1. 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)
I finished my PhD last year at the Technion in Israel, and about a year ago I started a post-doc at the MPIA.
This is a project I’ve been working on with Joe Hennawi and JU from the MPIA, and also with Aaron Barth from Irvine.
So the point is to use spectro-astrometry, which you already heard about in this conference, to constrain the BLR

2. What is the Quasar Broad Line Region (BLR)?
Richards+06
Spectral Energy Distribution
What is the Quasar Broad Line Region (BLR)?
Optical-UV spectrum
Vanden Berk+01
This is my sketch of a quasar.
It is not the standard sketch people usually use in quasar talks, but I think this sketch is more accurate, sorry I think it is less, less accurate than the standard sketch, so I’ll use this one.
We have a very small central source which emits ionizing photons
The ionizing photons travel freely until they reach a cloud of gas.
The clouds absorb the photons and reprocess the energy to emission lines and IR emission from the dust grains, and we see the reprocessed emission.
Now what’s special about quasars is that the absorbing clouds span a very large dynamical range in scales.
From clouds at sub-pc scales which emit broad lines, through 10pc scale clouds which emit mainly in the IR known as torus clouds, up to clouds at kpc scales which emit the narrow lines.
Note that I assume a continuous distribution of clouds vs distance, and I’m not making any strong claims on the 3D geometry of these clouds.
So the emission we see in quasars is a sum of direct emission from the central source and reflected emission from the clouds.
Let’s see how this looks like
<click> this is the SED of a RQQ, the bump at 1000A is the direct emission from the central source, and the IR bump is due to reprocessing by dust in the torus clouds and the NLR clouds
<click> If we zoom in on the optical-UV spectrum, we see that on top of continuum from the central source there are broad emission lines.
<if we zoom in on an emission line, say Ha, we see the lines is made of (at least) two distinct kinematical components, the strong BL, with width of 10,000 kms, and on top of it a narrow Ha, with width of 500 km/s
So the BL comes from the BLR, which is the focus of this talk
While the NL comes from the much more distant NLR,
What is the quasar broad line region?
This is a SED of a radio quiet quasar
This is an optical spectrum of an AGN, and you see that on top of the continuum emission these strong and wide emission lines.
So this is a zoom in on a broad H-a of a local quasar, you can see the Ha line is very broad, and on top of it some narrow lines with widths of 300 km/s.
The broad lines suggest the BLR is very close to the BH.
10,000 km/s
Hα spectrum
Broad Hα
Narrow Hα
Narrow [NII]

3. Why is the BLR interesting?
Part of the ∼10 3 𝑟 g accretion flow (e.g. Murray+1995, Czerny & Hryniewicz 2011)
𝑀 BH estimates, 𝑀 BH demographics vs. 𝑧 (e.g. Vestergaad+2004, Trakhtenbrot+2011, Shen & Kelly 2012)
Measurement of gravitational redshifts (Tremaine+14)
** add pictures
Why do we want to understand the BLR?
First, the BLR is one of the most popular ways to estimate M_BH. Since the BLR is deep within the gravitation potential of the BH, then if we assume the BLR velocities are virial, and we have an estimate of the distance to the BLR, then we can estimate the M_BH. Now, since quasars are so bright, we can then constrain the demographics of M_BH across cosmic history. In this study, for example, they plot the most massive M_BH as a function of redshift.
Also, since the blr is so close to the BH, it is likely part of the accretion flow, and therefore constraining the BLR geometry and kinematics can teach us about the physics of accretion
For example, this is work by Norm Murray which picture the BLR as some wind launched from the accretion disk
5. And recently, Scott Tremaine claimed that the BLR can also be used to measure gravitational redshifts.

4. How can we observe the ≲100𝜇as BLR?
𝑀 BH ~ M ⊙ , 𝑟 BLR ~ 𝑟 g , 𝑧~0.2 → 𝜃 BLR ~100𝜇as
…a factor of ~10 3 below 8m telescope diffraction limit

5. What do we know from Reverberation Mapping?
Hβ response from a narrow annulus
𝒓 𝑩𝑳𝑹 ≈𝟎.𝟎𝟏 𝑳 𝟒𝟒 𝟏 𝟐 𝐩𝐜
AGN Luminosity
Hβ lag (days)
Bentz+13
Bentz+10
Blackbody | Hβ
|IR (torus surface)
emissivity
𝑟 (pc)
10 − − −
** specify RM is at low luminosity
** add picture of line emissivity vs. r
Now that we understood what is the BLR, and why it is interesting, let’s see what we already know about it.
So currently, the main method to study the BLR is using reverberation mapping.
Reverberation mapping means that the continuum emission varies, and after some characteristic delay the broad line emission also varies
If we divide this lag by the speed of light, we get the distance to the BLR, r_BLR
Now we know two main things from reverberation studies
That in a single object the BLR comes from a small dynamical range in r_BLR, (maybe a factor of 5)
And that r_BLR scales as the sqrt of the AGN luminosity.
<click> And we also believe that we know the physical reason for these properties.
These two properties are explained with the line emissivity function.
This is the H-beta line emissivity per unit covering factor of the gas
You see the line emission peaks at a very narrow range in r, around the value observed in RM studies.
The drop at higher r is because once we pass the dust sublimation radius, dust dominates the opacity to ionizing photons and therefore the emission from the gas drops.
So dust suppression sets the outer limit of the BLR
At smaller r the gas densities are so high that the lines are collisionally de-excited.
So collosional de-excitation sets the inner limit of the BLR
dust suppression
Explained by line emissivity function:
collisional de-excitation
Baskin, Laor, and Stern (2014)

