortho/para=3 Upper spectra:NGC 4418

Lower spectra: Arp 220

LabelsRed: orthoBlue: para

NH3 lines Upper spectra:NGC 4418

Lower spectra: Arp 220

LabelsRed: orthoBlue: para

NH3: symmetric top (J,K)Red: detected in both sourcesBlue: detected only in NGC 4418Green: contaminated

Models for NH3 : ortho/para=1

Underestimating the ortho lines?Upper spectra:NGC 4418Lower spectra: Arp 220

LabelsRed: orthoBlue: para

NH3: symmetric top (J,K)Red: detected in both sourcesBlue: detected only in NGC 4418Green: contaminated

Models for NH3 : ortho/para=1. Gas thermometer

Arp 220:Tgas~150 KTdust~100 K

NH3/H2O~0.03-0.12

NGC 4418:Tgas~200 KTdust~140 K

NH3/H2O~0.05-0.15

Observations of H2O+

Upper spectra:NGC 4418Lower spectra: Arp 220

H2O+ is similar to H

2O but

with fine-structure level splitting (S=1/2)

Red: orthoBlue: para

ortho-H2O+ ground-state is lower in energy than para-H

2O+, so that OPR=3 at high

temperature and OPR>3 at low temperature.

Models for H2O+

ortho/para=3

Red: orthoBlue: para

Observations of H3O+

Upper spectra:NGC 4418Lower spectra: Arp 220

H3O+ and NH

3 are

isoelectronic, but with the pure inversion transitions of H

3O+ in

the far-IR (150-180 μm)

Population diagram of H3O+ (metastable levels) in Arp 220

Population diagram of H3O+ (metastable levels) in Arp 220

H2O+ + H

2 – H

3O+ + H + 1.7 eV T

form~ 800 K

Formation temperature: Tf o r m

defined as

in Bruderer et al. (2010, ApJ, 720, 1432)

...that's similar to what is found toward Sgr B2 and W31C (Lis+2014)

Models for H3O+ (ortho/para=1)

Tr o t

=170 K up to K=5, 500 K for K>5

HE: high-excitation

LE: low-excitation

Line shapes in Arp 220

2 velocity components:a) HE reflects an outflow (~75 km/s)b) Different regions of the quickly rotating disk(s)

Conclusions

* The OPRs of H2O, H218O, NH3, H2O+, and H3O+ are consistent with the high-temperaturelimit, or Tspin>~35 K. ps: this lower limit should be refined.

* Very high columns and abundances of H2O and NH3 are inferred in the nuclear regions of these sources, with X(H

2O)~10-5 and X(NH

3)~10-6.

* How is NH3 formed? And how is the gas heated? 2 main heating mechanism: -X/Cosmic rays (high columns of excited OH+ are found in both sources) -Mechanical heating (shocks, dissipation of turbulence) Main path for NH3 formation: -Gas-phase: N+ + 4H

2 → NH

4+ + H; NH

4+ + e → NH

3 + H (Herbst & Klemperer 1973)

ps: i thank Dr. Roueff for her correction on alternative pathways. -Dust mantles and then released to the gas phase by shocks (sputtering) or mantle evaporation (X(H

2O)~10-5 and X(NH

3)~10-6 in agreement with models for sputtering in

C-shocks, Flower+1995, apparently reflecting mantle composition). We indeed have evidence for shocks in Arp 220 (Martin+2011), and Tdust is high. However, we find no evidence for a previous “cold” phase in the OPR, so either NH3 can be formed on “warm” dust surfaces, or the release process equilibrates the OPR to higher temperatures, or the memory is “lost” due to subsequent proton exchange in the gas phase.