hi-to-h2 transitions in galaxy star-forming...
TRANSCRIPT
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HI-to-H2 Transitions in Galaxy Star-Forming Regions
Amiel Sternberg Sackler School of Physics & Astronomy Tel Aviv University ISRAEL
Molecules and Dust as Fuel for Star Formation Kavli Institute for Theoretical Physics June 2016
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HI-to-H2 Transitions in Galaxy Star-Forming Regions
Amiel Sternberg Sackler School of Physics & Astronomy Tel Aviv University ISRAEL
Molecules and Dust as Fuel for Star Formation Kavli Institute for Theoretical Physics June 2016
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The Multi-Phased Interstellar Medium (ISM) and Star-Formation:
hot ionized medium (HIM) T = 105 - 106 K
warm neutral medium (WNM) T ~ 104 K
<density> = 1 cm-3
but very inhomogeneous
warm ionized medium (WIM) T ~ 104 K
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The Multi-Phased Interstellar Medium (ISM) and Star-Formation:
hot ionized medium (HIM) T = 105 - 106 K
warm neutral medium (WNM) T ~ 104 K
<density> = 1 cm-3
but very inhomogeneous
CNM cold neutral medium T < 200 K
[multi-phased, pressure-equilibrium]
warm ionized medium (WIM) T ~ 104 K
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The Multi-Phased Interstellar Medium (ISM) and Star-Formation:
hot ionized medium (HIM) T = 105 - 106 K
warm neutral medium (WNM) T ~ 104 K
Interstellar gas exposed to
starlight shock waves energetic particles (cosmic-rays) magnetic fields
Global ISM heated and energized by stars (outflows, radiation, and supernova explosions).
Turbulent. Dusty.
Total Galactic ISM mass = 5x109 M¤
Mid-plane thermal pressure = 2.5x103 cm-3 K (at Solar circle)
most of the mass in cold neutral hydrogen phase most of the volume in warm/hot ionized phase.
<density> = 1 cm-3
but very inhomogeneous
CNM cold neutral medium T < 200 K
[multi-phased, pressure-equilibrium]
warm ionized medium (WIM) T ~ 104 K
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The Multi-Phased Interstellar Medium (ISM) and Star-Formation:
hot ionized medium (HIM) T = 105 - 106 K
warm neutral medium (WNM) T ~ 104 K
Interstellar gas exposed to
starlight shock waves energetic particles (cosmic-rays) magnetic fields
Global ISM heated and energized by stars (outflows, radiation, and supernova explosions).
Turbulent. Dusty.
Total Galactic ISM mass = 5x109 M¤
Mid-plane thermal pressure = 2.5x103 cm-3 K (at Solar circle)
most of the mass in cold neutral hydrogen phase most of the volume in warm/hot ionized phase.
<density> = 1 cm-3
but very inhomogeneous
stars form in cold molecular (H2) clouds: Galactic star-formation rate ~3 M¤ yr-1
CNM cold neutral medium T < 200 K
[multi-phased, pressure-equilibrium]
HI to H2 conversion: star-formation!
Herschel views Aquila
warm ionized medium (WIM) T ~ 104 K
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Talk Outline:
• brief history and motivation. • HI-to-H2 transition, some radiative transfer computations. • analytic formula for the HI column density. • self-regulated media. • observations: from Perseus to galaxies.
• “HI-to-H2 Transitions and HI Column Densities in Galaxy Star-forming Regions” Sternberg, Le Petit, Roueff, & Le Bourlot, 2014 ApJ 790 10
• “HI-to-H2 Transitions in the Perseus Molecular Cloud” Bialy, Sternberg, Min-Young, Le Petit, & Roueff, 2015 ApJ 809 122
• “Analytic HI-to-H2 Photodissociation Transition Profiles” Bialy & Sternberg, 2016 ApJ 822 83
see also “C+/H2 Gas in Star-Forming Clouds and Galaxies” Nordon & Sternberg arXiv:1603.02300
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1948 ApJ 107 6
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1948 ApJ 107 6
1971 ApJ 163 165
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1970 ApJ 161 L43
Dame, Hartmann & Thaddeus 2001 ApJ 546 792
Milky-Way in CO 1-0
H2 usually difficult to observe directly. CO is the tracer molecule.
