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Introduction to the Interstellar Medium

and Radio Astronomy

Bon-Chul Koo (SNU)

Radio Summer School

2019. 8. 26.

What is the ISM?– “The Interstellar Medium is anything not in stars.”

Donald Osterbrock

– gas, dust + radiation, B, cosmic rays

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Structure of the Milky Way

Within 15 kpc of the G.C.,

Mstar ~ 5 × 1010 Msun

Mdark matter ~ 5 × 1010 Msun

Mdust/Mgas ~ 1/160

Draine 2011

Dopita and Sutherland

ISM Gas Phase

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Contents

1. Dust

2. HII Gas

3. HI Gas

4. Molecular Gas

5. Hot Coronal Gas

Bible of the ISM

• Physics of the Interstellar and Intergalactic Medium

by B. T. Draine (2011)

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1. Dust

Dark Nebula• Bok globules, Barnard objects

– Bart J. Bok (1906-1983), Edward E. Barnard (1857-1923)

The Horsehead (B33 in Dark regions in the sky suggesting an obscuration of light by E. E. Barnard, 1913, ApJ, 38, 496)

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Interstellar Dust

• Extinction & Reddening

Kim, H.-J. + (2013)

observed

extinction corrected

(ESO)

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Gas-phase abundances (relative to the solar) in a diffuse cloud versus condensation temperature (Draine 2011)

• Composition

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Whirlpool galaxy M51

Hubble Hα/I(R)+V(G)+B(B) Image of M51 (Mutchler 2005). Field size = 7.′5 × 10.′2 distance to M51=7.1 Mpc

Galaxy in Visible Light

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Radio Astronomy I

Radio Wave and TransparencyRadio wave = l≥ 1 mm, 𝜈 ≤ 300 GHz

Seeds, 『Horizon』

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Radio Telescope

Dark Nebula

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Interstellar Extinction and FIR Dust Emission

IS Cloud (Gas + Dust)

Infrared

Visible light

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SED?

A model spectral energy distribution of a disk galaxy (Poposecu et al 2011)

FIR Emission from Interstellar Dust

Galactic Plane in FIR (≥25 μm)

Spitzer 24 um + Herschel 70 and 160 um gl=10.6 to 13.8 deg

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2. H II Gas

NGC 604 in M33 at 840 kpc (radius 250 pc)

HII Region

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Stromgren’s HII Region (1939)

History 1

H II region = Stromgren Sphere

https://ase.tufts.edu/cosmos/view_picture.asp?id=1417

Copyright 2010, Professor Kenneth R. Lang, Tufts University

• diffuse ionized nebula around OB stars

– T~10,000K.

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Photoionization

• If an H atom absorbs an UV photon with E> Eion= 13.6 eV(λ< 912 Å), the electron becomes free (photoionization)

– H0 + hν →H++ e-

• The excess energy of the photon above the ionization potential is carried away by the photoelectron as kinetic energy

– Ekin(e) = hν –13.6 eV

K.E.

Hα 6563Å

M51 in H𝜶

Hubble H𝛼 Image of M51 (Mutchler 2005). Field size = 7.′5 × 10.′2 distance to M51=7.1 Mpc

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• Ionized hydrogen is not confined to discrete regions, but at low surface-brightness is seen through the ISM.– 90% of the H+ in the Galaxy lies outside the classical HII regions,

making up the “Warm Ionized Medium (WIM).

Hα survey by Doublas Finkbeiner (2003, ApJS, 146, 407; Draine Plate 3)Surveys: WHAM + VTSS + SHASSA

Warm Ionized Gas

Radio Astronomy II

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Image courtesy of NRAO/AUI (http://www.nrao.edu/imagegallery/)

• 우주 전파의 발견

– 1932년 미국의 Karl Jansky에 의해 우연히 우리 은하의 중심으로부터 방출되는 전파 검출.

• 1936년 Grote Reber

– 9.5m 포물면 망원경을 제작하여 은하의 지도작성

Image courtesy of NRAO/AUI (http://www.nrao.edu/imagegallery/)

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Reber survey

Image courtesy of NRAO/AUI

(http://www.nrao.edu/imagegallery/php/level3.php?id=425)

• Henyey and Keenan 1940

Thermal Free-Free Emission

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Non-thermal Synchrotron Radiation

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3. H I Gas

HI 21cm Line

F=1

F=0

ΔE=0.068 K

ν=1,420.4058 MHz, λ=21.1 cm

A10=2.884x10-15 s-1

Lifetime= 1/A10= 1.1x107 yr

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The Milky Way Galaxy seen in HI 21 cm line

LAB survey, I-GALFA Survey

HI 21cm Line Detection (1951)• Predicted by van de Hulst in 1945 and first detected in

1951 by Ewen and Purcell

History 2

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HI Spiral Structure of the Milky Way

Westerhout 1958? Oort 1958

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Two Phase ISM Model (1969)

History 3

Radio Astronomy III

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대덕 14m 전파망원경

• 전파망원경

– 분해능 = 파장/망원경 직경

JCMT 15m 서울대 6m 전파망원경

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NRAO 300 ft Telescope

1988. 11. 15.

