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Why use [CII] as a SFR indicator?

Why use [CII] as a SFR indicator?. Represents ≈ 10 -3 of the FIR continuum C + is an important coolant of the ISM [CII] = generally strong line in all star-forming galaxies Observable diagnostic at high redshift  line shifts to submm λ (ALMA, IRAM, SMA, CARMA, …) 

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Why use [CII] as a SFR indicator?

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  1. Why use [CII] as a SFR indicator? • Represents ≈ 10-3 of the FIR continuum • C+ is an important coolant of the ISM • [CII] = generally strong line in all star-forming galaxies • Observable diagnostic at high redshift  line shifts to submm λ (ALMA, IRAM, SMA, CARMA, …) Need to calibrate the [CII] line as SFR tracer in the local Universe on metal-poor galaxies

  2. Previous calibrations Boselli et al. (2002) • calibration of SFR relation based on Hα+[NII] and [CII] ISO data  dispersion LHα – L[CII] relation: ≈ 3  uncertainty on SFR estimate: ≈ 10 Conclusion: large dispersion in SFR-L[CII] relation due to different contributions to [CII] emission in galaxies BUT: large uncertainty on [NII] correction and Hα extinction

  3. Previous calibrations De Looze et al. (2011) • calibration of SFR relation based on FUV+24 μm data  ISO [CII] data from Brauher+ 2008 • unresolved with respect to 75” ISO LWS beam no aperture correction required • No active galactic nucleus (AGN)  sample of 24 galaxies

  4. Previous calibrations De Looze et al. (2011) • tight correlation betweenSFR and L[CII] • less dispersion (0.27 dex) than in Boselli et al.(2002) BUT • Limited sample of merely metal-rich galaxies • No spatially resolved observations  no constraint on origin of C+ SFR [M/yr] = (L[CII] [erg/s])0.983 1.028x1040

  5. Previous calibrations Sargsyan et al. (2012) • [CII] can be used as SFR tracerin starburst galaxies • deficiency in [CII] for AGNsdue to contribution fromAGN to LIR (see also Herrera-Camus et al. in prep.) Conclusions: [CII] = reliable SFR indicator in normal star-forming galaxies, possibly also in AGNs

  6. DGS Survey • Sample of 48 dwarf galaxies • Covering wide range in 1/50 Z≤ Z ≤ Z • allows studying the effect of metal abundance on the ISM physics, SF, level of ionization, ISM filling factors, etc. • Subsample of 14 objects < 10 Mpc • high spatial resolution • disentangle ≠ ISM phases • study the resolved SF law

  7. Reference SFR tracer? • Below 12+log(O/H) < 8.1 need to trace unobscured + obscured SF FUV or Hα • 8 μm  PDR origin, absent in HII • 24 μm  peaks in HII • FIR (70, 100, 160 μm) • TIR luminosity drawbacks: - heating by old stellar population- submm excess MIR 24 μm is only SFR calibrator that also SPATIALLY correlates with SF

  8. Global [CII]-SFR relation • [CII]-SFR relation for DGS sample  ± consistent with relations for metal-rich samples (but larger dispersion) • Offset for subsample 12+log(O/H) ≥ 8.1 consistent with Sargsyan et al. (2012)

  9. Resolved [CII]-SFR relation All pixels rebinned to physical size of (436 pc)2,i.e. (12”) 2 at 7.5 Mpc  independent pixels of size ≈ beam at 158 μm  trace same regions within galaxies  proxy for surface density of gas/SFR • [CII]-SFR relation:Individual galaxies  • similar slopes • BUT with offset galaxies have - higher [CII] • OR • lower SFR

  10. Cause of offset in [CII]-SFR relation? • higher [CII]? = different [CII] behavior in galaxies? • lower SFR? =various SF conditions/efficiency?  Differences in: • Metal abundance? II. Photoelectric efficiency? III. Relative fraction of gas phases contributing to [CII]?

