star formation in galaxies along the huble sequence
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Star Formation In Galaxies Along The HubbleSequence
Robert C. Kennicut, Jr.(Annu. Rev. Astron. Astrophys. 1998. 36:189-231)
Christian Herenz
Extragalactic Science Club 2011
March 7th, 2012
Citations from ADS
The Author
Robert C. Kennicut, Jr.
Foto Source: http://www.flickr.com/photos/swilliams2001/4029565601/
Overview
Review is basically 2 Parts:I Diagnostic methods used to measure star-formation rates
(SFRs) in galaxies.I Systematics of SFRs along the Hubble Sequence.
This talk focuses on the first part, which is not merely asummary, but
“... a self-consistent set of SFR calibrations is presented as anaid to workers in the field.”
(from the Abstract)
Outline of this Talk
I Historical Overview of SFR MeasurementsI SFR from Evolutionary SynthesisI SFR Diagnostics
1. Continuum Luminosity - Color Scaling Relation2. UV Continuum Luminosity3. H Recombination Lines4. Forbidden Lines5. FIR Continuum Emission
I Summary
Historical Overview of SFR Measurements
I Late 1960s: First quantitative SFRs from evolutionarysynthesis models (e.g. Beatrice Tinsley 1968 - GalaxyColors)
I 1970s-80s: Development of precise direct SFR calibrators.I emission-line fluxesI (near) UV continuumI IR continuum
Application to large galaxy samples. Interpretation in termsof evolutionary properties.
I 1990s - present: Detection of star-forming galaxies athigh-z. Trace evolution of SFR density with look-back time(e.g. Madau plot)
SFRs from Evolutionary Synthesis
I Individual stars in galaxies typically unresolved−→ SFRs derive from integrated light (Colors, UV, IR,recombination lines).
I Basis of all calibrators: Evolutionary Synthesis Models(ESM)
I Grid of stellar evolution tracks = T?(t) & Lbol.? (t) for various
M?.I Stellar atmosphere modells or spectral libraries: T?(t) &Lbol.? → broadband colors or spectra (=Templates(t))
I∑
weighed by IMF Templates(t) = Luminosity, Color (andspectrum) for single age population
I For different SFH: Use linear-combination of single agepops.
I 4 free parameters (at least): age, metalicity /metal-abundance, IMF, SFH (constant / e−τ ...)
Synthesis modells can be downloaded / generated online(e.g. GALEXV - Bruzal & Charlot)
SFR Diagnostic I. – Continuum Luminosity - ColorScaling
I Color dictated by ratio of early to late-type starsI Color⇒ Fraction of young (t < 109y) massive starsI Knowledge of amount of massive stars: IMF→ SFRI Scales via broad band L to total stellar mass
109y old pop., Salpeter IMF, e−τ SFH
SFRs via Continuum Luminosity - Color Scaling
Pros:I Easy applicable for homogeneous sample, when no
absolute accuracy is required.
Cons / Gotchas:I IMF dependent, age, metalicity & SFH (holds for all
calibrators)I Reddening (!)I Imprecise & prone to systematic errors
SFR Diagnostic II. - UV Luminosity
Direct tracer: UV (1250 A – 2500 A) photons produced only bymassive O – B Type Stars (no attenuation by Lyα forest & nocontribution by old stars).
Fλ of O0 (red), B8 (blue) and G5×5000 (green) type stars
For Salpeter IMF 0.1 . . . 100M�, continuous SFH tPop = 108 yrs:
SFR[M�yr−1] = 1.4× 10−28 Lν [erg s
−1Hz−1] (1)
Pros:I Directly tied to photospheric emission of young-stellar
population.I Can be used for high-redshift galaxies in the optical.
Cons / Gotchas:I Not accesible from the ground for local galaxies.I Sensitive to extinction, form of IMF (large extrapolation,
since measured M? > 5M�)I Inappropriate e.g. for young (t ∼ 107yrs) star-burst (lower
SFR/Lν ratio, i.e. less luminos for same SFR)
SFR Diagnostic III. - H Recombination Lines
Direct Tracer: Only M? > 10M� stars (i.e. t? < 2× 107yr)contribute signifcantly to F (λ < 912 A).
200Å 2000Å 3000Å 4000Å912ÅΛ
FΛ
For Salpeter IMF 0.1 . . . 100M�, continuous SFH tPop = 108 yrs:
SFR[M�yr−1] = 1.08× 10−53Q(H0)[s
−1] (2)
Q(H0)= photons with λ < 912A
From Eq. (2), using your “favorite recombination scenario”,line-strengths can be derived (e.g. using tables compiled inOsterbrock’s monograph).
For example for Case B recombination with Te = 10, 000 K:
SFR[M�yr−1] = 7.9× 10−42L(Hα) [erg s−1] (3)
= 8.2× 10−53L(Brγ) [erg s−1] (4)= . . .
⇒ Recombination Lines = “Ionizing Photon Counters” (undercertain assumptions).
Parenthesis: Case B RecombinationGas region optically thick in Lyman-Lines (generally all gas regionsthat contain enough gas to be observable - because of high Lynline-absorption cross section - because ∝ 〈ψi| − er|ψf〉- because of . . . ,)
SFRs via hydrogen recombination lines
Pros:I High sensitivity, Hα easily measureable with small
telescopes. SFR measurable in individual regions innearby galaxies.
I Directly coupled to most massive stars.Cons / Gotchas:
I Escape of Q(H0) photonsI Extinction for Hα (but e.g. not H53α in the radio)I IMF & reliability of ESMI Hα - at high-z only with JWST.
Forbidden Lines ([OII])
I z ∼ 0.5 Hα λ6563 shifts to IR→ interest in strong bluerlines⇒ [OII] λ3727 (doublet).
I Excitation dependent on abundance and ionization state ofgas, i.e. not directly coupled to ionizing flux.
I Empirical calibration to Hα Eq. 3 (using a set of ∼ 170galaxies) yields:
SFR[M�yr−1] = (1.4 ± 0.4)× 10−41L([OII]) [erg s−1] (5)
Pros: bluer, stronger Cons: Less precise
FIR Continuum
I Simplest Case: Radiation field dominated by young stars,dust opacity high everywhere (dusty circumnuclearstarburst)
I Dust: Absorbs essentially bolometric luminosity andre-emits it as thermal emission (i.e. calorimetric SFRmeasure).
I Real Situation more complex - e.g. τ � 1 approximationnot valid, dust needs to depleted by stars (i.e. oldgeneration contributes to dust heating) - etc.
I Models from literature calibrated to IMF used for otherrelations (±30%):
SFR[M�yr−1] = 4.5× 10−44LFIRo(8− 1000µm) [erg s−1]
(6)10 – 100 Myr old starburst
Summary
Several tracers (UV, Lines, FIR) for SFR exist, but quantitativecalibration is tricky and requires a set of assumptions. Providedthat for a galaxy or sample of galaxies the assumptions given inthis review are valid, the formulas
SFR[M�yr−1] = 1.4× 10−28 Lν [erg s
−1Hz−1]
SFR[M�yr−1] = 7.9× 10−42L(Hα) [erg s−1]
= 8.2× 10−53L(Brγ) [erg s−1]
= . . .
SFR[M�yr−1] = (1.4 ± 0.4)× 10−41L([OII]) [erg s−1]
SFR[M�yr−1] = 4.5× 10−44LFIRo(8− 1000µm) [erg s−1]
can be used.
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