Download Kinematics of the galaxies

Radio Galaxies
Part 3
Gas in Radio galaxies
Why gas in radio galaxies?
Host galaxies  early-type: not supposed to
have much gas but….
 gas on small scales: connected with the
environment of the AGN (e.g. tori, but also messy gas,
fueling AGN?)
HI, CO, ……
 gas on large scales: can trace the origin of the
galaxy (more tomorrow); mainly HI
Merger origin of radio galaxies.
Evidence: mainly optical characteristics
(tails, counter-rotating cores, dust lanes)
Why neutral hydrogen?
LARGE-SCALE HI is known to be a good tracer for
merger (if detected) it can provide clues on the
origin of radio galaxies.
Interaction & mergers are often invoked
for the triggering of AGN
providing both the gas and the instability
to bring gas to the nuclear regions
Interaction between galaxies
Forming an elliptical galaxy from mergers
Kinematics of the interaction
Hibbard (VLA)
Kinematics of the interaction
Is there HI in early-type galaxies?
3 arcmin~ 54 kpc
(1”=0.3 kpc)
orbital time ~ 2x109 yrs
 Some elliptical galaxies have HI content and size similar to spiral galaxies
 Compare to the life of a radio source
21-cm emission line of neutral hydrogen
The ground state can undergo a hyperfine transition,
reverse the spin of the electron
Frequency of the transition: 1420.405752 MHz
+ 1420.40575180 MHz
proton
electron
proton
electron
The temperature Ts (spin or excitation temperature) account for the distribution of
the atoms between the two states. The population of the two states is determined
primarily by collisions between atoms  Ts equal to the kinetics temperature
(with some exceptions!)
narrow spectral line
(van de Hulst)
+ 1420.40575180 MHz
proton
electron
proton
electron
Doppler effect  kinematics!
Most common element in the universe  present “everywhere”!
Transparent
TB ( ) Tspin [1  e  ( ) ]
 = optical depth
Column density of HI, number of hydrogen atoms in the in a cylinder of
unit cross-section (in the low optical depth limit)
NH  1.82 1018 TB dV
3.1 1017 SdV  2
atoms/cm2
where  is beam size (arcmin)
dV km/s
S mJy/beam
To derive the mass of the neutral hydrogen
MHI  2.365 10 5 D 2 F (Msun )
where F ~ S dV Jy km/s
D distance in Mpc
(1 Jy = 10-26 W/m2/Hz)
D ~ cz/H0
Doppler effect
V>0
V=0
Frequency 
in emission and absorption!
V<0
HI cloud
HI emission
HI absorption
HI detected in absorption
Particularly common in radio galaxies given the strong
underlying radio continuum
Optical depth
Column density
S  Scont c f (1  e )
N H 1.823 1018 Tspin   dv
cm-2
Tspin accounts for the electrons that are in the upper state (i.e. those that do not absorb)
Higher Tspin
more electrons in the upper state
higher column density
From galactic studies, typical Tspin= 100 K
Typical column densities:
in emission
~1021 cm-2 in a disk of a spiral galaxy
in absorption
from 1019 cm-2 against the core of some radio galaxies
 What can produce HI absorption?
Observations of the neutral hydrogen
(line observations)
Distinguish between undisturbed and
interacting galaxies using the gas
Example of HI observation
Vhel  13500 km / s
Vhel
0  
c (
)

  1359.2 MHz
z  0.045
this will be the central frequency of your
band to be able to detected HI at z=0.045
The typical bandwidth of HI observation is 5, 10 or 20 MHz:
10MHz: 1354.2  1364.2
the range of velocities covered goes from
14665 to 12358 km/s
 for 10MHz ~2300 km/s velocity range covered
for 20MHz ~4600 km/s velocity range covered
Channel width
c

0
1 MHz  ~ 200 km/s
Kinematics of the galaxies
Case of an undisturbed galaxy:
rotating disk
H I observation (datacube)
of NGC 4414
A messy case