Spatially Resolving the Kinematics of the ≲
100𝜇as Quasar Broad Line Region
using Spectro-astrometry
Jonathan Stern (MPIA)
Patzer Colloquium, Nov. 2015
with:
Joseph Hennawi (MPIA), Jörg-Uwe Pott (MPIA), Aaron Barth (UCI)
Spectral Energy Distribution
What is the Quasar
Broad Line Region
(BLR)?
Richards+06
Optical-UV spectrum
Vanden Berk+01
Hα spectrum
Narrow Hα
Narrow [NII]
Broad Hα
10,000 km/s
Why is the BLR interesting?
1. Part of the ∼ 103 𝑟g accretion flow
(e.g. Murray+1995, Czerny & Hryniewicz 2011)
2. 𝑀BH estimates, 𝑀BH demographics vs. 𝑧
(e.g. Vestergaad+2004, Trakhtenbrot+2011, Shen & Kelly 2012)
3. Measurement of gravitational redshifts
(Tremaine+14)
How can we observe the ≲ 100𝜇as BLR?
𝑀BH ~109 M⊙ ,
→ 𝜃BLR ~100𝜇as
diffraction limit
iffraction limit
beration Mapping
𝑟BLR ~103 𝑟g ,
𝑧~0.2
What do we know from Reverberation Mapping?
1.
Hβ response from a narrow annulus
Hβ lag (days)
2. 𝒓𝑩𝑳𝑹 ≈ 𝟎. 𝟎𝟏 𝑳𝟒𝟒
𝟏
𝟐
𝐩𝐜
Bentz+13
Bentz+10
Blackbody | Hβ|IR (torus surface)
1042
1044
1046
Explained by line emissivity function:
collisional
de-excitation
emissivity
AGN Luminosity
dust
suppression
10−3.5 10−3 10−2.5 0.01
𝑟 (pc)
Baskin, Laor, and Stern (2014)
0.03
A New Method to Constrain the BLR: Spectroastrometry
Spectroastrometry: Measure photon centroid vs. wavelength
≈
PSF
•
Astrometric precision
•
BLR angular size of most luminous quasars:
•
PSF(8m, with AO) ≈ 0.1"
1/2
𝑁photons (λ)
→ ~𝟏𝟎𝟔 photons required
Systematics? Pontoppidan+11 achieved ~100𝜇as in YSOs
( m hr −1 103 km s −1
Photon flux
−2
Projected
BLR ring
Centroid offset
( 𝜇as )
Slit spatial direction
slit
−1
)
A Simplified Example: A Rotating Ring
Slit spectral direction
Velocity ( km s −1 )
BLR Characteristics
Centroid offset ( 𝜇as )
𝑣turbulent
𝑣rotation
Turbulence
𝑟 ≈ 𝑟BLR
r-distribution of
line photons
Centroid offset ( 𝜇as )
Velocity ( km s−1 )
𝑟 ≫ 𝑟BLR
1. Narrow lines need
to be masked
2. Offset detectable
on an 8m!
Centroid offset ( 𝜇as )
Expected signal
(𝑧 = 2)
Expected Signal vs. Redshift
Large symbols: 39m
Small symbols: 8m
redshift
Spectro-astrometry vs. RM
Reverberation Mapping:
Spectroastrometry:
• Response-weighted function
of BLR geometry
• Requires variability
→ low 𝑳𝐀𝐆𝐍
• Small response time
→ low 𝑳𝐀𝐆𝐍 , low z
• 𝒓-weighted function of
BLR geometry
• Large angular size
→ high 𝑳𝐀𝐆𝐍
• High photon count
→ high 𝑳𝐀𝐆𝐍
Spectroastrometry provides independent constraints
on the BLR, mainly at high 𝑳𝑨𝑮𝑵
Proposal Status
1. Gemini 2015A: Submitted and awarded 2 nights with
LGS-AO, eventually not scheduled
2. VLT P95: Submitted and awarded 3 nights, weather
permitted only 1 hour of LGS-AO
3. Gemini 2016A: submitted
4. VLT P97: submitted
Summary
Spectro-astrometry is applicable to the BLR.
→ A novel method to constrain 𝑴𝐁𝐇 at high-𝑳 and high-𝒛
→ Feasible with 8m telescopes (proposals submitted)
→ 30m telescopes: high 𝑣-resolution, 𝑧~5 quasars, AGN sub-classes
→ Need to reduce systematics to ≲ 30𝜇as
(Pontoppidan+11: achieved ~100𝜇as in YSOs)