The reliability of [CII] as a SFR indicator Ilse De Looze, Suzanne Madden, Vianney Lebouteiller, Diane Cormier, Frédéric Galliano, Aurély Rémy, Maarten Baes Workshop “C+ as an astronomical tool” 7 th February 2013, Leiden

Why use [CII] as a SFR indicator?  Represents ≈ 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

Previous calibrations Boselli et al. (2002)  calibration of SFR relation based on Hα+[NII] and [CII] ISO data  dispersion L Hα – 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

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

Previous calibrations De Looze et al. (2011)  tight correlation between SFR 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] ) x10 40

Previous calibrations Sargsyan et al. (2012)  [CII] can be used as SFR tracer in starburst galaxies  deficiency in [CII] for AGNs due to contribution from AGN to L IR (see also Herrera-Camus et al. in prep.) Conclusions: [CII] = reliable SFR indicator in normal star-forming galaxies, possibly also in AGNs

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

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

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)

Resolved [CII]-SFR relation  [CII]-SFR relation: Individual galaxies  - similar slopes - BUT with offset  galaxies have - higher [CII] OR - lower SFR 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

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

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? Global Resolved

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???

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 L TIR

Dispersion due to ε ≈ [CII]/TIR  Highest values L CII 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 Global Resolved

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)

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

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 n e !!  need for reliable estimate of electron density n e Croxall+ 2012

III. Ionized gas contribution to C + For n e ≈ 15 cm -3 n e ≈ 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

Origin of SFR-[CII] relation?  Σ SFR -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

Conclusions ①[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 ①Dispersion in [CII]-SFR relation for individual galaxies  driven by photoelectric efficiency: higher [CII] OR lower SF for 12 + log(O/H) ≥ 8.1 ①Metallicity has  influence on photoelectric effect  differences in grain abundance (PAHs, VSGs) ①Origin of SFR-[CII] relation  consistent with dominant PDR origin of C +

Future work ①Determine PAH-VSG abundance (use spectroscopy as well!) + quantify precise influence on dispersion of [CII]-SFR relation ①Identify best SFR calibrator for [CII]  FUV/Hα, 8/24/70/100/160 or TIR luminosity? ①Determine CII/TIR-dependent (or Z dependent) SFR-[CII] relation ①Extend [CII]-SFR relation to - more metal-rich objects (12 + log (O/H) ≥ 8.5) - high redshift objects ①Calibration of other FIR-lines ([OIII] 88,[OI] 63,… ) as possible SFR tracers