Kavli Affiliate: Claudio Ricci
| First 5 Authors: Catarina Aydar, Andrea Merloni, Tom Dwelly, Johan Comparat, Mara Salvato
| Summary:
We present one of the largest uniform optical spectroscopic surveys of X-ray
selected sources to date that were observed as a pilot study for the Black Hole
Mapper (BHM) survey. The BHM program of the Sloan Digital Sky Survey (SDSS)-V
is designed to provide optical spectra for hundreds of thousands of X-ray
selected sources from the SRG/eROSITA all-sky survey. This significantly
improves our ability to classify and characterise the physical properties of
large statistical populations of X-ray emitting objects. Our sample consists of
13079 sources in the eROSITA eFEDS performance verification field, 12011 of
which provide reliable redshifts from 0<z<5.8. The vast majority of these
objects were detected as point-like sources (X-ray flux limit F(0.5-2
keV)>6.5×10^-15 erg/s/cm^2) and were observed for about 20 years with fibre-fed
SDSS spectrographs. After including all available redshift information for the
eFEDS sources from the dedicated SDSS-V plate programme and archival data, we
visually inspected the SDSS optical spectra to verify the reliability of these
redshift measurements and the performance of the SDSS pipeline. The visual
inspection allowed us to recover reliable redshifts (for 99% of the spectra
with a signal-to-noise ratio of >2) and to assign classes to the sources, and
we confirm that the vast majority of our sample consists of active galactic
nuclei (AGNs). Only ~3% of the eFEDS/SDSS sources are Galactic objects. We also
show the diversity of the optical spectra of the X-ray selected AGNs and
provide spectral stacks with a high signal-to-noise ratio in various
sub-samples with different redshift and optical broad-band colours. Our AGN
sample contains optical spectra of (broad-line) quasars, narrow-line galaxies,
and optically passive galaxies. It is considerably diverse in its colours and
in its levels of nuclear obscuration.
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