The interstellar medium

Stars & Planets (week 1)

Literature: Schulz, chapter 3 + 4

(see also: slides and videos by Troels Haugbølle from last year)

Michael Küffmeier

kueffmeier@nbi.ku.dk

Niels Bohr Institute

You find the original slides with videos included here:
https://slides.com/kuffmeier/m

The interstellar medium

1500 \mathrm{pc} \approx 1500 \times 2.06\times 10^5 \mathrm{au} \approx 3 \times 10^8 \mathrm{au}

"parsec"

"astronomical unit"

(distance between Sun and Earth)

Hi-GAL: part of Galactic plane of the Milky Way

Interstellar medium is dynamic!

Brief overview of phases

  • gas and dust between the stars
  • building blocks of new stars

Gas appears in different phases (Schulz p. 36)

Q: Why is denser gas colder?

Interstellar Medium:

P= nk_{\rm B}T\approx \mathrm{const}

ISM

components & processes

  • gas: mostly H and He
  • dust: up to 75 % of heavier elements ("metals") are part of dust, carbon- and silicate-based, can be ice-covered, ~1% of gas mass

dust is important!

...and beautiful

ISM

components & processes

  • gas: mostly H and He
  • dust: up to 75 % of heavier elements ("metals") are part of dust, carbon- and silicate-based,
    in cold phase covered by ice
  • radiation: stars, cosmic microwave background, thermal emission of dust, molecular and atomic lines
  • cosmic-rays: high-energy particles produced in supernova shocks and of extra-galactic origin
  • magnetic fields: dynamically important for all phases
  • turbulence: gas moves subsonic in HIM, transsonic in WIM, supersonic in molecular clouds

ISM

energy density

  • thermal:
  • turbulence:
  • magnetic:
  • cosmic-rays:
  • stellar irradiation:
  • dust emission:
  • CMB:
3/2\, P = 0.4 \left(\frac{P}{3000 \, \mathrm{K}\, \mathrm{cm}^{-3}}\right) \mathrm{eV}\, \mathrm{cm}^{-3}
1/2\, \rho v^2= 2.1 \left(\frac{n_{\mathrm{H}}} {100 \mathrm{cm}^{-3}} \right) \left(\frac{v}{2\, \mathrm{km}\, \mathrm{s}^{-1}}\right)^2 \mathrm{eV}\, \mathrm{cm}^{-3}
1/2\, B^2 = 0.6 \left(\frac{B}{5\, \mu \mathrm{G}} \right)^2 \mathrm{eV}\, \mathrm{cm}^{-3}
\approx 1 \mathrm{eV}\, \mathrm{cm}^{-3}
0.5 \mathrm{eV}\, \mathrm{cm}^{-3}
0.3 \mathrm{eV}\, \mathrm{cm}^{-3}
0.25 \mathrm{eV}\, \mathrm{cm}^{-3}

all components have about the same energy density! 

...but there are huge fluctuations in contribution to various phases

ISM

energy density

  • non-thermal pressure contributes significantly
  • disk is in hydrostatic equlibrium
  • disk in Milky Way is thin because
v_{\mathrm{thermal}} \ll v_{\mathrm{rot}}

Element abundance

(in the Sun)*

  • Hydrogen (X=0.7381) and Helium (Y=0.2485) are produced in Big Bang
  • Other elements ("metals") (Z=0.0134) are produced by stars
  • Most abundant (in per mille for the Sun): O (5.74), C (2.37), Fe (1.29), Ne (1.26), Mg (0.71), N (0.69), Si (0.67), S (0.31)

Note! abundances by mass (e.g., used to estimate mean molecular mass)

Alternative: abundance by number (important for radiation transfer & collisions)

N_{\mathrm{H}}=0.921
N_{\mathrm{He}}=0.078
N_{\mathrm{Z}}=0.0008

To compute mean molecular mass:

\mu = \frac{\mu N_{\rm tot}}{N_{\rm tot}} = \frac{\Sigma \mu_i N_i + \mu_e N_e } {\Sigma N_i + N_e} = \frac{\Sigma \mu_i N_i } {\Sigma N_i + N_e} = \frac{\Sigma X_i } {\Sigma X_i/\mu_i + N_e/(\mu N_{\rm tot})}=\frac{1}{\Sigma X_i/\mu_i+N_e/(\mu N_{\rm tot})}
\mu = \frac{1}{0.7381/1 + 0.2485/4 + 0.0134/20 + 0.7381} = 0.65

Example: gas consisting of ionized hydrogen, helium and metals

*Asplund 2009

see also Schulz table 3.3, page 60

Hot ionized medium

T \approx 10^6 \, \mathrm{K}

Totally ionized gas

Tycho's SN in X-ray

  • supernovae heat up ISM to millions of K.
    Groups can produce super bubbles
  • cosmic rays in SNe shock waves provide extra pressure
  • plasma rises from disk in galactic fountains
  • above the plane, gas expands and cools,

the cold gas falls back on the disk

Milky Way: Filling factor: 20 to 70 % (?), mass (?)

