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)

What is the Quasar Broad Line Region

(BLR)?

Vanden Berk+01

Richards+06

Spectral Energy Distribution

Optical-UV spectrum

10,000 km/s

Hα spectrum

Broad Hα

Narrow Hα

Narrow [NII]

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~109M⊙, 𝑟BLR~10

3𝑟g, 𝑧~0.2

→ 𝜃BLR~100𝜇as

…a factor of ~103 below 8m telescope diffraction limit

• Alternative #1: Reverberation Mapping (e.g. Peterson+1993, 2004, Bentz+2004)

emis

sivi

ty

𝑟 (pc)

10−3.5 10−3 10−2.5 0.01 0.03

What do we know from Reverberation Mapping?

1. Hβ response from a narrow annulus

2. 𝒓𝑩𝑳𝑹 ≈ 𝟎. 𝟎𝟏 𝑳𝟒𝟒𝟏𝟐 𝐩𝐜

1042 1044 1046

AGN Luminosity

H

β la

g (d

ays)

Bentz+13

collisional de-excitation

dust suppression

Baskin, Laor, and Stern (2014)

Hβ

Bentz+10

Explained by line emissivity function:

|IR (torus surface) Blackbody |

A New Method to Constrain the BLR: Spectroastrometry

Spectroastrometry: Measure photon centroid vs. wavelength

• Astrometric precision ≈PSF

𝑁photons1/2

(λ)

• BLR angular size of most luminous quasars:

• PSF(8m, with AO) ≈ 0.1"

→ ~𝟏𝟎𝟔 photons required

Systematics? Pontoppidan+11 achieved ~100𝜇as in YSOs

A Simplified Example: A Rotating Ring

slit Projected BLR ring

Slit spectral direction

Slit

sp

atia

l dir

ecti

on

Ph

oto

n f

lux

( m

−2hr−

1103 km s−1

−1 )

Velocity ( km s−1 )

C

entr

oid

off

set

( 𝜇as )

BLR Characteristics C

entr

oid

off

set

( 𝜇as )

Velocity ( km s−1 )

𝑣turbulent𝑣rotation

Cen

tro

id o

ffse

t ( 𝜇as )

𝑟 ≈ 𝑟BLR 𝑟 ≫ 𝑟BLR

Turbulence

r-distribution of line photons

Expected signal (𝑧 = 2)

Cen

tro

id o

ffse

t ( 𝜇as )

1. Narrow lines need to be masked

2. Offset detectable on an 8m!

Expected Signal vs. Redshift

Large symbols: 39m Small symbols: 8m

redshift

Reverberation Mapping: • Response-weighted function

of BLR geometry

• Requires variability → low 𝑳𝐀𝐆𝐍

• Small response time → low 𝑳𝐀𝐆𝐍 , low z

Spectroastrometry provides independent constraints on the BLR, mainly at high 𝑳𝑨𝑮𝑵

Spectroastrometry: • 𝒓-weighted function of

BLR geometry

• Large angular size → high 𝑳𝐀𝐆𝐍

• High photon count → high 𝑳𝐀𝐆𝐍

Spectro-astrometry vs. RM

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)