The InterStellar Medium ISM(5-10% Mgal)

Composition:

•99% gas ( 90% H, 10% other elements ,He the most abundant)

•1% dust

Immersed in radiation, magnetic field and cosmic rays

So far we know about 160 molecules in the ISM. (for details see next

chapter)

They are complex (up to 13 atoms (linear cyanopolyyne=HC11N)), f.e.

formic acid, formaldehyde, ethyl alcohol, acetic acid.

Sugar was decteted in Sept. 2004 in the Galaxy centre.

Among the molecules detected in the interstellar space: glycine

(NH2CH2COOH) very imortant in astrobiology. It is the smallest of

the 20 amino acids commonly found in proteins.

In space glycine could be formed from ice containg methane and

ammonia trough UV radiation.

In the ISM about 1 atom/cm3 , but the distribution is not uniform: nebulae

In emission: Orion nebula (~10000K)

In absorption : Bernard 86 (~10K)

In reflection: light scattered from dust grain (Pleiades)

Hydrogen

• Neutral (HI) hyperfine transition at 21 cm T~100-3000K

• Ionized (HII) bremsstrahlung emission in the radio

range T~104 K

• Molecular (H2 ) studied trough CO

Phases of ISM

Phase ntot

(cm-3 )

T

(K)

M

(109Ms)

f

molecular >300 10 2.0 0.01

Cold

neutral

50 80 3.0 0.04

Warm

neutral

0.5 8000 4.0 0.30

Warm

ionized

0.3 8000 1.0 0.15

Hot

ionized

3x10-3 5x105 - 0.50

f=filling factor 1.00

The molecular component is not in pressure equilibrium with the other phases

Neutral H: 21 cm line (1)basic concepts

skTEen

/

yr7101.1 Median life time in the upper

level:

Collisions among atoms

decrease the transition time.

The population density of the two hyperfine levels depends on the energy (Boltzmann law):

Spin T: a way of measuring the T of HI in terms of the relative populations of its two

possible spin state.

Spin T is the T at which we would expect to find a system with this ratio of level

populations. It has no physical meaning except in local thermodynamical equilibrium.

S

H

S

HH

T

N

T

ln 1515 1058.21058.2~

n1 upper level, density 3 times grater than lower level

n0 lower level

nHl=density of H atoms, width along the l.o.s

=NH column density

The brightness T of the line:

)1( H

HeTT sB

where optical depth H

s

H

s kT

h

kT

EE

g

g

n

n exp3exp 01

0

1

0

1

(2)

(3)• If <<1

H HHSB NTTH

151058.2~

The brightness temperature gives an estimate of the total number of H

atoms along a given direction.

The collisions are the dominant factor to determine the spin state:

TS ~Tcin in the majority of the Galaxy

TS <Tcin for n<~5x103 m-3

TS >>Tcin when a powerful radiation at 21 cm is present, f.e. emitted from an

AGN

Absorption line ( source of synchrotron radiation behind an HI cloud)

Measure of the brightness temperature (with and without the source

behind the cloud) and knowing the radio spectrum can be computed

and then NH. From Tcin can be derived. H

H

(4)

• Natural width of 21 cm line = 10-16 km/s

• Observed width ~100 km/s 500 km/s in the Galactic centre

a) Systematic shift due to the velocity (vr ) along the l.o.s:

b) Broadening due to the thermal motions of the gas

Thermal/turbolent and/or systematic motions are studied using the 21 cm line, which has a gaussian profile (or superposition of clouds with gaussian profiles)

c

vvv r

HHoss1

Dust

• Dimension of the grain: 0.1 - 1 μm; core ~ 0.05 μm

• Density of grains in ISM ~ 100/km³

The core can be constituted by C o

Si, or more complex (f.e. olivine

Mg2SiO4)

HI kinematics in galaxies

Rotation curve of the Galaxy (1)

To determine the rotation curve, i.e. the rotation velocity for each galattocentric distance, the radial velocities vr for a large number of objects at different distances must be measured.

