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The Star Formation Indicators in diff. bands and on diff. scales Jun Yin / Oct. 24

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  1. The Star Formation Indicatorsin diff. bands and on diff. scalesJun Yin / Oct. 24 What indicators could be used in different bands? What are the counterpart? connections? What are their advantages and disadvantages? Limitations? What caused the uncertainty? And how much is it? Over- or under-estimate? Which method is best for your work? Are the data reliable? Calzetti 2012, arXiv:1208.2997 Kennicutt & Evans 2012, ARA&A,50,531 IAU XXVIII SpS8 “Calibration of SFR measurements” …….

  2. SF gas dust

  3. Different wavelength • In UV/optical/NIR(~0.1-5μm): direct stellar light • In MIR/FIR (~5-1000μm): stellar light processed by dust • Ionizing photo rate (gas ionized by massive stars) • Hydrogen recombination lines: opt – NIR – Radio • Forbidden metal lines • Free-free emission: millimeter • X-ray emission: high-mass X-ray binaries • Synchrotron emission: radio

  4. Different scales: • the accretion of gas (Mpc); • the cooling of this gas to form a cool neutral phase (kpc); • the formation of molecular clouds (∼ 10−100 pc); • the formation of denser structures such as clumps (∼ 1 pc) and cores (∼ 0.1 pc) • the contraction of the cores to form stars (R⊙) and planets (∼ AU). “global” SF -- whole galaxies -- kpc scale -- timescales ~ 10-100 Myr “local” SF -- Regions or structrures within galaxies -- sub-kpc scale -- timescales ~ 1-10 Myr

  5. Content • Introduction • Indicators based on direct stellar light • Indicators based on dust-processed stellar light • Indicators based on ionized gas emission • Indicators based on mixed processes • Indicators based on other processes • Indicators on different scales • Summary

  6. Direct stellar light • Count young stellar objects (YSOs) <M>: mean YSO mass, depend on IMF τ: the life time of a YSO, ~2 Myr • Resolve individual YSOs  very close, MW, MCs

  7. Direct stellar light • UV continuum (0.0912μm<λ<0.3μm,) • Timescale: 100-300 Myr (shorter τ for shorter λ) • Sources: O & B stars (M>~5M⊙) ,  • Calibration: SED from Starburst99, Z=Z⊙,Kroupa IMF (Calzetti 2012) (Kennicutt & Evans 2012)

  8. Direct stellar light • UV continuum (0.0912μm<λ<0.3μm,) • Uncertainty: • Accuracy of calibration constant: 15% • 2 SFHs, similar UV SED over 0.13-0.25 μm • GALEX, XMM Optical Monitor, Swift UV/Optical Tel., HST(NUV),Ultraviolet Imaging Telescope • Adv: • FUV: more accurate and sensitive at low SF levels • Disadv.: • dust attenuation • Av=0.9, UV  1/10 @0.13μm • FUV-NUV color, UV spectral slope β, IR/UV ratio (IRX) – β rel.  estimate dust attenuation, presume intrinsic color, extinc. curve… • Balmer decrement (Hα/Hβ)  dust attenuation correction normal galaxies √, circumnuclear starbursts or dusty galaxies × • Depend on IMF

  9. Dust-processed stellar light • IR (~5-1000 μm) • Depend on dust content & heating rate provided by stars • Foundation: • The dust heated by UV-luminous, young stellar population will produce an IR SED that is more luminous and peaked at shorter wavelengths (observationally ~60μm) than the dust heated by UV-faint, old or low-mass stars (~100-150μm) • UV-bright stars will likely heat the surrounding dust to relatively high effective temperatures • Spizer(3.6-180 μm), Herschel (60-670 μm), AKARI (2.4-160 μm), WISE (3.4-22 μm), Planck (350-850 μm)

