J/A+A/701/A283 Misclassified galaxy clusters in eRASS1 (Balzer+, 2025)
The First SRG/eROSITA All-Sky Survey: Characterization of clusters of galaxies
misclassified in the eRASS1 point source catalog.
Balzer F., Bulbul E., Kluge M., Liu A., Salvato M., Fabricius M., Seppi R.,
Artis E., Bahar Y.E., Bender R., Clerc N., Comparat J., Ghirardini V.,
Grandis S., Krippendorf S., Lamer G., Malavasi N., Merloni A., Nandra K.,
Ramos-Ceja M.E., Sanders J.S., Zelmer S., Zhang X.
<Astron. Astrophys. 701, A283 (2025)>
=2025A&A...701A.283B 2025A&A...701A.283B (SIMBAD/NED BibCode)
ADC_Keywords: Clusters, galaxy ; X-ray sources
Keywords: galaxies: clusters: general - galaxies: groups: general -
X-rays: galaxies: clusters
Abstract:
The detection of the extended X-ray emission of the intracluster
medium by the first SRG/eROSITA All-Sky Survey (eRASS1), combined with
optical and near-infrared follow-up, resulted in the identification of
more than 12000 galaxy clusters, yielding precise constraints on
cosmological parameters. However, some clusters of galaxies can be
misclassified as point sources by eROSITA's source detection
algorithm due to the interplay between the point-spread function, the
shallow depth of the survey, compact (cool core) X-ray emission, and
bright active galactic nuclei hosted in their centers or their
vicinity. To identify such misclassified galaxy clusters and groups,
we apply optical follow-up to the eRASS1 X-ray point sources
analogously to the treatment of the extent-selected catalog. After
rigorous filtering to ensure purity, we find a total of 8347 clusters
of galaxies, of which 5819 are novel detections, in a redshift range
0.05<z≲1.1. This corresponds to a 70% discovery rate, a fraction
similar to that of the extent-selected sample. To facilitate finding
new exceptional clusters such as the Phoenix cluster (which is
recovered in our sample), we divide the clusters into five classes
based on the optical properties of likely single-source counterparts
to the X-ray emission. We further investigate potential biases in our
selection process by analyzing the optical and X-ray data. With this
work, we provide a catalog of galaxy clusters and groups in the eRASS1
point source catalog, including their optical and X-ray properties
along with a meaningful classification.
Description:
This catalog presents X-ray, optical, and single-source counterpart
properties of misclassified galaxy clusters and groups in the eRASS1
point source catalog. It was obtained by running the
red-sequence-based cluster finding algorithm eROMaPPer at the
locations of the eRASS1 X-ray point source detections using the
optical and near-infrared source catalogs provided by the Legacy
Surveys DR9 and DR10. It contains 8347 clusters of galaxies, of which
5819 are novel detections, in a redshift range 0.05<z≲1.1. The
catalog includes the classification of clusters into five classes
based on optical counterpart properties identified via NWAY.
File Summary:
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FileName Lrecl Records Explanations
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ReadMe 80 . This file
catalog.dat 886 8347 X-ray and optical properties of the misclassified
clusters in the eRASS1 point source catalog
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See also:
J/A+A/682/A34 : SRG/eROSITA all-sky survey catalogs (eRASS1) (Merloni+, 2024)
J/A+A/683/A130 : eRASS. Western Galactic Hemisphere Superclusters (Liu+, 2024)
J/A+A/685/A106 : eRASS. Galaxy clusters and groups in WGH (Bulbul+, 2024)
J/A+A/688/A210 : eRASS. Optical properties of clusters & groups (Kluge+, 2024)
Byte-by-byte Description of file: catalog.dat