Molecular lines or line radiation of neutral hydrogen can be used.

)(R

)(R

Hypothesis: all motions are axially symmetric if seen from the galactic centre;

circular motions have differential galactic rotation, i.e. the angular velocity

decreases as a function of R

)2/cos()2/cos( 00 lRlRv

v=component of along the l.o.s.

– component of along the l.o.s.

R

00R

On the equator plane (b=0):

l= longitude

= galattocentric azimuth

skmRkpcR /220;5.8 000 IAU data

lRllR sin)sincoscos(sin 00

(2)

sinsin Rlr

lrRR coscos 0

lRRv sin)( 00

Fundamental equation to obtain the

rotation curve

For each v it is possible to derive

then knowing R

R is available for bright stars, HII

regions, planetary nebulae. No

information for the HI line or

molecular lines.

)(R

RRR )()(

(3)

If R is not available and all motions are axially symmetric if seen from the galactic centre, circular motions have differential galactic rotation, i.e. the angular velocity decreases as a function of R:

along the l.o.s defined by l v depends on r ( , that is proportional to 1/R). v has the max value for R min:

)(R

lRR sin0 lRr cos0

The point with the minimum distance is

called subcentral (or tangential) point. The

velocity of this point is called terminal velocity

and for the previous eq. is:

R

000max )(sin)( RRlRRv

(4)

or

In the line profile the maximum velocity is measured for

each l and it is considered as the velocity corresponding

to the point.

The curve obtained with this method can be

interpolated to obtain .

R

)(R

)( R

(5)

lRRv sin)( 00 From the value of can be derived

for each v, then from the curve obtained from the interpolation of

the corresponding value of R is determined.

The distance from the galactic centre is assigned for each cloud.

)(R

)( R

The rotation velocity of the Galaxy in the terminal points can be inferred:

lRvRvRRR sin)()( 00max0max

The rotation curve of the Galaxy :RRR )()(

This method can be applied only if HI or molecules are present in the

subcentral point.

(6)

Limits of the methods:

• l< 20 deg in case of pure circular motions the Doppler is not measurable

• 75<l<90 deg and 270<l<295 deg R and v vary slowly with l and it is difficult to associate a distance to vmax

• 90<l<270 deg R increases with r and turns more and more negative then v increases (or decreases depending on the sign of sin l) and there is no univocal correspondence between velocity and distance

• It is not possible to obtain the rotation curve for R>R0

0)( R

(7)

lRRv sin)( 00

• The rotation curve is flat and the mass derived from all the available data is greater than the mass estimated from the HI measurements.

• HI is distributed in filaments with 1-102 atomi/cm3 in between regions with ~10-1 atomi/cm3

(spiral structure)

• M(tot)~5-7x1011 M(sol)

(8)

Example: HI data observed with the

Dominion Radio Astrophysical

Observatory (Canada) (Higgs+

2005).

Note the stream with anomalous

velocity at latitude ~5deg. And

velocity between -60 and -130

km/s.

HI in the Galaxy

Oort constants (1)

To determine the rotation curve ( rotation velocity of the Sun with respect to the Galactic Centre) and R0 (distance of the Sun with respect to the Galactic Centre) must be known.

• R0 can be measured from masers (VLBI), RRLyrae, globular clusters: 8.5kpc

• can be computed as follows:

This eq. gives the radial velocity for circular motions:

0Considering the Taylor series of ....)( 00

0

RR

dR

d

R

and (R-R0 ) small:2

1)/()(

RdR

d

RdR

Rd

dR

Rd

0

0

=velocita’ angolare

=velocita’ lineare

lRRv sin)( 00

From (1):

lRRRdR

dv

R

R sin)( 00

0

In the vicinity of the Sun (R-R0) ~ r cos l

llrdR

d

Rv

R

R sincos

00

0

00

0

2

1

RdR

d

RA

lrdR

d

Rv

R

R 2sin2

1

00

0

sin 2Rv Ar lOort constant is:

0

0

1

2 R

dA R

dR

A can be written as:

