galaxias5

Medio interestelar
en galaxias
Ejemplo: galaxia del Sombrero, polvo y gas.
•  El ISM es:
–  La materia entre estrellas
–  La “atmósfera” de una galaxia
•  El ISM contiene información sobre temperatura,
presión, etc. de una galaxia
–  Define el tipo morfológico de una galaxia
•  La distribución del en una galaxia define en gran
medida su tipo de Hubble.
Normalmente, hablar del ISM implica el ISM local o de la Vía
Láctea. Se supone entonces que el ISM en otras galaxias es similar,
aunque hay bastantes datos sobre el ISM en otras galaxias.
El ISM es crucial en la evolución de una galaxia en cuanto que está
supuestamente abastecido por el material proveniente de formación
estelar y en su seno (nubes densas de gas y polvo) se siguen
produciendo nuevas estrellas.
Constituyentes principales del ISM:
•  Gas y polvo que representa ~1% de la masa de una galaxia
como la nuestra.
•  El gas está en fases distintas que se supone están en equilibrio
(o cuasi) de presión;
• 
• 
• 
• 
• 
Gas frío y neutro (CNM)
Caliente y neutro (WNM)
Caliente y ionizado (WIM)
Muy caliente y ionizado (HIM)
Nubes moleculares, pero éstas no están en equilibrio (MM)
•  Además:
•  Campos magnéticos con ~1/3 de la densidad de energía del ISM
•  Rayos cósmicos, que probablemente representan otro ~1/3 de la
densidad de energía del ISM
Propiedades estándard
Fase
Estado del H
T (K)
n (cm-3)
f, mf
HIM
H II
106
~ 10-3
25-65%, traza
WIM
H II
8000-104
0.3
25%, 15%
WNM
HI
3-8 x 103
0.4
35%, 35%
CNM
HI
20-100
1-50
3%, 10%
MM
H2
10
102-106
1%, 40%
Modos de detección
•  Trazadores a lo largo del espectro:
–  Líneas de emisión:
•  e.g. Hα (óptico), HI (radio), CO (milimétricas), líneas de recombinación
(H109α en radio)
–  Líneas de absorción
•  e.g. HI, Ca, Na, Fe
–  Emisión térmica (contínuo)
•  e.g. PAH emisión (12µm), regiones HII (radio, infrarrojo, óptico, mm,
…), plasma difuso caliente (rayos X)
–  Emisión no térmica (contínuo)
•  e.g. radiación sincrotrón del medio magnetoiónico
–  Absorción y scattering
•  e.g. granos de polvo (rayos X, UV, óptico)
–  Reflexión
•  e.g. polvo (óptico)
–  Dispersion y scintillation
•  e.g. señales dispersadas de pulsares
Ecuación de transferencia radiativa:
dIν
−τ ν
−τ ν
+ Iν = Sν ⇒ Iν (τν ) = Iν (0)e + Sν (1 − e )
dτν
Iν = intensidad de radiacion (W Hz -1 m -2 sterad -1 )
τν = espesor optico
Sν = funcion fuente (p.ej. si el material esta en
equilibrio termico a la temperatura T,
Sν = Bν (T ) = cuerpo negro)
Hot Ionised Medium
•  "Coronal gas
–  n ~ 0.003 cm-3
–  T ~ (5-10) x 106 K
–  f ~ 0.40? - hard to know
•  First observed in O VI
absorption lines towards
stars
•  Also X-ray/UV emission
(but absorbed by gas)
•  Where does it come from:
–  hot interiors of supernova
remnants?
NGC 4631: X-rays (blue)
UV from stars & H II regions
(orange)
Warm Ionised Medium
•  Two main components:
–  Clumped medium in H II regions
•  Confined to the disk with a scale height of 100 - 200 pc,
conincident with the stars
•  Ionised bubbles produced by UV photons around hot
stars
–  The diffuse medium, or “Reynolds Layer”
•  Faint Ha emission over the entire sky, main scale height
1 kpc
•  T~8000 K, ne = nH+ ~ 0.1 cm-3
•  How is this medium ionised? Is the O star strong flux
enough?
