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 Diffuse matter in the Universe:
[Interplanetary medium]
[Solar/Stellar neighborhood]
InterStellar Medium (ISM) ­ Milky Way and galaxies in general
­ composition, physical conditions
­ special locations (e.g. SFR)
InterGalactic Medium (IGM) &
IntraCluster Medium (ICM) ­ origin, composition, physical conditions [The diffuse matter in the early Universe]
­ Dopita & Sutherland: ''Astrophysics of the Diffuse Universe'',
A&A Library, Springer ­ Padmanabhan:
'' Theoretical Astrophysics'', Vol I, Cambridge University Press
Some info on the medium: it is mainly gas (atomic and/or molecular, often ionized), dust and DARK MATTER
Condensed diffuse
ISM
ICM/IGM
From Dopita & Sutherland:
ISM in our and spiral galaxies (1):
– disk distribution, with inhomogeneities
– mostly gas and dust + DM.
ISM in our and spiral galaxies (3):
– – ISM in our and spiral galaxies (4):
Multi­wavelength image of the Spiral Galaxy Messier 101
Three color composite image of the nearby spiral galaxy M101. The green color represents emission from neutral hydrogen (HI), emitted at 21 cm. The HI observations are part of VLA The HI Nearby Galaxy Survey (THINGS) which
is based at MPIA (image credit: Fabian Walter, MPIA). Blue shows UV emission due to recent (<108 yr) star formation as seen by the Galaxy Evolution Explorer (GALEX). Red indicates warm dust emission as traced by infrared emission at 24 microns as seen by SPITZER (image credit: Karl Gordon, Steward Observatory).
ISM in our and spiral galaxies (4):
Multi­wavelength image of the Spiral Galaxy Messier 101
Three color composite image of the nearby spiral galaxy M101. The green color represents emission from neutral hydrogen (HI), emitted at 21 cm. The HI observations are part of VLA The HI Nearby Galaxy Survey (THINGS) which
is based at MPIA (image credit: Fabian Walter, MPIA). Blue shows UV emission due to recent (<108 yr) star formation as seen by the Galaxy Evolution Explorer (GALEX). Red indicates warm dust emission as traced by infrared emission at 24 microns as seen by SPITZER (image credit: Karl Gordon, Steward Observatory).
ISM in our and spiral galaxies (4):
Left: optical image (HST) of M51
Right: CO(J 1­0) image of M51
Dust and molecular gas are projected on the same space, and trace the spiral structure of the galaxy.
ISM in our and spiral galaxies (5):
It accounts for about 5­10% of the total galaxy mass with higher
fractions for the latest galaxy types.
Its distribution/properties are function of distance from the galaxy center.
HI (and dust)
ISM in our and spiral galaxies (6):
It may be converted into stars in particular locations (SFRegions)
where cold molecular
gas collapse; massive
(proto­)stars provide photons
to ionize the debris, shocks
are formed during the collapse
ISM in our and spiral galaxies (7):
Chemical composition: gas = H (90%), He(9%), ...(1%)[relative abundances: Cowie & Songaila ARAA1996, V. 24, p.499] Mg, Al, Na, K, Ca, Ti underabundant wrt the solar composition
as atoms & molecules (cold)
ions +electrons (hot)
average number density = 1 cm­3
quantum processes then line emission/absorption
~80% of the interstellar space filled with cold, high density atomic and molecular gas.
denser clouds < 0.5% of space
low density HI/H2 is in cold, mixed with other molecules (CO, HCN,NH3, H2O, CH3OH...)
These regions may appear dark/luminous, depending on the observing
wavelength (radio to the X­rays), physical condition (SFR, SNR, GMC,
RN,...) and chemical composition
ISM in our and spiral galaxies(8):
Chemical composition(2): dust = graphite, silicates, olivine,... (~1% of the total ISM mass)
temperatures in the range 30 to 100 K
small sizes (few µm)
effective absorber (v dependent ­ reddening) of radiation
with λ < of their typical size
re­radiates as a black body (bulk at IR wavelengths)
fundamental for molecule formation
molecules = in GMC, emit/absorb in the mm/submm range as a
consequence of roto­vibrational transitions (optical in electronic transitions)
ISM phases: summary
Hot Ionized Medium (HIM) [T= 106 ~ 107 K]: Warm Ionized Medium (WIM) [T ~ 104 K]: Warm Neutral Medium (WNM) [T= 100 ~ 10000 K]: Cold Neutral Medium (CNM) [T= 10 ~ 100 K]: ISM phases(1)
temperature and density define the status of the ISM:
Hot Ionized Medium (HIM) [T > 106 to 107 K]: ISM heated by shocks originated in SN esplosions.
­ Initial cooling mainly via bremsstrahlung.
­ Secondary process (at a later epoch): recombination (line emission
