Introduction to the Interstellar Medium

Bon-Chul Koo (SNU)

Radio Winter School2018. 2. 21.

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

Donald Osterbrock– gas, dust + radiation, B, cosmic rays

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

General Properties of the ISM• Different `Phases’

– Diffuse HI, HII, hot coronal gas + `Dense’ H2– pressure equilibrium: nT~3,000 cm-3 K– thermal equilibrium: heating rate = cooling rate– Energy density ≤ 1 eV/cm3

+ UV rad from OB stars, SN, stellar wind, … + star formation Æ Violent ISM

Ferriere 2001

Bible of the ISM• Physics of the

Interstellar and Intergalactic Mediumby B. T. Draine (2011)

Contents

1. Dust2. HII Gas3. HI Gas4. Molecular Gas5. Hot Coronal Gas

1. Dust

Galaxy in Visible Light

Trumpler Effect (in 1930)

Photometric Distance

History 1

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)

Interstellar Dust• Extinction & Reddening

Kim, H.-J. + (2013)

observed

extinction corrected

(ESO)

Dark Nebula

Interstellar Extinction and IR Dust Emission

IS Cloud (Gas + Dust)

Infrared

Visible light

SED?

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

IR 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

From Kim Hyun-Jeong

Infrared Dark Nebula

Cosmic (Solar) AbundancesElement Abundance Element Abundance

H 1.00 Mg 3.4x10-5

He 0.085 Al 2.3x10-6

C 2.5x10-4 Si 3.2x10-5

N 6.0x10-5 S 1.4x10-5

O 4.6x10-4 Ca 2.0x10-6

Na 1.5x10-6 Fe 2.8x10-5

• Mass fraction of H, He, and heavier elements “metals”– X=0.71, Y=0.27, Z=0.02

Solar photospheric abundances: astro-ph/0410214

By number

18

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

Depletion of heavy elements

2. H II Gas

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

HII Region

Stromgren’s HII Region (1939)History 2

Photoionization

• HII (H+) region: “diffuse nebula” around OB stars– T~10,000K.

• 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

• 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

Whirlpool galaxy M51

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

Radio Emission from HII Region

Radio continuum map of the HII region of the Orion Nebula (20 cm, 6.2" resolution) (Felliet al. 1993).

Orion HII region (Shu 1991)

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

Æ Lifetime= 1/A10= 1.1x107 yr

The Milky Way Galaxy seen in HI 21 cm line

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 3

gl=49 deg

Russeil (2003)

Difficulty in HI Study• Strong Galactic Background HI emission

LAB HI Sky (Kalberla et al. 2005)

HI Spiral Structure of the Milky Way

Westerhout 1958? Oort 1958

Levine+ 2006Koo+ 2017

Simonson 1970

New Face-on Map of Dense HI Concentrations

Two Phase ISM Model (1969)History 4

Radio Astronomy I

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/)

전파천문학

GBT 100-m telescope (WV, USA) Effelsberg 100-m telescope (FRG) 대덕 14m 전파망원경

• 전파망원경

– 분해능 = 파장/망원경 직경

Arecibo telescope

Arecibo telescope

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

FAST (Five-hundred-meter Aperture Spherical Telescope)

FAST

서울전파망원경

• 지름 6미터의 밀리미터파 전파망원경

• 1999-2001년에 건설

4. Molecular Gas

Energy Levels of Molecules

R : internuclear distance

Energy

Energy levels of CO

CO 3 mm Line Detection (1970)History 5

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

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

51

Galactic MC distribution Dame et al (2001, ApJ, 547, 792)

Galactic Ring Survey

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

Star Formation

N604 in the SMC

• Galactic SFR: M* ~ 1 M⊙ yr–1

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

Æ SFE ~ 1%1200

yrMM

M sunff

totff

y

year104.4323 7

ffHnG

t

M51 (Schinnerer et al. 2013)

• Macrophysics of Star Formation

Schmidt-Kennicutt law (Bigiel et al. 2008, Motte 2017)

• Microphysics of Star Formation

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

Radio Astronomy II

InterferometryThe 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

• 전파간섭계

VLA (NM, USA)

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

ALMA (Atacama Large Millimeter/submillimeter Array)

ALMA Science

생성 중인 태양계의 상상도. (출처: NASA)

5. Hot Gas

X-ray Emission from Hot Gas

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

Spitzer’s Coronal Gas (1956)History 6

Collisional Ionization

• Supernova remnant: remnant of SN explosion - T~106-107K- Bright in X-rays

• 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-

SNR Morphological Claissification• shell-type (77%), filled-center (4%),

composite (12%), ? (7%)

68

Cas A

SN Rate of the Milky Way

• Historical SN 9 SN1006, Crab (1054), 3C58 (1181), Tycho (1572), Kepler (1604)

Æ 5 SN/1000 yrs × (10/3)2 ~ 5 SN/100 yrsMethod ccSN SNIa All SN Authors

Historical SN 3.4+7.3-2.6 1.4+1.4

-0.8 4.6+7.4-2.7 Adams+ 2013

SN statistics 2.30±0.48 0.54±0.12 2.84±0.60 Li, W. et al. 2011SFR 1-2 … … Reed+ 200526Al 1.9±1.1 … … Diehl+ 2006

pulsar 3.2-3.7 … … Faucher-Giguere & Kaspi 2006

No neutrino burst

≤ 11.4 … …

Three Phase ISM Model (1977)History 7

Norman & Ikeuchi (1989)

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

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

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

Numerical Models

Hill et al. (2012) 512 x 512 x 2048 pc3 (Kim+ 2013)

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