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Bytes Format Units Label Explanations
--------------------------------------------------------------------------------
1- 32 A32 --- DetUId Unique eSASS detection ID (DETUID)
34- 56 A23 --- IAUName Point source detection name in the
IAU format (IAUNAME)
58- 65 F8.4 deg RAdeg Right Ascension J2000 of the point
source detection (RA)
67- 75 F9.5 deg DEdeg Declination J2000 of the point
source detection (DEC)
77- 84 F8.4 deg RAOdeg Right Ascension J2000 of the cluster
optical center (RA_OPT)
86- 94 F9.5 deg DEOdeg Declination J2000 of the cluster
optical center (DEC_OPT)
96-103 F8.4 deg RABdeg Right Ascension J2000 of the
brightest member (RA_BCG)
105-113 F9.5 deg DEBdeg Declination J2000 of the
brightest member (DEC_BCG)
115-126 F12.8 deg RAXdeg Central Right Ascension J2000 of the
mbProj2D X-ray fit (RA_XFIT)
128-140 F13.9 deg DEXdeg Central Declination J2000 of the
mbProj2D X-ray fit (DEC_XFIT)
142-152 F11.5 --- DetLike0 X-ray detection likelihood
(DETLIKE0)
154-159 F6.3 mag RefMagBCG Galactic extinction-corrected z-band
magnitude (AB) of the brightest
member (REFMAG_BCG)
161-167 F7.5 --- zBest Best available cluster redshift
(BEST_Z)
169-175 F7.5 --- e_zBest Error of best available cluster
redshift (BESTZERR)
177-187 A11 --- zBestType Type of best available cluster
redshift (BESTZTYPE)
189-195 F7.5 --- zlambda Uncorrected cluster photometric
redshift (Z_LAMBDA)
197-203 F7.5 --- e_zlambda Error of uncorrected cluster
photometric redshift (ZLAMBDAERR)
205-211 F7.5 --- zLit ?=- Literature cluster redshift
(LIT_Z)
213-219 F7.5 --- e_zLit ?=- Error of literature cluster
redshift (LITZERR)
221-238 A18 --- r_zLit Reference for the literature redshift
(LITZSRC)
240-246 F7.5 --- zspBoot ?=- Bootstrap estimate of biweight
spectroscopic redshift (SPECZBOOT)
248-254 F7.5 --- e_zspBoot ?=- Error of bootstrap estimate of
biweight spectroscopic redshift
(SPECZBOOT_ERR)
256-257 I2 --- Nmembers Number of members used in biweight
redshift and velocity dispersion
estimate (N_MEMBERS)
259-265 F7.5 --- zspCG ?=- Spectroscopic redshift of the
central galaxy (CGSPECZ)
267-273 F7.5 --- e_zspCG ?=- Error of spectroscopic redshift
of the central galaxy
(CGSPECZ_ERR)
275-281 F7.5 --- zspBCG ?=- Spectroscopic redshift of
brightest member (BCGSPECZ)
283-289 F7.2 km/s VdispBoot ?=- Bootstrap estimate of velocity
dispersion (VDISP_BOOT)
291-297 F7.2 km/s e_VdispBoot ?=- Error of bootstrap estimate of
velocity dispersion (VDISPBOOTERR)
299-301 I3 --- f_VdispBoot ?=- Mean velocity dispersion flag for
bootstrap estimate (VDISPBOOTFLAG)
303-310 A8 --- VdispBootType Estimator used to determine velocity
dispersion. Gapper or Biweight
depending on N_MEMBERS (VDISP_TYPE)
312-317 F6.2 --- lambdaNorm Richness normalized to the definition
of the grz run (LAMBDA_NORM)
319-323 F5.2 --- e_lambdaNorm Error of richness normalized to the
definition of the grz run
(LAMBDANORMERR)
325-330 F6.2 --- lambdaOptNorm Richness (optical center) normalized
to the definition of the grz run
(LAMBDAOPTNORM)
332-336 F5.2 --- e_lambdaOptNorm Error of richness (optical center)
normalized to the definition of the
grz run (LAMBDAOPTNORM_ERR)
338-344 F7.5 --- ScaleVal Richness scale factor, Eq. 2 of
Rykoff et al. (2016) (SCALEVAL)
346-353 F8.6 --- MaskFrac Fraction of cluster area which is
masked (MASKFRAC)
355-383 A29 --- Run RedMaPPer calibration:
surveybandsrefband_iteration (RUN)
385-392 F8.4 --- Lmax Optical maximum likelihood (LMAX)
394-400 F7.5 10+21cm-2 NHI HI column density from HI4PI,
NSIDE = 1024 (NHI)
402-406 F5.2 mag gmagLim Limiting g band galaxy magnitude (AB)
(LIMMAG_G) (1)
408-412 F5.2 mag rmagLim Limiting r band galaxy magnitude (AB)
(LIMMAG_R) (1)
