# The nature of the X-ray sources in dwarf galaxies in nearby clusters from
the KIWICS

Şeyda Şen,1 Ersin Göğüş,1 Reynier F. Peletier,2 Nelvy Choque-Challapa3 and
Amirnezam Amiri4 5 6 7 8

1Faculty of Engineering and Natural Sciences, Sabancı University, İstanbul
34956, Turkey
2Kapteyn Astronomical Institute, University of Groningen, P. O. Box 800, 9700
AV Groningen, The Netherlands
3Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama,
Copayapu 485, Copiapó, Chile
4Dipartimento di Fisica e Astronomia, Università di Firenze, Via G. Sansone 1,
50019, Sesto Fiorentino (Firenze), Italy
5Iranian National Observatory, Institute for Research in Fundamental Sciences
(IPM), Tehran, Iran
6Departamento de Astrofísica, Universidad de La Laguna, E-38200, La Laguna,
Tenerife, Spain
7Instituto de Astrofísica de Canarias E-38205, La Laguna, Tenerife, Spain
8Department of Physics, University of Arkansas, 226 Physics Building, 825 West
Dickson Street, Fayetteville, AR 72701, USA
E-mail<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

We present a deep search for and analysis of X-ray sources in a sample of
dwarf galaxies (Mr < -15.5 mag) located within twelve galaxy clusters from the
Kapteyn IAC WEAVE INT Cluster Survey (KIWICS) of photometric observations in
the r and g using the Wide Field Camera (WFC) at the 2.5-m Isaac Newton
telescope (INT). We first investigated the optical data, identified 2720 dwarf
galaxies in all fields and determined their characteristics; namely, their
colors, effective radii, and stellar masses. We then searched the Chandra data
archive for X-ray counterparts of optically detected dwarf galaxies. We found
a total of 20 X-ray emitting dwarf galaxies, with X-ray flux ranging from
1.7$\times 10^{-15}$ to 4.1$\times 10^{-14}$ erg cm-2 s-1 and X-ray
luminosities varying from 2$\times 10^{39}$ to 5.4$\times 10^{41}$ erg s-1.
Our results indicate that the X-ray luminosity of the sources in our sample is
larger than the Eddington luminosity limit for a typical neutron star, even at
the lowest observed levels. This leads us to conclude that the sources
emitting X-rays in our sample are likely black holes. Additionally, we have
employed a scaling relation between black hole and stellar mass to estimate
the masses of the black holes in our sample, and have determined a range of
black hole masses from 4.6$\times 10^{4}$ to 1.5$\times 10^{6}$ M$\sun$.
Finally, we find a trend between X-ray to optical flux ratio and X-ray flux.
We discuss the implications of our findings and highlight the importance of
X-ray observations in studying the properties of dwarf galaxies.

###### keywords:

galaxies: clusters: general – galaxies: dwarf – galaxies: evolution – X-rays:
galaxies: clusters - astronomical data bases: surveys

††pubyear: 2023††pagerange: The nature of the X-ray sources in dwarf galaxies
in nearby clusters from the KIWICS–A

##  1 Introduction

Galaxy clusters are the largest and most massive gravitationally-bound
structures in the Universe. According to current cosmological theories of
large scale formation, galaxy clusters have grown hierarchically by the
merging of smaller virialized haloes (Press & Schechter, 1974; White & Rees,
1978; Blumenthal et al., 1984). They allow us to study numerous astrophysical
phenomena. In particular, the investigations of the formation of early-type
galaxies whose fraction is much higher in clusters as determined by the
morphology-density relation (Dressler, 1980). This relation also reveals that
the fraction of dwarf galaxies is even higher.

Dwarf galaxies constitute the most numerous subset of galaxies in the Universe
(Binggeli et al., 1988). However, the observed number of dwarfs is lower by
about two orders of magnitude than expected from the current models of galaxy
formation (Moore et al., 1999). This is the so-called missing satellite
problem. Moreover, the processes driving the formation of dwarf galaxies and
how the environment affects their evolution are still poorly understood.

The problem of the dwarf invisibility is generally attributed to feedback
processes which ejects most of the baryons, thus, making them difficult or
impossible to be detected. Silk & Mamon (2012) review that three fundamental
mechanisms for dwarf galaxy feedback are reionization of the Universe at early
epochs, supernovae (SNe) and (ram pressure and tidal) stripping.
Alternatively, active galactic nuclei (AGN) driven outflows from black holes
(BHs) could contribute to the feedback mechanism. This scenario has been
gaining support through in depth studies in recent years (Silk & Nusser,
2010). However, none of these models has so far has provided clear a solution.
More accurate census of dwarf galaxies and revealing their properties will
allow us to better understand their formation mechanisms and can be a
benchmark for our cosmological models.

In massive galaxies, AGN feedback mechanism has been added as a regular
component of the galaxy formation model since it seems that every massive
galaxy has a supermassive black hole (SMBH) in its centre (Kormendy &
Richstone, 1995). In dwarf galaxies, on the other hand, the presence of
massive BHs (mBH; $M_{BH}$ = 104 $\sim$ 106 $M_{\sun}$; Greene et al., 2020;
Mezcua, 2017) is becoming increasingly evident, thanks to studies of
individual galaxies to large scale surveys (e.g. Reines et al., 2011; Reines
et al., 2013; Moran et al., 2014; Mezcua & Domínguez Sánchez, 2020; Birchall
et al., 2020). However, the occupation fraction of BHs in dwarf galaxies is
still debated. The best evidence for BHs in dwarf galaxies comes from X-ray
(Kormendy & Ho, 2013; Pardo et al., 2016) besides optical emission line
studies (Baldassare et al., 2016). These investigations indicate that AGNs are
found in roughly one percent of dwarf galaxies.

X-ray observations are ideal for the detection of galaxy clusters since the
X-ray emission is proportional to the square of the gas density, making it
easier to identify clusters with cool cores. In addition, the projection
effects can be completely overcome in the X-ray band. The first cluster
samples were compiled from the first all-sky X-ray survey with Uhuru and
subsequent further observations were found more objects with HEAO-1, Ariel-V,
Einstein, EXOSAT and a centerpiece ROSAT. Later deep pointed observations with
the current generation of X-ray satellites XMM-Newton, Chandra111To see a list
of recent and ongoing Chandra and XMM Newton surveys
http://cxc.harvard.edu/xraysurveys/surveys.html and Suzaku have remarkably
changed our X-ray view of clusters and their galaxies to investigate their
evolutionary properties (see Rosati et al. 2002, for a review).

