Dual Supermassive Black Holes as Tracers of Galaxy Mergers

Julie Comerford

NSF Astronomy and Astrophysics Postdoctoral Fellow University of Texas at Austin

Collaborators: Michael Cooper, Marc Davis, Mike Eracleous, Karl Gebhardt, Brian Gerke, Jenny Greene, Roger Griffith, Fiona Harrison, Greg Madejski, Kristin Madsen, Claire Max, Rosalie McGurk, Jeff Newman, Dave Pooley,

Dan Stern, Ben Weiner, Joan Wrobel

Dual Supermassive Black Holes Are the Smallest Separation Black Hole Pairs that

Can Be Confirmed (Currently)

“Binary Quasars”

“Dual SMBHs” or “Dual AGN” “Binary SMBHs”

There Should Be Many Dual AGN at Kpc-scale Separations

Can identify dual supermassive black holes observationally if they power dual AGN

Simulations of galaxy mergers show there should be lots of dual AGN visible at < 10 kpc separations

Van Wassenhove et al. 2011

We Expect Dual AGN in Merger-remnant Galaxies, Yet

Very Few Have Been Found

AGN AGN

Mrk 463 z = 0.05 !x = 4 kpc Bianchi et al. 2008

NGC 6240 z = 0.02 !x = 0.7 kpc Komossa et al. 2003

0402+379 z = 0.06 !x = 7 pc Rodriguez et al. 2006

A Large Observational Sample of Dual AGN Would Measure:

NASA

ACS/NASA

What is the galaxy merger rate?

How much do supermassive black holes grow in mass through gas accretion during galaxy mergers?

What is the supermassive black hole merger rate?

A Large Observational Sample of Dual AGN Would Measure:

Gültekin et al. 2009

What is the galaxy merger rate?

How much do supermassive black holes grow in mass through gas accretion during galaxy mergers?

What is the supermassive black hole merger rate?

A Large Observational Sample of Dual AGN Would Measure:

C. H

en

ze/N

ASA

What is the galaxy merger rate?

How much do supermassive black holes grow in mass through gas accretion during galaxy mergers?

What is the supermassive black hole merger rate?

Gravitational waves produced by supermassive black hole binaries…

…should be detected by pulsar timing arrays

NR

AO

Goal: Build a Large Catalog of Dual AGN to Use as Tracers of Galaxy Mergers, Black

Hole Growth, and Black Hole Mergers

Need systematic approach to advance from individual systems to assembling a large catalog of dual AGN

Begin by selecting candidate dual AGN in large spectroscopic surveys or imaging surveys of galaxies

Three Steps to Finding Dual AGN in Spectroscopic Surveys

1. Select Dual AGN Candidates as Galaxies with Double-peaked

AGN Emission Lines

[O III] H"

Smith et al. 2010 Liu et al. 2010 Wang et al. 2009

340 double-peaked AGN in SDSS

But, Double Peaks Can Also Be Produced by AGN Outflows

Mrk 78 at z=0.04 (Fischer et al. 2011)

[O III] #5007 [O III] #4959

Mrk 78 at z=0.04 (Fischer et al. 2011)

But, Double Peaks Can Also Be Produced by AGN Outflows

[O III] λ5007

Determining the Nature of Double-Peaked AGN Emission Lines

2. Obtain follow-up longslit spectroscopy to determine the spatial extent of the AGN emission and help determine the source of the emission

[O III] in AGN outflow:

Fischer et al. 2011

[O III] in dual AGN:

Comerford et al. 2011a

[O III] λ5007

Use Longslit Spectroscopy to Identify the Most Promising Dual AGN Candidates

Obtained Lick, Palomar, and MMT longslit spectroscopy for 81 of the 340 SDSS double-peaked AGN (0.03 < z < 0.69)

10”

Measure Physical Separation and Orientation of the Two Emission

Components on the Sky

x2

x1 ]

]

separation x1 at position angle θ1 separation x2 at position angle θ2

full separation on the sky Δx position angle on the sky θsky

!

x1 cos " sky #"2( ) = x2 cos " sky #"1( )

!

