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Register a new userVarstrometry for Off-nucleus and Dual sub-Kpc AGN (VODKA): Methodology and Initial Results with Gaia DR2
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Gaias milli-arcsec (mas) astrometric precision allows systematic identification of optically-selected sub-kpc dual active galactic nuclei (AGN), off-nucleus AGN, and small-scale lensed quasars by `varstrometry -- where variability-induced astrometric jitter, i.e., temporal displacements of photocenter in unresolved sources, can be reasonably well detected or constrained. This approach extends systematic searches for small-scale ($gtrsim$ mas) dual and off-nucleus AGN to poorly explored regime between $sim 10$ pc and $sim 1$ kpc, with Gaias full sky coverage and depth to $Gsim 21$. We outline the general principles of this method and calculate the expected astrometric signals from the full time series of photocenter measurements and light curves. We demonstrate the feasibility of varstrometry by using Gaia DR2 data on a sample of variable pre-main sequence stars with known close companions. We find that extended host galaxies have a significant impact on the accuracy of astrometric and photometric variability in Gaia DR2, a situation to be improved in future Gaia releases. Using spectroscopically confirmed SDSS quasars, we present several examples of candidate sub-kpc off-nucleus or dual AGN selected from Gaia DR2. We discuss the merits and limitations of this method and follow-up strategy for promising candidates. We highlight Gaias potential of systematically discovering and characterizing the sub-kpc off-nucleus and dual AGN population in the entire optical sky.
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Off-nucleus active galactic nuclei (AGN) can be signposts of inspiraling supermassive black holes (SMBHs) on galactic scales, or accreting SMBHs recoiling after the coalescence of a SMBH binary or slingshot from three-body interactions. Because of the stochastic variability of AGN, the measured photocenter of an unresolved AGN-host system will display astrometric jitter that depends on the off-nucleus distance of the AGN, the total photometric variability of the system, and the AGN-host contrast. Here we use the precision astrometry from Gaia DR2 to constrain the off-nucleus population of a low-redshift (0.3<z<0.8) sample of unobscured broad-line AGN drawn from the SDSS with significant host contribution and photometric variability. We find that Gaia DR2 already provides strong constraints on the projected off-nucleus distance in the sub-kpc regime at these redshifts: 99%, 90% and 40% of AGN must be well-centered to <1 kpc, <500 pc and <100 pc, respectively. Limiting the sample to the most variable subset constrains >99% of AGN to be well-centered below 500 parsec. These results suggest that genuine off-nucleus AGN (offset by > a few hundred pc) must be rare at low redshift. Future Gaia releases of time series of photocenter and flux measurements, improved treatments for extended sources and longer baselines will further tighten these constraints, and enable a systematic full-sky search for rare off-nucleus AGN on ~ 10-1000 pc scales.
Dual supermassive black holes (SMBHs) at $sim$kpc scales are the progenitor population of SMBH mergers and play an important role in understanding the pairing and dynamical evolution of massive black holes in galaxy mergers. Because of the stringent resolution requirement and the apparent rareness of these small-separation pairs, there are scarce observational constraints on this population, with few confirmed dual SMBHs at $<10$kpc separations at $z>1$. Here we present results from a pilot search for kpc-scale dual quasars selected with Gaia Data release 2 (DR2) astrometry and followed up with Hubble Space Telescope (HST) Wide Field Camera 3 dual-band (F475W and F814W) snapshot imaging. Our targets are quasars primarily selected with the varstrometry technique, i.e., light centroid jitter caused by asynchronous variability from both members in an unresolved quasar pair, supplemented by sub-arcsec pairs already resolved by Gaia DR2. We find an overall high fraction of HST-resolved pairs among the varstrometry-selected quasars (unresolved in Gaia DR2), $sim 30-50%$, increasing toward high redshift ($sim 60-80%$ at $z>1.5$). We discuss the nature of the 43 resolved sub-arcsec pairs based on HST and supplementary data. A substantial fraction ($sim 40%$) of these pairs are likely physical quasar pairs or gravitationally lensed quasars. We also discover a triple quasar candidate and a quadruply lensed quasar, which is among the smallest-separation quadruple lenses. These results provide important guidelines to improve varstrometry selection and follow-up confirmation of $sim$kpc-scale dual SMBHs at high redshift.
