Evidence for the ISM

• How do we know there is an interstellar medium (ISM)?

1)

The Oort Limit

• Hydrostatic equilibrium, but for the whole Galaxy!

- gravity of Galactic disk balanced by "pressure" (= individual velocities) of stars - measure velocities of stars → density of disk • Total density:

ρ 0

 0.08 M  /pc 3 • Density of stars:

ρ stars

 0.06 M  /pc 3 mass density • What's left?

ρ ISM

number   0.02 M  /pc 3 1.3 x 10 -24 g/cm 3 density

n ISM

 0.8 H atoms / cm 3 (but in very few places is the actual value close to this average!) Astronomy 16: The Interstellar Medium 1

Extinction – Discrete Clouds

2)

Extinction

• Clearly present in discrete clouds spread throughout Galaxy Dark cloud Barnard 68 (ESO / VLT ANTU) Horsehead Nebula (Nigel Sharp / NOAO / NSF; © AURA) http://www.astro.lu.se/Resources/Vintergatan/ Astronomy 16: The Interstellar Medium 2

Extinction – Diffuse Gas

• Robert Trumpler (1930) : - catalog of 100 open clusters spread throughout Galaxy - cluster fitting: distance estimates for each cluster → "photometric distance" - nearby clusters: diameter depends on concentration, number of stars → "diameter distance" - plot "photometric" vs "diameter" distance: photometric distance equals diameter distance photometric distance more than diameter distance Distant clusters are fainter than they should be!

→ ~0.7 mag/kpc (modern value: ~2 mag/kpc) of extinction No globular clusters or background galaxies close to Galactic plane ("zone of avoidance") Astronomy 16: The Interstellar Medium 3

Reddening & Spectra

3)

Reddening

- stars in same MK class have different

B – V

;

B – V

increases with overall extinction → ISM also makes stars redder 4)

Interstellar absorption lines

- in binary systems, some lines do

not

show Doppler shift due to binary motion Astronomy 16: The Interstellar Medium 4

Trumpler’s “Reddening”

Astronomy 16: The Interstellar Medium 5

Extinction & Dust

• Extinction is due to small

dust particles

in the ISM - combination of

absorption

and

scattering

Absorption: Scattering: • At a given distance, a star appears fainter than implied by its

distance modulus

: extinction in

m



M

 5 log 10

d

 5 

A

magnitudes (

A

> 0) “

A V

dust = 3” means star is 3 magnitudes fainter in

V

filter due to Towards Galactic center,

A V

 30 !

A λ

=

k λ d

, where

k λ

mag/pc is wavelength λ

extinction coefficient

at Astronomy 16: The Interstellar Medium 6

Optical Depth & Cross Section

Recall

optical depth

,

τ λ

: (stopped at this slide Tuesday)

I

(

L

) 

I

( 0 )

e

  

L

In "stellar structure", we wrote:      

r

Same situation here, but we convert

ρ

, to number density,

n

. We thus now write:     

nd

where

σ λ

is

cross section

(units m 2 or cm 2 ) of each dust grain.

If dust grains were hard spheres of radius

a

bullets, then

σ λ

= π

a

2 . But if light & photons were

diffracts, σ λ

=

Q λ

π

a

2 , where

Q λ

is "extinction efficiency factor" at wavelength λ.

E.g. graphite grains of various radii Note:

Q ext = Q abs + Q scat

from Draine & Lee,

The Astrophysical Journal

,

285

, 89 (1984) Astronomy 16: The Interstellar Medium 7

Optical Depth & Cross Section

Q λ

is "extinction efficiency factor" at wavelength λ.

or, a cartoon view…

Astronomy 16: The Interstellar Medium 8

Extinction & Optical Depth

A m observed



m without dust

   2 .

5 log 10  2 .

5 log 10

F observed F without dust e

    1 .

086 

k

  1 .

086  

n

But how do we measure

A λ

(or equivalently

τ λ

) ?

V



M V

 5 log 10

d

 5 

A V

Direct observation:

V

Spectrum:

M

}

A V

Parallax:

d

But if star is close enough for parallax,

A V

is probably small!

If we don't know

d

, we can't get

A V

!

Can resolve this because dust produces

selective extinction

- blue light gets scattered more than red light (blue skies, red sunsets) - more extinction → more

reddening

Astronomy 16: The Interstellar Medium 9

Extinction curve:

Reddening

red

V B

blue

note: inverse wavelength units!

So

longer

wavelengths show

less

extinction. Thus extinction not only changes magnitude, it changes

color index

also!

"color

E B



V

 (

B



V

)  (

B



V

) 0 intrinsic excess" observed color index, equal to

M B – M V

color index From shape of extinction curve, can show that (roughly!):

R V



A V E B



V

 3 .

