The CNM – How Much, How Cold, and Where?

John DickeyUniversity of Tasmania

4 February 2013C+ as an Astronomical Tool

What is the Cool Neutral Medium?

The Cycle of Galaxy EvolutionAtomic

hydrogen emission

Atomic hydrogen absorption +

diffuse OH emission

OH masers

OH masers

Synchrotron emission

Image credit: Bill Saxton (NRAO)

Dalgarno & McCray 1972 ARAA 10, 375

Field, Goldsmith & Habing 1969, Ap. J. 155, 149.

WNM

CNM

How Cold is the Cool Neutral Medium (CNM)?

Murray, Stanimirovic, Goss,Heiles, JD, Begum, Hennebelle 2013 in prep.

The 21-SPONGE Project (EVLA)

Gaussian fitting gives:Tsp = 178 KTsp = 2280 K

Tsp distribution in the 21-SPONGE survey based on Gaussian Fitting (Murray et al. 2013)

Spin temperature distribution from Heiles and Troland (2003, Ap. J. 586, 1067) based on Gaussian fitting techniques.

Spin temperature distribution from Dickey et al. (2003, Ap. J. 585, 801) using data from Heiles and Troland (2003).

Tkin distribution in the 21-SPONGE survey based on line widths (Murray et al. 2013)

Stanimirovic and Heiles (2006 Ap. J. 631, 371), HI emission/absorption spectra from Arecibo.Using very bright background sources at high latitude (both at +81o latitude).

Moving from high latitude directions to low latitudes the CNM begins to cover a wider velocity range, due to differential Galactic rotation. But the CNM linewidths are still much narrower than the WNM emission, and there is lots of WNM emission at low levels that has no corresponding CNM.

Low Galactic latitude 21-cm emission-absorption spectra from Strasser et al. 2007 (A. J. 134, 2252)

Tsp temperature distribution of CNM clouds vs. RG

from Strasser (2006) based on SGPS, CGPS, VGPS data.

Dickey et al. 2000, Ap. J. 536, 756.

Higher optical depths are confined to the midplane.

21-cm absorption can be visible againstthe bright background 21-cm emission in the Galactic plane. Recent surveys are by Gibson et al. (2005, Ap. J. 626, 195) and Kavars et al. (2005, Ap. J. 626, 887).

They find that half of all lines of sight at low latitudes contain some HISA, and roughly half of the detected HISA clouds have corresponding CO detections.

HISA – H I Self-Absorption

Analysis

An example τ vs. Ts curve

21-cm emission and absorption and self-absorptionfrom the Southern Galactic Plane Survey (HISA)

McClure-Griffiths et al. 2004

Leafy Sea Dragon (Australian marine animal)photo by Victoria Graham

McClure-Griffiths et al.(2007, Ap. J. 652, 1339)

The Riegel-Crutcher Cloud

How much CNM is there?

Even counting the HISA gas, there is less CNM than WNM (maybe 1:3 ratio).

This ratio stays roughly constant with Galactic radius outside the solar circle.

Longitude Velocity diagrams in emission and absorption:

opacity, , binned in l and vdensity, n, measured at thesame locations and binnedin the same way as for

Data from Strasser et al. 2007

The radial dependence of the density, nH.

The radial dependence of the opacity, .

The radial dependence of the spin temperature, T.

The structure of the CNM in the far outer Galaxy is interesting. It is located as cores or a network inside large (106 Msun) HI clouds.

The mixture of WNM, CNM, and molecular gas in these outer Galactic plane clouds resembles that in the Magellanic Clouds.

Strasser et al. 2007 A.J. 134, 2252.

This cloud is at a distance of 16 to 17 kpc from the Galactic center.

In the central 2 or 3 kpc of the Galaxy, there is CNM in most of the features that can be seen in CO and HI emission. But there is not as much CNM as we might expect, particularly in the structures inside the 3 kpc arms.

HI from McClure-Griffithset al. 2012

CO from Dame and Thaddeus 2008

Red dots: 21-cm absorption towardGalactic HII regions (Jones et al. 2013)

Jones, Dickey, Dawson, McClure-Griffiths, Anderson, Bania, 2013 in prep.

Galactic HII region recombination linevelocities (Jones et al. 2013)

CNM 21-cm absorption toward HII regions

How does C+ align with the CNM on a Galactic scale?

Very well.

As well as any other tracer?

Maybe.

What does Herschel see?

Langer et al. 2010

Nearby SGPS HI emission and absorption spectra (Strasser 2006)note the HISA features

Conclusions:• The CNM temperature is mostly in the range 15K to 250K. The median is about 50K.• The CNM is about 25% to 35% of the total HI mass. Thus only about 1/4 to 1/3 of the atomic medium is in the cool phase.• The CNM keeps a ~constant ratio with the WNM on a large scale in the outer MW. Some structures in the inner Galaxy have less CNM than we might expect given recent SF.

CGPS absorption and emission combined, from Strasser & Taylor 2004, Ap. J. 603, 560.

Cool phase clouds plus warm phase diffuse gas.

Data for emission/absorption spectral channels in the CGPS I+II survey (Strasser Ph.D. thesis 2006).

SGPS dataVGPS data

In the inner Galaxy something similar appears, but even more pronounced.

Monte Carlo model with random column density of HI (normal sigma 35K absolute value)...

Threshold column density of warm gas (50K + normal sig 10 K) for existence of cool gas...

All HI above the threshold is cool gas, with Tcool = 70 K...

Half of the warm gas emission is absorbed by the cool gas.