Adam LeroyNRAO Charlottesville Hubble Fellow

Alberto BolattoKarl GordonErik RosolowskySnezana StanimirovicFrank IsraelCaroline BotAndreas SchrubaKarin SandstromNorikazu MizunoEvidge CorbelliElias BrinksFabian Walter

Using IR Emission to Trace H2 in Galaxies

Dust emission from the SMC:SAGE-SMC (PI: K. Gordon), S3MC (Bolatto+ 07)

CO emission from the SMC (Mizuno+ 01)

Ways to Get at H2 (in Other Galaxies)…The H2 that makes up most of the mass in GMCs is practically invisible.Indirect tracers have to be used, which may have their own environmental biases.

CO

Dust

H2UV absorption studies: great! But requires background source, probe pencil beam.Rotational line emission (IRS), sensitive only to gas down to T~100K (most H2 much colder).

Next most common molecule after H2. Standard tracer of H2 in high-mass galaxies. At low metallicities C and O less abundant and dust shielding weak. CO suppressed?

Comes with velocity information - dynamics offer another way to trace mass.

Probes total gas column modulo dust-to-gas ratio. Absorption tough because of lack of clear background screen. Emission limited by finite resolution of IR telescopes.

rays Cosmic ray hits nucleus, produces ray… modeled to yield total nuclei column density.Major resolution and sensitivity challenge beyond Milky Way, modeling complex…

Why We Think CO May Break in Low Mass Galaxies…

CO is faint, but when it can be measured (roughly the Local Group) these galaxies have low CO-to-SFR, CO-to-HI and CO-to-starlight ratios.

Is this a real effect on the H2 or a breakdown in our tracer?

CO per B-band Luminosity

Stellar Luminosity [Magnitudes]

Dots: relatively massive star-forming galaxies.

Local Group Dwarfs(very low CO / SFR)

CO per unit area

SFR per unit area

Dots: measurements from various local galaxies.

The CO-to-H2 Conversion Factor…

XCO

Ratio of H2 column density to CO intensity.

Certainly a function of scale studied.

In this talk: shorthand for “how much H2 is there?”

Milky Way value ~1.5 - 3.0 1020 cm-2 (K km s-1)-1

Recast as , equivalent parameter in mass units.

Is the amount of H2 inferred from various independent tracers consistent? e.g., CO brightness, dynamics, and dust emission…

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Using CO Dynamical Masses to Calibrate XCO…

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CO-to-H2 Conversion Factor

Metallicity Metallicity

CO-to-H2 Conversion Factor

Wilson ‘95 Arimoto+ ‘96

Early (often interferometer) studies used dynamical masses and found a moderate increase in the amount of H2 per CO as metallicity decreases.

GMCs seen in CO in IC 10

Metallicity [12+log O/H]

X CO from Virial Mass

(units of Milky Way)

See also: Walter et al. (2001, 2003) in NGC3077, 4214 and Israel et al. (2003) in Magellanic Clouds

Using CO Dynamical Masses to Calibrate XCO…

Milky WaySMC

In subsequent studies at higher resolution and sensitivity this effect has diminished.

Linear

Milky Way range

Points are galaxies, error bars show full range of XCO

Bolatto et al. (2008)

Using Infrared Emission Instead of CO…

o Dust traces the total gas (HI + H2) column.

o Better than CO? at least different biases…

o In the Galaxy, matches rays and CO well.

H2 = (dust × DGR-1)- HI

Measure the dust-to-gas ratio from the ratio of dust to atomic gas away from the molecular line emission but near enough to calibrate out galactic variations.

Subtract the already known distribution of atomic gas.

Estimate dust surface density from IR (need at least two bands to make a temperature estimate).

Dame+ 01 (see also Bloemen ‘90)

Observed CO

Predicted from IRAS Dust Emission

Comparison in Profile (pretty good match!)

Dust emission offers another independent check on the H2 distribution.

o IRAS towards many galaxies

o temperature correction based on 60/100

o very steep dependence, XCO Z-2.7

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Using Infrared Emission Instead of CO…CO-to-H 2 Conversion Factor

Metallicity

IR-to-HI map of the SMC

Suggests strong XCO variations in dwarfs.

Israel (1997)

Using Infrared Emission Instead of CO in the SMC…

See also: Rubio+ 04, Bot+ 07; LMC study by Bernard+ 08

Higher resolution application to the SMC finds large XCO with H2 in roughly the same spot as the CO peaks but more extended.

Dust45 pc resolution to match NANTEN CO

H2 peaks match CO, but H2 more extended

Total H2 mass 3 ×107 Msun

much more than implied by CO.

H2 ~ 10% of the HI mass.

Roughly in line with SFR and stellar content.