414-418 F5.2 mag imagLim ?=- Limiting i band galaxy magnitude
(AB) (LIMMAG_I) (1)
420-424 F5.2 mag zmagLim Limiting z band galaxy magnitude (AB)
(LIMMAG_Z) (1)
426-430 F5.2 mag W1magLim ?=- Limiting W1 band galaxy magnitude
(AB) (LIMMAG_W1) (1)
432-604 A173 --- MatchNames The names of cross- matched clusters
(see Sect. 4), comma-separated
(MATCH_NAMES)
606-617 E12.6 mW/m2 FX300 X-ray flux in 0.2-2.3keV within
300kpc (XF300)
619-630 E12.6 mW/m2 e_FX300 Lower error of X-ray flux in
0.2-2.3keV within 300kpc (XF300_L)
632-643 E12.6 mW/m2 E_FX300 Upper error of X-ray flux in
0.2-2.3keV within 300kpc (XF300_U)
645-656 E12.6 ct/s CRX300 ?=- X-ray count rate in 0.2-2.3keV
within 300kpc (XCR300)
658-669 E12.6 ct/s e_CRX300 ?=- Lower error of X-ray count rate
in 0.2-2.3keV within 300kpc
(XCR300_L)
671-682 E12.6 ct/s E_CRX300 ?=- Upper error of X-ray count rate
in 0.2-2.3keV within 300kpc
(XCR300_U)
684-695 E12.6 ct ctX300 ?=- Total X-ray counts in 0.2-2.3keV
within 300kpc (XCTS300)
697-708 E12.6 ct e_ctX300 ?=- Lower error of total X-ray counts
in 0.2-2.3keV within 300kpc
(XCTS300_L)
710-721 E12.6 ct E_ctX300 ?=- Upper error of total X-ray counts
in 0.2-2.3keV within 300kpc
(XCTS300_U)
723-734 E12.6 10-7W LX300 X-ray luminosity in 0.2-2.3keV within
300kpc (XL300)
736-747 E12.6 10-7W e_LX300 Lower error of X-ray luminosity in
0.2-2.3keV within 300kpc (XL300_L)
749-760 E12.6 10-7W E_LX300 Upper error of X-ray luminosity in
0.2-2.3keV within 300kpc (XL300_U)
762 I1 --- SplitNeigNum Amount of neighboring X-ray point
sources pointing to the same cluster
(SPLITNEIGHBORNUM)
764 I1 --- hasClosExtNEig Whether the cluster is in the
vicinity of an extent-selected
(Kluge et al., 2024, Cat.
J/A+A/688/A210) cluster
(HASCLOSEEXT_NEIGHBOR)
766-777 F12.8 deg RACdeg ?=- Right Ascension J2000 of the
counterpart in LS DR 10
(RACTPLS10)
779-791 F13.9 deg DECdeg ?=- Declination J2000 of the
counterpart in LS DR 10
(DECCTPLS10)
793-810 A18 --- CtpLS10FullId DECaLS DR 10 ID, consisting of
releaseIDbrickIDobjID
(CTPLS10FULLID)
812-823 E12.6 --- CtpNWAYPAny ?=- The NWAY probability of the
counterpart (CTP) being correct
(CTPNWAYP_ANY)
825-826 I2 --- CtPClasGalExgal [-5/4]?=- The Galactic/extragalactic
CTP class (CTPCLASSGAL_EXGAL) (2)
828-839 E12.6 --- CtPNWAYCompur8 ?=-99 p_any completeness/purity
crossover at DETLIKE0=8
(CTPNWAYCOMPUR8) (2)
841-843 A3 --- CtpLS10Type ?=- CTP source type in LS DR10
(PSF, EXP, DEV, REX, SER)
(CTPLS10TYPE)
845-851 F7.5 --- CtpzBest ?=- Best available counterpart
redshift (CTPBESTZ)
853-860 F8.5 --- e_CtpzBest ?=- Error of best available
counterpart redshift
(CTPBESTZ_ERR)
862-870 A9 --- CtpzBestType ?=- Type of best available
counterpart redshift (SPEC or PHOT)
(CTPBESTZ_TYPE)
872 I1 --- CtpzBsetUnrel [0/1] Whether the best CTP redshift
estimate has a high relative error
(CTPFLAGBESTZUNRELIABLE)
874 I1 --- CtpNWAYhasOMat [0/1] There exists a secondary (less
likely) CTP that was discarded
(CTPNWAYHASOTHERMATCH)
876 I1 --- CtpisGalactic [0/1] CTP is considered Galactic
(derived from CTPCLASSGAL_EXGAL)
(CTPISGALACTIC)
878 I1 --- CtpinHECATE [0/1] CTP is in range of a HECATE
galaxy (CTPINHECATE_RANGE)
880 I1 --- CtpisSecure [0/1] CTP is considered secure
(derived from CTPNWAYCOMPUR8)
(CTPISSECURE)
882 I1 --- CtpZMatClz [0/1] CTP and cluster redshift match
(Eq. 3.1) (CTPZMATCHESCLUSTERZ)
884 I1 --- CtpisClMemb [0/1] CTP is member galaxy of the
cluster (CTPISCLUSTER_MEMBER)
886 A1 --- ClusterClass [012345] The Cluster Class assigned
to this cluster (CLUSTER_CLASS) (3)
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Note (1): from Rykoff et al. (2015arXiv150900870R 2015arXiv150900870R) depth model.
Note (2): Salvato et al. (2025, in prep.).