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1977 ApJS 34 405
...HI-to-H2 initiates chemistry...
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1996 ApJ 468 269
Improved H2 line transfer:
1988 ApJ 332 400
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Global Galaxy Properties:
2010 ApJ 709 308
2014 ApJ 790 10
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M83 Crosthwaite+ 2002 ApJ 123 1892
HI as a Photodissociation Product in Molecular Spiral Arms:
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Conversion to molecular gas in high redshift galaxies: Tacconi et al. 2010 Nature 463 781
Genzel+ 2013 EGS13011166 z=1.5
PdBI NOEMA
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Kennicutt-Schmidt Relation:Schmidt 1959 ApJ 129 253 Kennicutt 1998 ApJ 498 541 Genzel, Tacconi, Sternberg, et al. 2010 MNRAS 407 2091 [SINS/KMOS(VLT) IRAM projects]
e.g., Bigiel+ 2008
apparent threshold: 10 M☉ pc-2
see also Semenov et al. poster
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I am going to be juggling many parameters - keep your eye on them!
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I am going to be juggling many parameters - keep your eye on them!
“...A perspicuous representation produces just that understanding which consists of ‘seeing connections’. Hence the importance of finding and inventing intermediate cases. The concept of a perspicuous representation is of fundamental significance for us. It earmarks the form of account we give, the way we look at things...”
Part I, paragraph122 Philosophical Investigations Ludwig Wittgenstein
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HI to H2 Transition in Dense Star-Forming Molecular Clouds.
Stellar ultraviolet “Lyman-Werner” radiation 11.2 - 13.6 eV
H2 formation (by grain catalysis) versus far-UV photodissociation.
Shielding required.
parameters:
• “effective” LW-band photon flux: wF0 cm-2 s-1 • gas density: n = n1 + 2n2 cm-3 • H2 (grain) formation rate coefficient: R cm3 s-1 • dust-grain attenuation cross section: σg cm2 • metallicity Z’ (Z’=1 is “solar”) •
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Effe
ctiv
e P
oten
tial E
nerg
y
Internuclear Separation
Ene
rgy
H2 Photodissociation in the Lyman-Werner bands (912-1108 Å):
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LW
Pumping
“Spontaneous Radiative Dissociation”
Effe
ctiv
e P
oten
tial E
nerg
y
Internuclear Separation
Ene
rgy
H2 Photodissociation in the Lyman-Werner bands (912-1108 Å):
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LW
Pumping
“Spontaneous Radiative Dissociation”
Effe
ctiv
e P
oten
tial E
nerg
y
Internuclear Separation
Ene
rgy
(Cosmologists call this “feedback”!)
H2 Photodissociation in the Lyman-Werner bands (912-1108 Å):
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LW
Pumping
“Spontaneous Radiative Dissociation”
Effe
ctiv
e P
oten
tial E
nerg
y
Internuclear Separation
Ene
rgy
(Cosmologists call this “feedback”!)
H2 Photodissociation in the Lyman-Werner bands (912-1108 Å):
H2 dissociation rate: D0 = 5.8 x 10-11 s-1
in the unit “free-space” (Draine) interstellar field.
Computed to 1% accuracy in S+ 14
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Lyman-Werner Radiative Transfer:
S+ 2014 [“Meudon PDR code”]
Characteristic multi-line H2 absorption spectrum, and “self-shielding”.
Dust absorption cross-section per hydrogen nucleus:
Numerical radiative transfer on a fine frequency grid with a spectral resolution ~ 105.
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“Universal” Total H2 Dust Limited Dissociation Bandwidth:
Universal: independent of radiation field intensity, or cloud gas density, etc.
high Z’low Z’solar
“Effective Dissociation Flux”
Total LW Flux:
S+ 2014
“little w”
The bandwidth (Hz) of radiation absorbed in H2 line dissociations in a dusty and fully molecular cloud.
“H2 dust” versus “H2 lines”
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dust grain
H
H
H2
H2 Formation:
Z′ is the “metallicity” (Z′=1 is solar)
H + e → H- + photon
H- + H → H2 + e
time scale for equilibrium
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dust grain
H
H
H2
H2 Formation:a black box... and another talk!