Targeted searches for dual active galactic nuclei (AGN), with separations 1 -- 10 kpc, have yielded relatively few successes. A recent pilot survey by Satyapal et al. has demonstrated that mid-infrared (mid-IR) pre-selection has the potential to significantly improve the success rate for dual AGN confirmation in late stage galaxy mergers. In this paper, we combine mid-IR selection with spatially resolved optical AGN diagnostics from the Mapping Nearby Galaxies at Apache Point Observatory (MaNGA) survey to identify a candidate dual AGN in the late stage major galaxy merger SDSS J140737.17+442856.2 at z=0.143. The nature of the dual AGN is confirmed with Chandra X-ray observations that identify two hard X-ray point sources with intrinsic (corrected for absorption) 2-10 keV luminosities of 4*10^41 and 3.5*10^43 erg/s separated by 8.3 kpc. The neutral hydrogen absorption (~10^22 cm^-2) towards the two AGN is lower than in duals selected solely on their mid-IR colours, indicating that strategies that combine optical and mid-IR diagnostics may complement techniques that identify the highly obscured dual phase, such as at high X-ray energies or mid-IR only.
The Gaia mission has opened a new window into the internal kinematics of young star clusters at the sub-km/s level, with implications for our understanding of how star clusters form and evolve. We use a sample of 28 clusters and associations with ages from 1-5 Myr, where lists of members are available from previous X-ray, optical, and infrared studies. Proper motions from Gaia DR2 reveals that at least 75% of these systems are expanding; however, rotation is only detected in one system. Typical expansion velocities are on the order of ~0.5 km/s, and, in several systems, there is a positive radial gradient in expansion velocity. Systems that are still embedded in molecular clouds are less likely to be expanding than those that are partially or fully revealed. One-dimensional velocity dispersions, which range from 1 to 3 km/s, imply that most of the stellar systems in our sample are supervirial and that some are unbound. In star-forming regions that contain multiple clusters or subclusters, we find no evidence that these groups are coalescing, implying that hierarchical cluster assembly, if it occurs, must happen rapidly during the embedded stage.
We make use of APOGEE and $Gaia$ data to identify stars that are consistent with being born in the same association or star cluster as the Sun. We limit our analysis to stars that match solar abundances within their uncertainties, as they could have formed from the same Giant Molecular Cloud (GMC) as the Sun. We constrain the range of orbital actions that solar siblings can have with a suite of simulations of solar birth clusters evolved in static and time-dependent tidal fields. The static components of each galaxy model are the bulge, disk, and halo, while the various time-dependent components include a bar, spiral arms, and GMCs. In galaxy models without GMCs, simulated solar siblings all have $J_R < 122$ km $rm s^{-1}$ kpc, $990 < L_z < 1986$ km $rm s^{-1}$ kpc, and $0.15 < J_z < 0.58$ km $rm s^{-1}$ kpc. Given the actions of stars in APOGEE and $Gaia$, we find 104 stars that fall within this range. One candidate in particular, Solar Sibling 1, has both chemistry and actions similar enough to the solar values that strong interactions with the bar or spiral arms are not required for it to be dynamically associated with the Sun. Adding GMCs to the potential can eject solar siblings out of the plane of the disk and increase their $J_z$, resulting in a final candidate list of 296 stars. The entire suite of simulations indicate that solar siblings should have $J_R < 122$ km $rm s^{-1}$ kpc, $353 < L_z < 2110$ km $rm s^{-1}$ kpc, and $J_z < 0.8$ km $rm s^{-1}$ kpc. Given these criteria, it is most likely that the association or cluster that the Sun was born in has reached dissolution and is not the commonly cited open cluster M67.
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