It is the increase of the rotation velocity

per radius unit in the vicinity of the Sun

The tangential component of the differential rotation velocity is:

vT=component along the l.o.s. – component along the l.o.s. 0

0 0 0 0cos cos cos ( ) cos ( )Tv R l r R l l R R l R rR

Taylor series expansion in the vicinity of the Sun: 0~r r

(2)

0

0 0

0 0

(1 cos(2 )2R

d rl r

R dR R

0

20 0

0 0

cosT

R

dv r l r

R dR R

0

0

0

cos(2 )2

T

R

d rv Ar l

R dR

The Oort constant B is:0

0

0

1

2 R

dB

R dR

cos(2 )Tv r A l B

The Oort constants describe the effects of the differential rotation of the Galaxy

inside ~1 kpc from the Sun.

The measurements confirm the behaviour described by the equations.

The velocities are referred to a local reference system that is defined by a motion of the Sun with

220 km/s towards alfa, delta=18h, 30deg.

(3)

• From measurements of open star clusters:

1 114A km s kpc

1 112B km s kpc

Considering that : 0

0

0

A BR

0R

dA B

dR

From the first eq. with R0 =8.5 kpc and the values of A and B we have:

One complete rotation of the Sun around the g.c. in ~ 2.4x108 years

0 220 /km s

Observations at 21cm and in CO give of estimates AR0.

(4)

Limits of the differential rotation model

Hypothesis of the model are: • Axial symmetry

• Rotation as function of R

• No macroscopic radial motions

• HI present in each point

Deviations:• Indications of non circular motions, in particular expansion of

the 3kpc arm with velocity ~50km/s

• Irregularities in the rotation curve, probably radial motions with velocities ~ 5-10 km/s

• Expansion with v ~7km/s in the vicinity of the Sun

Distribution of HI

• Emission decreased for R< 4kpc

• Density almost constant (0.4 cm-3 ) between 5 and 13 kpc in the

direction perpendicular to the galactic plane

• Galactic plane well defined with thin width ~220pc up to R0

• From R0 up to ~25 kpc the width becomes thicker and distorted

S (warp) similar in other galaxies

• At ~ 2 R0 the thickness is ~2 times the normal disk

Continuum at 8.3 GHz. Resolution

~2 arcsec. The gray-scale

representation indicates the H92a

line emission.

VLA HI map

(Taramopoulos etal. 2001)

superimposed to the

optical map

NGC 2146

CO kinematics in galaxies

Velocity moments and CO data cubes

Position

Intensity maps = zeroth-order moment =

I(CO) = ∫ Tb

dv

Velocity field maps =

first-order moment =

∫ Tb

v dv

Intensity-weighted velocities;if optically thin line, then would be mass weighted.

velocity -240 km/s

velocity +240 km/s

Channel map: 1 channel = 10 km/s

Iso-velocity (“spider”) diagrams

NGC 2782 (Hunt+ 2008)Schematic of iso-velocity contours superposed on intensity contour map

Disk must be inclined to line of sight

But do NOT forget CO is optically thick

NGC 5953 (Casasola+ 2009)

major axis (line of nodes)

nuclear gas bar

Cuts along the major and minor axes...

major axis (line of nodes)

minor axis (rotation axis)

... give position-velocity diagrams (Casasola+ 2009)

NGC 5953

Distance in arcsec along major (top) minor (bottom)

Position-velocity (PV) diagrams

Simulations of beam-smearing on a major-axis PV diagram.

Top: Assumed “true” rotation curve (thick) with a central core, bulge, disk, and halo

Middle: “Observed” CO PV diagram

Bottom: “Observed” HI PV diagram

High resolution + high sensitivity necessary to detect central high velocities and steep rise

(taken from Sofue & Rubin 2001)

bulge

core

disk

halo

Observed major-axis PV diagrams

Signature of rotating disk clear in both galaxies

NGC 5953NGC 2782

CO(1-0)

CO(2-1)

Observed major-axis PV diagrams

Signature of rotating disk clear in both galaxies, but non-circular streaming motions evident in NGC 2782

NGC 5953NGC 2782

CO(1-0)

CO(2-1)

Rotation curves

Rotation (or “circular-velocity”) curves are the trace of velocities on a major-axis PV diagram, correcting for the angle between the line of sight and the galaxy disk (inclination).

