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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
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