Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.
-
Upload
bathsheba-dean -
Category
Documents
-
view
212 -
download
0
Transcript of Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.
![Page 1: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/1.jpg)
Star Formation: Near and Far
Neal J. Evans II
with Rob Kennicutt
![Page 2: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/2.jpg)
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
![Page 3: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/3.jpg)
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
![Page 4: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/4.jpg)
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
![Page 5: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/5.jpg)
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
![Page 6: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/6.jpg)
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)
![Page 7: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/7.jpg)
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)
![Page 8: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/8.jpg)
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
![Page 9: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/9.jpg)
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)
![Page 10: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/10.jpg)
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)
![Page 11: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/11.jpg)
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)
![Page 12: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/12.jpg)
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
![Page 13: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/13.jpg)
CO, SF continue into HI region
Schruba et al. 2011
SFR ~ I(CO) even in HI dominated outer parts
![Page 14: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/14.jpg)
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
![Page 15: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/15.jpg)
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
![Page 16: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/16.jpg)
The Predictions
![Page 17: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/17.jpg)
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)
![Page 18: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/18.jpg)
Whole Clouds (2-16 pc)
Heiderman et al. 2010
Almost all clouds within 300 pcTotal SFR from YSO counting /areaTotal mass/area
![Page 19: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/19.jpg)
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
![Page 20: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/20.jpg)
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)
![Page 21: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/21.jpg)
Really Near: Within Clouds
Heiderman et al. 2010
![Page 22: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/22.jpg)
Less Near: Add Clouds to 1 kpc
Gutermuth et al. subm.
N = 2.67
N = 1.87
![Page 23: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/23.jpg)
Cep OB3
Gutermuth et al. subm.
![Page 24: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/24.jpg)
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.
![Page 25: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/25.jpg)
Dense Clumps on Sgas-SSFR
Using LFIR to get SFR, likely underestimates.Includes fit from Wu et al.
![Page 26: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/26.jpg)
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.
![Page 27: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/27.jpg)
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
![Page 28: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/28.jpg)
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?
![Page 29: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/29.jpg)
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
![Page 30: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/30.jpg)
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
![Page 31: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/31.jpg)
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
![Page 32: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/32.jpg)
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
![Page 33: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/33.jpg)
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.
![Page 34: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/34.jpg)
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
![Page 35: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/35.jpg)
Backup Slides
![Page 36: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/36.jpg)
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
![Page 37: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/37.jpg)
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
![Page 38: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/38.jpg)
Nor in YSO Counting
Yellow stars are from Class I and Flat SED SFRs in c2d+GBClouds.
![Page 39: Star Formation: Near and Far Neal J. Evans II with Rob Kennicutt.](https://reader030.fdocuments.in/reader030/viewer/2022032606/56649e875503460f94b8acb5/html5/thumbnails/39.jpg)
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)