  10. Dust-processed stellar light

  11. Dust-processed stellar light Small dust grain heated by intense radiation fields Stars Small, PAH dust grain • λ↑,diffuse component↑, physical change, not due to low resolution • The conversion of LIR to SFRs must change for diff. λ diffuse dust emission ↑ (Kennicutt & Evans 2012)

  12. Dust-processed stellar light • Thermal IR (TIR) emission (~5-1000 μm) • Timescale: 100 Myr • Assumption: all of the stellar emission is absorbed by dust and re-emitted in the IR  lower limit • Calibration: Starburst99, Z=Z⊙ • Applicable in the limits of complete dust obscuration and dust heating fully dominated by young stars

  13. Dust-processed stellar light • PAH (3.3, 6.2, 7.7, 8.6, 11.3 μm) • Heated mainly by UV photons produced by massive stars. • PAH luminosity scales relatively well with the SFR in metal-rich luminous SF galaxies. • Adv. • High-z: PAH@z=1-3 24 μm • Disadv. • Strong dependence on Z: weak when Z< 1/4~1/3 Z⊙ • May be better tracers of B stars than current SFR

  14. Dust-processed stellar light • 24 μm • Local calibration (~500 pc) : non-linear • Uncertainty of calibration constant: 15% • Global calibration: linear SF regions (~200-600 pc) in 33 nearby galaxies (Calzetti +2007) (Rieke +2009)

  15. Dust-processed stellar light • 70 μm • Local calibration (~1 kpc) : non-linear • Uncertainty of calibration constant: 2% • Global calibration: linear • Smaller region, higher constant shorter SF timescales ~190 galaxies (Calzetti+ 2010) 550 regions (0.05- 2 kpc) (Li + 2010) (Calzetti 2012)

  16. Dust-processed stellar light • Systematic effects • UV: miss radiation that has been attenuated by dust; IR: miss the starlight that is not absorbed by dust; “missing” component – underestimate • Zero in dusty starburst galaxies • ~100% in dust-poor dwarfs and metal-poor regions • Fraction of dust heating from young stars – overestimate • ~100% in extreme circumnuclear starburst galaxies or individual SFing regions • ~10% in evolved galaxies with low sSFRs • IR SFR conversion factor is not fixed • Fortunately, for most galaxies with moderate to high sSFRs, appear to compensate roughly for each other • Avoid using IR-based indicators • In low-Z and other largely dust-free galaxies • In galaxies with low specific SFRs and a strong radiation field from more-evolved stars

  17. Ionized gas emission • Hydrogen recombination lines Barmer series Only stars M> ~15-20M⊙ produce a measurable ionizing photon flux

  18. Ionized gas emission • Hα • Band: opt. red (6563Å) • Timscale: ≥ 6 Myr • Calibration: Kroupa IMF, Te=10000K, ne=100 cm-3 • Uncertainty: • Te=5000-20000K: 15% • ne=100 cm-3: <1% • Adv. • Present SFR, independent of long timescale & SFH

  19. Ionized gas emission • Hα • Disadv. • sensitive to the effects of dust • Av=1  Hα is depressed by a factor ~2 • Recomb. line decrement (Paα/Hα, Hα/Hβ) dust correction Scales smaller than resolution  underestimate of the extinction • direct absorption of Lyman continuum photons(Lyc) by dust  No emission from either recomb. lines or ffcontin. emission • Normal disk galaxies: low Lycabsoprion, <15-20% • LIRGs, ULIRGs, high-density central regions: significant • leakage of ionizing photons • SF regions: lose about 25-40% of their ionizing photons • biased downwards by about 1/3 of their true value (not included) • Local and distant galaxies (z<0.5) • Sensitive to upper IMF in low SFR region depletion of massive stars  substantially underestimate the actual SFR

  20. Ionized gas emission • Lyα • Band: redshifted (λrest=1216Å) • Adv. • Strength > 9Hα • Tracer of SF galaxies at high redshift • Disadv. • Absorpted by dust Lyα escape fraction (order 0.1 to 1) • Large scatter and systematic uncertainties