If the photometry of a counterpart is considered unreliable, it will only be
labeled as Galactic if CtPClasGalExgal=-5; otherwise, we label it as
Galactic if CtPClasGalExgal<3.
Note (3): Classes as follows:
0 = This class contains the 213 cluster candidates for which no NWAY
counterpart is available but which still reflect X-ray point sources for
which eROMaPPer has identified an optical cluster counterpart in the
LS DR 9 north data. Class 0 candidates could thus belong to any of the
other classes, but we cannot distinguish them. We leave these in the
sample for completeness but note that they are not as well characterized
as the rest of the sample that contains single-source counterpart
information.
1 = This class contains cluster candidates for which the X-ray source is in
range of a HECATE galaxy or for which the secure counterpart association
is likely to be Galactic and not identified as a member of a cluster
(see also the definition of Class 4). For the point sources of Class 1
clusters, any X-ray emission from the optically identified cluster is,
therefore, likely to be contaminated. We provide an example for such a
cluster in the upper left corner of Fig. 6, where the optical cluster
detection is clearly offset from the Galactic point source detection.
The contours show some faint extended emission at the location of the
cluster, but that was not enough to warrant a detection. In total, this
class contains 291 cluster candidates, of which only 15 are in the range
of a HECATE galaxy.
2 = This class is used for cluster candidates for which the NWAY counterpart is
securely associated, is extragalactic, is not a member galaxy of the
cluster, and has good photometry. In addition to that it is required to
have either a different redshift than the cluster (see Eq. 3.2) or an
unreliable redshift estimate. The criteria lead to a total of 2698 Class 2
sources, of which 512 have an unreliable redshift estimate or bad
photometry with only photometric redshift available. Physically, these
objects correspond to X-ray point sources for which the majority of the
measured X-ray emission is expected to stem from AGN along the line of
sight to the optically identified clusters; there are 2305 sources with a
counterpart redshift higher than the cluster, and 390 sources where the
counterpart appears to be in the foreground the cluster. An example of
such a cluster is shown in the upper central panel of Fig. 6. Here the
blue counterpart is very close to the peak of the X-ray emission and seems
to be behind the cluster given zctp.
3 = This class indicates sources for which the NWAY counterpart is secure, is
extragalactic, eROMaPPer has not identified it as a member galaxy of the
cluster, and the counterpart redshift and cluster redshift match according
to Eq. (3.2). We find 970 sources that fulfill the criteria for Class 3.
These objects are likely to represent AGNs that reside in clusters, and
the detected X-ray signal might be the cluster's emission boosted by the
point source or the faint extended emission from the ICM gas dominated by
the X-ray emission from the central AGN. An example of such a cluster is
shown in the upper right panel of Fig. 6; the counterpart looks similar to
the one of the Class 2 example, but its spectroscopic redshift pinpoints
it to be part of the cluster.
4 = This class represents sources for which the NWAY counterpart is secure and
is a member of the cluster as assigned by eROMaPPer and the colors of the
counterpart are consistent with an early type red galaxy (see the details
in Sect. 5.2). We find a total of 1073 sources fulfilling these criteria.
We expect the sources in this class to be galaxy clusters which might be
more compact than the extent-selected ones, usually having a cool core.
An example of such a cluster is shown in the lower left panel of Fig. 6.
There is clearly no AGN candidate that could be attributed to the
emission, and NWAY selects a member of the cluster to be the most likely
single-source association. We decided to prioritize the membership
decision point both over the Galactic/extragalactic and the matching
redshift criteria after realizing in visual inspection that those
clusters are a better fit for this class than Class 1 or Class 2. We note
that there are 12 counterparts classified as Galactic that have also been
identified as cluster members. We include these in Class 4 after a visual
inspection showed that their Galactic classification might be in-accurate.
Also, there are 188 counterparts that seemingly conflict with the cluster
redshift estimate, although they are found to be members. We discuss the
latter cases in Sect. 3.2.3.
5 = This class represents sources for which no reliable NWAY counterpart to the
X-ray point source could be identified. For sources in Class 5, the
measured X-ray flux likely stems from the clusters. We find a total of
3102 clusters in this class. These clusters have been misclassified in
part due to having emission comparable to the PSF of eROSITA, in part due
to having another point source close by, and in part due to the nature of
the Lext=3 cut, where sources with only a slight extent can end up being
classified as a point source. The latter might be the case for the example
shown in Fig. 6. Close to the reported coordinates of the X-ray emission,
there seems to be no single source that could account for it, so NWAY
assigns the closest (faint) galaxy with pany∼0. The cluster is slightly
offset.
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Acknowledgements:
Fabian Balzer, fbalzer(at)mpe.mpg.de
(End) Patricia Vannier [CDS] 02-Jun-2025