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12. Megamasers

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12. Megamasers 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 H2O at λ = 1.35cm Milky Way: - masers in immediate neighborhood of young stellar objects or circumstellar envelopes of evolved stars - isotropic luminosities up to 1 Lsun (integrated over all lines) or up to 0.1 Lsun in one line (example star formation region W49N) 1 Rolf Kudritzki SS 2015 External Galaxies: - H2O maser 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 2 4 6 - isotropic luminosities ~ 10 - 10 Lsun à 10 brighter than MW maser 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⌫ gl ⌫ = φ(⌫)Blu nl nu à 4⇡ − gu < 0 line emission exponentially amplified ✓ ◆ 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 H2O (An Asymmetric Top molecule) J K+K– E(616)/k = 640 K 616-523: λ = 1.35 cm ν = 22.235 GHz “Maser Galaxy” NGC 4258 7.6 Mpc distance accurate to 3% Humphreys et al., 2013 New anchor galaxy of extragalacticMIAPP 2014 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 H2O emissions lines • three radial velocity systems vrad ~ -470 km/s (systemic velocity) and vrad ~ -470 ± 1000 km/s (blue and red shifted) • 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 RolfHumphreys, Kudritzki SS2014, 2015 MIAPP WS NGC 4258 – spectrum at 1.35 cm Strong nuclear water maser at 22 GHz 31& vsys&±&1000&kms MIAPP,&28&May&2014& GBT&(Modjaz&et&al.&2005)&&7 RolfHumphreys, Kudritzki SS2014, 2015 MIAPP WS 2. VLBA Mapimaging structure and spectroscopy of AGN of disks 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&May&2014& 8 Rolf Kudritzki SS 2015 2. VLBA imaging and spectroscopy • individual masers resolved and vrad assigned to individual components • objects in front of AGN have vrad ~ -470 km/s (systemic velocity) the inclination angle relative to AGN is i ~ 82° • objects at the edges have vrad ~ -470 ± 1000 km/s (blue and red 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 RolfHumphreys, Kudritzki SS2014, 2015 MIAPP 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 RolfHumphreys, Kudritzki SS2014, 2015 MIAPP WS Red-shifted emission MIAPP,&28&May&2014& 11 Argon et al. (2007) RolfHumphreys, Kudritzki SS2014, 2015 MIAPP WS Systemic Maser Emission MIAPP,&28&May&2014& Argon&et&al.&(2007)& 12 RolfHumphreys, Kudritzki SS2014, 2015 MIAPP WS Blue-shifted Emission MIAPP,&28&May&2014& Argon&et&al.&(2007)& 13 Rolf Kudritzki SS 2015 Masers probe sub-pc portion of nuclear disk 0.15&pc& 40&000&Rsch& Almost perfect Keplerian rotation Miyoshi&et&al.&(1995)& to ~ 1% MIAPP,&28&May&2014& 14 Herrnstein&et&al.&(1999)& Rolf Kudritzki SS 2015 more detailed discussion of model will follow later 15 Rolf Kudritzki SS 2015 3. Time dependence and observed rotational motion • the individual components of the vrad ~ -470 km/s system in front of the AGN show a common change with time in vrad and position dvrad • average accelleration = a =9 . 3 km/s/yr dt d⇥ • average change of impact parameter position = 31 . 5 mas/yr dt • the ± 1000 km/s components at the edges do not show such a drift • this can be perfectly understood as the result of Keplerian rotation 16 Rolf Kudritzki SS 2015 dv rad = a =9.3 vrad drift dt km/s/yr Herrnstein et al., 1999 17 Rolf Kudritzki SS 2015 d⇥ position drift = 31 . 