Leroy et al. (2007)

Using Infrared Emission Instead of CO in the SMC…

Mizuno et al. (2001) - NANTEN CO map of the SMC

CO45 pc resolution to match NANTEN CO

H2 peaks match CO, but H2 more extended

Total H2 mass 3 ×107 Msun

much more than implied by CO.

H2 ~ 10% of the HI mass.

Roughly in line with SFR and stellar content.

Leroy et al. (2007)

Why the Discrepancy Between Dust and CO Dynamics?

As metallicity and dust-to-gas ratios decrease…… Av~1 mag moves deeper into a given clump.

CO disappears when Av<2 mag through a clump… … but H2 exists to much lower extinctions

CO kinematics pick up only inner (CO-bright) part of cloud

Maloney & Black (1988), Lequeux et al. (1994), Bolatto et al. (1999), Röllig et al. (2006)

H2C+CO

Z

Clumps at decreasing (left to right) metallicity…

The discrepancy between CO, dynamics, and IR may be largely a function of PDR structure, driven by dust shielding…

S. GloverPoster

10 parsec resolution (previous map ~45 pc)

Infrared Emission and CO at High Resolution…

CO H2 from Dust

A similar approach at high (10 pc) resolution in the SMC Wing appears to work…

Leroy et al. (2009)

Line-of-Sight Extinction (Estimated From IR Emission)

CO Intensity

Infrared Emission and CO at High Resolution…On 10 pc scales, the data appear consistent with dust shielding playing a key role in whether or not CO is present…

Leroy et al. (2009)

This Experiment…

Estimate the dust surface density (or optical depth) from FIR emission.

Need at least two bands (here 70, 160) to make a temperature estimate.

1 Measure HI, CO, and IR (24+70+160) over matched lines sight.

Try to focus on areas where HI ~ H2.

2

3 Vary XCO and measure scatter about the bet-fit dust-to-gas ratio.

Look for XCO that minimizes scatter or maximizes dust-gas correlation.

M33

HI: WSRTDeul & van der Hulst ‘87

IR: SpitzerK. Gordon see Verley+ ‘07, Tabetabei+ ‘07

CO: FCRAO + BIMARosolowsky+ ‘07Heyer, Corbelli et al. ‘04

IR: SpitzerGordon et al. ‘05

CO: IRAM 30mNieten et al. ‘06

HI: WSRTBrinks & Shane ‘84

M31

The Two Nearest Spirals…

LMC

IR: Spitzer SAGEMeixner et al. ‘06

CO: NANTENFukui et al. ‘99

HI: ATCAKim et al. ‘98

IR: Spitzer S3MC and SAGE-SMCBolatto et al. ‘07, Gordon et al. ‘09, ‘10

CO: NANTENMizuno et al. ‘01

HI: ATCA + ParkesStanimirovic et al. ‘99, ‘04

The Two Nearest Star-Forming Dwarfs…

SMC

Step 1. Grab regions where HI and H2 both contribute to the ISM…M31

M33 LMC

SMC

CO Maps(roughly) Region Considered

Step 2: Minimize Scatter in (XCO CO + HI) vs. Dust

o Error bars: bootstrapping

o Not independent

o Bar: 1 range for minimum

Step 3: Compare Results Among Galaxies (XCO)CO-to-H 2 Conversion Factor

Galaxy (Decreasing Metallicity)

Each point a different methodology…

Colors: how to solve

o Correlationo Rank correlationo Scattero Median Abs. Dev.

Error bars: Bootstrapping

Point Shape: dust map

o Draine & Li 2007o Modified BB (=1.5)o 70 m = 50% VSGo 160 m intensity

Milky Way

Draine et al. (2007)Using Dust in SINGS

Conclusions

IR emission from dust can be used as an ISM tracer.

Gives a way to estimate otherwise invisible H2.

Significant discrepancies from interferometric CO dynamics.

Dust shielding key? CO only at core of clouds?

Local Group XCO(IR) ~ constant in M31, M33, LMC; SMC high

Follow-Up

How does dust-to-gas ratio and emissivity of H2 relate to HI?

How does CO as a function of AV vary with location and U?

Herschel: Expand Local Group experiment to many galaxies.

Sanity Check on Dust-to-Gas Ratio

Draine & Li (2007) models

May be a bit biased low? (i.e., too much dust)

Trend about right (a bit steep?)

Super-linear agrees with Munoz-Mateos 2009

Gas-

to-Dus

t Rati

o (Dra

ine &

Li 200

7)

Milky Way

AV Estimates in the Magellanic Clouds from FIR

~10 parsec resolution

The 160 Micron Band and Temperature

Temperature [K]

Emission at 160 per unit optical depth