Brant Robertson and Andrey Kravtsov- The Molecular ISM, Star Formation, and Dwarf Galaxies
Transcript of Brant Robertson and Andrey Kravtsov- The Molecular ISM, Star Formation, and Dwarf Galaxies
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8/3/2019 Brant Robertson and Andrey Kravtsov- The Molecular ISM, Star Formation, and Dwarf Galaxies
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The Molecular ISM,
Star Formation, andDwarf Galaxies
Brant Robertson (Spitzer Fellow, KICP/UChicago/EFI)
Andrey Kravtsov (KICP/UChicago/EFI)
NASA/ESA/STScI
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McGaugh (2005)[
Madgwick et al. (2002)
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van den Bosch et al. (2007)
i i i i i i i
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Blanton et al. (2007)
Challenges for Low-Mass Galaxy Formation
1) The faint-end slope of the
luminosity is flat,
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Star formation rates in
disks: the standard lore n ~ 1.4
Kennicutt (1998)
SMD
SFR
Schmidt (1959): Star formation rate has a power-law dependence on the local gas density
n
g
SFR 1.40.15gas
Kennicutt (1989,1998): Disk-averaged starformation rate surface density has a power-lawdependence on the total gas mass surface density
Theoretical prescriptions for star formation thatconvert gas mass into stars on a dynamicaltimescale date at least to Larson (1969). Insimulations of galaxy formation, Katz (1992) andNavarro & White (1993) were among the first toadopt such prescriptions.
Normal Disks,bivariate least-squares
fit: n~2.470.39
Starbursts, bivariateleast-squares fit:
n~1.400.13
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Spatially-dependent
determinations ofthe Schmidt Law
--
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--
. . .
,, .
.,
i . . ri i r r i r r i r i ir
r ri r i i . i i r
. i i i .
Boissier et al. (2003)
Wong &Blitz (2002)
Heyer et al. (2004)
CO-Bright M33
16 systems with abundancegradients, vcirc~100-300 km/s
SFR (1.963.55)gas
SFR
1.360.08
H2
SFR 3.3 0.1gas
SFR 1.7 0.3gas
Wong & Blitz (2002), CO-Bright:
Heyer et al. (2004), M33 Total Gas:
Heyer et al. (2004), M33 H2:
Boissier et al. (2003), Z vs.NH:
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Is thewhole story?
Kennicutt et al. (2007)
g/tdyn
M51
* HI
H2Perhaps its time toconsider a model for starforming gas in simulationsof galaxies that explicitlyfollows the evolution ofthe molecular ISM and tiesthe SFR to the moleculargas properties.
Observations suggest:
SFR vs. Molecular Gas:
1) The total gas Schmidt Law
for quiescent galaxies is scale-dependent.
ngas 1.29 3.55
nH2 1.3 1.7
2) SF in molecular gas +efficiency tdyn-1 is consistent.
SFR vs. Total Gas:
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A cartoon of molecular
ISM processes
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At sufficiently high gas densities, low-temperature coolants will allow molecular gasto condense from the hot ambient medium. The molecular gas fraction isdetermined by the equation of molecular equilibrium.
A cartoon of molecular
ISM processes
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Stars form from the molecular clouds, and the local interstellar radiation fieldincreases. Soft UV photons in the ISRF can begin to photodissociate the molecularclouds.
A cartoon of molecular
ISM processes
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In the presence of an ISRF, the molecular density at moderate ISM densities is suppressed. In someregions of the ISM, the local ISRF can destroy all molecular gas, removing low-temperaturecoolants and increasing the gas temperature. The destruction of H2 by the ISRF acts as a feedbackmechanism to regulate star formation, and is efficient even as the local cooling time is short.
A cartoon of molecular
ISM processes
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Additional feedback mechanisms, such as supernovae from massive stars, may still operate.
A cartoon of molecular
ISM processes
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After the young stars die the ISRF may abate, allowing the molecular ISM to reform andthe star formation cycle to start again.
A cartoon of molecular
ISM processes
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A new model for the molecular
ISM and star formation
fH2 = fH2(gas,T ,Z ,U ISRF)
net = net(gas,T ,Z ,U ISRF)
= CfH2(1 )1.5
gas gasdudt
= SN net
UISRF = U()
SFR
SFR,
Robertson & Kravtsov (2007, in prep)
Star formation is tied to the localmolecular density and dynamical time
The molecular fraction is a function ofdensity, temperature, metallicity, and
ISRF strength
The local ISRF strength tracks thelocal star formation rate density
The thermal evolution of ISM gas dependson the supernovae heating and the net
atomic and molecular cooling rates
The net atomic and molecular coolingrates depend on density, temperature,
metallicity, and ISRF strength
Implemented in the N-body/SPH code GADGET2
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Robertson & Kravtsov (2007, in prep)
T,, Z, and ISRF-dependent Cooling +Heating rates calculated with thephotoionization code Cloudy
Tabulate the molecular fraction fH2, theionization fraction, and the molecularweight + interpolate
Molecular gas may be photodissociatedby soft UV photons. Include thepresence of an interstellar radiationfield (Mathis et al. 1983), and vary itsstrength with the local SFR density.