from free­bound transitions in the soft X­rays [CIV, OVI],
bound­bound in the optical [H, He,C, O])
(in SNR also synchrotron radio emission from freshly accelerated electrons)
ISM phases(1bis):
Hot Ionized Medium (HIM) [T > 106 to 107 K] in our galaxy:
ROSAT all­sky (soft) X­rays (0.1­2.2 keV), after removing point sources (from Snowden et al. 1997)
ISM phases(2):
Bok's globules in IC2944
Warm Ionized Medium (WIM) [T ~ 104 K]: – Around massive and hot stars (OB) capable of strong
UV emission (also around galactic centres, where the UV radiation field is strong). OB associations are very often found in SFR (e.g. Orion).
– Equilibrium between photo – ionization and recombination [T ~ 104 K]
– Bremsstrahlung + line emission mainly of H and O [HII regions]
– detectable via
optical (line) and radio/IR/submm (bremsstrahlung continuum)
ISM phases(3)
Warm Neutral Medium (WNM) [T ~103 to 104 K]: low densities (0.1 cm­3), mainly atomic H (HI) heated by ­diffuse UV and/or X­ray radiation
­interaction of low energy Cosmic Rays detected via absorption of 21cm radiation (1)
(opacity)
ISM phases(3): Warm Neutral Medium (WNM) in spirals detected via emission
of 21cm radiation (2)
ISM phases(3)
Herschel view of the Orion Nebula (FIR spectroscopy 160­630 m)
ISM phases(4)
Cold Neutral Medium (CNM) [T= 10 ~ 100 K]: Neutral atomic hydrogen HI (densities 1 to 10 cm­3, T= 100 K), often
distributed on regions larger that those with stars Molecular hydrogen H2 (densities > 103 cm­3, T= 10 K) CO,other
molecules, in SFR (spiral arms and where interaction condensed matter)
Revealed by mm/submm/radio observations (generally line emission)
from Dame, Hartmann, & Thaddeus 2001 ISM phases: Cold Neutral Medium (CNM) in M31 ISM phases: Cold Neutral Medium (CNM) in M31 (with rotation curve info!)
ISM phases: Cold Neutral Medium (CNM) in 3C31 q
ISM phases: summary
Hot Ionized Medium (HIM) [T= 106 ~ 107 K]: Warm Ionized Medium (WIM) [T ~ 104 K]: Warm Neutral Medium (WNM) [T= 100 ~ 10000 K]: Cold Neutral Medium (CNM) [T= 10 ~ 100 K]: ISM detectability
we define the “Emission Measure” E.M. representing the amount of radiation emerging from a region (cloud)
E.M.=∫ n2e dl
which is usually averaged over the region of interest, with size l
E.M.= 〈 ne 〉2 l
Examples:
­Nova shell ejected from a WD when it becomes optically thin:
ne ~ 107 cm­3 ; l ~ 10­5 pc then E.M. ~ 109 pc cm­6
­Planetary nebula (ionized envelope of a dying star [from RG to WD])
ne ~ 104 cm­3 ; l ~ 10­1 pc then E.M. ~ 107 pc cm­6
­HII region
ne ~ 10 cm­3 ; l ~ 102 pc then E.M. ~ 104 pc cm­6
­Diffuse (ionized) ISM
ne ~ 0.1 cm­3 ; l ~ 103 pc then E.M. ~ 10 pc cm­6
The ISM planetary nebulae
ISM in elliptical and irregular galaxies
Early­types (elliptical galaxies) are known to posses much less gas and dust than late­types (spiral galaxies)
– result of formation process (?)
– low star formation rates
– different origin of ISM matter (from stellar mass loss: i.e. winds and SN explosions)
– metal enrichment via stellar evolution
In Irregulars the fraction of total mass in gas is higher than in spirals
and ellipticals and the SFR is high (per unit mass).
General consequence con “colors”:
Ellipticals are generally redder than spirals
Irregulars (and dwarfs) are bluer than spirals
Large scale environment have strong influence on gas/dust content in galaxies
(e.g. isolated – groups – clusters)
The ISM in irregular galaxies
Sextant A
Ngc1427A
The intergalactic medium(IGM / ICM): very hot (107­108) and sparse The intergalactic medium(IGM / ICM): very hot (107­108) and sparse The intergalactic medium(IGM / ICM): very hot (107­108) and sparse non­thermal plasma (contours)
synchrotron emission
cluster wide H field
shock acceleration at cluster periphery
thermal plasma (colors)
elliptical shape ⇨ unrelaxed
no counterpart to the SE extended radio emission
Brunetti et al. 2009, Nature
End of description of the ISM (and ICM) ATOMIC (molecular) SPECTRA
Atomic spectra basic concepts ­ Energy levels as according to quantum mechanics
­ “orbits” correspond to different energies
­ Radiative process (emission/absorption) during quantum transition
­ must obey to firm selection rules
­ Ground (fundamental) state as minimum electron energy
­ also defines the ionization energy
(e.g. for H atom 13.6 eV [UV photons])
Atomic spectra basic concepts (2)
Quantum numbers:
n – main number 1,2,3,.... defines the energy and the size of the “orbit”
2
n
an = ao
ao = 0.53 A
Z
l – azimutal number (angular momentum, related to eccentricity) 0,1,2,..., n­1
l 1
b
2
1
−
e
=
=
≤1