The resolving power of Chandra and the low background of its ACIS instrument
provide us an ideal combination to detect faint sources in nearby galaxies.
Several studies have taken this advantages to study the fraction of AGNs in
nearby late-type galaxies and have successfully detected X-ray nuclei in star-
forming galaxies (Ghosh et al., 2008; Grier et al., 2011; She et al., 2017a),
late-type bulgeless spirals (She et al., 2017b) and dwarf irregular galaxies
(Lemons et al., 2015). For the local AGNs selected from the Sloan Digital Sky
Survey (SDSS), more attention in X-rays has been devoted to perform detailed
spectral and timing analysis using sufficiently deep observations (e.g. Moran
et al. 2005; Greene & Ho 2007; Dong et al. 2012; Jin et al. 2016). In
addition, based on mainly archival observations, detailed characterizations of
X-ray nuclei in nearby, lower mass early-type galaxies in the Virgo cluster
(Ghosh et al., 2008; Gallo et al., 2010), and the Fornax cluster (Lee et al.,
2019) were performed.

In this paper, we performed one of the first investigations to unveil X-ray
sources within dwarf galaxies in nearby galaxy clusters. Our search for X-ray
emission using deep archival Chandra observations focused on dwarf galaxies up
to a redshift of z $\sim$0.03. These dwarf galaxies were identified with
optical data in order to bring some light into galaxy evolution and
transformation processes. It is clear that studying dwarf galaxies in cluster
environments of different properties in both optical and X-rays would yield
deeper understanding about their evolution.

Here, we use a sample of twelve nearby clusters from Kapteyn IAC WEAVE INT
Cluster Survey (KIWICS) and present the results of our comprehensive
investigations to uncover X-ray emission from dwarf galaxies in these galaxy
clusters. In the next section, we describe optical and X-ray observations and
explain our methodology to reduce them. In Section 3, we present the main
results. We conclude, in section 4, with a discussion of these results and a
comparison of X-rays versus optical data.

Throughout this paper, we assume a $\Lambda$CDM cosmology with $\Omega_{m}$ =
0.3, $\Omega_{\lambda}$ = 0.7, and H0 = 70 kms-1Mpc-1.

##  2 Observations and Data Reduction

###  2.1 Optical Observations

Our optical observational data come from a deep photometric survey of galaxy
clusters: The Kapteyn IAC WEAVE INT Cluster Survey (KIWICS, PIs R. Peletier
and J.A. Lopez-Aguerri). This survey consisted in imaging of 47 X-ray
selected, nearby (0.02 < z < 0.04) galaxy cluster (Piffaretti et al., 2011) in
the Northern hemisphere, and will be ideal for studying dwarfs and low-
surface-brightness galaxies (LSB). All observational data were obtained by
using the Wide Field Camera (WFC) at the 2.5-m Isaac Newton Telescope (INT) in
La Palma, Spain. The full KIWICS sample was selected as a preparation for the
future spectroscopic WEAVE Cluster Survey to be carried out with WHT Enhanced
Area Velocity Explores (WEAVE) spectrograph (Dalton et al., 2016). We use the
two broadband Sloan filters g and r with total integration times of $\sim$
1800 s and $\sim$ 5400 s, respectively. The observations cover each cluster up
to at least 1 $R_{\textit{200}}$ with a dithering pattern of individual
exposures of 210s. A comprehensive description of the observational strategy
and data reduction processes can be found in Mancera Piña et al. (2018, 2019)
and Choque-Challapa et al. (2021). Here, we briefly summarise the main
aspects.

The data reduction was done by using the Astro-WISE (McFarland et al., 2013)
environment following the same routine as explained in there. The data
reduction was performed in two main steps; the first step contains the
standard instrumental corrections, namely, applying bias subtraction and flat-
fielding corrections. At this stage, weight maps were also generated for each
frame. These weight maps contain information about bad pixels or saturated
pixels (from hot and cold pixel maps), as well as the expected noise
associated with each pixel and cosmic rays. The second step deals with the sky
subtraction, after which astrometric and photometric corrections were applied.
For this purpose, a set of standard stars were observed during each night of
the observations with the SDSS DR14 catalogue (Abolfathi et al., 2018).
Finally, all the cluster frames corrected for bad pixels and cosmic rays. The
astrometric solutions were computed by making use of the publicly available
software SCAMP (Bertin, 2006). The astrometry of our final mosaic has an rms
of $\sim$ 0.2′′. They are median-stacked to produce a deep coadded mosaic with
re-sampled to a scale of 0.2′′ per pixel.

Figure 1: Sky map of all of the clusters being surveyed in KIWICS and those
studied in this work represent in red star symbols.

The galaxy clusters studied here come from the sample of Choque-Challapa et
al. (2021) that have a seeing < 1.6′′ FWHM with redshift lower than 0.03. In
this study, we analyse dwarf galaxies in these clusters by combining optical
and X-ray data. To classify objects as dwarf galaxies, we employed a criterion
of Mr > $-$19.0 mag. This threshold adheres to the established convention, as
defined by Binggeli et al. (1988), which designates dwarf galaxies as those
with MB > $-$18.0 mag and assumes a color index B - r $\sim$ 1.0 mag for such
galaxies. Additionally, we implemented a lower luminosity cutoff at Mr =
$-$15.5 mag. Galaxies fainter than this limit were excluded from our analysis
to reduce potential contamination from background objects.

Table 1: Properties of our sample of galaxy clusters.