"x = x1 /cos # sky $#1( )

]!v

10” PA=170.5°

PA=80.5°

The Double Peaks Are Produced by Kpc-scale Dual AGN or Outflows

Comerford et al. 2011b

0.2 kpc < Δx < 5.5 kpc

Median Δx = 1.1 kpc

This means that the mechanism(s) producing double-peaked AGN are neither very small-scale nor very large-scale effects

! Rotation of gas in disks <100 pc, small-scale AGN outflows"

# kpc-scale dual AGN or kpc-scale AGN outflows

! AGN pairs or AGN outflows on scales ~10 kpc"

Spatial Extent of AGN Emission 58% exhibit spatially-compact emission components

42% exhibit spatially-extended emission components

Spatially-extended AGN Emission Reminiscent of AGN Outflows

AGN outflow in Mrk 78 (z=0.04):

AGN outflow in SDSS J1517+3353 (z=0.14):

Fischer et al. 2011 Rosario et al. 2010

[O III] λ5007

Use Orientation of Double AGN Emission Components within Host Galaxy to

Identify Promising Dual AGN Candidates

"sky

!"

Use Orientation of Double AGN Emission Components within Host Galaxy to

Identify Promising Dual AGN Candidates

Dual AGN expected to be aligned with host galaxy major axis (seen in simulations of dual AGN)

In contrast, AGN outflows have a range of orientations within host galaxy

Blecha et al. 2012

15 kpc

15 kpc

The Most Promising Candidates for Dual AGN

Good Candidates The 14 objects with Δθ consistent with zero, since we expect dual AGN in the plane of the host galaxy

Best Candidates The 3 objects with double AGN emission components with the same spatial separation and orientation on the sky as the double stellar nuclei seen in adaptive optics image (McGurk et al. 2011; Fu et al. 2011)

McGurk et al. 2011

Comerford et al. 2011b

Chandra observations upcoming!

Fu et al. 2011

3. For Definitive Proof of Dual AGN, Need X-ray or Radio Detections

X-ray

Upcoming Chandra observations for 13 dual AGN candidates

Radio

Jansky VLA proposals submitted for 20 dual AGN candidates

Hubble Space Telescope

Upcoming HST observations for 10 of the Chandra targets; will reveal double stellar nuclei, tidal features, stellar populations

Komossa et al. 2003

HST

X-ray

Rodriguez et al. 2006

Radio

Barrows et al. 2012

z=1.2

Proof of Concept: X-ray Confirmation of Dual AGN

SDSS J171544+600835 z=0.16 Δv=350 km/s, Δx=1.9 kpc (0.68”), PA=147° E of N

Double emission components in longslit observations coincide with double X-ray sources in Chandra observations

Comerford et al. 2011a

Chandra/ACIS

Progress towards Building a Large Catalog of Dual Supermassive Black Holes

2008: 3 known dual supermassive black holes, discovered serendipitously Systematic search begins

2012: 40 known dual supermassive black holes, most discovered through systematic searches Observers: Comerford et al. 2009ab, 2011ab; Wang et al. 2009; Smith et al. 2010; Liu et al. 2010ab; Shen et al. 2011; McGurk et al. 2011; Fu et al. 2011ab; Fu et al. 2012; Rosario et al. 2011; Tingay & Wayth 2011;

Fabbiano et al. 2011; Barrows et al. 2012; Koss et al. 2011, 2012 Theory and simulations: Shen & Loeb 2010; Yu et al. 2011; Van Wassenhove et al. 2011; Blecha et al. 2012

2013 - 2014: >100 known dual supermassive black holes, given the dozens of strong candidates in the queue for confirmation

A Systematic Search for Dual AGN 1. Identify dual AGN candidates as double-peaked AGN in spectroscopic surveys (e.g., SDSS, DEEP2)

2. Obtain follow-up observations (e.g., longslit spectroscopy, imaging) to identify most promising dual AGN candidates

3. Obtain follow-up X-ray/radio observations to confirm or refute dual AGN nature (13 candidates approved for Chandra, 10 candidates approved for HST, 20 candidates submitted to Jansky VLA)

Use large catalog of dual AGN to probe galaxy merger rate, supermassive black hole growth, and supermassive black hole merger rate

Result from DEEP2 dual SMBHs: ~3 mergers/Gyr for red galaxies at 0.3 < z < 0.8 (Comerford et al. 2009a)

SDSS DEEP2

HST