  11. I. Metal abundance Offset in [CII]-SFR relation  clearly influenced by metal abundance! How does metallicity influence- ISM conditions (ISRF, density)? - PAH/VSG abundance? - star formation efficiency? Resolved Global

  12. Influence of Z? • Lower Z  less dust longer free path lengths FUV photons enlarge C+ emitting zone higher [CII]/TIR ≈ photoelectric efficiency (PE) • Lower Z peculiar grain properties dearth of PAHs: due to grain charging/PAH destruction high abundance of very small grains (VSGs):large grain fragmentation due to shocks in turbulent ISM  PAHs/VSGs dominate PE  outcome on PE???

  13. II. Photoelectric efficiency ε ≈ [CII]/TIR • Large spread in PE  up to 2 orders of mag • No clear dependence of metal abundance effect of metallicity on [CII]/TIR is non-trivial • Declining [CII]/TIR & [CII]+[OI]/TIR ratio within galaxies for higher LTIR

  14. Dispersion due to ε ≈ [CII]/TIR Global Resolved • Highest values LCII for [CII]/TIR  • Obvious trend of [CII]/TIR with offset in SFR-L[CII] relation ! lower photoelectric efficiency = lower [CII] luminosity • Lower metallicity • no clear trend with [CII]/TIR

  15. Photoelectric efficiency ε’ ≈ [CII]/PAH • Little spread in PE  < 1 order of mag • Constancy of [CII]/PAH throughout galaxies [CII] emission & PAHs physically linked through photoelectric effect, better than for all grains(see also Croxall et al. 2012, Lebouteiller et al. 2012)

  16. Dispersion due to ε’ ≈ [CII]/PAH • Offset in SFR-LCII relation for metal-poor galaxies with CII/PAH ≥ 0.05 • Higher CII/PAH  decreased LCII for a certain level of SF  photo-electric effect on PAHs more efficient @ low-Z ? •  rather reflects lower PAH emission (≈ abundance)

  17. III. Ionized gas contribution to C+ Most common diagnostic  [CII]158/[NII]205 BUT no [NII]205 detections Alternative diagnostic  [CII]158/[NII]122  12/48 galaxies with [NII]122 observations Drawback  [CII]158,ion/[NII]122 depends on ne!!  need for reliable estimate of electron density ne Croxall+ 2012

  18. III. Ionized gas contribution to C+ For ne ≈ 15 cm-3 ne ≈ 100 cm-3 • On global scales: ionized gas contribution < 20 % • Locally: ionized gas phases might dominate C+ contribution in diffuse regions  BUT globally [CII]diffuse/[CII]HII << 1

  19. Origin of SFR-[CII] relation? • ΣSFR-Σ[CII] law shows Schmidt-like behavior? more gas ≈ higher SFR?BUT absence of [CII] in quiescent molecular clouds trend = different beam filling factors of PDRs in regions • OR • Spatial link between CII emission from PDRs and adjacent HII regions global CII emission in galaxies is dominated by PDRs cooling efficiency in PDRs is linked to star formation activity(higher SFR  increased input for gas heating  more cooling through [CII] •  • resulting in tight correlation of [CII] with SFR

  20. Conclusions • [CII]-SFR relation DGS sample more or less consistent with De Looze et al.(2011) and Sargsyan et al.(2012) holds for lower-metallicity galaxies • Dispersion in [CII]-SFR relation for individual galaxies driven by photoelectricefficiency: higher [CII] OR lower SF for 12 + log(O/H) ≥ 8.1 • Metallicity has influence on photoelectric effect differences in grain abundance (PAHs, VSGs) • Origin of SFR-[CII] relation consistent with dominant PDR origin of C+

  21. Future work • Determine PAH-VSG abundance (use spectroscopy as well!) + quantify precise influence on dispersion of [CII]-SFR relation • Identify best SFR calibrator for [CII] FUV/Hα, 8/24/70/100/160 or TIR luminosity? • Determine CII/TIR-dependent (or Z dependent) SFR-[CII] relation • Extend [CII]-SFR relation to - more metal-rich objects (12 + log (O/H) ≥ 8.5)- high redshift objects • Calibration of other FIR-lines ([OIII]88,[OI]63,…)as possible SFR tracers

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