Name | RA | Dec | Redshift | $\sigma_{\text{e}}$ | Seeing r-band | Seeing g-band | $M_{500}$ | $R_{500}$
---|---|---|---|---|---|---|---|---
| (deg) | (deg) | z | (km s-1) | (") | (") | 1014 $M_{\sun}$ | Mpc
(1) | (2) | (3) | (4) | (5) | (6) | (7) | (8) | (9)
A1367 | 176.152 | 19.759 | 0.0214 | 581 $\pm$ 64 | 1.5 | 1.6 | 2.14 | 0.90
A262 | 28.188 | 36.157 | 0.0163 | 402 $\pm$ 24 | 1.5 | 1.4 | 1.19 | 0.74
RXJ0123.2+3327 | 20.801 | 33.461 | 0.0146 | 483 $\pm$ 95 | 1.6 | 1.3 | 0.36 | 0.50
RXJ0123.6+3315 | 20.921 | 33.261 | 0.0164 | 483 $\pm$ 95 | 1.6 | 1.3 | 0.61 | 0.60
RXCJ0751.3+5012 | 117.844 | 50.213 | 0.0228 | 360 $\pm$ 24 | 1.3 | 1.3 | 0.42 | 0.52
RXCJ1206.6+2811 | 181.656 | 28.184 | 0.0283 | 381 $\pm$ 42 | 1.5 | 1.4 | 0.42 | 0.52
RXCJ1223.1+1037 | 185.777 | 10.624 | 0.0258 | 302 $\pm$ 12 | 1.6 | 1.5 | 0.56 | 0.58
RXCJ1715.3+5724 | 258.841 | 57.408 | 0.0276 | 475 $\pm$ 29 | 1.3 | 1.5 | 0.87 | 0.67
RXCJ0919.8+3345 | 139.955 | 33.760 | 0.0230 | 299 $\pm$ 19 | 1.4 | 1.4 | 0.26 | 0.45
RXCJ1714.3+4341 | 258.578 | 43.690 | 0.0276 | 176 $\pm$ 22 | 1.3 | 1.5 | 0.31 | 0.48
RXCJ2214.8+1350 | 333.720 | 13.847 | 0.0253 | 351 $\pm$ 07 | 1.3 | 1.3 | 0.32 | 0.48
ZwCL1665 | 125.798 | 04.356 | 0.0293 | 382 $\pm$ 15 | 1.3 | 1.4 | 0.73 | 0.63

Note. (1) galaxy cluster name; (2) and (3) right ascension and declination in
J 2000; (4) and (5) redshift and velocity dispersion from Choque-Challapa et
al. (2021); (6) and (7) mean seeing during the observation in the r and g
band; (8) and (9) mass ($M_{500}$) and radius ($R_{500}$) from Piffaretti et
al. (2011).

####  2.1.1 Identification of dwarf galaxies

We use SExtractor (Bertin & Arnouts, 1996; Holwerda, 2005) based on the
stellarity CLASS${}_{\text{--}}$STAR and FLAG parameters to detect potential
dwarf candidates. The CLASS${}_{\text{--}}$STAR parameter ranges from 0 to 1;
objects close to 0 are likely to be extended objects and close to 1 are more
likely to be point sources. Galaxies are defined as object with
CLASS${}_{\text{--}}$STAR $\leq$ 0.2 in both g and r filters, and with
FLAGS${}_{\text{r}}$ = 0. This criterion is used to exclude objects that are
either blended with other nearby objects or have poor quality photometry.

To effectively eliminate background objects from the SExtractor catalogue, we
firstly performed a cut based on the color g - r $\leq$ 1.0 mag to exclude all
galaxies redder than this limit. This limit corresponds to a 12 Gyr old
stellar population with supersolar metallicity, respectively (Worthey, 1994).
As a final step of our selection of the dwarf candidates, a visual cleaning
was done in order to remove artefacts if present. We exclude all the objects
if they are background, artefacts, interaction, or fainter than
m${}_{\text{r}}$ = 20 mag, as their visual selection become inaccurate (see
Section 3 in Choque-Challapa et al., 2021, for details). In the end, we find
that there are overall 2720 dwarf galaxies detected in all of these 12
clusters of the KIWICS survey fields. As an example, we present the dwarf
galaxies in A1367 in Figure 2.

In order to obtain more accurate photometric measurements for the identified
galaxies, we used GALFIT (Peng et al., 2010) via a fit to their light
profiles. The process was carried out according to the methodology outlined in
Venhola et al. (2018) for all probable cluster members, excluding objects with
an SExtractor ISORAREA-IMAGE value of less than 200 pixels. This exclusion was
primarily due to the fact that smaller objects tend to result in poor fits due
to their faintness (apparent r-band magnitudes fainter than 22 mag). We used
the central coordinates, isophotal magnitudes, and semi-major axis lengths
(all obtained using SExtractor) as inputs for the photometric investigation.
We adopt the non-circularized effective radius, which represents the length of
the semimajor axis of an ellipse that best fits the isophotes enclosing half
of the total light emitted by a galaxy. A single Sérsic function was applied
to each object in both the r and g bands. However, due to the limited
resolution of our images, it was not possible to discern a nucleus in the
galaxies. For the objects that have not a clear nucleus, the center
coordinates are kept fixed when determining the SExtractor magnitudes.

Figure 2: (Left) Map of the A1367 cluster. The black symbols correspond to
dwarf galaxies identified in optical. The red stars represent matching
galaxies with X-rays. The green cross shows the X-ray center from Ebeling et
al. (1998), the green dotted circle indicates the estimated R200 radius, and
the blue area indicates the combined field covered by multiple Chandra
observations. (Right) Zoomed in view of the X-ray centre of the galaxy
cluster.

To provide a comprehensive analysis, we also estimate the stellar masses of
the sample galaxies. To accomplish this, we use the relation observed between
the (g$-$r) color and mass to-light (M/L) ratio by Roediger & Courteau (2015):

$\log(M_{*}/L_{r})=1.629\times(g-r)-0.792$ (1)

Our all dwarf samples and X-ray emitting dwarf samples have a median (mean)
stellar mass of 4.93 (5.33) $\times 10^{8}$ $M_{\sun}$ and 7.16 (10.90)
$\times 10^{8}$ $M_{\sun}$, respectively.

###  2.2 X-ray Observations

We searched the Chandra data archive for X-ray observations covering the
fields of our sample of 12 galaxy clusters with minimum exposure times of 10
ks in order to ensure significant detection. We find that one cluster
(RXCJ1714.3+4341) was not observed with Chandra. The exposure times of X-ray
observations of two clusters, namely RXCJ0919.8+3345 and ZwCL1665 were 3 and 2
ks, respectively. Therefore, these two observations were not used. The
remaining nine clusters were found to have deep enough Chandra observations in
one or multiple pointings. We list the observation IDs of these data sets in
Table 3.

The Chandra data were reduced using the Chandra Interactive Analysis of
Observations (ciao) software version 4.13 with CALDB version 4.9.5. As the
focal point instruments, we employed data recorded with a back-illuminated
ACIS-S chip (S3) or the four front-illuminated chips of ACIS-I (I0-I3). Note
that the other chips on ACIS-S were generally not useful for our purposes due
to the fact that the point spread function (PSF) becomes large at large off-
axis angles.