1 Astronomy 16: The Interstellar Medium 10

Color Excess & Dust/Gas Ratio

Example: O6III star is observed with

V

= +12.4 &

B

= +13.8

From HR diagram, we know that O6III stars have

(B – V) 0

= -0.30 and

M V

= -5.5

What is distance to star?

Relation between dust & gas: • Star's color excess gives amount of extinction • Star's spectrum shows ISM absorption lines of H, from which equivalent width gives column density,

N H =

∫

n dl

from Diplas & Savage,

The Astrophysical Journal

,

427

, 274 (1994) •

E B-V

vs

N H

gives straight line:

N H

 5 .

8  10 21

E B



V

cm -2 mag -1

Comparison to dust in this room?

Astronomy 16: The Interstellar Medium 11

Dust Properties & Formation

• Size of interstellar dust grains: 50 Å – 0.25 μm (cf. sand: 50-2000 μm, silt 2-50 μm, toner ~10 μm) • Tiny part of ISM – 1 dust particle every 10 6 m 3 !

- by mass, ISM is 99% gas, 1% dust • Temp: absorbs photons, reradiates as 20-40 K blackbody • Composition: silicates, graphite, water ice • Formation: need high pressure, temperature steadily falling - condensation in winds of cool giants & of AGBs - expanding/cooling ejecta of novae & supernovae • Critical role in

astrochemistry

: site of

molecule formation -

e.g. H

t

2 molecule can never form by 2 H atoms colliding:

collision

 10 -13 sec,

t bond formation

 → so atoms will usually just rebound 10 -9 sec But H atoms can stick to dust grain & bond, then escape Astronomy 16: The Interstellar Medium 12

Grain Shape & Polarization

• Reddened light is

polarized -

grains preferentially absorb one pol. and leave other - need something to break symmetry → dust grains are elongated, not round!

from Worm & Blum,

The Astrophysical Journal

,

529

, L57 (2000) • But only works if all grains aligned in same direction

-

global Galactic

magnetic field

causes alignment from Han & Wielebinski,

Chinese Journal of Astronomy & Astrophysics

,

2

, 293 (2002) Astronomy 16: The Interstellar Medium 13

The Gaseous ISM

• Dust is important, but remember that 99% is gas!

• Abundances: 85% H, 10% He, 5% rest (by number) • Gaseous ISM exists in (at least) five

phases -

molecular medium (MM) - cold neutral medium (CNM) - warm neutral medium (WNM) - warm ionized medium (WIM) - hot ionized medium (HIM) (aka "coronal gas") Astronomy 16: The Interstellar Medium 14

Molecular Medium (MM)

• Grouped into "clouds" – ill-defined variety of structures •

M

~ 1 – 10 6 M  ; GMCs have

M

• size ~ 1-100 pc ;

T ~

10 K ;

n H

• Opaque!

E.g. n H = 10 4 cm -3

> 10 4 M  ~ 10 2 – 10 6 cm -3

and L ~ 1 pc. What is A V ?

• 1% of ISM volume (

f

= 0.01), 50% of ISM mass!

• Almost entirely

molecular hydrogen

(H 2 ), but H 2 emission lines at low

T

& so is hard to see has few • Best tracer:

carbon monoxide -

only 0.01% by number, but has

rotational transition

at λ = 2.6 mm (ν = 115.27 GHz) Not absorbed by dust – can see through whole Galaxy!

from Dame et al,

The Astrophysical Journal

,

547

, 792 (2001) • 100s of molecules now detected: C 2 H 5 OH, C 24 H 12 , glycine… • Only known site of star formation Astronomy 16: The Interstellar Medium 15

The Molecular Ring

• CO observations show inner Galaxy dominated by

molecular ring

at

R

~ 4 kpc • Many supernovae, H II regions, open clusters here also from Clemens et al,

The Astrophysical Journal

,

327

, 139 (1988) Astronomy 16: The Interstellar Medium 16

Neutral Medium

• Cold Neutral Medium (CNM) - atomic hydrogen,

n H

~ 20 cm -3 ,

T

~ 100K,

f

~ 0.02

• Warm Neutral Medium (WNM) - atomic hydrogen,

n H

~ 0.3 cm -3 , T ~ 6000K,

f

~ 0.5

• Seen through "spin-flip" or "hyperfine" transition of H -

λ

= 21.1 cm,

ν

I = 1420.4 MHz (discovered at Harvard, 1951) - not absorbed by dust; most useful tracer of ISM (spontaneous transition from high to low occurs once every 11 million years!) CNM Astronomy 16: The Interstellar Medium 17

Warm Ionized Medium

• Ionization potential of H in ground state = 13.6 eV - photon w. E > 13.6 eV (UV:

λ

< 911 Å) can ionize H - H will

recombine

→ Hα seen at 656.3 nm

(why not Lyα?)

