12. Megamasers

41 Pages·2015·7.11 MB·English

Rolf Kudritzki SS 2015 12. Megamasers
Maser sources in the ISM of galaxies

amplification of ISM molecular line radiation at microwave and radio
wavelengths

classical maser lines

OH at λ = 18cm
H O at λ = 1.35cm
2

Milky Way:

- masers in immediate neighborhood of young stellar objects or
circumstellar envelopes of evolved stars

- isotropic luminosities up to 1 L (integrated over all lines) or
sun
up to 0.1 L in one line
sun
(example star formation region W49N)

1
Rolf Kudritzki SS 2015
External Galaxies:

- H O maser
2
in high density molecular gas located in circumnuclear disks
within parsecs of AGN cores (Seyfert2, LINER, spirals, ellipticals)

- OH maser
within 100 pc of nuclear starburst regions

- isotropic luminosities ~ 102 - 104 L à 106 brighter than MW maser
sun
à Megamaser

- archetype megamaser galaxy is NGC 4258 at 7.6 Mpc

- most distant megamaser detected at red shift z = 0.66
(Barvainis & Antonucci, 2005, ApJ 628, L89)

Megamaser are potential distance indicators in the Hubble flow

key publications: K.Y. Lo, 2005, Annu.Rev.Astron.Astrophys. 43, 625
Herrnstein, J.R. et al., 1999, Nature 400, 539
Humphreys, E.M.L. et al., 2013, ApJ 775, 13
2
Rolf Kudritzki SS 2015
Maser mechanism
most likely process:
- collisional excitation from below (symbolic level a)
to upper (symbolic) level b

- spontaneous transitions from b downwards
overpopulate level u relative to level l
-  level inversion
à line absorption coefficient negative
h⌫ g
l
 = �(⌫)B n n
⌫ lu l u < 0 à line emission exponentially amplified
4⇡ � g
u
✓ ◆
by stimulated emission
L
0 0  dL
I I = I e ⌫
⌫ � 0
⌫ ⌫
R
note: exponent is positive
exponential amplification
3
© Fred Lo, 2011
Rotational Transitions of H O
2
(An Asymmetric Top molecule)
J
K+K–
E(6 )/k = 640 K
16
6 -5 :
16 23
λ = 1.35 cm
ν = 22.235 GHz
NGC 4258 7.6 Mpc
4
“ ”
1Maser Galaxy
0
distance accurate to 3%
2
Humphreys et al., 2013

P
P
A
I
New anchor galaxy of
M
extragalactic distance scale
using PLR of Cepheids
Riess et al., 2011
Rolf Kudritzki SS 2015
NGC 4258 – observations

1. spectrum at 1.35 cm of the central region
•  strong H O emissions lines
2
•  three radial velocity systems
v ~ -470 km/s (systemic velocity)
rad
and
v ~ -470 ± 1000 km/s (blue and red shifted)
rad

•  for each systems many different components
•  obvious interpretation: we look almost edge on at a circumnuclear disk
which is rotating with ~1000 km/s
6
RHolufm Kpuhdrrietyzksi, S2S0 1240, 1M5 IAPP WS
NGC 4258 – spectrum at 1.35 cm
Strong nuclear water maser at 22 GHz
v &±&1000&kms31&
sys
MIAPP,&28&May&2014& 7
GBT&(Modjaz&et&al.&2005)&&
RHolufm Kpuhdrrietyzksi, S2S0 1240, 1M5 IAPP WS
Map structure of AGN disks
2. VLBA imaging and spectroscopy of NGC 4258
< 1 pc from SMBH
VLBA: Angular resolution = 200 µas (0.006 pc at ~ 7.0 Mpc)
Spectral resolution < 1 kms-1
MIAPP,&28&M ay&2014& 8
Rolf Kudritzki SS 2015
2. VLBA imaging and spectroscopy
•  individual masers resolved and v assigned to individual components
rad
•  objects in front of AGN have v ~ -470 km/s (systemic velocity)
rad
the inclination angle relative to AGN is i ~ 82°

•  objects at the edges have v ~ -470 ± 1000 km/s (blue and red
rad
shifted)

•  a rotating Keplerian disk provides a perfect fit to each individually
observed components of the three radial velocity systems with an
accuracy better than 1%
9
RHolufm Kpuhdrrietyzksi, S2S0 1240, 1M5 IAPP WS
This Dataset: 18 epochs
1&mas&@&7.2&Mpc&!&0.04&pc&
•  Insert the time seried
1220&km&s31& 1650&km&s31& 3525&km&s31& 3280&km&s31&
Argon, Greenhill, Reid, Moran & Humphreys (2007)
VLBA + VLA +EFLS
MIAPP,&28&May&2014& 10