To classify optical counterparts to our Chandra X-ray data, we place both the
Chandra and KIWICS data onto the International Celestial Reference System
(ICRS) by finding matches between stars or background galaxies in the Two
Micron All-Sky Survey Point Source Catalog (2MASS-PSC, Skrutskie et al.,
2006). We have already performed astrometry for KIWICS data (see Section 2.1).
In order to improve the astrometry of Chandra/ACIS images relative to the
KIWICS images, we use our own pipeline which aligns the images using the
astroalign Python package (Beroiz et al., 2020) and the CIAO task wcs-match
script. Using the transformation matrices obtained from the analysis, the
aspect solution files and the coordinate parameters are updated in all of the
X-ray event files. We set an upper bound for the rms residual between optical
and X-ray positions as $\sim$ 0.02′′.

We identified X-ray sources within our aligned Chandra images using the
wavdetect tool of CIAO. It is a Mexican-Hat wavelet-based source detection
algorithm on the full energy band (0.5$-$10 keV) with the false detection
threshold value set to 10-6. The wavelet scales are set to 1.0, 2.0, 4.0 and
8.0 pixels. We detected 487 X-ray sources in all nine fields that we
investigated. Finally, we obtained the matching pairs of the coordinates of
Chandra X-ray sources and optical dwarf source lists by allowing the two to be
at most 0.5′′ apart from each other. In total, 20 dwarf galaxy sources were
detected in X-rays. We list these 20 sources in Table 2. We also illustrate
the optical and X-ray paired galaxies in A1367 in Figure 2.

Table 2: Details of Chandra X-Ray Observations for the 20 X-ray Emitting
Galaxies

Name | ObsID | Start Date and Time | Exposure | Count Rate(1)
---|---|---|---|---
|  | (UTC) | (ks) | (10-4 cts s-1)
A1367-1 | 4189 | 2003-01-24 10:29:13 | 48 | 25.2 $\pm 3.0$
| 17199 | 2015-01-30 21:05:47 | 38 |
| 17201 | 2016-01-31 13:37:06 | 61 |
A1367-2 | 514 | 2000-02-26 10:44:03 | 41 | 18.6 $\pm 3.6$
| 17199 | 2015-01-30 21:05:47 | 38 |
| 17200 | 2015-11-05 00:56:42 | 40 |
| 17201 | 2016-01-31 13:37:06 | 61 |
A1367-3 | 17199 | 2015-01-30 21:05:47 | 38 | 6.4 $\pm 2.5$
| 17200 | 2015-11-05 00:56:42 | 40 |
A1367-4 | 17199 | 2015-01-30 21:05:47 | 38 | 6.2 $\pm 2.4$
| 17200 | 2015-11-05 00:56:42 | 40 |
| 17201 | 2016-01-31 13:37:06 | 61 |
A1367-5 | 17199 | 2015-01-30 21:05:47 | 38 | 10.8 $\pm 3.0$
| 17200 | 2015-11-05 00:56:42 | 40 |
| 17201 | 2016-01-31 13:37:06 | 61 |
A262-1 | 7921 | 2006-11-20 03:35:12 | 111 | 54.4 $\pm 3.6$
A262-2 | 7921 | 2006-11-20 03:35:12 | 111 | 8.8 $\pm 2.0$
A262-3 | 7921 | 2006-11-20 03:35:12 | 111 | 7.4 $\pm 3.1$
RXJ0123-1 | 2882 | 2002-01-08 04:47:16 | 44 | 42.6 $\pm 5.0$
RXCJ0751-1 | 15170 | 2013-05-14 07:59:03 | 98 | 7.6 $\pm 1.2$
RXCJ0751-2 | 15170 | 2013-05-14 07:59:03 | 98 | 2.9 $\pm 1.3$
RXCJ0751-3 | 15170 | 2013-05-14 07:59:03 | 98 | 2.3 $\pm 0.9$
RXCJ1206-1 | 6939 | 2006-02-16 02:40:12 | 36 | 20.6 $\pm 3.7$
RXCJ1223-1 | 3232 | 2003-02-04 16:09:47 | 30 | 15.6 $\pm 3.6$
RXCJ1715-1 | 4194 | 2003-09-17 08:33:38 | 47 | 2.5 $\pm 2.4$
RXCJ1715-2 | 4194 | 2003-09-17 08:33:38 | 47 | 3.8 $\pm 2.0$
RXCJ1715-3 | 4194 | 2003-09-17 08:33:38 | 47 | 13.1 $\pm 2.3$
RXCJ2214-1 | 6392 | 2006-01-12 22:33:07 | 33 | 6.1 $\pm 3.0$
RXCJ2214-2 | 6392 | 2006-01-12 22:33:07 | 33 | 8.3 $\pm 2.4$
RXCJ2214-3 | 6392 | 2006-01-12 22:33:07 | 33 | 9.8 $\pm 2.2$

Note. (1) Background subtracted count rates in the 0.5$-$10 keV band.

