Ay126: Fine Structure Line Emission from the Galaxy S. R. Kulkarni

Cooling of CNM and WNM In both components H is largely neutral Cooling is primarily by lines which are easily excited (relative to kT) After H, He the most abundant elements are O, N, C The ionization potentials (IP) are: 11.26(C), 14.53 (N), 13.6181 (O) eV Species [CI], [CII], [OI], [SiII], SII] and [FeII] are present in the neutral medium. They are also present in the ionised medium, but generally in smaller amounts than more ionised species. Ground state of NI is 4S (so no FSL)

Fine structure lines are forbidden Arise in the same electronic configuration Magnetic or Quadrupole transitions Small A coeffecients Need to compute critical density Sometimes self absorption can be important

Ion / spectrum Transition Collider ΔΕ/k [K] λ [μm] C+ / [CII] O0 / [OI] Si+ / [SII] 2P3/2 - 2P1/2 H2, H, e- 92 157.7 3P1 - 3P2 H, e- 228 63.2 3P0 - 3P1 99 146 (H), e- 413 34.8 Of the most important forbidden lines in the ISM, only [CI], [CII], [OI], [SiII], SII] and [FeII] are present in the neutral medium. They are also present in the ionised medium, but generally in smaller amounts than more ionised species. .

The most important forbidden lines in the interstellar medium The most important forbidden lines in the interstellar medium. Only CI, CII, OI, SiII, SII, and FeII are present in the neutral medium. They are also present in the ionised medium, but generally in smaller amounts than more ionised species. The collision strengths Ωul are for collisions with electrons at a temperature of 104 K. The critical densities ncrit = Aul/⟨σulv⟩ correspond to collisions either with electrons (for Te ≃ 104 K), or with H2 molecules when between round brackets (for Tk ≃100K).

OII OI

CII 157 micron line

Observations of CII line HII regions studied with KAO First sensitive all-sky survey was carried out by FIRAS (Far-infrared Spectrometer) on COBE Wide spread [CII] and [NII] 205 micron was detected Correlation between [CII], [NII} and FIR was found Herschel provided both high angular resolution (12 arcsec) and high velocity (0.1 km/s) observations of [CII]. Five hundred lines of sight in the Galaxy were observed

Contribution to the C+ line HII regions Photo-dissociation regions (PDR) Interface between HII regions and molecular clouds CNM WNM WIM

References Herschel Survey in [CII] line J. L. Pineda t al. A&A 554, A103 (2013) J. L. Pineda et al. A&A 570 A121 (2014) Physics & Inference of [CII] line P. F. Goldsmith et al. ApJS, 203, 13(2012) Survey in the [NII] line P. F. Goldsmith et al., ApJ 814, 133 (2015) From ratios of fine structure lines concludes that this line does not arise in the WIM

Inferences from Analysis of Galactic Plane Observations of [CII] Inferences from Analysis of Galactic Plane Observations of [CII] .. Pineda et al. 2013 [CII] emission arises from PDR (47%), ”CO-dark” H2 gas (28%), CNM (21%) and ionized gas (4%). Global Inference: