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

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

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

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])0.983

1.028x1040

Previous calibrations

Sargsyan et al. (2012) [CII] can be used as SFR tracer

in starburst galaxiesdeficiency in [CII] for AGNs

due 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

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 ofionization, 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 LTIR

Dispersion due to ε ≈ [CII]/TIR

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

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 linkedthrough 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-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)

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

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

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

Conclusions① [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

① Dispersion in [CII]-SFR relation for individual galaxies driven by photoelectric efficiency: higher [CII] OR lower SF for 12 + log(O/H) ≥ 8.1

① Metallicity has influence on photoelectric effect differences in grain abundance (PAHs, VSGs)

① Origin of SFR-[CII] relation consistent with dominant PDR origin of C+

Future work① Determine PAH-VSG abundance (use spectroscopy as well!)

+ quantify precise influence on dispersion of [CII]-SFR relation

① Identify best SFR calibrator for [CII] FUV/Hα, 8/24/70/100/160 or TIR luminosity?

① Determine CII/TIR-dependent (or Z dependent) SFR-[CII] relation

① Extend [CII]-SFR relation to - more metal-rich objects (12 + log (O/H) ≥ 8.5)- high redshift objects

① Calibration of other FIR-lines ([OIII]88,[OI]63,…)as possible SFR tracers