Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.
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Transcript of Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.
Star Formation: Near and Far
Neal J. Evans II
with Rob Kennicutt
Far: Whole Galaxy RelationsSolid circles are disk-averaged normal spiralsOpen circles are central regions of normal disksSquares are circumnuclear starbursts
Slope is 1.4±0.15
Kennicutt 1998, ARAA 36, 189
StarburstsSpirals
black: normal galaxiesred: starburstsgreen: circumnuclear starburstsblue open: Low metals (<~1/3 solar), mostly dwarfsBlue line: slope of 1.4, not a fit
RCK, in preparation
SFR/Mass Increases with SFR
SFR/Mass of molecular gas increases with SFR
Factor of ~ 100 “Efficiency” increasing But what does this really
mean?
Solomon & Vanden Bout (2005 ARAA)
Sta
r fo
rmat
ion
effic
ienc
y
Star formation Rate
The Dense Gas SF Relation
LFIR correlates better with L(HCN)
Smaller scatter Higher rate SFR rate linearly
proportional to amount of dense gas
“Efficiency” for dense gas stays the same
Gao & Solomon (2004) ApJ 606, 271
Amount of dense molecular gas
Sta
r fo
rmat
ion
rate
Whole Galaxy Prescriptions
Kennicutt (1998) SFR(Msun yr–1 kpc–2) = 2.5x10–4
gas(Msun pc–2) Gao and Solomon (2004)
SFR (Msun/yr) ~ 1.8 x 10–8 M(dense) (Msun) SFR(Msun yr–1 kpc–2) = 1.8x10–2 0
dense(Msun pc–2)
What Does SSFR Mean?
SSFR is grand average over: Whole galaxy, with huge variations in
SFR, Sgas, metallicity, …
Time ~5 Myr for Ha ~ 30-100 Myr for UV, 5-100 Myr for FIR (short for starbursts)
What Does Sgas Mean?
“Sgas” is not the mean surface density of any structure.
At best, the filling factor x mean cloud emission times X(CO)
Higher “Sgas” really means more clouds in beam
CO: Limited Dynamic Range
Heiderman et al. 2010
CO can be off by large factors in some regions. It clearly fails for AV > 10 mag.
Need AV >0.4 mag for CO, but issues below 3 mag (Pineda et al. 2010)
Not so Bad on Average
12CO underestimates AV at gas > 200 M pc–2
by 30%
Constant value of 13CO vs gas, underestimating gas
by factors of 4-5
Correcting for 12CO, would flatten the slope of the Kennicutt-Schmidt relation (but does not explain big offset)
Intermediate: Resolved Studies
Radial cuts or averages Martin and Kennicutt (2001): threshold Schruba et al. (2011): SF continues even
when HI > H2
Pixel by pixel: e.g., Kennicutt et al. (2007) Bigiel et al. (2008) Blanc et al. (2009)
Sub-kpc scales
Bigiel et al. 2008
Study of 18 nearby galaxies with sub-kpc resolution in HI, CO. SFR from UV+24 micronThreshold around 10 Msunpc–2 in total gas: transition from HI to H2
CO, SF continue into HI region
Schruba et al. 2011
SFR ~ I(CO) even in HI dominated outer parts
Star Formation Prescriptionsfor sub-kpc scales
Kennicutt et al. (2007) M51 SFR(Msun yr–1 kpc–2) = 1.7x10–4 37
mol(Msun pc–2) Bigiel et al. (2008)
SFR(Msun yr–1 kpc–2) = 7.9x10–3 0mol(10 Msun pc–2)
SFR(Msun yr–1 kpc–2) = 7.9x10–4 0mol(Msun pc–2)
Blanc et al. (2009) M51 SFR(Msun yr–1 kpc–2) = 5.1x10–2 0.82
mol(Msun pc–2) Includes 0.43 dex scatter in SFR and includes limits
Issues of tracer, diffuse emission, fitting method
Star Formation PrescriptionsTheory
Schmidt (1959) SFR ~ n, n = 1 or 2 (or Sn, 2009)
Krumholz et al. (2009) SFR = f(gas, f(H2), Z, clumping) Nearly linear with mol below ~ 100 Msun pc–2
Steepens above 100 Msun pc–2
Other dynamical relations
The Predictions
17
Very Near: Clouds in Solar Neighborhood
Spitzer Programs
c2d + Gould Belt:20 nearby molecular clouds (blue circles)
Cluster Project:35 young stellar clusters(red circles)
90% of known stellar groups and clusters within 1 kpc(complete to ~ 0.1 MSun)
Whole Clouds (2-16 pc)
Heiderman et al. 2010
Almost all clouds within 300 pcTotal SFR from YSO counting /areaTotal mass/area
Clouds within 1 kpc
Adds Orion, Mon R2, S140, Cep OB3, all forming more massive stars, and North America nebula, less active
not complete to 1 kpc, but representative
It’s Worse than that…
Gray is extinction, red dots are YSOs, contours of volume density (blue is 1.0 Msun pc–3; yellow is 25 Msun pc–3)
Really Near: Within Clouds
Heiderman et al. 2010
Less Near: Add Clouds to 1 kpc
Gutermuth et al. subm.