Table 3: Results of X-ray Spatial and Spectral Investigations Name | RA | Dec | $N_{H}$ | Photon Index | Flux | $logL_{x}$ |  $\chi^{2}$ / dof
---|---|---|---|---|---|---|---
| (deg) | (deg) | (1020 cm-2) |  $\Gamma$ | (10-14 erg cm-2 s-1) | (erg s-1) |
A1367-1 | 176.152 | 19.893 | 1.80 | $1.77^{+0.33}_{-0.30}$ | 4.05 $\pm 0.25$ | 40.62 | 26.45/36
A1367-2 | 176.160 | 19.735 | 1.80 | $1.72^{+0.29}_{-0.26}$ | 2.07$\pm 0.18$ | 40.33 | 65.18/69
A1367-3 | 176.257 | 19.764 | 1.80 | 1.75 | 1.42 $\pm 0.34$ | 40.17 | 4.85/5
A1367-4 | 176.298 | 19.702 | 1.80 | 1.75 | 1.19$\pm 0.26$ | 40.09 | 16.38/13
A1367-5 | 176.070 | 19.738 | 1.80 | 1.75 | 2.37$\pm 0.27$ | 40.39 | 26.79/26
A262-1 | 28.210 | 36.155 | 6.80 | $2.17^{+0.21}_{-0.20}$ | 2.94$\pm 0.18$ | 40.25 | 128.19/72
A262-2 | 28.170 | 36.198 | 6.80 | 2.00 | 0.56 $\pm 0.10$ | 39.53 | 3.82/5
A262-3 | 28.261 | 36.160 | 6.80 | $1.92^{+1.65}_{-0.98}$ | 0.67$\pm 0.14$ | 39.60 | 43.86/50
RXJ0123-1 | 20.892 | 33.252 | 5.26 | $2.39^{+0.49}_{-0.40}$ | 2.74$\pm 0.30$ | 40.12 | 35.22/24
RXCJ0751-1 | 117.609 | 50.156 | 5.61 | $0.96^{+0.66}_{-0.60}$ | 1.66$\pm 0.21$ | 40.29 | 6.54/7
RXCJ0751-2 | 117.864 | 50.169 | 5.61 | $1.54^{+1.45}_{-1.11}$ | 0.59$\pm 0.16$ | 39.84 | 2.71/6
RXCJ0751-3 | 117.671 | 50.175 | 5.61 | 1.50 | 0.17$\pm 0.03$ | 39.30 | 5.79/3
RXCJ1206-1 | 181.712 | 28.108 | 1.72 | $1.02^{+0.67}_{-0.58}$ | 2.93$\pm 0.25$ | 40.73 | 10.85/9
RXCJ1223-1 | 185.742 | 10.630 | 2.71 | $2.49^{+1.20}_{-1.00}$ | 0.85$\pm 0.14$ | 40.11 | 1.98/5
RXCJ1715-1 | 258.892 | 57.450 | 2.12 | 1.61 | 0.29$\pm 0.04$ | 39.71 | 3.28/5
RXCJ1715-2 | 258.768 | 57.447 | 2.12 | 1.61 | 0.41$\pm 0.11$ | 39.85 | 5.29/3
RXCJ1715-3 | 258.818 | 57.337 | 2.12 | $1.61^{+0.65}_{-0.56}$ | 1.66$\pm 0.28$ | 40.46 | 1.74/5
RXCJ2214-1 | 333.788 | 13.972 | 4.31 | 1.50 | 0.93$\pm 0.32$ | 40.13 | 7.17/9
RXCJ2214-2 | 333.640 | 13.810 | 4.31 | $1.26^{+1.18}_{-1.00}$ | 1.33$\pm 0.25$ | 40.29 | 7.21/5
RXCJ2214-3 | 333.815 | 13.915 | 4.31 | $1.58^{+0.91}_{-0.79}$ | 1.55$\pm 0.29$ | 40.35 | 7.29/6

For each of these X-ray emitting systems, we performed X-ray spectral analysis
to determine their X-ray flux as follows: We extracted source spectra for 18
sources from a circular region of 10′′ radius centered at their X-ray
positions listed in Table 3. For two sources, in particular A262$-$2 and
RXCJ2214$-$1, the radius was set to be 12′′ due to their extended nature. We
then grouped these source spectra in order for each spectral bin to contain 10
source counts. The background spectra were extracted from circular aperture of
the same radii from nearby source-free regions. To take into account time-
dependent and position-dependent ACIS responses, the corresponding response
and ancillary response files are also extracted per observation. For the five
sources in the A1367 galaxy cluster for which there were multiple Chandra
observations, we modeled these multiple spectra for each source simultaneously
by linking the power law index parameters so that the fit would yield a joint
power law index for each source.

We fit each background subtracted spectrum with the power law model attenuated
by the interstellar hydrogen population using XSPEC (Arnaud, 1996). We fix the
HI column density NH with corresponding value for each cluster that is the
Galactic absorption towards these clusters222Obtained from NASA’s HEASARC NH
tool, https://heasarc.gsfc.nasa.gov/cgi-bin/Tools/w3nh/w3nh.pl. We initially
allowed the power law index to vary to perform the fits. We find that the
spectra of 12 sources can be represented with the power law model whose
indices range from about 1 to 2.5 (see Table 3). However, the index parameter
for the spectra of eight X-ray source cannot be constrained. In those cases,
we fixed the power law index at the average value of constrained indices of
the other cluster members. We finally used the best fit parameters to
calculate the X-ray flux in the band 0.5$-$10 keV (see Table 3). We list g $-$
r color, Mr magnitudes, effective radius as well as stellar mass (M∗)
estimates of these 20 X-ray emitting dwarf galaxies in Table 4.

Table 4: Optical Properties of the 20 X-ray Emitting Galaxies Name | g $-$ r | Mr | $R_{e}$ | log $M_{*}$
---|---|---|---|---
| (mag) | (mag) | (kpc) | ($M_{\sun}$)
A1367-1 | 0.68 $\pm 0.04$ | -18.40 $\pm 0.09$ | 1.02 $\pm 0.01$ | 9.55
A1367-2 | 0.68 $\pm 0.03$ | -16.08 $\pm 0.22$ | 0.88 $\pm 0.01$ | 8.73
A1367-3 | 0.70 $\pm 0.03$ | -15.56 $\pm 0.25$ | 2.39 $\pm 0.07$ | 8.62
A1367-4 | 0.69 $\pm 0.03$ | -16.34 $\pm 0.13$ | 1.31 $\pm 0.02$ | 8.48
A1367-5 | 0.68 $\pm 0.06$ | -18.96 $\pm 0.14$ | 0.71 $\pm 0.01$ | 8.43
A262-1 | 0.76 $\pm 0.04$ | -18.65 $\pm 0.11$ | 0.93 $\pm 0.00$ | 9.78
A262-2 | 0.55 $\pm 0.05$ | -15.93 $\pm 0.24$ | 0.70 $\pm 0.01$ | 8.83
A262-3 | 0.63 $\pm 0.03$ | -16.81 $\pm 0.16$ | 1.11 $\pm 0.09$ | 8.35
RXJ0123-1 | 0.69 $\pm 0.04$ | -16.39 $\pm 0.20$ | 0.85 $\pm 0.01$ | 8.75
RXCJ0751-1 | 0.79 $\pm 0.04$ | -16.95 $\pm 0.12$ | 1.23 $\pm 0.01$ | 9.35
RXCJ0751-2 | 0.40 $\pm 0.05$ | -17.88 $\pm 0.19$ | 0.83 $\pm 0.01$ | 9.14
RXCJ0751-3 | 0.78 $\pm 0.03$ | -17.49 $\pm 0.22$ | 2.23 $\pm 0.01$ | 8.88
RXCJ1206-1 | 0.60 $\pm 0.03$ | -16.17 $\pm 0.25$ | 1.56 $\pm 0.07$ | 8.52
RXCJ1223-1 | 0.63 $\pm 0.08$ | -16.67 $\pm 0.13$ | 1.11 $\pm 0.03$ | 8.78
RXCJ1715-1 | 0.87 $\pm 0.05$ | -18.22 $\pm 0.10$ | 1.33 $\pm 0.01$ | 9.67
RXCJ1715-2 | 0.80 $\pm 0.04$ | -17.98 $\pm 0.15$ | 1.27 $\pm 0.01$ | 9.79
RXCJ1715-3 | 0.82 $\pm 0.08$ | -18.15 $\pm 0.17$ | 1.55 $\pm 0.04$ | 9.57
RXCJ2214-1 | 0.82 $\pm 0.07$ | -17.84 $\pm 0.11$ | 1.44 $\pm 0.01$ | 9.43
RXCJ2214-2 | 0.74 $\pm 0.06$ | -17.88 $\pm 0.11$ | 1.50 $\pm 0.02$ | 9.55
RXCJ2214-3 | 0.72 $\pm 0.04$ | -15.73 $\pm 0.23$ | 0.91 $\pm 0.02$ | 8.54