A new model for the
molecular ISM andstar formation
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Robertson & Kravtsov (2007, in prep)
Isolated disk starformation efficiency
Characterize the SF efficiency in diskswith vcirc~50-300 km/s, modeled afterDDO154, M33, and NGC 4501.
Compare and contrast three ISM + SFmodels:
#1) Standard atomic cooling + totalgas density SF scaling + SF thresholdmodel
#2) New atomic & molecular cooling +molecular density SF scaling w/o ISRF
#3) New atomic & molecular cooling +molecular density SF scaling w/ ISRF
log10T
5
2
#1, 300 km/s #1, 125 km/s
#2, 125 km/s
#3, 125 km/s
#1, 50 km/s
#2, 50 km/s
#3, 50 km/s
#2, 300 km/s
#3, 300 km/s
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Robertson & Kravtsov (2007, in prep)
Results: Total Gas
Schmidt Law
#3, 300 km/s = 2.0
#2, 300 km/s = 1.9
#1, 300 km/s = 1.8
#1, 50 km/s = 2.4
#1, 125 km/s = 2.3
#2, 50 km/s = 2.2
#3, 50 km/s = 4.8
#3, 125 km/s = 3.3
#2, 125 km/s = 2.5
~ NGC 4501 ~M33 ~DDO154
SFR density vs. total gas surface densityaveraged over annuli
SF timescale chosen to match disk-averaged Schmidt Law (Kennicutt1998, dashed line) at high densities.
Molecular ISM model shows scale-dependence owing to H2(gas)
Molecular ISM + ISRF model producesa much stronger dependence of SFefficiency on the galaxy mass scale.
SFR
[h2Msun
yr-1k
pc-2]
gas [h Msun pc-2]
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Robertson & Kravtsov (2007, in prep)
Results: Molecular
Gas Schmidt Law
SFR density vs. molecular gas surfacedensity averaged over annuli
Molecular ISM + ISRF model produces
a slightly shallower dependence of SFRon molecular gas surface density thandoes the model without an ISRF.
The scale dependency of SF efficiencyvs. molecular gas surface density ismuch weaker than for the total gasdensity Schmidt Law, as required by
observations.
SFR
[h2Msun
yr-1k
pc-2]
#3, 300 km/s = 1.45
#2, 300 km/s = 1.48
#2, 50 km/s = 1.44
#3, 50 km/s = 1.22
#3, 125 km/s = 1.24
#2, 125 km/s = 1.44
~ NGC 4501 ~M33 ~DDO154
mol [h Msun pc-2]10-2 102
10-5
101
10-2 102
10-3
10-1
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Blitz & Rosolowsky (2006)
fH2-Pressure
Correlation In 1993, Elmegreen calculated the relationbetween pressure, ISRF emissivity j, andmolecular fraction as
fH2 P2.2j1
P
2Ggasgas +
gas
Modeling the disk midplane pressure as
Wong & Blitz (2002) and Blitz & Rosolowsky(2006) find that
fH2 P0.80.9
This proportionality is similar to the predictedproportionality if the ISRF emissivity scaleswith the stellar surface mass density.
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Blitz & Rosolowsky (2006)
Robertson&Kravtsov(2007,
inprep)
H2
/HI
H2
/HI
PEXT/k [cm-3 K]
fH2 P0.4
fH2 P0.92
MolecularISM + ISRF
MolecularISM
101 10710-4
10210-4
102fH2-Pressure
Correlation
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The disk galaxy models simulated with themolecular ISM + ISRF model successfullyreproduce the Blitz & Rosolowsky (2006)pressure-molecular fraction trend.
The disk galaxy models simulated withoutan ISRF do not reproduce the trend, butfollow a weaker trend that reflects the lackof molecular photodissociation at low ISMdensities.
Given the scalings of the radiation fieldstrength with the SFR surface density in our
simulations, one expects that
fH2 P0.87
which closely matches the observe trend.
Robertson&Kravtsov(2007,
inprep)
H2
/HI
H2
/HI
PEXT/k [cm-3 K]
fH2 P0.4
fH2 P0.92
MolecularISM + ISRF
MolecularISM
101 10710-4
10210-4
102fH2-Pressure
Correlation
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Results: Dwarf Galaxy
Gas Disk Structure
H-Cooling H2-CoolingH2-Cooling + ISRF
50 km/s Dwarf
1kpc
1kpc
1kpc
See also James Bullocks talk and Kauffman, Wheeler, and Bullock (2007)
Mean Temperature of the ISM104 K 102 K
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