A Search for water masers in High-redshift unbeamed AGNs: T.Ghosh, S.Mathur (OSU), C.J. Salter
Motivation: Water maser in NGC 4258 -A Seyfert2 galaxy
Green – Optical
Blue – Radio
Red - X-ray
Distance = 7.2 Mpc
BH Mass = 3.78 x 107 Mo
Search & follow-up observations in external galaxies have now become key
projects for some existing (GBT, EFF, EVLA, VLBI) and upcoming telescopes (ASKA
& SKA) e.g. the Megamaser Cosmology Project -- Braatz et al., WMCP -- Greenhill et
al.
--Motivation- contd....
Detection Statistics: As of Jan, 2014, 22-GHz maser emission has been
detected in 162 galaxies, out of ~2700 searched (detection rate of ~6%).
Most of these are Seyfert-2 type galaxies.
Most are nearby objects within velocities ~10,000 km/s (z=0.03). Only
three “high-z” objects, at z=0.6, 2.2 & 2.4 were added recently, all in
Gravitational-lens systems.
Goal: We wanted to extend the search to redshift ranges of 1.22 – 1.78
and 2.71 – 4.5 for which the rest-frame transition frequency of 22-GHz
would fall within the pass-bands of Arecibo’s X- and C-band receivers.
For any detected objects, follow up observation would help in extending
direct exploration of AGN models and cosmological parameters to
higher z.
Conditions for 22-GHz Water maser emission
•
•
The transition is between the energy levels of 610 -- 523
These states lie ~645 K above the ground state,and the
kinetic temperature of the gas should be > 300K to
populate these. Collisional population
inversion requires densities >107 /cc.
Such conditions can occur close to
the AGN in obscuring molecular
torus.
•
Likely to be seen in Type2 AGNs
Sample Selection & Observation:
We selected sources satisfy one or more of the following criteria:
(1) Soft X-ray absorption,
(2) Narrow optical emission lines (these narrow line radio galaxies are the radioloud counterparts of Seyfert 2 galaxies),
(3) Steep radio spectrum,
(4) Compact edge-brightened (FRII) radio morphology (angular size < 30′′, so that
they are not heavily resolved by the Arecibo X-band beam).
Together, these criteria ensure that these radio AGNs are viewed roughly
edge-on. Also included were 3 other objects of special interest (NLRG, Grav.
Lens, & ERO), making 46 galaxies in our sample.
Our total-power position-switched observations used the InterimCorrelator,
recording 50-MHz bandwidth centered at the redshifted frequency of the 22GHz transition. Integration times varied due to scheduling constraints. The
brightness scale was established via the noise-diode calibration and the
telescope's standard Gain curves (K/Jy). Most of the spectra were RFI-free.
Results -1
No detected emission:
Results – 1 (Contd: with updated z)
Updated z for 11/46 galaxies implies that the line was outside the passband.
Result – 2: Absorption (?)
Jet-Torus Interaction?
Conclution & Future Work
• Poor detection rate may be mainly due to a
combination of poor sensitivity and water-maser
luminosity function.
• Need to extend the search down to a uniform
sensitivity limit over the z-range. The new wider-band
spectrometer – new C-band Rx, better source selection
via improved redshifts including gravitational lenses
should be useful.
• Absorption in B2209+152 will be verified (new
proposal awaiting review) – if true and truly variable
– imaging with VLBI will help study this possible jettorus interaction, a far off analog of NGC660 ?