##  3 Results

We present the color-magnitude diagram (CMD) of all optically detected systems
in Figure 3. We also indicate those 20 galaxies emitting X-rays on the same
Figure. We find that X-ray emitting dwarfs cover this range of magnitude
almost uniformly. On the other hand, the g$-$r colors of these dwarfs range in
a rather narrow interval of 0.55 and 0.85. Note that the g$-$r color outlier
with 0.40 (RXCJ0751$-$2), the estimated effective radius is one of the
largest. Excluding this system, we observe a nearly linear trend between the
g$-$r color and absolute magnitude. A linear fit to those 19 yields a slope of
$-0.52\pm 0.02$ which is the red sequence. We compare these relations with
early- and late-type dwarf galaxies from the Fornax Deep Survey (FDS, Venhola
et al., 2018). One clearly sees that early type dwarfs and our X-ray emitting
samples form a red sequence, while the late type dwarfs are situated in a blue
cloud below it. Our outlier dwarf galaxy is located within the region occupied
by FDS late-type dwarf galaxies in Figure 3.

Figure 3: Color$-$Mr, Color$-$Re and Re$-$Mr diagram for the identified dwarf
galaxies in all 12 clusters. Red points represent those with paired X-ray
emission. Turquoise and blue symbols show the FDS early- and late-type dwarfs,
respectively.

Figure 4: Color$-$stellar mass (top) and stellar mass$-$Mr diagrams (middle)
for the identified dwarf galaxies in all 12 clusters. Red points represent
those with paired X-ray emission. Black hole mass as a function of stellar
mass (bottom) with three different assumptions.

We present the color vs. effective radius behavior of the optically identified
dwarf galaxies in Figure 3 (middle). We find that 80% of the objects detected
in X-ray have a size smaller than 1.5 kpc. They appear to be also clustered in
$R_{e}$ $\sim$ 1.2 kpc, that is in general agreement with the size
distribution of dwarf. Also in Figure 3 (bottom), we show effective radius
against absolute magnitudes of galaxies. It is noteworthy that the clustering
of dwarfs towards smaller $R_{e}$ and high Mr is not followed by those dwarfs
identified in X-rays, indicating that the detected objects have a range in
surface brightness (or density).

We plot stellar mass as a function of color, Mr in the top and middle panels
of Figure 4, respectively. Our X-ray emitting dwarf samples mass range from
$\sim$ 2 $\times 10^{8}$ $M_{\sun}$ to 6 $\times 10^{9}$ $M_{\sun}$. We find
no X-ray emitting dwarf galaxies below the galaxy mass of $M_{*}$ $\sim
10^{8.3}M_{\sun}$.

We also studied the X-ray to optical emission (X/O) ratio as introduced by
Maccacaro et al. (1988) as

$X/O=\log(f_{X}/f_{opt})=\log(f_{X})+C+m_{r}/2.5$ (2)

where, fX is the X-ray flux in given energy band in ergs cm-2 s-1, mopt is the
magnitude at the chosen optical wavelength and C333It is taken as 5.57 for r
band from Haggard et al. (2010). is a constant which depends on the specific
filter used in the optical observations. We present X/O values of these 20
systems against their X-ray flux in the 0.5$-$10 keV band in Figure 5. We find
that all X/O ratios in our sample are less than $-0.25$. Moreover, there is a
positive correlation between X/O values and flux with Spearman’s rank order
correlation coefficient, r of 0.70 and chance occurrence probability, P of
4.5$\times 10^{-4}$. A linear trend fit to these data points yields a slope of
0.69$\pm$0.19. We also investigated the trends of X/O and O/X with their
corresponding optical flux values. We find that both ratios vary significantly
with increasing optical flux as well. Therefore, X/O vs. X-ray flux is an
indicative probe for both X-ray and optical emission.

Figure 5: Plot of X-ray to optical flux ratio vs. the X-ray flux in the 0.5-10
keV band. The solid line is the best fit linear trend to these data points.
The horizontal dotted line marks the level of equal X-ray and optical fluxes.

An important advantage of BH-galaxy scaling relationships is their ability to
estimate the mass of a black hole from readily measurable galaxy properties.
This feature makes them a valuable tool for investigating BH properties and
their role in galaxy evolution. In our case, it is, however, not possible to
estimate the mass of a black hole (MBH) using the relationship between MBH and
stellar velocity dispersion ($\sigma_{*}$) (e.g., Ferrarese & Merritt 2000;
Gültekin et al. 2009; Beifiori et al. 2012) due to the lack of available
$\sigma_{*}$ measurements in our samples. Previous studies have shown a strong
positive correlation between MBH and M∗ (Reines & Volonteri, 2015; Shankar et
al., 2016; Suh et al., 2020). Here, we evaluate the relationship between MBH
and M∗ with three different assumptions. Reines & Volonteri (2015) suggested a
relation between the masses of the black holes and their host galaxies (see
their equation 5) using a sample of nearby inactive early-type galaxies and
local AGN. They report that their MBH measurements have errors of $\sim$0.5
dex. Shankar et al. (2016) used a combination of dynamical modeling and the
virial method to calculate black hole masses (see their equation 6). Suh et
al. (2020) investigate the relationship between black hole mass and galaxy
total stellar mass up to a redshift z $\sim$ 2.5 for 100 X-ray-selected AGN
sample and provides a relation via their equation 2.

Note that their sample of galaxies has total stellar masses of
$10^{11-12}M_{\sun}$. We present $M_{BH}$ estimates within these three
approaches for the sample of 20 X-ray emitting galaxies in the bottom panel of
4. It is important to note that the $M_{BH}$ vs. $M_{*}$ relation for dwarf
galaxies is not established well. Nevertheless, the sample of Reines &
Volonteri (2015) is the only one that includes dwarf galaxies and extends to
lower galaxy masses. Therefore, we employed the relation of Reines & Volonteri
(2015) and obtain an average black-hole-mass of 2.4$\times 10^{5}M_{\sun}$ for
our 20 galaxy sources (see Figure 6).

