Radio Galaxies Part 3 Gas in Radio galaxies

Why gas in radio galaxies? Merger origin of radio galaxies. Evidence: mainly optical characteristics (tails, counter-rotating cores, dust lanes) 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

Why neutral hydrogen? Interaction & mergers are often invoked for the triggering of AGN providing both the gas and the instability to bring gas to the nuclear regions 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 between galaxies

Forming an elliptical galaxy from mergers

Hibbard (VLA) Kinematics of the interaction

Is there HI in early-type galaxies?  Some elliptical galaxies have HI content and size similar to spiral galaxies  Compare to the life of a radio source 3 arcmin~ 54 kpc (1”=0.3 kpc) orbital time ~ 2x10 9 yrs

The ground state can undergo a hyperfine transition, reverse the spin of the electron Frequency of the transition: MHz The temperature T s (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  T s equal to the kinetics temperature (with some exceptions!) 21-cm emission line of neutral hydrogen MHz proton electron

Most common element in the universe  present “everywhere”! Transparent narrow spectral line (van de Hulst) Doppler effect  kinematics! MHz proton electron

where F ~ S dV Jy km/s (1 Jy = W/m 2 /Hz) D distance in Mpc D ~ cz/H 0 Column density of HI, number of hydrogen atoms in the in a cylinder of unit cross-section (in the low optical depth limit) atoms/cm 2 where  is beam size (arcmin) dV km/s S mJy/beam  = optical depth To derive the mass of the neutral hydrogen

Doppler effect Frequency  V = 0V < 0V > 0 in emission and absorption!

HI cloud HI emission HI absorption

T spin accounts for the electrons that are in the upper state (i.e. those that do not absorb) Higher T spin more electrons in the upper state higher column density Particularly common in radio galaxies given the strong underlying radio continuum Optical depth Column density cm -2 From galactic studies, typical T spin = 100 K Typical column densities: in emission ~10 21 cm -2 in a disk of a spiral galaxy in absorption from cm -2 against the core of some radio galaxies HI detected in absorption

 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 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:  the range of velocities covered goes from to 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

H I observation (datacube) of NGC 4414 Kinematics of the galaxies Case of an undisturbed galaxy: rotating disk

A messy case

The radio galaxy B