  21. Ionized gas emission • Recombination lines at longer wavelenghth • Brγ (n=4,m=7, 2.16μm) • Timescale: ≥6Myr • Calibration: Kroupa IMF, Te=10000K, ne=100 cm-3 • Uncertainty: • Te=5000-20000K: 35% • ne=100-106cm-3: 4% • Brα (n=4,m=5, 4.05μm) • Uncertainty: • Te=5000-20000K: 58% • ne=100-106 cm-3: 13% • Adv. • Lower sensitivity to dust attenuation (λ↑attenuation↓) • Av=1  Brγ is depressed by 11% • Disadv. • Progressively fainter (Brγ is ~1/100th of Hα) • More sensitive to the physical conditions of the gas (Te, ne)

  22. Ionized gas emission • Recombination lines at longer wavelenghth • Millimeter and/or radio recombination lines (RRL) • ALMA, EVLA • Adv. • Unimpeded by effects of dust attenuation • Disadv. • Extremely weak • n>80-200, stimulated emission is no longer negligible • Line luminosity is dependent on Te, SFR(RRL) • Te measurement: uncertain

  23. Ionized gas emission • Forbidden metal line • [OII]λ3727 doublet • Adv. • z up to 1.6 in optical. • Disadv. • Same limitations as H recombination lines • Dependent on the metal content and ionization condition • Uncertainty is larger than Hα

  24. Ionized gas emission • IR fine-structure cooling lines (in HIIregions or PDRs) • [NeII] 12.8μm & [NeIII] 15.6μm • [CII] 158μm • Disadv. • Same limitations as H recombination lines • Dependent on the metal content and ionization condition • Uncertainty larger than Hα

  25. Mixed processes 97 galaxies (Hao + 2011) • Direct stellar light + dust-processed light ~200 galaxies (Kennicutt & Evans 2012)

  26. Other processes • Radio • Synchrotron radiation ~ SN explosions ~ SFR radio ∝ IR ∝ SFR • Calibration: strongly wavelength dependent • Log SFR(1.4GHz)=log L(1.4GHz) – 28.20 • Adv. • High-z • Disadv. • Depend not only on the mean cosmic ray production per SN, but also on the galaxy’s magnetic field • AGN contribution

  27. Other processes • X-ray emission • Sources: high-mass X-ray binaries, massive stars, Sne • 2-10 keV correl. with IR and radio continuum fluxes Log SFR(Xray)=log L(X-ray) – 39.77 • Disadv. • non-negligible contributions from low-mass X-ray binaries • AGN?

  28. On different scales – CHILI & MaNGA • Caution in using standard calibrations of the SFR(UV) in spatially-resolved studies of disks, especially when the targeted regions include inter-arm areas that have not been independently confirmed to be actively SF. SFR measurements at any wavelength could be tricky. • Leakage of ionizing photons from SF regions • Weak recombination line emission will appear in regions that are not SFing

  29. On different scales – CHILI & MaNGA • At smaller linear scales, nearly all the statistical approximations begin to break down • Measuring bias • SFR<~0.001-0.01M⊙yr-1incomplete sampling of IMF large fluctuation in the tracer L (scales: 0.1-1 kpc) • The spatial resolution of the SFR measurements encompass single young clusters • The resolution < the diameters of HII regions or corresponding dust-emission nebulae (scales: <500 pc) • Hα and dust emission, 30-60% by diffuse ionized gas and dust located hundreds pc away false-positive signal

  30. On different scales – CHILI & MaNGA • Radial profile • Use the integrated SFR calibrations • 2D map • Short-lived tracer: Hα (very young region) • Direct stellar photospheric tracers: UV continuum • Resolved stellar tracers: YSOs or deep CMDs • Dust-corrected map: pixel-resolved SED in UV

  31. Summary All the calibrations are sensitive to metallicity Abundance 1dex ↓, L(FUV) ~0.07±0.03dex↑, L(ion)~0.4±0.1dex↑