Z′ is the “metallicity” (Z′=1 is solar)
H + e → H- + photon
H- + H → H2 + e
time scale for equilibrium
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Basic Theory Questions:
• What do the depth-dependent HI-to-H2 transition profiles look like for far-UV irradiated systems?
• What is the resulting total HI column densities (cm-2) or HI mass surface densities (M¤ pc-2) ?
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Dimensionless Parameter:
Physical Meaning:
self-shielded H2 dissociation rate H2 formation rate
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Dimensionless Parameter: Sternberg 1988; McKee & Krumholz 2010; Sternberg+ 2014
Physical Meaning:
HI-dust absorption rate of the effective dissociation flux free space H2 photodissociation rate
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Dimensionless Parameter: Sternberg 1988; McKee & Krumholz 2010; Sternberg+ 2014
...so can be small “weak-field” or large “strong-field”
Physical Meaning:
HI-dust absorption rate of the effective dissociation flux free space H2 photodissociation rate
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H2 lines H2 lines + H2 dust
HI dustαG >> 1 “strong-field”
αG << 1 “weak-field” Z’ ≳ 1 “high-metallicity”
αG << 1 “weak-field” Z’ ≲ 1 “low-metallicity”
Three-Way Competition for the FUV Absorption:
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HI-to-H2 Transition Profiles: S+14 Bialy & Sternberg 2016 arXiv:1601.02608
The profile shapes depend primarily on the dimensionless parameter αG.
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HI-to-H2 Transition Profiles: S+14 Bialy & Sternberg 2016 ApJ 822 83
low-metallicity high-metallicity
weak-field
strong-field
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HI-to-H2 Transition Profiles: S+14 Bialy & Sternberg 2016 arXiv:1601.02608
low-metallicity high-metallicity
weak-field
strong-field
weak-field: most of the HI built up inside the H2 zone, “gradual profiles”
FUV attenuation dominated by H2 lines plus “H2 dust”
strong-field: most of the HI built up in an outer layer, “sharp profiles”
FUV attenuation dominated by “HI-dust”
The profile shapes, including the HI-to-H2 transition points, depend primarily on the dimensionless parameter αG.
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General Purpose Analytic Formula for the Total HI Column Density:
Valid for all regimes:
• weak and strong fields • gradual to sharp transitions • arbitrary metallicity
[note: no reference to the H2 line photodissociation cross sections!]
weak-field strong-field
[derived in two ways in S+14]
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Weak-Field Limit:
weak-field strong-field
formation rate per unit area = effective dissociation flux
(a “Strömgren Relation”)
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Strong-Field Limit:
weak-field strong-field
(neglecting the logarithmic term)
makes sense: When HI-dust dominates the attenuation of the far-UV field, the HI-column is “self-limited” and
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Heavy-Element Abundances “Metallicity”:
H2 formation rate coefficient and dust absorption cross-section both proportional to metallicity.
weak-field strong-field
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Heavy-Element Abundances “Metallicity”:
H2 formation rate coefficient and dust absorption cross-section both proportional to metallicity.
weak-field strong-field
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General Purpose Analytic Formula for the Total HI Column Density: S+14
• useful for interpreting 21cm observations
• incorporation into hydrodynamics simulations
• application to “self-regulated” media
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HI Thermal Phases in Self-Regulated Media:
reduced SFR
P/k = nT
enhanced SFR
Field, Goldsmith & Habing 1969 ApJ 155 149
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HI Thermal Phases in Self-Regulated Media:
reduced SFR
P/k = nT
WNM Lyα T=8000°K
CNM [CII] 157 µm T≤200°K
enhanced SFR
Field, Goldsmith & Habing 1969 ApJ 155 149
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HI Thermal Phases in Self-Regulated Media:
P/k = nT
WNM Lyα T=8000°K
Field, Goldsmith & Habing 1969 ApJ 155 149
field strength
metallicity
Wolfire, McKee & Hollenbach 2003 Sternberg, McKee & Wolfire 2002
Ansatz: Krumholz McKee & Tumlinson 2009
multiphase
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HI Thermal Phases in Self-Regulated Media:
P/k = nT
WNM Lyα T=8000°K
CNM [CII] 157 µm T≤200°K
Field, Goldsmith & Habing 1969 ApJ 155 149
field strength
metallicity
Wolfire, McKee & Hollenbach 2003 Sternberg, McKee & Wolfire 2002
Ansatz: Krumholz McKee & Tumlinson 2009
multiphase
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HI Column Density for Self-Regulated Media:
remarkable!
