Radio Galaxies

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Radio Galaxies Part 3

Gas in Radio galaxies
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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)
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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
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Interaction between galaxies
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Forming an elliptical galaxy from mergers
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Kinematics of the interaction
Hibbard (VLA)
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Kinematics of the interaction
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Is there HI in early-type galaxies?
3 arcmin 54 kpc (10.3 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

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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
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!)
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• narrow spectral line (van de Hulst)

Doppler effect ? kinematics!

• Most common element in the universe ? present
  everywhere!
• Transparent

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? 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)
atoms/cm2
where ? is beam size (arcmin) dV km/s
S mJy/beam
To derive the mass of the neutral hydrogen
where F S dV Jy km/s (1 Jy 10-26
W/m2/Hz) D distance in Mpc
D cz/H0
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Doppler effect
V 0
V lt 0
V gt 0
Frequency ?
in emission and absorption!
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HI cloud
HI emission
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HI detected in absorption
Particularly common in radio galaxies given the
strong underlying radio continuum
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
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• What can produce HI absorption?

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Observations of the neutral hydrogen (line
observations)
Distinguish between undisturbed and interacting
galaxies using the gas
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Example of HI observation
this will be the central frequency of your band
to be able to detected HI at z0.045

• The typical bandwidth of HI observation is 5, 10
  or 20 MHz
• 10MHz 1354.2 ? 1364.2 the range of
  velocities covered goes from
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  14665 to 12358 km/s
• for 10MHz 2300 km/s velocity range covered
• for 20MHz 4600 km/s velocity range covered
• Channel width 1
  MHz ? 200 km/s

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Kinematics of the galaxies
Case of an undisturbed galaxy rotating disk
H I observation (datacube) of NGC 4414
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A messy case
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The radio galaxy B2 025835