Radio Summer School 2019. 8. 26.

Introduction to the Interstellar Medium and Radio Astronomy

Bon-Chul Koo (SNU)

What is the ISM? – “The Interstellar Medium is anything not in stars.” Donald Osterbrock – gas, dust + radiation, B, cosmic rays

1 Structure of the Milky Way

Within 15 kpc of the G.C., 10 Mstar ~ 5 × 10 Msun 10 Mdark matter ~ 5 × 10 Msun

Mdust/Mgas ~ 1/160

Draine 2011

ISM Gas Phase

Dopita and Sutherland

2 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)

3 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)

4 Interstellar Dust • Extinction & Reddening

extinction corrected

observed

(ESO) Kim, H.-J. + (2013)

• Composition

Gas-phase abundances (relative to the solar) in a diffuse cloud versus condensation temperature (Draine 2011) 10

5 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

6 Radio Astronomy I

Radio Wave and Transparency Radio wave = l≥ 1 mm, 휈 ≤ 300 GHz

Seeds, 『Horizon』

7 Radio Telescope

Dark Nebula

8 Interstellar Extinction and FIR Dust Emission

Visible light IS Cloud (Gas + Dust)

Infrared

9 FIR Emission from Interstellar Dust

SED?

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

Galactic Plane in FIR (≥25 μm)

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

10 2. H II Gas

HII Region

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

11 History 1 Stromgren’s HII Region (1939)

H II region = Stromgren Sphere • diffuse ionized nebula around OB stars – T~10,000K.

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

Copyright 2010, Professor Kenneth R. Lang, Tufts University

12 Photoionization

K.E. • If an H atom absorbs an UV

photon with E> Eion= 13.6 eV(λ< 912 Å), the electron Hα 6563Å 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

M51 in H휶

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

13 Warm Ionized Gas • 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

Radio Astronomy II

14 • 우주 전파의 발견 – 1932년 미국의 Karl Jansky에 의해 우연히 우리 은하 의 중심으로부터 방출되는 전파 검출.

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

• 1936년 Grote Reber – 9.5m 포물면 망원경을 제작하여 은하의 지도 작성

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

15 Reber survey

Image courtesy of NRAO/AUI (http://www.nrao.edu/imagegallery/php/level3.php?id=425)

Thermal Free-Free Emission

• Henyey and Keenan 1940

16 Non-thermal Synchrotron Radiation

17 3. H I Gas

HI 21cm Line

-15 -1 A10=2.884x10 s 7  Lifetime= 1/A10= 1.1x10 yr

ν=1,420.4058 MHz, λ=21.1 cm

F=1

ΔE=0.068 K

F=0

18 The Milky Way Galaxy seen in HI 21 cm line

LAB survey, I-GALFA Survey

History 2 HI 21cm Line Detection (1951) • Predicted by van de Hulst in 1945 and first detected in 1951 by Ewen and Purcell

19 HI Spiral Structure of the Milky Way

Westerhout 1958? Oort 1958

20 History 3 Two Phase ISM Model (1969)

Radio Astronomy III

21 • 전파망원경 – 분해능 = 파장/망원경 직경

대덕 14m 전파망원경 서울대 6m 전파망원경 JCMT 15m

22 NRAO 300 ft Telescope

1988. 11. 15. Image courtesy of NRAO/AUI

Green Bank Telescope

23 Arecibo telescope

Arecibo telescope

24 FAST

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

FAST (Five-hundred-meter Aperture Spherical Telescope)

4. Molecular Gas

25 Energy Levels of Molecules

K&K

K&K

26 History 4 CO 3 mm Line Detection (1970)

X factor

• N(H2)=XCOWCO – W (K km s-1) integrated intensity of CO J=1-0 line 20 -2 -1 -1 – XCO ≃ 2 × 10 cm (K km s ) (Bolatto et al. 2013)

Solomon et al. 1987

27 Large CO Surveys

Heyer and Dame 2015, ARAA

Galactic MC distribution

Heyer and Dame 2015, ARAA

28 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

29  –1 • Galactic SFR: M* ~ 1 M⊙ yr (cf) SFR if MCs form stars in free-fall time  M tot 1 M ff   200M sun yr  SFE ~ 1%  ff 3 4.4 107 tff   year 32G nH

M51 (Schinnerer et al. 2013)

• Microphysics of Star Formation

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

30 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

31 • 전파간섭계

VLA (NM, USA)

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

32 ALMA (Atacama Large Millimeter/submillimeter Array)

ALMA Science

생성 중인 태양계의 상상도. (출처: NASA)

33 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

34 5. Hot Gas

X-ray Emission from Hot Gas

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

35 History 5 Spitzer’s Coronal Gas (1956)

Collisional Ionization

• 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-

• Shock wave

= 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.

36 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.)

History 6 Three Phase ISM Model (1977)

37 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 Norman & Ikeuchi (1989) (http://www.nasa.gov/mission_pages/herschel/ multimedia/pia15254.html)

SN Feedback Simulation

Goldbaum, N. J.

38 How does the Galaxy work?

Key words: ISM, star formation, supernova, supernova remnants, supershells, IS shocks, IS dust, galactic structure, …

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

39