Gas - NAOC Colloquium Program

Star Formation In Nearby
“Extreme” Galaxies
Yong Shi (施勇，Nanjing University)
Collaborators: Lee Armus (Caltech, US), Yu Gao (PMO, China), Qiusheng
Gu (NJU, China), George Helou (Caltech, US), Sabrina Stierwalt (U.of
Virginia, US), Junzhi Wang (SHAO, China), Lin Yan (Caltech, US), Zhi-Yu
Zhang (Edinburgh, England)
Outline
• Introduction
• Star formation at extreme low stellar densities:
extended Schmidt law.
• Star formation at extreme low metallicities.
• Star formation in other nearby “extreme”
galaxies.
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1. Introduction
Gas  Stars
Stellar Mass
Growth
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Planet
formation
Chemical
Enrichment
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Super-massive
blackhole growth
1. Introduction
Star formation is a complicated physical process.
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1. Introduction
For extragalactic studies of star formation, a
powerful empirical tool is to investigate
Star Formation Rate
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V.S.
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Gas Mass
1. Introduction
SFR Surface Density [M/yr/kpc2]
Pioneered by M. Schmidt in 1959, and R. Kennicutt in
1989 (Kennicutt-Schmidt relation).
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Gas Mass Surface Density [M/pc2]
1. Introduction
The importance of SFR-gas relationships (star
formation law):
 Observational constraints on how stars form.
ILLUSTRIS
 Tools to making stars in
cosmological simulations.
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SFR surface density
1. Introduction
Daddi+2010
Bigiel+2008
Leroy+2008
Kennicutt+1998
Gao+2004
Zhang+2014
Low-SurfaceBrightness
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Gas Surface Density
Star Formation Threshold
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1. Introduction
Over the space and time, galaxies present a large
range of properties.
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1. Introduction
More studies are underway to explore star formation in
the full range of galaxy properties.
 Extreme low stellar mass densities (thanks to CFHT
through China Telescope Access Program).
(Shi et al. 2011, ApJ, 733, 87; Shi et al. in preparation).
 Extreme low metallicities (Shi et al. 2014, Nature,
514, 335; Shi et al. in preparation).
 Other “extreme” galaxies in the literature.
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2. Extended Schmidt law
CFHT deep imaging of nearby galaxies.
NGC 5194
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NGC 5194
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2. Extended Schmidt law
Kennicutt-Schmidt relation:
1.4 0.15
SFR (gas)
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Stellar gravity:
Help gas collapse
Remove gas angular
momentum
Metal Enrichment:
ISM cooling
Dust catalyzes H2 formation
Dust shields H2 from radiation
Stellar Radiation:
Compress Gas to increase SFRs
Heat gas to decrease SFRs
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2. Extended Schmidt law
New approach:
ΣSFR as a dependent variable
Σgas and Σstar as independent variables
Diverse Galaxy Samples:
• Nearby Spiral Galaxies (NNG)
• Low-Surface-Brightness (LSB) Galaxies
• Local LIRGs
• High-z BzK SFGs and SMGs
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2. Extended Schmidt law
ΣSFR as a function of Σgas and Σstar
A unity index on Σgas gives a clear physical
implication of the relation:
Star-Formation-Efficiency (SFE)=SFR/Mgas is
a function of the stellar density, i.e., the stellar
density controls how efficient new stars form.
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SFR Surface Density
Extended Schmidt Law
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KS Law
Z=0 and high-z LIRGs
Nearby NGs
LSB
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Star Formation Efficiency
Extended Schmidt Law
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KS Law
Stellar gravity:
Help gas collapse
Remove gas angular
momentum
Metal Enrichment:
ISM cooling
Dust catalyzes H2 formation
Dust shields H2 from radiation
Stellar Radiation:
Compress Gas to increase SFRs
Heat gas to decrease SFRs
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2. Extended Schmidt law: Physical Origin
Stellar Gravity And Radiation:
 Free Fall In Stellar Potential
 Pressure-Supported Star Formation
 Stellar Torque [for merging cases]
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Star formation as gas collapse per
timescale:
SFR 
gas

A free-fall timescale in a stellar
gravitational potential:

   ff 
1
 0.5
h 0.5
 0.5 ; if Σstar >> Σgas
star
This explains the extended Schmidt law if Σstar >> Σgas
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Poutflow = Pgravity
Poutflow » PSN + PRAD µ SSFR
(Thompson et al. 2005)
Pgravity  gas( 
0.5
star

0.5
  )  gas star
4
h 0.5
0.5
gas
(Blitz & Rosolowsky et al. 2004)

