Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.naic.edu/~astro/frontiers/talks/SS_JE_ISM.pdf Дата изменения: Tue Sep 18 03:18:00 2007 Дата индексирования: Sat Dec 22 11:28:23 2007 Кодировка: Поисковые слова: superbubble

A Hitchhiker's Guide to Galactic Gastrophysics
ґ Snezana Stanimirovic & John Everett
University of Wisconsin­Madison

With insights from the Madison `ISM Lunch' gang:
Bob Benjamin, Ed Churchwell, Don Cox, Jay Gallagher, Alex Lazarian, John Mathis, Dan McCammon, Ron Reynolds, Blair Savage, Linda Sparke, Eric Wilcots, Ellen Zweibel

September 13th, 2007

ISM Major Questions - p. 1


Motivation q Why Study Galactic Gas Dynamics? Major ISM Questions: Where do the models stand?

Motivation

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ISM Major Questions - p. 2


Why Study Galactic Gas Dynamics?

Motivation q Why Study Galactic Gas Dynamics? Major ISM Questions: Where do the models stand?

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How does Galactic Ecology work? We need to build a detailed foundation for understanding our Galaxy, other galaxies, and galaxy formation & evolution. The Milky Way is the "Rosetta Stone" where we learn to translate plasma physics into the larger-scale observables we see elsewhere.

Carina Nebula, Hubble Heritage
September 13th, 2007 ISM Major Questions - p. 3


Why Study Galactic Gas Dynamics?

Motivation q Why Study Galactic Gas Dynamics? Major ISM Questions: Where do the models stand?

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How does Galactic Ecology work? We need to build a detailed foundation for understanding our Galaxy, other galaxies, and galaxy formation & evolution. The Interstellar Medium (ISM) is a ready-made laboratory for plasma physicists: many of the processes plasma physicists study are exhibited (in their simpler extremes!) in the ISM. Understanding the ISM can also help in those studies for which the Galaxy is a foreground, such as CMB studies.

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WMAP K-band
September 13th, 2007

(23 GHz)
ISM Major Questions - p. 4


Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

Major ISM Questions: Where do the models stand?

September 13th, 2007

ISM Major Questions - p. 5


How important is IGM accretion onto galaxies? Does accreting gas remain cold, or does it virialize?

IGM
erbubbles

Galactic Halo

Does the ram pressure of the IGM affect the ISM?

To what extent are there phases in the ISM? Are those phases in equilibrium, or more transient? What sets the "equipartition" b etween the comp onents? What sets the pressure, and is it constant? What is the distribution function of matter in the phases? How wide is that distribution?

How is the halo formed? Under what conditions do galaxies launch winds? Is there a Galactic fountain? Wind? Does the halo have cooler inclusions/phases?

superbubbles, winds

"Hot Phase"

sup

What produces the soft X-ray background? What happens to the bulk of SNe energy? What is the plasma metallicity distribution?

bubbles

"Warm Phase"

Do these phases represent a unique solution? How does plasma move between these phases? What is the filling factor of the phases?

SNe
ISRF

Stars
Collapse mediated by turbulence + B-fields? What sets the stellar IMF? How is SF triggered? How do star clusters form?
winds

"Cold Phase"

How do GMCs What sets the What sets the What sets the How does cool

form/evolve/die? cloud core IMF? population of GMCs? scales of GMCs? gas feed an AGN?

Rad. Pressure

Does radiation pressure play a significant role in pressurizing gas in galaxies? How important is radiation pressure on dust as a pressure component of the ISM? Can rad. pressure dominate stellar feedback?

Dust
bip olar
SNe sho cks

How does dust form? Does the dust distribution give hope of the ISM in some sort of equilibrium?

ws outflo

B Fields

How are primordial magnetic fields amplified? What sets the structure of magnetic fields? Does B tension help with pressure equilibrium? What role do ordered vs. random B fields have?

Cosmic Rays

How are CRs accelerated? What are the highest energy Galactic CRs? How do CRs act hydrodynamically?

Turbulence

What drives turbulence? How does turbulent energy cascade? How important is turbulence to cloud core support and collapse?


Overview of Salient Questions

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

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What is the topology of the ISM, and what are the filling factors of the various phases? How does material transition between phases? Are there really "phases"? How does MHD turbulence work? What is the source of turbulent energy, and where does it dissipate? What happens on the smallest scales? What is the structure of the Galaxy's Halo? What is the nature of the disk/halo interface? Does matter accrete onto the Milky Way? Is there a large-scale Galactic fountain and/or wind? What are High Velocity Clouds? How much does the IGM affect the Milky Way, and vice-versa? Where's the edge of the Galaxy? How does star formation work? How do Giant Molecular Clouds form? How did the magnetic field grow to its observed strength?
ISM Major Questions - p. 7

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September 13th, 2007


How is the Interstellar Medium Structured?

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

We have only a very rough understanding of the filling factors of the various "components" of the ISM. Knowledge of those components can strongly constrain models:
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McKee & Ostriker (1977) hypothesized that fh 0.7 - 0.8. McKee (1995) sets fh 0.6.