N = 2.67
N = 1.87
Cep OB3
Gutermuth et al. subm.
Still Less Near: Dense Clumps
L(HCN J = 1-0)
L(IR
)
Wu et al. (2005)
Survey of dense clumps across MW.(n ~ 105 to 106 cm–3)Birthsites of large clusters.
Follow linear relation very similar to dense gas relation for starbursts, as long as LFIR > 104.5 Lsun.
Dense Clumps on Sgas-SSFR
Using LFIR to get SFR, likely underestimates.Includes fit from Wu et al.
Combine with Nearby CloudsFit with broken powerlaw with slopes of 4.6 below and 1.1 above a turnover Sgas = 129+-14 Msun pc–2.(see Lada et al. 2010)
Gutermuth et al. favor continued rise withSSFR ~ Sgas
2 throughout.
All agree: well above all exgal relations except for dense gas relation.
Lessons from Nearby Clouds
SFR >10 times prediction of relations for galaxies
SFR determined on sub-pc scales << exgal resolution
On scales where SF actually happens… Dependence on Smol may be very strong, at least up to
Smol~ 100 Msun pc–2
Speculation
The underlying SF law is linear in Sgas above a noisy threshold ~ 100 Msunpc–2
10 times exgal relations around threshold.
Fraction of gas above threshold (fdense) increases with <S> as S0.5 for <S> >100 Msunpc–2
When <S> ~ 100 Sth, fdense ~1 KS prescription and Dense gas prescription agree
What about linear relations in resolved studies of non-starbursts? fdense ~ constant below <S> ~ 100 Msunpc–2?
Issues for Resolved Studies
SFR have be restricted to local SF Remove diffuse emission Use tracer with short timescale
Clouds are not resolved, much less clumps “Sgas” is still not that of any structure
Small number statistics cause larger spread
Massive stars can destroy clouds SF tracers and gas may even anti-correlate
30
Observe the Solar Neighborhood from Outside
Size and location of beam/pixel causes huge variations
All centered on Sun100 pc: No SF, no CO300 pc: SF, CO, but no Ha, little 24 mm500 pc: SF, Ha, CO
What would we see?
300 pc, count YSOs,
500 pc, count YSOs
300 pc, using Ha, remove diffuse emission
500 pc, using Ha, remove diffuse emission, assume standard L(Ha) to SFR
Bigiel et al. 2011
The Larger Context of MW
Surveys in mm continuum finding 1000’s of dense clumps Bolocam Galactic Plane Survey (>8000 sources) http://irsa.ipac.caltech.edu/data/BOLOCAM_GPS/ ATLASGAL survey from APEX Future SCUBA2 survey Herschel Galactic Plane Survey (HIGAL)
Infrared Dark Clouds (IRDC) MSX, GLIMPSE, MIPSGAL
New models of Galaxy, VLBA distances, … Provide link to extragalactic star formation
The Improved Milky Way Model
Green and blue dots show VLBA measurements of distance, which align star-forming regions along spiral arms much better than previous distances.
Summary
Star formation highly concentrated to dense regions Steep increase in SSFR to at least Sgas > 120 Msun pc–2
10-20 x more SF than predicted by any prescriptions SFR ~ Mass of gas above a threshold density Non-linear nature of KS relation:
A consequence of fdense ~ <Sgas>0.5? Resolved studies of galaxies must watch for systematic
issues
Backup Slides
A Popular Explanation for Non-linear Relation
Free-fall time depends on volume density tff ~ r–0.5
Common theoretical approach Krumholz and Thompson Narayanan et al. SFR ~ Mass/tff
dr*/dt ~ r/r–0.5 ~ r1.5
Local version of Kennicutt relation
Any evidence for this?
Mean density from virial mass and radius of well-studiedsample of dense clumps. <n> ~ M/r3 (Wu et al. 2010)
~ S
FR
Nor in YSO Counting
Yellow stars are from Class I and Flat SED SFRs in c2d+GBClouds.
Milky Way Estimates
Volume filling factor of molecular gas (as traced by CO) is about 0.005 (Heyer, prelim estimate)
Volume filling factor of clumps (density of few x 103 cm–3) < 10–5 (M. K. Dunham, prelim estimate)