NGC 5953

Intensity-weighted velocities:

Vint

= ∫ I(v) v dv / ∫ I(v) dv

approximated by Gaussian fits for each radial bin (and take the velocity with the most intense component in messy situations with multiple velocity peaks)

Velocity (km/s)

Rotation curves

Average of postive and negative velocities about the rotation axis

CO rotation curves useful only for circumnuclear region where the molecular is the dominant component; but high resolution has diagnostic power.

HI traces the outer disk more effectively (extending the flat part of the rotation curve).

1 kpc

NGC 5953pc (28 Mpc, 1”=136pc)

Rotation curves

NGC 2782 (35 Mpc, 1” = 171 pc)

Circumnuclear gas may not always trace circular motions; in NGC 2782, streaming motions between 200 and 800 pc...

theoretical Vrot

empirical Vrot

theoretical Vrot

empirical Vrot

CO(1-0)

CO(2-1)

Masers

Masers: Schematic model (1)(non-LTE)

Consider a molecule or atom in a gas with 3 energy levels with relative population n1, n2 ,n3. If the gas is in thermodinamical equilibrium with T the population of each level depends on the energy E:

The ratio between the population 2 and 1 (E2 > E1) is:

where is the frequency of the radiation emitted in the transition 21. At the equilibrium the levels with high energy are less populated.

/E kTn e

2 1 12( )/ /2

1

1E E kT h kTne e

n

12

Schematic model (2)

As long as n2/n1 <1, the probability of a photon to be absorbed and to produce the transition (12) or to stimulate the transition (21) is the same.

If an inversion in the populations is present (n2 > n1) the incident radiation with frequency stimulates in the gas many more transitions 21 than 12. The number of photons with this frequency increases, triggering a cascade process.

To have a population inversion a maser pump is necessary: radiation (or chemical, magnetic origin).

12

Characteristics

• Coherent radiation

• High degree of circular polarization (~ 100%) (Zeeman effect) B ~ mG

• Strong intensity of the lines (>103 Jy)

• Diameter ~ 10-3 arcsec (VLBI observations) Brightness temperatures, many around 109 K, up to 1012 K and even

1014K

• OH – H2O – SiS are the most common masers in HII regions or

supergiant red stars

• Others: CH - H2CO - NH3 - 15NH3 - CH3OH - - HC3N –SiO -29SiO – 30SiO – HCN - H13CN - H

Characteristics

VLBI data imply:

•Small spot of emission (~10-4 pc) clustered in regions of ~1016 cm

•Density up to 109 cm-3 (H2O)

•High luminosity up to 1 Lsol for a single line

Prominent Maser lines in SF Regions

Molecule Wavelength

(cm)

n

(cm-3)

T

(K)

OH 18 105 -107 100-200

H2O 1.35 107 -109 300-1000

CH3OH 0.83

4.49

104 –105

104 -105

20-100

20-100

SiO 0.35 109 -1010 700-1000

NH3 1.25 104 -105 60-150

H2CO 6.3 104 -105 20-40

SiO masers are much less common in SF regions than in the evelope of late-type, evolved stars.

Distribution of maser emission in different molecular species in the neighbourhood of the ultracompact HII region W3(OH), which dominates the 15 GHz radio continuum map shown in the center panel. A blow-up is shown in the upper right panel with maser spots observed in the 1665 MHz OH and the 6.7 GHz CH3OH lines marked as black dots and open circles, respectively. Blow-ups of the two regions within the dotted rectangles are shown in the upper left panels, which also contain, as crosses, positions of maser spots identified in the 12 GHz CH3OH maser line. Very weak non-thermal continuum emission, marked TW, is seen approximately 6'' east of W3(OH); it is coincident with a compact dust emission core.