Figure 6: Plot of X-ray luminosity in the 0.5$-$10 keV band against the
estimated mass of corresponding dwarf galaxies (lower scale), as well as black
hole mass (upper scale).

##  4 Discussion and Conclusions

Through our investigation of the correlation between optical and X-ray data of
a sample of nearly galaxy clusters, we identified 2720 dwarf galaxies, twenty
of which were found to emit also in X-rays. Earlier multiband investigations
revealed by Pardo et al. (2016) revealed X-ray emission from ten dwarf
galaxies at z < 1 out of 605 in the optical. They estimate an AGN fraction of
$\sim$ 1% for their sample of dwarf galaxies. They identified AGN candidates
based on their hardness ratios and a hard X-ray photon index in the energy
range between 2 and 10 keV. Reines et al. (2013) studied a sample of 151 dwarf
galaxies (mass range from $10^{8.5}$ to $10^{9.5}$ M$\sun$, z < 0.055) with
narrow and/or broad emission line signatures indicating the presence of an AGN
and identified 10 dwarf galaxies with both narrow and broad emission line AGN
signatures. Lemons et al. (2015) cross-matched the Reines et al. (2013) parent
sample with the Chandra Source Catalog and found 8 systems with nuclear hard
X-ray emission at levels higher than expected from low-mass and high-mass
X-ray binaries. Miller et al. (2015) is another study that used a combination
of optical and X-ray data to identify AGN in dwarf galaxies. They conducted a
study on approximately 200 early-type dwarf galaxies in the local universe.
These galaxies were selected optically using Hubble Space Telescope imaging as
part of the AGN Multi-wavelength Survey of Early-type galaxies (AMUSE) surveys
(see Gallo et al. 2008; Miller et al. 2012, for details on these surveys).
They identified a total 23 AGN in their samples with mass range from about
$10^{7}$ to $10^{9}$ M$\sun$. Recently, Chen et al. (2017) identified ten AGNs
in low-mass galaxies from the NuSTAR serendipitous survey (Lansbury et al.,
2017), which is capable of probing hard X-ray emission up to 24 keV.

The X-ray flux of our sample of 20 galaxies range from 1.7$\times 10^{-15}$
erg cm-2 s-1 to 4.1$\times 10^{-14}$ erg cm-2 s-1. The corresponding X-ray
luminosities (considering isotropic emission at their corresponding distances)
vary from 2$\times 10^{39}$ erg s-1 to 54$\times 10^{40}$ erg s-1. Note that
even the lowest X-ray luminosity exceeds the Eddington luminosity limit for a
typical neutron star (mass of 1.4 M$\sun$ and radius of 10 km). Mezcua et al.
(2016) also analyzed star-forming dwarf galaxies in the COSMOS field up to z =
1.5, and found an excess of X-ray emission that is attributed to a population
of accreting BHs, accounting for expected contributions from low-mass and
high-mass X-ray binaries (LMXBs and HMXBs, respectively) as well as X-ray
emission from hot gas.

X-ray binaries (XRBs) are expected to make a significant contribution,
primarily through the presence of young, HMXBs and and the latter by longer-
lived low-mass XRBs. This is because LMXBs are typically found in early-type
galaxies with very low star formation rates and ages exceeding a few Gyr (see
review by Fabbiano 2006). The luminosity of LMXBs can range up to $10^{38}$
erg s-1, while HMXBs can have luminosities higher than $\sim 10^{39}$ erg s-1.
We can consider the contribution of HMXBs according to our samples luminosity
range. On the other hand, we expect that galaxies with larger SFR will have
more significant contributions from HMXBs but dwarf galaxies are mostly
passive and old systems.

Lehmer et al. (2016) reported an intriguing observation that a larger
population of HMXBs is found in regions with lower metallicity and the
luminosity function of HMXBs at lower luminosity (Lx < 1038 erg s-1) does not
show significant sensitivity to changes in metallicity. With the purpose of
determining the impact of metallicity, we calculate the gas-phase
metallicity444Here, we adopt the term ”metallicity” to refer to the gas-phase
oxygen abundance, which is measured in units of 12 + log (O/H). The ratio of
O/H represents the abundance of oxygen relative to hydrogen by number. The
solar metallicity is defined in this scale as 8.69 (Allende Prieto et al.,
2001), while the metallicities of the Large Magellanic Cloud (LMC) and the
Small Magellanic Cloud (SMC) are 8.4 and 8.0, respectively (Garnett, 1999).
for three out of the total of twenty galaxies in our study. This limited
number of calculations was due to the lack of optical spectral observations
for the majority of the galaxies. The calculated gas-phase metallicity values
for the three galaxies are as follows: 8.48 for A1367-1, 8.43 for RXCJ0751-1,
and 8.47 for RXCJ2214-2. A detailed descriptions of analysis is given in
Appendix A.

The relation between size and color in dwarf galaxies in cluster environments
is similar to that observed in the general population of dwarf galaxies.
According to the analysis conducted, the dwarf galaxies in the cluster
environment are on the red sequence, which means that they are relatively old
and passive objects. It does not appear that the contribution of HMXBs plays a
significant role in our sample.

In order to investigate the effect of AGN, we calculated the black hole masses
of galaxies using three approaches in the literature.

The virial method has been a useful tool in estimating the masses of these
objects in quiescent galaxies. Reines & Volonteri (2015) employed this method
to estimate black hole masses and found a linear $M_{BH}$–$M_{*}$ relation,
indicating that the black hole mass scales proportionally with the host
galaxy’s stellar mass. However, they did not take into account the bias
introduced by resolving the black hole’s sphere of influence. Shankar et al.
(2016) adopted a combination of dynamical modeling and the virial method to
estimate black hole masses in galaxies. Moreover, they accounted for the bias
introduced by resolving the black hole’s sphere of influence, resulting in a
lower normalization than that derived by Reines & Volonteri (2015). Note that
the highest stellar mass in our sample is 1.4$\times 10^{10}$ M$\sun$.
However, the de-biased $M_{BH}$–$M_{*}$ relation by Shankar et al. (2016) was
derived for galaxies with $M_{*}$ > 2$\times 10^{10}$ M$\sun$. This relation
yields significantly lower mass for the low mass systems in our sample, down
to stellar mass black hole regime. To further investigate the $M_{BH}$–$M_{*}$
relation, Suh et al. (2020) employed a Bayesian approach to estimate black
hole masses in early-type galaxies up to z $\sim$ 2.5. Considering the stellar
mass range and redshifts, the scaling relation by Reines & Volonteri (2015) is
the best resemblance to our sample among the three approaches. In this
framework, the highest $M_{BH}$ reaches the level of 10${}^{6.2}M\sun$. Note
that even higher black hole masses were reported in dwarf galaxies using
optical diagnostics (Reines et al., 2013), X-ray observations (Mezcua et al.,
2018, 2023) and radio observations (Reines et al., 2020).