McKee & Ostriker (1977)
September 13th, 2007 ISM Major Questions - p. 8


How is the Interstellar Medium Structured?

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

We have only a very rough understanding of the filling factors of the various "components" of the ISM. Knowledge of those components can strongly constrain models:
s s s

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McKee (1995) set fh 0.6 from McKee & Ostriker. Slavin & Cox (1993) predicted fh 0.2 Ferriere (1998) developed analytical models and found fh 0.2 at R . Hypothesized that fractions may change with Galactic radius. Simulations by Avillez & Breitschwerdt give fh 0.2.

September 13th, 2007

ISM Major Questions - p. 9


New Turbulent ISM Simulations

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

Avillez & Breitschwerdt (2005)
September 13th, 2007 ISM Major Questions - p. 10


How is the Interstellar Medium Structured?

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

We have only a very rough understanding of the filling factors of the various "components" of the ISM. Knowledge of those components can strongly constrain models:
s s s

s s

McKee (1995) sets fh,McKee & Ostriker 0.6. Slavin & Cox (1993) predicted fh 0.2 Ferriere (1998) developed analytical models and found fh 0.2 at R . Hypothesized that fractions may change with Galactic radius. Wolfire et al. (1995) assumed constant fractions. Simulations by Avillez & Breitschwerdt give fh 0.2. Simulations by Mac Low and collaborators yield fh 0.4.

There are not yet strong observational constraints on this, but the best constraints on gas filling factors are in the radio: H I holes in M31 (Brinks & Bajaja 1986) and the warm neutral medium observations of Heiles & Troland (2003). Pulsar dispersion measurements of ionized medium?
September 13th, 2007 ISM Major Questions - p. 11


How does MHD Turbulence work?

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

The ISM is dynamic! This dynamic nature is rooted in magnetohydrodynamic (MHD) turbulence: the ISM is not in a static equilibrium (at least in the warmer, less dense, components). Turbulence is impor tant for: s Supplying turbulent mixing & diffusion s Providing pressure suppor t for the gas in the disk s Turbulent models can explain H I obser vations of Giant Molecular Clouds (Ballesteros-Parades, Avillez, & Mac Low (2000)). So, globally, models seem to be able to explain cloud suppor t. s On ver y small scales, turbulence drives compressions that can trigger star formation. s Turbulence may also explain the wide range of pressures observed (Jenkins & Tripp 2006): Gas can exist in the thermally unstable regime.
ISM Major Questions - p. 12

September 13th, 2007


How does MHD Turbulence work?

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

Of course, still many open questions about turbulence: s What drives the turbulence? How does the energy cascade to smaller scales? How does it dissipate? Spectrum fit with incompressible, hydrodynamic (Kolmogorov) turbulence... but that doesn't seem to fit what we know about the ISM! B-fields? Anisotropy? Armstrong et al. (1995)

September 13th, 2007

ISM Major Questions - p. 13


Understanding the Milky Way's Halo

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

Does the Milky Way need an Environmental Impact Statement ? Many ISM models show gas flowing into the halo (see Mac Low et al. 2000).
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Does that material fall back to the disk, as a galactic fountain? Might this explain High Velocity Clouds? Can plasma escape the Galaxy as a wind with help from Cosmic Rays?

Snowden et al. (1997) Everett et al. (submitted)

September 13th, 2007

ISM Major Questions - p. 14


Understanding the Milky Way's Halo

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

Characteristic of the ISM, studies of the Galactic Halo are related back to our two other questions! Phases: Avillez & Breitschwerdt (2005) claim that the hot gas filling fraction (fh ) is low due to hot gas `venting' into the halo. In addition, gas in the halo seems to have multiple phases (McKee 1995), and is surrounded by a more pervasive hot gas. Turbulence: if the clouds then fall back to the disk, they can impact the disk, feeding back on the ISM and helping to drive turbulence. The ISM is a Complex System: the current models clearly need observational help to distinguish between them, to provide checks the models.

September 13th, 2007

ISM Major Questions - p. 15


Overview of Salient Questions

Motivation Major ISM Questions: Where do the models stand? q Overview of Salient Questions q How is the Interstellar Medium Structured? q How does MHD Turbulence work? q Understanding the Milky Way's Halo q Overview of Salient Questions

s

What is the topology of the ISM, and what are the filling factors of the various phases? How does material transition between phases? Are there really "phases"? How does MHD turbulence work? What is the source of turbulent energy, and where does it dissipate? What happens on the smallest scales? What is the structure of the Galaxy's Halo? What is the nature of the disk/halo interface? Does matter accrete onto the Milky Way? Is there a large-scale Galactic fountain and/or wind? What are High Velocity Clouds? How much does the IGM affect the Milky Way, and vice-versa? The Milky Way is a much better computer than any we have available! Arecibo can read important parts of it with great sensitivity.

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September 13th, 2007

ISM Major Questions - p. 16