A schematic diagram showing the origin of 6.7 GHz methanol (CH3OH) maser emission. The exact location of the maser emitting regions is still a topic of current research, but hypotheses include disks and outflows around young forming massive stars, and shock fronts near massive young stellar objects.

Masers and

star formation

Circumstellar masers

Old stars (Mira, giant red stars) expel material in the ISM in the last phases of their evolution. Masers are present at different distances from the star. These stars have strong IR emission from dust IR is probably the maser pump.Masers are probably in a shell of expanding gas.

VLBI data show expansion velocity 10-40 km/sconsidering the velocity constant the electron density of ~ 104 cm-3

expelled materialFrom the radio angular dimension of the shell and the distance of the star the mass loss is ~ 10-5 Msol/yr

Circumstellar masers

VLBA image of the star TX Cam: spots of maser emission from SiO forming a shell around the star. Approximate size of the star is indicated by red circle (artifically added to image). The maser spots are where the SiO molecules are amplifying radio emission at a frequency of 43 GHz. In separate images made at different times, these maser spots are seen to move.

Spiral structure with masersStandard model obtained from HII regions (Georgelin & Georgelin, 1976). After decades little agreement on this structure (i.e. number of spiral arms). Primary

reason: lack of accurate distance measurements throughout the Galaxy.

Trigonometric parallaxes and proper motions of masers in 18 massive SF regions with VLBA (4 periods of observations, Reid + 2009). VLBI yields accuracy of ~ 10 microarcsec (parallax) and a few km/s (proper motion).

Combination of distances, proper motions and radial velocities yields complete 3-dimensional kinematic information.

Construct a model of the MW and adjust the model parameters to fit the data.

Results:MW probably Sb Sc galaxy

0 8.4 0.6R kpc 0 254 16 /km s

the rotation curve of the MW is similar to that of Andromeda, suggesting that the dark matter halos are comparably massive.

Distances using masers in proto-stars

The lower panel, shows H2O maser proper motion vectors overlaid on the continuum map. The H2O center of expansion, marked by the cross, is coincident with the non-thermal radio source.

Accurate VLBI observations (for example two observations with Delta t=6months) give information on proper motions (projected angular motion). With statistical consideration it is possible to obtain a medium value of the deprojected angular displacement.

Using the radial velocity from the Doppler shift of the line a typical deprojected velocity can be obtained the linear displacement of the maser between the 2 measurements.

Comparing the linear and the angular displacement distance

Distances using circumstellar masers

OH emission vary with time: the red part and the blue one are well correlated but they have a delay time.

If this is interpreted has the time implied to cross the shell the linear dimension of the shell is

measuring the angular dimension (VLBI) the distance is obtained.

L c t

Mega-masers

• Masers are detected in external galaxies, up to ~1 Mpc

• They are called mega-masers (few tenth stronger than masers)

• Maser pump probably is in the central regions of these galaxies (high star formation)

• They could be used to measure the distance of the galaxies then H0 (the small angular dimensions can be measured with the actual radio telescope)

Water maser emission from nearby

galaxies(Braatz&Gugliucci, 2008)

• Survey of 611 nuclei of

galaxies: 8 detected, both

megamasers associated with

AGN and relatively low

luminosity masers probably

associated with star formation.

Isotropic luminosity:

solLL 7.0370

•Cosmic masers, which are very bright radio sources, arepumped by energy from the central object (e.g. a young star,red giant or active galactic nucleus).

•The masers are clouds of gas and dust, containing excitedmolecules of hydroxyl (OH), water (H2O), methanol (CH3OH),silicon monoxide (SiO) and formaldehyde (CH2OH). The spectralline emission is typically amplified thousands of times by themaser process, making them visible over much larger distances.

•Sites of maser emission include: comets, star-forming regions,circumstellar envelopes of red giant stars, supernova remnantsand discs in the centres of active galaxies.

More in “The formation of stars” Stahler & Palla