n
a
m – magnetic number (orbit orientation in case of magnetic field)
­l,­l+1,...,0,...l­1,l
at a given main number (n), there is a tiny difference in energy between
levels with different combination of l,m
this is not true (first order) for hydrogen, where there is a high degree
of energy degeneracy (fine structure)
further extremely small energy structures (hyperfine structure) can be
defined by the spin number – s (±1/2)
contrary to classical theory of charges in motion, the electrons
do not radiate in their curved orbits, except during a “transition”
Atomic spectra basic concepts (3)
Hydrogen:
easiest example to study lines between various energy levels:
if RH is Rydberg constant, the frequency v of a photon emitted/absorbed
during a transition between two levels m and n is
2
RH
nm
=
=
c RH

4
2 e m e
3
h c
1
m2
−
1
n2
=
1.1⋅10 cm− 1

Hz
5
;
n

m

0
⇒ with increasing n, lines get closer to a limiting frequency v=cRH/m2
⇒ if m is large, lines of different series start at very close frequencies and
produce a spectrum similar to a continuum emission (but it is not!)
[hydrogen­like atoms – He+, Li++, Be+++ – are obtained attributing the energy En at each “level”]

En = − Z
2
R H hc
n
2
n ,l
Atomic spectra basic concepts (3b)
Hydrogen transitions in terms of energy:
Ry
hmn
=
=
22 e4 me
2
h c
Ry

1
n2
−
=
13.6 eV
1
m2

eV
 called 1 Rydberg
;
m

n

0
For a given pair of quantum numbers, decay and excitation can either
produce (emission) or cancel (absorption) photons at the appropriate frequency/energy
Atomic spectra Hydrogen
transitions obey to well known selection rules: n, l, m,
Atomic spectra Hydrogen: Grotrian diagrams
UV, visible, IR
Atomic spectra Hydrogen: main series
Atomic spectra Hydrogen at visual wavelengths
397 434 486 656
410 Atomic spectra typical spectra of stars
for an extremely detailed spectrum of the sun see
http://chinook.kpc.alaska.edu/~ifafv/lecture/fraunhofer.htm
Molecular spectra(1):
Molecules have 3–D structures capable to oscillate around the equilibrium distance, and may also change their rotation axis/velocity moving through energy levels defined by quantum mechanics
electronic (~eV energies, optical)
Electrons in individual atoms may move to a different energy level
vibrational (~0.1 – 0.01 eV, IR)
n is the vibrational quantum number
 
2
 
2
ho 
1
1
E n= n ho − n
2
2
4De
2
ho 
E n1 − E n = ho − n 1 
2De
With the quantum harmonic oscillator, the energy between adjacent levels is constant, hv0. With the Morse potential, the energy between adjacent levels decreases with increasing n as is seen in nature. It fails at the value of n where En+1 − En is calculated to be zero or negative. The Morse potential is a good approximation for the vibrational fine structure at n values below this limit. Molecular spectra (2):
rotational (~meV, submm, mm & cm wavelengths) rotational levels lie within vibrational levels
The rotational quantum number J is related
to the moment of inertia I, to the distance r
and to the rotational energy Erot:
2
J  J 1 =
2
8 I r E rot
2
h
I = m1 r 21 m2 r 22
Molecular spectrum: rotatiotal transitions
Molecular spectrum: NIR & MIR, from vibrational transitions