The 158 Micron [C II] Line: A Measure of Global Star Formation Activity in Galaxies Stacey et al. (1991) ApJ, 373, 423

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1 The 158 Micron [C II] Line: A Measure of Global Star Formation Activity in Galaxies Stacey et al. (1991) ApJ, 373, 423 Presented by Shannon Guiles Astronomy 671 April 24, 2006 Image:[C II] map of the galaxy NGC 6946 as made with FIFI on the KAO from S. Madden.

2 Reasons to Be Happy About Some C+ 's C+ 158 um ([C II]) line accounts for 0.1 1% of the FIR luminosity in the nuclear regions of galaxies [C II] is produced in photodissociation regions (PDRs). PDRs may contain up to 40% of the total gas mass in starburst (SB) nuclei [C II] increases with increasing FIR continuum intensity > it measures the excitation of the neutral ISM by the average external UV radiation field [C II]/12CO(1-0) indicates the global star formation activity in galaxies

3 Brief Review of Photodissociation Regions UV warm (Tgas ~ K) heated by photoelectric emission from grains & large molecules in low density regions collisional de excitation of UV pumped vibrationally excited H2 in high density regions cooled by FIR fine structure lines of Oo, Si+, & C+ if density high, also by molecular rotational emission & gas grain collisions G. Stacey's lectures 2 &3 for Astro 671, and Hollenbach & Tielens (1999) RvMP, 71, 173

4 [C II] in Six Galaxies (Crawford et al. 1985) First Detection of [C II] in Extragalactic Objects (1) [C II] line bright, ~ 0.5 % of the bolometric luminosity { (2) [C II] and 12CO(1-0) integrated line intensities and spatial distribution correlated (3) [C II] not correlated with 21 cm atomic H emission (4) [C II] correlated with FIR continuum [C II] intensity traces local UV field strength [C II] is associated with molecular clouds (not with atomic gas or the intercloud medium) CO(1-0) might also be strongly affected by local UV fields & thus may not reliably trace molecular 12

5 [CII] in a Range of Galaxies (Stacey et al. 1991) looked at 13 new galaxies 11 detected in [C II] 4 mapped in [C II] Observations done with the KAO image credit: TRW Inc. Fabry Perot spectrometer on the 91.4 cm telescope on the Kuiper Airborne Observatory (KAO) 12 flights over 4 years Line flux calibration accuracy of +/ 30% absolute pointing accuracy of +/ 20'', relative accuracy < 10'' Beam size 55'' FWHM

6 [CII] in the Starburst NGC 2146 GMOS N image of NGC 2146 in g, r, i, and H alpha Figure 2 of Stacey et al. (1991). side channels located +/ 55'' from the nucleus total integration time 22 minutes superposed 12CO(1-0) spectra in a 45'' beam note the good agreement between [C II] and 12 CO(1-0) in spectral line profile and spatial distribution

7 Integrated [C II] and Velocity Channel Maps of M82 Nucleus [C II] traces rotation curve along major axis blue shifted in West, red shifted in East consistent with 12CO(1-0) rotation most of [C II] in nuclear regions of M82 associated with rotating molecular disk or ring Extended [C II] may be associated with gas expelled from nucleus Figure 13 of Stacey et al. (1991). M82 image, purple represents the ionized gas. Credit: Mark Westmoquette (University College London), Jay Gallagher (University of Wisconsin Madison), Linda Smith (University College London), WIYN//NSF, NASA/ESA

8 Correlation between [C II] & the FIR Continuum Figure 17 of Stacey et al. (1991).

9 Correlation between [C II] & the FIR Continuum Nonlinear correlation of [C II] to FIR continuum As the FIR intensity increases [C II]/FIR decreases FIR continuum intensity is proportional to the UV field intensity As UV intensity goes up, the efficiency of converting UV radiation to [C II] emission goes down Why?

10 Correlation between [C II] & the FIR Continuum Looking at theoretical models of [C II] in PDRs: When FIR continuum BIG, [C II] saturates because then [C II] column density is determined by dust penetration depth of UV photons which depends only logarithmically on the UV field intensity, and cooling by [O I] 63 m emission dominates at high density and high UV field strength. When FIR continuum small, [C II] increases quickly with increasing FIR continuum because [C II] column density determined by [C II] formation rate which is proportional to the UV field intensity; also as UV field increases, an increasing fraction of [C II] emitting gas is at a temperatures above the transition temperature of the 158 m line

11 Correlation Between [CII] and 12CO (1-0) SB galaxies have tight correlation with [C II]/12CO(1-0) ~ 6300 normal galaxies have more scatter and [C II]/12CO(1-0) ~ 2000 galactic OB star forming regions fall along SB line galactic molecular clouds fall with normal galaxies Figure 15 of Stacey et al. (1991).

12 [CII] versus Atomic Hydrogen Density atomic hydrogen density derived from 21 cm HI lines represent calculated [C II] for the indicated volume densities in the infinite temp. limit (T >> 91 K) standard HI clouds have nh ~ 30 cm 3, Tkin ~ 100 K HI intercloud medium has nh ~ 0.1 cm 3, Tkin ~ 1000 K Bulk of [C II] from galaxies does not come from standard HI clouds or intercloud medium Figure 16 of Stacey et al. (1991).

13 Summary [C II]/12CO(1-0) same in SB nuclei and OB star formation regions in the Milky Way. Thus most of CO from SB nuclei arises from warm molecular gas exposed to elevated UV fields, not from cold disk clouds CO may trace both mass & excitation in SBs large molecular mass deduced for SB nuclei could be due to excitation For normal galaxies, [C II]/12CO(1-0) =1/3 that of SBs due to many non UV exposed disk molecular clouds more like the Milky Way Since [C II] associated with OB star formation [C II]/12CO(1-0) tests star formation activity

14 References The 158 Micron [C II] Line: A Measure of Global Star Formation Activity in Galaxies, Stacey et al. (1991) ApJ, 373, 423 [C II] map of the galaxy NGC 6946 as made with FIFI on the KAO. Picture courtesy of S. Madden (adaptation from Madden et al., 1993, ApJ 407, 579). Found on the web at: Image of KAO Credit TRW Inc. Image found on the web at: Image of NGC 2146 from gemini.noao.edu/gallery/gallery_software/index.php%3fset_albumname=albu Image of M82 from