• Discrete component: "H II regions" (

f

~ 0.03) - ionized bubbles produced by UV photons around hot stars - seen in Hα, in IR (hot dust), in radio ("free-free" emission) • Orion Nebula (Messier 42) - top left: Hα - top right: infrared - bottom left: radio N.B.: extinction seen in optical, but not in IR/radio Astronomy 16: The Interstellar Medium 18

H II Regions

Astronomy 16: The Interstellar Medium 19

Str ömgren Spheres

• Theorist's H II region: "Strömgren Sphere" - "photoionization equilibrium" between ionizations & recombinations

S

  4 3 

R

3

n

2

H



B

- LHS = total no. of ionizations per second - RHS = total no. of recombinations per second -

S *

= no. of ionizing photons emitted per second (can be derived from Planck equation) e.g. O5 star:

S *

B1 star:

S *

  5 x 10 49 3 x 10 45 photons/sec photons/sec -

R

= radius of H II region (cm) -

n H

= density of gas being ionized (cm -3 ) -

α B

= "recombination coefficient"  2.5 x 10 -13 cm 3 /sec • UV has short mean free path: H II regions have sharp edges - 100% ionized inside, 0% ionized outside • Oxygen and nitrogen ions in H II regions act as thermostat:

T

~ 8000-10000 K regardless of central star Astronomy 16: The Interstellar Medium 20

Hypothetical & Real HII Regions

Str ö mgren Says “Spheres,” with Radii…: Nature says… O Star will destroy it’s birthplace rather thoroughly.

NGC 3603

QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture.

The Rosette Nebula

Astronomy 16: The Interstellar Medium 21

Diffuse WIM

• Diffuse component of WIM recently identified (

f

~ 0.20 ?) - aka "Diffuse Ionized Gas" (DIG) or "Reynolds Layer" - faint Hα from recombinations over entire sky (hard to map) -

T

~ 8000 K,

n e

=

n H+

~ 0.1 cm -3 - H II regions confined to thin disk of height ~100-200 pc, but DIG is in disk of height ~ 1000 pc - ionization source unknown: escaped photons from O stars?

Astronomy 16: The Interstellar Medium 22

Hot Ionized Medium

• "Coronal gas" -

n

~ 0.003 cm -3 ;

T

~ (5-10) x 10 6 K ;

f

~ 0.40?

- first seen in O VI absorption lines towards stars - also seen in X-ray/UV emission (but absorbed by

gas

) - hot interiors of supernova remnants?

• Left: optical image of edge-on spiral galaxy NGC 4631 • Right: X-rays (blue), UV from stars & H II regions (orange) Astronomy 16: The Interstellar Medium 23

The Multi-phase ISM

• 1960s: "two phase ISM" (Field, Goldsmith & Habing 1969) - cold (neutral) clouds, embedded in warm (10% ionized) intercloud medium; two phases in

pressure balance P



nkT



P

/

k

 1000 K cm  3

-

occasional hot cavities produced by SNe, but not part of big picture • 1970s: "3 phase ISM" (Cox & Smith 1974; McKee & Ostriker 1977) - hot cavities left by old SNRs merge & interconnect → HIM is persistent & pervasive phase of ISM - CNM=clouds; WNM/WIM=cloud envelopes; HIM=cavities - pressure balance:

P

/

k

~ 2500  3000 K cm  3 - probably not completely correct, but useful complete picture from McKee & Ostriker,

The Astrophysical Journal

,

218

, 148 (1977) Astronomy 16: The Interstellar Medium 24

Recycling in the ISM

• Over billions of years, gas moves through all phases!

cooling SNRs recomb ination star light dust SNRs starlight

Adadpted from Dopita & Sutherland, "Astrophysics of the Diffuse Universe" (Springer, 2003) Astronomy 16: The Interstellar Medium 25

Clustered Supernovae

• Basic three-phase picture assumes SNe are randomly located - but in reality SN progenitors found in "OB associations" - clustered SNe: >100 stars, all going SNe within ~ 1 Myr!

→ "supershell" : similar evolution to SNR, but 100x energy → can escape from Galaxy's gravity to form "chimney" HIM 21cm H I (WNM) ~ 1 kpc !

cooling?

from McClure-Griffiths et al,

The Astrophysical Journal

,

594

, 833 (2003) Astronomy 16: The Interstellar Medium 26