10^{-4} \mathrm{cm}^{-3} < n < 10^{-2} \mathrm{cm}^{-3}
10^{5} \mathrm{K} < T < 10^{7} \mathrm{K}

Warm ionized medium

T \approx 8000 \, \mathrm{K}

HII - ionized gas

massive O and B stars emit plenty UV

  • origin of Strömgren spheres & large regions of ionized H
  • hydrogen recombines to higher electron energy levels and emits blue H-alpha emission
  • large cross-section for ionising photons causes sharp transition to WNM in HII regions

Milky Way: Filling factor: 20 to 50 % (?), mass > 

0.2 \, \mathrm{cm}^{-3} < n < 0.5 \, \mathrm{cm}^{-3}
T \approx 8000 K
1.6 \times 10^9 \, M_{\odot}

Warm ionized medium

T \approx 8000 \, \mathrm{K}

HII - ionized gas 

Milky Way: Filling factor: 20 to 50 % (?), mass > 

1.6 \times 10^9 \, M_{\odot}

seen via H-alpha line

Finkbeiner 2003

0.2 \, \mathrm{cm}^{-3} < n < 0.5 \, \mathrm{cm}^{-3}
T \approx 8000 K

Neutral medium

T \sim 80 \, \mathrm{K}

HI - neutral gas 

Milky Way: Filling factor: 10 to 20 %, mass > 

6 \times 10^9 \, M_{\odot}

seen via 21 cm line

Finkbeiner 2003

0.2 \, \mathrm{cm}^{-3} < n < 100 \, \mathrm{cm}^{-3}
8000 \, \mathrm{K}

-

50 \, \mathrm{K} < T < 8000 \, \mathrm{K}

ISM

structure & thermodynamics

Tielens 2013

  • absorption and shock wave -> heating
  • emission due to collisions -> cooling
  • chemical energy (e.g., recombination of hydrogen) -> thermostat!
  • almost pressure equilibrium between the phases as long as gravity is negligible

ISM

structure & thermodynamics

Schulz, page 37

in reality: 

structure and environment are more complex

ISM

dynamic and very active

gas density

gas temperature

ISM

Molecular gas and dust

T \sim 10 \, \mathrm{K}
  • stars form in such molecular clouds
  • molecular hydrogen is the densest component of the ISM
  • difficult to observe cold molecular hydrogen directly, but possible to trace dust (see JWST image in near-IR on the left) and/or CO molecules in sub-mm to radio 

Milky Way: Filling factor: <1 %, mass ~ 

2.5 \times 10^9 \, M_{\odot}
10 \, \mathrm{K} < T < 20 \, \mathrm{K}
10^{2} \mathrm{cm}^{-3} < n < 10^{6} \mathrm{cm}^{-3}

ISM

Molecular gas and dust

T \sim 10 \, \mathrm{K}

Dust

  • responsible for reddening; depending on dust particle size
  • emits and/or absorbs light, reemission in IR to sub-mm
  • can cool or heat gas through collisions
  • catalyst for chemistry in molecular clouds
  • dust-to-gas ratio is approximately 1:100

ISM

Molecular gas and dust

T \sim 10 \, \mathrm{K}

Dust

  • responsible for reddening; depending on dust particle size
  • emits and/or absorbs light, reemission in IR to sub-mm
  • can cool or heat gas through collisions
  • catalyst for chemistry in molecular clouds
  • dust-to-gas ratio is approximately 1:100

Reddening by dust

reddening happens throughout all observational wavelengths

  • reddening depends on dust size distribution & chemical composition and it can vary significantly
  • measurable by comparing observed magnitude with expected magnitude in two filters:
E(B-V) = (B-V)_{\rm obs} - (B-V)_{\rm expected} = A_{\rm B} - A_{\rm V}
  • visual extinction can be directly linked to difference in V (
R_{\rm V}=3.1

)

A_V=(3.1\pm0.1)E(B-V)

Reddening by dust

computing E(B-V)

Depletion of metals in ISM

Reddening by dust

Dust and gas are generally distributed similarly

correlation between reddening by dust and gas (Schulz eq. 3.25)

A_{\rm V}=5.6 \left( \frac{N_{\rm H}}{10^{22} \mathrm{cm}^{-2}} \right) + 0.23

As hydrogen is everywhere in the Milky Way, and extragalactic observations require to "look" through it, reddening is essential for almost all observations!

Accounting for reddening the magnitude-distance formula is:

m_{\rm v} - M_{\rm V} = 5 \mathrm{log}_{10} \left( \frac{d}{10 \, \mathrm{pc}} \right) + A_{\rm V}

GAIA 3D extinction map

GAIA DR3: Delchambre 2022

GAIA 3D distances

By measuring reddening and distance to stars, we can also determine distribution of dust and distances to molecular clouds 

Zucker 2019

Take-away

  • Interstellar medium (ISM) consists of multiple phases:
    hot ionized medium (HIM), warm ionized medium (WIM), warm neutral medium (WNM), cold neutral medium (CNM), molecular clouds (MCs)
  • Approximately pressure equilibrium on large scales, but ISM is very dynamic with variations (thermal, chemical, radiative)
  • Pressure depends on mass of gas in Milky Way's disk: other galaxies have varying pressures!
  • The Milky Way is an open system; warm gas can be ejected while new gas can fall in
  • Presence of dust requires us to go to large wavelengths to observe molecular clouds 
  • Dust occurs in all phases, but is most relevant in the cold phase and for reddened light