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HI Column Density for Self-Regulated Media:
remarkable!
independent of radiation field intensity and/or gas density.
characteristic HI photodissociation mass surface density in self-regulated systems
“transition” total gas mass surface density... star-formation threshold...galaxies.
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HI-to-H2 in the Perseus Molecular Cloud:
Giovanni Antonio Vanosino’s celestial globe 1567 Vatican Museum
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HI-to-H2 in Perseus: Perseus Cloud distance 300 pc
2MASS AV and ICO CfA & COMPLETE surveys Dame+ 2001 Ridge+ 2006
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HI-to-H2 in Perseus: Perseus Cloud distance 300 pc
2MASS AV and ICO CfA & COMPLETE surveys Dame+ 2001 Ridge+ 2006
dark clouds & low-mass star-forming regions:
B1 B1E B5 NGC1333 IC348
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10 20 30 40 50 60 70 8010−3
10−2
10−1
100
101
102
B1
R=Σ
H2/Σ
HI
10 20 30 40 50 60 70 80
B1E
10 20 30 40 50 60 70 80
B5
ΣHI+ΣH2 ( M /pc2)⊙
10 20 30 40 50 60 70 8010−3
10−2
10−1
100
101
102
IC348
ΣHI+ΣH2 ( M /pc2)⊙
R=Σ
H2/Σ
HI
10 20 30 40 50 60 70 80
NGC1333
ΣHI+ΣH2 ( M /pc2)⊙
HI-to-H2 in Perseus: Lee+ 2012 Peek+ 2011 GALFA HI Survey (Arecibo 4arcmin)
Lee+ 2012
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10 20 30 40 50 60 70 8010−3
10−2
10−1
100
101
102
ΣHI=6.79 M /pc2⊙
χ2min=0.52
B1
R=Σ
H2/Σ
HI
10 20 30 40 50 60 70 80
ΣHI=6.62 M /pc2⊙
χ2min=0.79
B1E
10 20 30 40 50 60 70 80
ΣHI=5.82 M /pc2⊙
χ2min=0.27
B5
ΣHI+ΣH2 ( M /pc2)⊙
10 20 30 40 50 60 70 8010−3
10−2
10−1
100
101
102
ΣHI=6.50 M /pc2⊙
χ2min=0.82
IC348
ΣHI+ΣH2 ( M /pc2)⊙
R=Σ
H2/Σ
HI
10 20 30 40 50 60 70 80
ΣHI=8.16 M /pc2⊙
χ2min=0.34
NGC1333
ΣHI+ΣH2 ( M /pc2)⊙
Fits using S+14:
HI-to-H2 in Perseus: Bialy, Sternberg, Lee, Le Petit, & Roueff 2015 ApJ 809, 122
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HI-to-H2 in Perseus: Bialy+ 2015
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HI-to-H2 in Perseus: Bialy+ 2015
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HI-to-H2 in Perseus: Bialy+ 2015
CNM
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HI-to-H2 in Perseus: Bialy+ 2015
Galactic interstellar LW radiation field: F0 ≈ 2 x 107 photons cm-2 s-1
Conclusions: αG ≈ 5 to 50
• FUV absorption dominated by HI-dust
• nHI ≈ 10 to 2 cm-3
• probably a UNM/CNM multiphase
CNM
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HI-to-H2 in Perseus: Bialy+ 2015
Galactic interstellar LW radiation field: F0 ≈ 2 x 107 photons cm-2 s-1
Conclusions: αG ≈ 5 to 50
• FUV absorption dominated by HI-dust
• nHI ≈ 10 to 2 cm-3
• probably a UNM/CNM multiphase
CNM
consistent with spin-temperatures Stanimirovic et al. 2015
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Galaxies:
e.g. Bigiel+ 08
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Galaxies:
e.g. Bigiel+ 08
Caveat: This interpretation requires typically “one” primary cloud per line-of-sight.
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To Conclude:
2014 ApJ 790 10 arXiv:1404.5042