This
explains
the extended Schmidt
law if Σstar >> Σgas
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2014
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Hopkins et al. (2009)
Stars:
 collisionless [relaxing violently]
 angular momentum removes quickly
Gas:
 collisional
 conserves most of its angular momentum.
 due to the lag of stellar motion, stellar gravity pulls back gas
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and
remove ang. mom.
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fstarburst = fgas
f starburst / f gas µ f star
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2. Extended Schmidt Law: Summary
Stellar masses may play an important role in
regulating the star formation efficiency.
SSFR µ Sgas S
0.5
star
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2. Extended Schmidt law: follow up works
SFR Surface Density
Extended Schmidt Law
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KS Law
DEEP CFHT observations of galaxy outer disks
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3. Star formation at low metallicity:
Introduction
Cosmic Re-ionization: one of
the most important modern
astrophysical problems.
Pop. III stars initiate the reionization; Pop. II stars
complete the re-ionization.
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3. Star formation at low metallicity:
Introduction
However, the basic properties of formation of Pop.
III and Pop. II stars are far from known:
Transition metallicity.
Star formation efficiency (SFR/gas-mass).
Lyman photon escape fraction.
Structures of ISM and IGM.
….
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3. Star formation at low metallicity:
Introduction
• Severe limitations of current capabilities in
probing the above properties.
• Using nearby metal poor galaxies as local
laboratories.
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3. Star formation at low metallicities:
Introduction
Existing studies of star formation law are limited
to galaxies with Z > 20%Z:
Kennicutt et al. 2007
Bigiel et al. 2010
Bolatto et al. 2011
….
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3. Star formation at low metallicity:
Introduction
However,
Pop. II Stars:
1%-10% Z
Wise et al. (2012)
Wise et al. 2012 ApJ, 745, 50
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3. Star formation at low metallicity:
Herschel Observations
Sample (PI: Y. Shi) :
Solar 12+log(O/H)=8.65( Pettini & Pagel 2004)
Broad-band Images:
PACS (70 and 160 um) + SPIRE (250, 350, 500 um)
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3. Star formation at low metallicity:
Herschel Observations
Sextans A
Blue: HI gas, Green: Far-UV, RED: 70+250 um
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3. Star formation at low metallicity:
why spatially-resolved
• SFR: Relatively Easy!
• Cold Gas:
CO is a poor tracer of H2 in EMP
galaxies: faint and unknown CO-to-H2
factor.
 Employ dust to trace the cold gas
content.
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3. Star formation at low metallicity: why
spatially-resolved
Spatially Resolved Dust Map!
Diffuse Region!
Star Forming Regions!
GDR x Dust-Mass=Gas
Mass
GDR=HI-gasmass/dust-mass
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3. Star formation at low metallicity:
why spatially-resolved
Gas-to-dust ratio is constant within 0.3 dex if taking out the
metallicity gradient (Sandstrom et al. 2013, ApJ, 777, 5)
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3. Star formation at low metallicity: Multiband Images
Sextans A
Blue: HI gas, Green: Far-UV, RED: 70+250 um
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3. Star formation at low metallicity: :
Draine-Li Model Fitting
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3. Star formation at low metallicity: :
gas-to-dust ratio
Sextans A
 diffuse 1: 6900
 diffuse 2: 8600
 diffuse 3: 6600
 total diffuse: 14000
ESO 146-G14:
 total diffuse: 4400
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 Spiral galaxies (Solar metallicity）
3. 结果与讨论。
Oct 14, 2014, 北京
先导b：宇宙结构起源
 Spirals
3.
结果与讨论。
 HI gas of metal poor galaxies
Oct 14, 2014, 北京
先导b：宇宙结构起源
 Spirals
结果与讨论。
 HI gas of 3.
metal
poor galaxies
 Dust-based total gas of metal poor galaxies
Oct 14, 2014, 北京
先导b：宇宙结构起源




Spirals
HI gas of 3.
metal
poor galaxies
结果与讨论。
Dust-based total gas of metal poor galaxies
Theoretical models
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先导b：宇宙结构起源
A much higher molecular gas fraction is seen than
3. 结果与讨论。
models’ predictions.
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先导b：宇宙结构起源
Nature News & Views:
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3. Star formation at low metallicity: :
planned projects.
What suppresses star formation?
 A large reservoir of molecular gas.
 Searching for CO emission (IRAM 30 m).
 Searching for warm H2 emission (Subaru 8 m?).
 Characterizing the kinematics.
 Ionized gas kinematics.
 molecular gas kinematics.
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4. Other “extreme” galaxies
A large reservoir of warm molecular gas in
3C 326 (Ogle et al. 2007, ApJ, 668, 699).
3C 326
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4. Other “extreme” galaxies
Jet driven turbulence may inhibit star formation
(Guillard et al. 2014, arXiv:1410.6155)
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4. Other “extreme” galaxies
ALMA observations CO (6-5) of a merger NGC 1614.
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4. Other “extreme” galaxies
Xu, C. K. et al. 2014, ApJ, arXiv:1411.1111
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A summary
• Besides gas, factors such as existing stars,
metallicities, radio jets and mergers, also
likely play roles in regulating star
formation.
• Star formation law is not a universal
“law”.
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