5 mas/yr dt 18 Rolf Kudritzki SS 2015 4. A simple model thin rotating Keplerian disk - AGN with mass M at center - maser at angle φ, radius R - projected angular distance of maser from center (impact parameter) is Θ - D is distance of galaxy to observer - projected linear distance of maser from the center is D⇥ cos(φ)= D⇥ = Rcos(φ) R 1 M 2 v = 2G rot R Keplerian velocity ✓ ◆ observed radial velocity vrad = cos(φ)vrot 19 Rolf Kudritzki SS 2015 5. Likelihood to detect masers at φ = 0° and 90° for strong maser amplification one needs a long light path along which Doppler shifts of photons through differential rotational velocity are dl smaller than a thermal line width ∆v ∆v ∆⌫ = ⌫ rad ∆⌫ = ⌫ th 0 c th 0 c or ∆vrad vth 1km/s 0 ∆lc dl ⇡ I = I e 0 ⌫ ⌫ ⌫ − The longest coherence lengths R ∆lc are obtained for φ = 90° (no Doppler shifts) and φ = 0° (smallest gradient of vrad along light path) 20 Rolf Kudritzki SS 2015 calculation of maser coherence length ∆lc 1 M 2 vrad = cos(φ)vrot v = 2G rot R ✓ ◆ dv dv dcos(φ) rad = cos(φ) rot + v dl dl rot dl dv 1 1 dR rot = v dl −2 rot R dl dcos(φ) dφ = sin(φ) dl − dl dv dφ 1 dR | rad| = v sin(φ)( )+ cos(φ) dl rot dl 2R dl (1) ✓ ◆ 21 Rolf Kudritzki SS 2015 (R + dR)2 =(Rcos(φ))2 +(Rsin(φ)+dl)2 2RdR =2Rsin(φ)dl + dl2 dl dR dl sin(φ)+ ⇡ 2R (2) ✓ ◆ applying the law of sines and noting that the angle between vrot and vrad is φ sin(dφ) sin( ⇡ + φ) cos(φ) = 2 = dl R + dr R + dr cos(φ) dφ dl (3) ⇡ R combining (2) and (3) with (1) we obtain… 22 Rolf Kudritzki SS 2015 dvrad 1 3 dl | | = v cos(φ) sin(φ)+ or dl R rot 2 4R ✓ ◆ 2 ∆vrad 3 4R = ∆lccos(φ) 4R sin(φ)+∆lc (4) v 2 rot ✓ ◆ 1 2 for the tangential angle φ = 0° we obtain from (4) ∆vrad ∆l0 =2R and we then re-write (4) as v ✓ rot ◆ 2 1 ∆l v 2 ∆l 1 c +3 rot sin(φ) c =0 ∆l ∆v ∆l − cos(φ) ✓ 0 ◆ ✓ rad ◆ 0 which has the solution 1 ∆l 3 v 2 9 v 1 c = rot sin(φ)+ rot sin2(φ)+ ∆l −2 ∆v 4 ∆v cos(φ) (6) 0 ✓ rad ◆ s rad 23 Rolf Kudritzki SS 2015 with vrot ≈ 1000 km/s and Δvrad ≈ 1 km/s 1 2 ∆lc 3 vrot 9 vrot 2 1 = sin(φ)+ sin (φ)+ ∆l −2 ∆v 4 ∆v cos(φ) 0 ✓ rad ◆ s rad the solution varies dramatically as soon as sin(φ) ≠ 0, as shown in the figure on the next page. ∆lc is different from zero only at φ = 0° and 90°. ∆l0 For vrot ≈ 1000 km/s and Δvrad ≈ 1 km/s we have ∆ l 0 =0 . 06 R . For R = 0.2 pc this corresponds to 3.7 1016 cm. This explains why we see masers mostly at φ = 0° and 90°. 24 Rolf Kudritzki SS 2015 ∆lc ∆l0 25 Rolf Kudritzki SS 2015 6. Discussion of the model and distance 1 M 2 D⇥ v = 2G cos(φ)= rot R R ✓ ◆ vrad = cos(φ)vrot in front of AGN with φ ≈ 90° we see the systemic components which have slightly different cos φ according to their different Θ. D à v = v ⇥ linear behaviour in vrad vs impact parameter figure. rad R rot obviously, the components are at roughly the same radius. D The average slope b can be measured from this b = vrot R (7) figure 26 RolfHumphreys, Kudritzki SS2014, 2015 MIAPP WS Masers probe sub-pc portion of nuclear disk 0.15&pc& 40&000&Rsch& Almost perfect Keplerian rotation Miyoshi&et&al.&(1995)& to ~ 1% MIAPP,&28&May&2014& 27 Herrnstein&et&al.&(1999)& Rolf Kudritzki SS 2015 for these components we can also use the observed drifts in vrad and Θ dv d dφ rad = a = v cos(φ)= v sin(φ) dt rot dt − rot dt the sin(φ) factor explains why no drift at φ = 0°. vrot For φ ~ 90°, sin(φ) ~ 1 and with φ(t)=!t ! = we obtain 2 R vrot a = R (8) R the spatial drift is the change of ⇥ = cos ( φ ) with time D d⇥ R dφ R vrot = sin ( φ ) = sin ( φ ) and for φ ~ 90°, sin(φ) ~ 1 dt −D dt −D R d⇥ vrot = (9). The measurements (7), (8), (9) already dt D yield R, D, vrot from the systemic components. 28 Rolf Kudritzki SS 2015 In addition, we can use the vrad shifted components at φ ~ 0° and 180° .
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