Clumped Warm Ionised Medium
•  N44C is an H II region
around a 75,000K star
•  Wolf-Rayet star
•  Red at the bottom is the
superbubble N44
HST H! , O III
Credit:D. Garnett &
the Hubble Heritage Team
15 pc
Diffuse (Warm) Ionised Medium
Wisconsin H! mapper (WHAM)
Atomic Hydrogen
•  By number, atomic hydrogen is the most pervasive of component of the
ISM, >90%
•  Detect H I in both absorption (tracing the CNM) and in emission (tracing
the WNM)
–  These two phases can co-exist over a narrow range of pressures
•  H I is an excellent tracer of dynamics in a galaxy
–  Bulk motions, like spiral arms, are traced
–  Pressure driven regions, like H II regions, H I shells, etc. are traced
•  H I structure on all size scales
–  HI emission structures observed with scales of ~0.1 pc to few kpc
–  HI absorption observed with scales of ~few AU to tens of pc
•  A problem with H I is that it is difficult to determine true gas density and
temperature
–  H I emission traces the column density if the gas is optically thin (and it isn’t
always!)
–  H I absorption measures the temperature weighted by the column density
Cold Hydrogen
•  Cold hydrogen
clouds (dark and
purplish) detected
via self-absorption
•  Hydrogen
emission in the
background is
absorbed by colder
hydrogen in the
foreground
•  Temperatures as
low as ~ 20 K
(Dickey et al.
2003)
Cold H I Clouds
Molecular Gas
•  Most of the molecular gas is made up of H2, which is
difficult to detect directly
–  Most H2 detections are via absorption lines in the far-UV
•  Generally infer its presence from observations of
12CO, which emits readily detected spectral line
2.6mm
•  One then assumes a conversion factor to calculate the
amount of H2
–  X ~ 2.3 x 1024 for the M.W.
–  X increases with metallicity as
X ∝ [12 + log10 (O / H )]
Molecular Gas
•  Most molecular gas is found in molecular clouds
–  These tend to be clumpy and have very large internal
turbulent pressure
–  They are often gravitationally bound rather than pressure
confined
–  Require a weak UV radiation field so that molecules can
form faster than they are photodissociated
•  Typical molecular clouds are:
–  r ~ 6 – 60 pc
–  n ~ 102 - 106 cm-3
–  M ~ 104 - 106 M¤
–  T ~ 10 K
Molecular Gas in the Milky Way
from Dame et al, ApJ, 547, 792 (2001)
Detecting Dust
•  Observed via extinction of optical and UV, dark
clouds, i.e. the Coalsack
–  Depending on the grain size, a, and wavelength, !, the
dust grains have some efficiency, Q, for scattering and
absorbing
–  The attenuation in magnitudes of the incoming radiation
field is therefore:
I
A! = #2.5log = 1.086" $ a 2Q(a)n(a)da
Io
•  Can study this via reddening EB-V = AB-AV
•  In the Milky Way l-o-s average Av is 1.8m kpc-1
ISM in Other Galaxies
•  The ISM of a galaxy largely defines its Hubble type
–  Spiral, disk galaxies have similar components to the Milky
Way
•  But, Mgas / Mdyn increases from ~0.03 at Sa to ~0.3 for late-type
spirals like Scd
•  MH2/ MHI decreases from ~ 3 for S0/Sa to ~0.06 for later-type
spirals like Sd/Sm
•  Elliptical galaxies have very different ISMs
–  Dominated by hot, T~106 K plasma
–  many have small amounts of HI, 40% detected by IRAS
(Knapp et al 1989), CO detected in several (Knapp 1990)
Large-scale distribution of phases
M51
Rand, Kulkarni & Rice (1992)