1 / 21

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, …) 

gafna
Télécharger la présentation

Why use [CII] as a SFR indicator?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related