Image courtesy of NRAO/AUI

Green Bank Telescope

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Arecibo telescope

Arecibo telescope

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– FAST (≥2016): sky-coverage ZA=±40 deg (cf) Arecibo ±20 deg

FAST (Five-hundred-meter Aperture Spherical Telescope)

FAST

4. Molecular Gas

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Energy Levels of Molecules

K&K

K&K

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CO 3 mm Line Detection (1970)

History 4

X factor

• N(H2)=XCOWCO

– W (K km s-1) integrated intensity of CO J=1-0 line

– XCO ≃ 2 × 1020 cm -2 (K km s-1) -1 (Bolatto et al. 2013)

Solomon et al. 1987

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Large CO Surveys

Heyer and Dame 2015, ARAA

Heyer and Dame 2015, ARAA

Galactic MC distribution

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Galactic Ring Survey

http://www.bu.edu/galacticring/new_index.htm

Molecular Gas• Temperature 10-20 K, number density >100 cm-3

• Many 100s of complex molecules including CO, HCN, NH3, H2O, CH3OH are known so far. Complex carbon compounds like PAH (Polycyclic Aromatic Hydrocarbon) and HC3N, CH3CHO important for forming amino acid exist as well.

Orion 230 GHz survey

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• Galactic SFR: M* ~ 1 M⊙ yr–1

(cf) SFR if MCs form stars in free-fall time

SFE ~ 1%1200

yrMM

M sun

ff

tot

ff

year104.4

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3 7

ff

HnGt

M51 (Schinnerer et al. 2013)

• Microphysics of Star Formation

Figure Credit: Hogerheijde Hogerheijde,1998,Ph.D.thesis)

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Radio Astronomy IV

Interferometry

The resolving power of a telescope depends on diameter D:

amin = 1.22 l/D

This holds true even

if not the entire

surface is filled out.

→ Combine the

signals from

several smaller

telescopes to

simulate one big

mirror →

Interferometry

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• 전파간섭계

VLA (NM, USA)

European VLBI (http://www.jodcast.net/archive/200605/)

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ALMA (Atacama Large Millimeter/submillimeter Array)

ALMA Science

생성 중인 태양계의 상상도. (출처: NASA)

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Twenty nearby protoplanetary discs, as imaged by the Atacama Large Millimeter/submillimeter Array, all show rings and gaps that indicate the formation of planets on shorter-than-expected time scales. Image: ALMA (ESO/NAOJ/NRAO), S. Andrews et al.; NRAO/AUI/NSF, S. Dagnello

거리 16.8 Mpc, 질량 6.5E9 Msun

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5. Hot Gas

X-ray Emission from Hot Gas

NGC 604 in X-ray + Optical (Tuellmann, R. et al, 2008, ApJ 685, 919)

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Spitzer’s Coronal Gas (1956)

History 5

Collisional Ionization

• Shock wave

• If an H atom collides with an energetic electron of E> Eion= 13.6 eV (T=1.5x105K), it can be ionized (collisional ionization)

– H0 + e- →H++ e- + e-

= A thin layer where the ordered kinetic energy is dissipated into heat, so that the hydrodynamic variables such as density and pressure of the flow increases abruptly. The change is irreversible.

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Shocks in Interstellar Space

– Shock occurs when an object moves supersonically. The sound speed in interstellar space is ~ 1 km/s, and there are diverse phenomena where the objects move much faster than this.

– HII regions (~ 10 km/s), stellar winds (10 to 1,000 km/s), SNRs (10 to 10,000 km/s), outflows, jets, …

(Image courtesy of Andrew Fruchter.)

Three Phase ISM Model (1977)

History 6

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Norman & Ikeuchi (1989)

Supershells, worms, chimneys – 75-90% of SNe are core-collapse SNe, and they

are correlated in space and time. superbubbles instead of isolated old SNRs.

LMC seen by Herschel and Spitzer (http://www.nasa.gov/mission_pages/herschel/multimedia/pia15254.html)

Goldbaum, N. J.

SN Feedback Simulation

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How does the Galaxy work?

Key words: ISM, star formation, supernova, supernovaremnants, supershells, IS shocks, IS dust, galacticstructure, …

Summary

• ISM – Gas, dust + radiation, B, cosmic rays

– Dust: extinction, depletion of heavy elements, IR emission

• IS Gas – Phases: HII, HI, (molecular gas), hot gas

– photo-/collisional ionization

– two-phase, three-phase ISM model

• Molecular gas – Energy levels, radio emission from rotational transitions

– X factor

– GMCs, Star formation