6. A New Method to Constrain the BLR: Spectroastrometry
Spectroastrometry: Measure photon centroid vs. wavelength
Astrometric precision ≈ PSF 𝑁 photons 1/2 (λ)
BLR angular size of most luminous quasars:
PSF(8m, with AO) ≈0.1"
→ ~𝟏𝟎 𝟔 photons required
Systematics? Pontoppidan+11 achieved ~100𝜇as in YSOs
So enough with the introduction
We’re proposing a new method to constrain the BLR, using spectroastrometry
The point of spectroastrometry is that you can measure the centroid of photons from an object to a precision which increases as the sqrt of the number of photons.
Now if the BLR flow is ordered, then you expect the redshifted photons to be offset from the blueshifted photons by the scale of the system
And if you extrapolate the r_BLR vs L relation derived from RM studies to the most luminous quasars in the sky, you get that the largest angular sizes are roughly 100muas.
So with a million photons, we should be able to detect the offset between the red and blue BL photons

7. A Simplified Example: A Rotating Ring
slit
Projected BLR ring
( m −2 hr − km s −1 −1 )
Photon flux
Slit spatial direction
So for example, lets assume the BLR comes from a ring of gas which is rotating
This is the ring of gas which is projected on the sky, so it looks like an ellipse
The gas is rotating counter-clockwise so the red wing comes from here, and the blue wing comes from here
Now we put a slit on this object. This is the spatial direction of the slit, and this is the velocity direction of the slit.
So you see that the red wing is offset in the spatial direction from the blue wing.
This is the spectral profile of the broad line in our simple picture
And is this is the spectroastrometric signal, the blue wing is offset to one side by a 100 muas and the red wing is offset to the other side by 100muas.
So What we are trying to detect is the difference between the centroid position of the blue and red wings
Centroid offset ( 𝜇as )
Slit spectral direction
Velocity ( km s −1 )

8. BLR Characteristics Turbulence r-distribution of line photons
𝑣 turbulent 𝑣 rotation
Turbulence
Centroid offset ( 𝜇as )
𝑟≫ 𝑟 BLR
𝑟≈𝑟 BLR
Velocity ( km s −1 )
Broad line profiles don’t look like expected from a pure Keplerian motion.
If you add turbluence, you get a more realistic profile.
In turbulence I mean some profile-broadening mechanism at each location in the ring
The turbulence weakens the signal, but as long as the turbulent velocity is not larger than the rotation velocity, then we still get a signal
r-distribution of line photons
Centroid offset ( 𝜇as )

9. Expected signal (𝑧=2) Narrow lines need to be masked
Offset detectable on an 8m!
Centroid offset ( 𝜇as )
What is the expected signal?
This is the spectrum
This is the theoretical signal
You can see that the location of the NLs because they come from much larger scales, they entirely dominate the signal, so these wavelengths need to be masked out
This is a simulated observation with an AO-assisted 8m telescope.
The red and the blue wings are clearly offset
This is the signal with a 40m telescope.

10. Expected Signal vs. Redshift
This is the expected signal vs. z
These are the expected angular sizes of the most luminous quasars at each z. again, we derive the angular size by extrapolating the r vs L relation found in RM studies to high L.
You can see that the largest angular size only weakly depends on z, with a maximum of 130muas at z~3, but you can observe angular sizes above 50muas from redshift 0 to redshift 6.
And these are expected S/N for the offset of the red and blue wing, with an 8m and a 39m.
We can detect the offset at z up to 2-3 with an 8m, and up to 5 with a 40m
Large symbols: 39m
Small symbols: 8m
redshift

11. Spectro-astrometry vs. RM
Reverberation Mapping:
Response-weighted function of BLR geometry
Requires variability → low 𝑳 𝐀𝐆𝐍
Small response time → low 𝑳 𝐀𝐆𝐍 , low z
Spectroastrometry:
𝒓-weighted function of BLR geometry
Large angular size → high 𝑳 𝐀𝐆𝐍
High photon count → high 𝑳 𝐀𝐆𝐍
Spectroastrometry provides independent constraints on the BLR, mainly at high 𝑳 𝑨𝑮𝑵

12. Proposal Status
Gemini 2015A: Submitted and awarded 2 nights with LGS-AO, eventually not scheduled
VLT P95: Submitted and awarded 3 nights, weather permitted only 1 hour of LGS-AO
Gemini 2016A: submitted
VLT P97: submitted

13. Summary Spectro-astrometry is applicable to the BLR.
A novel method to constrain 𝑴 𝐁𝐇 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)