Brant Robertson and Andrey Kravtsov- The Molecular ISM, Star Formation, and Dwarf Galaxies

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


2/22

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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

i i i , i i

, i i i i i

i i i

i

Blanton et al. (2007)

Challenges for Low-Mass Galaxy Formation

1) The faint-end slope of the

luminosity is flat,


3/22

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.


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.


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.


ISM processes


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Additional feedback mechanisms, such as supernovae from massive stars, may still operate.


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.


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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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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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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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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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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Documents

Brant Robertson and Andrey Kravtsov- The Molecular ISM, Star Formation, and Dwarf Galaxies