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Gaia as an AGN discovery machine

Lovro Palaversa Ruđer Bošković Institute lp@irb.hr	14th Serbian Conference on Spectral Line Shapes in Astrophysics Bajina Bašta, Serbia 22 Jun 2023

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The Gaia space mission, is one of the most ambitious and spectacular projects in astronomy from the last 20 years. The observations carried out by this satellite have radically transformed the base of knowledge upon which the astronomy community relies to explore and understand the Milky Way as well as the extragalactic Universe. The primary target of Gaia is the stellar content of the Milky Way but the satellite also observes extragalactic sources like quasars and galaxies. In this way, Gaia provides the first opportunity to analyse extragalactic sources from space, with the instrument reaching an exquisite spatial resolution of ∼180 mas (by the end of the mission), and to produce the first all-sky and space-based catalogues of extragalactic objects and of their properties in optical wavelengths. Gaia allows us to gain insight into a population of the local Universe that is mostly unresolved by ground-based facilities and has not been studied on such scale.

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Gaia astrometry & photometry

2 fields of view 2 Star Mappers 9x7 CCDs time delay integration	rectangular windows 2D & 1D windows gate configurations images not downloaded

DR3: 1.8B source with mean G-band photometry
G ≲ 20.7 mag
bandwidth limited (5Mbit/s, ~14h daily)
epoch photometry for ~ 11.75M sources designated as variable or in the GAPS survey
90% with 20-244 FP transits over 705-1038 days

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Angular coverage

the scanning law determines the frequency at which an object is observed
objects observed at different transit angles
a large fraction of objects are almost completely covered
surface brightness pipeline can reproduce morphology of object
caveats:

this can also produce a spurious variability signal
VPA has a strong impact on the type of observed sources, disk-like galaxies are disfavoured

source located at (l, b) = 0°, 0°

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Gaia XP spectra

2 photometers 330-680 nm 640-1050 nm 2 fields of view 7 CCDs/photometer	2 windowing strategies 5 gate configurations time-dependent a total of >25k effective instruments

DR3: 220M (G<17.65 mag) mean spectra
all sky coverage
published data depend on quality…
internally calibrated mean spectra represented as a continuous function defined by an array of coeff. applied to a set of basis functions

2D spectrum, binning in AC & AL

De Angeli +22, Montegriffo +22

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Data & Tools

Gaia Archive:

survey results and ancillary data products
ADQL, TAP/+, python, TOPCAT…
cross-matching & pre-computed cross-matches
in particular the extragalactic candidates tables

gaiaxpy:

converter -> to sampled spectrum
calibrator -> to absolute system
generator -> synthetic photometry
linefinder -> finds lines, extrema & their basic properties in the internally calibrated mean spectra (default: Hα, Hβ, CIV, CIII], MgII, Lyα, also user-defined lines)

extinction tools, SOMs…

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Provenance of extragalactic objects in DR3 tables

definition of “extragalactic”: unresolved/barely resolved objects beyond 50 Mpc
several modules operating mostly independently
CU3: astrometry, used external cat’s of point-sources to define the astrometric reference frame
CU4 uses SBP to look for physical extension (pre-determined source list)
CU7 uses photometric light curves
CU8 uses astrometry, photometry, BP/RP to estimate redshifts

Lindegren+21

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Extragalactic Gaia DR3 tables

two “main” extragal. tables: qso_candidates & galaxy_candidates
6.6M & 4.8M sources, respectively; flags available
a compilation of the results from all the processing modules

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Extragalactic Gaia DR3 tables

additional tables provide specific information: astrophysical_parameters, vari_classification_result…
vari_agn: ~900k AGN candidates s derived from photometric variability, have LCs
gaia_agn_cross_id: 1.6M celestial reference frame sources (Gaia-CRF3))
quasar candidates, majority with redshift estimates
1.1M analysed quasars with 60k host galaxies detected
15k galaxy surface brightness profiles
4.8M galaxy candidates with 1M redshift estimates
900k galaxies with surface brightness profiles

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Completeness & purity of the extragalactic tables

completeness > 90%
90% and better purity can be achieved with appropriate filtering

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Results and validation of galaxy

profiles - internal consistency

Ducourant+23

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Results and validation of galaxy profiles

seeing effects for the ground-based surveys
large galaxies may be too large for the assigned windows
inconsistencies between ground-based surveys

Ducourant+23

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Example light curves

Type 1 Seyfert galaxy PG 0921+525
blazar CTA 102 during 2016-2017 outburst
quasar B2 0945+22

Carnerero+23

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Example light curves and BP/RP spectra

BL Lac-type source 5BZBJ0035+1515

FSRQ 5BZQJ1549+0237

radio-quiet quasar

FBQS J163709.3+414030

Carnerero+23

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Composite quasar spectra

composites from 43k BP/RP spectra with spectroscopically confirmed redshifts from Flesch+21 & 111.5k BP/RP with QSOC redshifts
Vanden Berk+01 SDSS QSO composite spectrum in orange

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