VLBI observations of the water
megamaser in the nucleus of the
Compton-thick AGN IRAS15480-0344
Paola Castangia (INAF-OA Cagliari)
Collaborators:
Alessandro Caccianiga (INAF-OA Brera)
Paola Severgnini (INAF-OA Brera)
Andrea Tarchi (INAF-OA Cagliari)
Roberto Della Ceca (INAF-OA Brera)
Disk-masers
• PV diagram  θS,VR
• Velocity drift  RS
D = RS / θ S
DU3789 = 49.6±5.1 Mpc
MBH = (1.16±0.12) × 107MSUN
(Reid et al. 2013)
DN4258 = 7.2±0.5 Mpc
MBH = (3.9±0.3) × 107MSUN
(Herrnstein 1999)
(Reid et al. 2009)
Jet-masers
• Jet-cloud interaction
Mrk 348
(Peck et al. 2003)
•Continuum and line
flux densities are
correlated
•Reverberation maps
give:
Vshock, ρj, ρ0
Outflow-masers: Circinus
• H2O masers trace a warped edge-on accretion disk and
a wide-angle outflow
–Outflow l.o.s velocity:
± 160 km/s
–The warps collimate the
outflow
(Greenhill et al. 2003)
IRAS 15480-0344
An S0 galaxy at ~130 Mpc
WISE Multi-Colour image
The nucleus:
• Sy2 with broad Hα in pol. (weak)
(Young et al. 1996)
• Unresolved radio source (VLA
A, 8.4 GHz)
(Schmitt et al. 2001)
• Radio quiet (L5GHz/L2-10keV = -5)
10 kpc
(Terashima & Wilson 2003)
Compton-thick (NH > 1024 cm-2 ) AGN sample
• Selection method: FIR and X-ray flux and colours (Severgnini et al. 2012)
• Ongoing H2O maser survey
Single-dish observations
Line 2
GBT observations, April 7, 2012
• Narrow (FWHM < 1 km/s)
• Systemic (V=9106 km/s)
Vsys
Line 1
• Broad (FWHM ~ 90 km/s)
• Blueshifted (V=8960 km/s)
• LH2O ~ 200 Lsun
Channel spacing 0.3 km/s
CO
• LH2O ~ 15 Lsun
Single-dish observations
May 12, 2012
The broad line is variable:
Peak and int. flux density
decreased to ½ of their initial
values in ~1 month
The properties of the
narrow component
remained relatively
constant
April 7, 2012
Channel spacing
3.3 km/s
VLBI observations
Details of the observations
• Telescope: VLBA
• Dates: June 2 and 3, 2012 (2 x 5 hr)
• Bands: 2 x 16 MHz IFs
• Correlation: 128 ch per IF and pol.  ch spacing 1.7 km/s
“spectral zooming mode”  ch spacing 0.2 km/s
22 GHz maps
• Line low res. (rms = 2 mJy/beam)
• Line high res (rms = 5 mJy/beam)
• Continuum (from the line-free channels, rms = 0.1 mJy/beam)
VLBI observations
VLBA, 22 GHz
Tentative
Peak ~ 0.5 mJy
Slightly resolved
?
VLA, A-array, 8.4 GHz
(Schmitt et al. 2001)
Peak = 0.7 ± 0.1 mJy
A
TB > 3.6 x 106 K
P22 ~ 1.4 x 1021 W Hz1
0.5”
300 pc
6 pc
B
Tentative!
Peak ~ 12 mJy
VLBI observations
Peak ~ 42 mJy
Line parameters =
single-dish spectrum
Ch. 1.8 km/s
?
Ch. 0.2 km/s
Tentative!
Peak ~ 10 mJy
A
Ch. 1.8 km/s
3 pc
B
Investigating the obscured
nucleus of IRAS 1548-0344
Radio continuum emission
• TB of sources A and B suggest a non-thermal origin
• P22 > 1021 W Hz-1  unlikely to be SNRs
• P22 larger than average P5 in a sample of Sy (Panessa & Giroletti 2013)
• If the tentative source is real  10-20 pc linear structure
Origin of the maser?
• Absence of the triple-peak profile  jet/outflow maser
• Systemic and blueshifted emission separeted by 15-20 pc
 disk-maser unlikely (R ≤ 1 pc)
• Not associated with any compact continuum source at 22 GHz
Conclusions and future plans
Radio continuum emission: VLBA 22 GHz image show 2 (possibly
3) compact continuum sources in a linear structure of tenths of
parsecs  ejection processes in the nucleus of IRAS 15480-0344?
The water megamaser in IRAS 15480-0344 Systemic and
(tentative) blueshifted emission are separated by 15-20 pc
 jet/outflow origin still the most likely
Multifrequency (L and C bands) EVN observations have been approved
and await scheduling. These will be fundamental to establish the
nature of the radio continuum emission from the nucleus
Single dish monitoring of the water maser lines with Effelsberg will
help shedding light on the origin of the maser