Models and Observations of the Warm Ionized ^ Magnetised Medium (WI M M) of the Galaxy

LOFAR-MKSP workshop • Robert Benjamin (U of Wisconsin) • Apr 6, 2011

Models and Observations of the Warm Ionized

^Magnetised Medium

(WIMM) of the Galaxy

Talk Outline

1. WHAM update

2. Modeling WHAM

3. Magnetised HVCs!

DM = neo

d

∫ l( )dl cm-3pc

EM = ne2

o

∞

∫ l( )dl cm-6pc

ne(l)=fcn(IHα ,v)

Dame & Thaddeus 2011 submitted


Galactic Diffuse Ionized Gas (DIG) Layers Milky Way (Reynolds 1989): Pulsar DM in globular clusters [at known distance]

External galaxies NGC 891 (1990): Hα emission from edge-on spirals

Dettmar 1990 Rand et al. 1990

Low frequency free-free absorption of radio synchrotron halo (Hoyle & Ellis 1963)


Wisconsin Hα Mapper goes to Chile

• Ground-based high throughput velocity resolved survey

• 15 cm aperature dual-etalon Fabry-Pérot spectrometer—largest used in astronomy—attached to 0.6m aperature sidereostat

• Velocity resolution: 12 km/s • Spectral window: 4800 to 7300 Å • Angular resolution: 1o

• 3sensitivity: 0.15 Rayleigh (106/4 ph cm-2 s-1 sr-1 = 2.4 x 10-7 ergs cm-2 s-1 sr-1 at Hα)


[SII] 6716 Galactic plane observations (Gostisha et al 2011)

330o

320o

€

Δvh =12.8km⋅ s−1

€

Δvs = 2.27km⋅ s−1

Ionizing source: two late-type O stars near the center of the bubble HD 135240= Cir (l=319.69, b=-2.91, O8.5 V, mV=5.07) and HD 135591 (l=320.13, b=-2.6, O8 IIIp, mV=5.43). Log LLC= 48.72 and 49.22, consistent with Hα emission. Distance greater than 430 pc.


Mysteries of the WiMMQ: What powers it? A: Ionizing photons from O stars that apparently leak from disk… —Puts constraints on spatial organization of ISM, but attempts to “photoionize” Joung & MacLow (JM06) simulations don’t create our WIM (Wood et al. 2010, ApJ, 721, 1379).

Q: What stirs it? A: High latitude EM distribution very well fit by log-normal function (Berkhuijsen & Fletcher 2008, MNRAS, 390, L19; Hill et al 2008, ApJ, 686, 363). Idealized models of (M)HD turbulence that is mildly supersonic (Ms~2) but sub-Alfvénic (MA~0.6) can match (1) EM distribution, (2) Hα line widths, and (3) pulsar DM|sin b| distribution. Q: What supports it? A: Scaleheight of 1-2 kpc much thicker than the scaleheight of 104 K gas in the gravitational potential. Cox (2005, ARAA, 43, 337) shows that the observed vertical distribution is consistent with hydrostatic equilibrium given observational estimates of magnetic, CR, turbulent pressures. But what is the physics that establishes this equilibrium and how would it change with SFR, gas, stars?

Q: What temperature is it? A: A mixture? Dong & Draine, 2011, ApJ 727:35 address question of discrepancy of optical (8000 K) vs. free-free (3000 K) by appealing to mix of components/mechanisms.


Can we create a numerical WImM (and then photoionize it?)

A: We will see…

Previous results: Hydrodynamic simulations (JM06) + Monte Carlo photoionization (Wood et al 2010) show photons escape clumpy ISM, but do not produce an extended thick ionized layer and produce much broader distributions of EM than observed.

Hypothesis: Addition of magnetic field will provide additional pressure support, producing a more vertically extended layer with less compression (and narrower EM distribution).

Simulations: FLASH v2.5 adaptive-mesh hydrodynamic model of a turbulent, supernova-driven ISM (Joung & MacLow 2006,2009). A. Hill has added the MHD solver of Waagan, Federath & Klingenberg (2011, astro-ph/1101.3007).

Basic characteristics: 1 kpc x 1 kpc x [2 x 20 kpc], Δxres(z)=2-30 pc, B x, ini=0, 6.5, 13 G evolved for 400 Myr. Galactic gz, gas, SNe rate fixed. Found sensitivity to (a) grid size, (b) Thalo, i , (c) Δxres.


Medium resolution results: Δxres=8-64 pc

Initial conditions that definitely (or may) matter

? Thalo,i

xrange ≥ 1 kpc

yrange ≥ 1 kpc

zrange≥ 20 kpc

tmax≥400 Myr

fSN

? Bx,i


Vertical profiles (after 100 Myr)

Dickey-Lockman HI + Reynolds WIM


Midplane slices

Will definitely change with higher resolution runs…


An oscillatory surprise! Similar to that predicted by

Walters & Cox (2001, ApJ 549, 353)

Models of Vertical Disturbances in the Interstellar Medium


Magnetized HVCs?

McClure-Griffiths et al (2010, ApJ, 725,275)

Measurement of Magnetic Field in a [Magellanic Stream] Leading Arm High-Velocity Cloud

RM reversal coincident with HI HVC (from Taylor et al 2009 RM catalog from NVSS 1364.9 MHz/1435.1 MHz)

Using an estimate of neL:

BP ≥6 G

Br / BP < 0.8

PB comparable to estimated ram pressure.

Ordered field higher than in SMC/LMC.

Unsolved Mysteries of High Velocity Clouds —Benjamin (2004)


Smith Cloud (Smith 1963)

Lockman et al 2008, ApJL 679, L21 d=10.5-14.5 kpc; 12.4±1.3 kpc z=-3 kpc Projected size=3x1 kpc

Main body has two kinematic components vspace=296±20/ 271±6 km/s vISM=130±14/130±5 km/s NHI(cloud)=1-5 x 1020 cm-2

Assuming cloud moves as a coherent linear object projection on our line of sight--> L


Ionized component of Smith Cloud

Hill et al. 2009, ApJ 703, 1832

Brightest region of Hα at the tip of HI cloud, but an almost equally bright Hα cloud with low N(HI).


An extended search?

Another clump of RM reversal suggests a deeper search for high velocity gas.


Conclusions

—M. Haffner, A. Hill, K. Barger, R. Reynold, M. Gostisha, R. Benjamin

• Evidence for a second magnetized high velocity cloud where key parameters (position, velocity, neutral/ionized mass) are known. Significance?

—R. Benjamin, A. Hill, ?

• 3D adaptive mesh MHD simulations underway to explore the vertical structure, porosity/ionization, and dynamics of warm ionized medium. First two papers by this summer. Questions on B field evolution/topology welcomed!

• Magnetic fields definitely provide additional vertical support, narrow range in PDF than hydro-only models. Match to our galaxy not yet clear.

• Under certain conditions, simulations result in a damped oscillatory motion, with a period predicted by Walters & Cox (2001).

—A. Hill, R. Joung, M. Mac Low, K. Wood, K. Waggan, C. Federath, C. Klingenberg, R. Benjamin

• WHAM all-sky velocity-resolved Hα survey almost complete. Data reduction should be done by end of the spring.

• Several surprises include a nearby off-plane HII region and increased depth through the Galaxy.


Please Remove All Slides After This


The problems with averages

Volume vs. mass weighted?

Very resolution dependent Δxres=8-64 pc

Very height dependent

Black <125 pcRed 0.4-0.6 kpcGreen 0.8-1.2 kpc

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Models and Observations of the Warm Ionized ^ Magnetised Medium (WI M M) of the Galaxy

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