Abstract
The Euclid space mission will cover over 14 000 deg2
with two optical and near-infrared spectro-photometric instruments, and
is expected to detect around ten million active galactic nuclei (AGN).
This unique data set will make a considerable impact on our
understanding of galaxy evolution in general, and AGN in particular. For
this work we identified the best colour selection criteria for AGN,
based only on Euclid photometry or including ancillary
photometric observations, such as the data that will be available with
the Rubin Legacy Survey of Space and Time (LSST) and observations
already available from Spitzer/IRAC. The analysis was performed
for unobscured AGN, obscured AGN, and composite (AGN and star-forming)
objects. We made use of the spectro-photometric realisations of
infrared-selected targets at all-z (SPRITZ) to create
mock catalogues mimicking both the Euclid Wide Survey (EWS) and the
Euclid Deep Survey (EDS). Using these mock catalogues, we estimated the
best colour selection, maximising the harmonic mean (F1) of: (a)
completeness, that is, the fraction of AGN correctly selected with
respect to the total AGN sample; and (b) purity, that is, the fraction
of AGN inside the selection with respect to the selected sample. The
selection of unobscured AGN in both Euclid surveys (Wide and Deep) is possible with Euclid
photometry alone with F1 = 0.22–0.23 (Wide and Deep), which can
increase to F1 = 0.43–0.38 (Wide and Deep) if we limit out study to
objects at z > 0.7. Such a selection is improved once the Rubin/LSST filters, that is, a combination of the u, g, r, or z filters, are considered, reaching an F1 score of 0.84 and 0.86 for the EDS and EWS, respectively. The combination of a Euclid
colour with the [3.6]−[4.5] colour, which is possible only in the EDS,
results in an F1 score of 0.59, improving the results using only Euclid filters, but worse than the selection combining Euclid and LSST colours. The selection of composite (fAGN = 0.05–0.65
at 8–40 μm) and obscured AGN is challenging, with F1 ≤ 0.3 even when
including Rubin/LSST or IRAC filters. This is unsurprising since it is
driven by the similarities between the broad-band spectral energy
distribution of these AGN and star-forming galaxies in the wavelength
range 0.3–5 μm.
| Original language | English |
|---|---|
| Article number | A1 |
| Journal | Astronomy and Astrophysics |
| Volume | 691 |
| Number of pages | 24 |
| ISSN | 0004-6361 |
| DOIs | |
| Publication status | Published - 2024 |
Keywords
- Galaxies: active
- Galaxies: evolution
- Galaxies: photometry
- Galaxies: statistics