The X-ray to optical luminosity ratio is a useful tool for characterizing the
properties of dwarf galaxies, and there have been several studies in the
literature that have explored this ratio for these systems. X/O can provide
information about the source of the X-ray emission, such as hot gas, star
formation, or an AGN. In general, dwarf galaxies exhibiting high X-ray to
optical emission ratios are presumed to contain abundant amounts of hot gas,
which is likely due to a plethora of phenomena such as supernova explosions,
tidal interactions, or other factors. Dwarf galaxies with low X-ray to optical
emission ratios, on the other hand, may contain little hot gas and may be
dominated by star formation or AGN activity (Jeltema et al., 2005; Mineo et
al., 2012). Nonetheless, it’s worth mentioning that the X-ray to optical
emission ratio can fluctuate extensively between diverse dwarf galaxies and
can also be affected by a range of parameters, such as the star formation
history, the presence of an AGN, and the environment. For example, dwarf
galaxies in dense cluster environments may have lower X-ray to optical
emission ratios due to ram pressure stripping, which removes hot gas from
these systems (Liu et al., 2019).

We find that the dwarf galaxies in our sample have X/O ratios less than -0.25
and there exists a positive correlation between the X/O values and the flux of
these galaxies. X-rays in galaxy scales are usually produced through accretion
processes onto the central black holes. An increased mass accretion rate would
yield an increase in X-rays (from the inner portions of the disk), as well as
and increase in optical (from the outer accretion disk). Therefore, the
positive correlation between X/O values and X-ray flux levels observed could
be due to mass transfer rate onto the central compact object.

The use of multiwavelength observations, especially targeting the optical and
X-ray wavelength ranges, has emerged as a potential probe for elucidating the
intrinsic characteristics of X-ray sources. Notably, Ultra-Luminous X-ray
Sources (ULXs) display X/O ratios primarily spanning the range of 1.5 to 2.5
indicating that the X-ray flux is much higher than the optical flux in these
objects (Feng & Kaaret, 2008). With the resulting values of our
X-ray–to–optical flux ratios, we can conclude that our X-ray sources do not
indicate the characteristics typically associated with ULXs. On the other
hand, an X-ray source with X-ray-to-optical flux ratios (considering the
R-band as the reference for optical flux) ranging from -1.0 to 1.7 is
characteristically identified as an AGN (Maccacaro et al., 1988; Lehmer et
al., 2016). Such a wide range of values suggests that AGN generally exhibits a
broader distribution of X-ray to optical flux ratios, encompassing cases where
the optical emission can exceed that of X-rays. Moreover, based on the
position of our limited sample in the BPT diagram (Baldwin et al. 1981; see
Fig. 7), a commonly employed tool for classifying galaxies based on the
primary sources of ionizing radiation, one would conclude that they would be
classified as AGN rather than star-forming galaxies.

In order to achieve a more comprehensive understanding of the characteristics
of dwarf galaxies, it is crucial to have extensive and detailed surveys that
can provide a significant number of objects to study their kinematics, stellar
populations, and metallicity. These surveys can enable the study of properties
such as substructure within the dwarf population, which can obtain insights
into how the cluster environment influences the formation and evolution of
these galaxies and the reasons behind their X-ray emissions.

Further observations and studies are essential to better understand the nature
of these X-ray emitting components. Nonetheless, hope is on the horizon as the
imminent WEAVE nearby spectroscopic cluster survey holds the promise of
providing us with a wealth of data. This survey will comprehensively cover all
the clusters of our photometric survey and endow us with thousands of
kinematic properties, such as velocity dispersion per cluster. With the help
of this upcoming data, we could unravel the intricate properties of dwarf
galaxies and gain invaluable insights into the role of their environment in
shaping their evolution.

## Acknowledgements

We thank the anonymous referee for valuable comments and suggestions that
improved the clarity and impact of our results. Ş. Ş. (Aydemir) acknowledges
support from through 2218-National Postdoctoral Research Fellowship Program
under project number 118C553 from the The Scientific and Technological
Research Council of Turkey (TÜBİTAK). A.A. acknowledges support from the
ACIISI, Consejería de Economía, Conocimiento y Empleo del Gobierno de Canarias
and the European Regional Development Fund (ERDF) under the grant with
reference PROID2021010044. NCC acknowledges support from 'Proyecto comité
mixto ESO - Gobierno de Chile', N° 21119.

## Data Availability

Upon request to the corresponding authors, the optical data supporting this
article will be made available. The original, unprocessed data is currently
stored in the Isaac Newton Group Archive. The X-ray data underlying this
article are available in the Chandra Data Archive
(https://cxc.harvard.edu/cda/) by searching the observation identifiers
(ObsID) listed in Table3 in the Search and Retrieval interface, ChaSeR
(https://cda.harvard.edu/chaser/).

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##  Appendix A Measuring gas-phase metallicity

In order to characterise the origin of the X-ray fluxes we detected in dwarfs,
we analyze their optical spectra. Unfortunately, the availability of optical
spectra has been limited for the sources in our study. We were constrained to
employ the SDSS-DR10 spectroscopic main galaxy sample data set, which provided
information for only six galaxies (A1367-1, RXCJ0751-1, RXCJ1715-1,
RXCJ1715-2, RXCJ2214-1, and RXCJ2214-2). To measure the emission-line fluxes,
we utilize the penalized pixel-fitting (pPXF) (Cappellari & Emsellem, 2004)
method and fit the observed spectra using a combination of templates from the
E-MILES stellar library (Vazdekis et al., 2016). These templates cover the
wavelength range of 3525$-$7500 Angstrom with a constant spectral resolution
of 2.50 Angstrom FWHM, similar to the SDSS spectral resolution. Among these
six galaxies, only three (A1367-1, RXCJ0751-1, and RXCJ2214-2) had a good
continuum fit thereby enabling us to calculate their emission lines and
metallicity with reasonable accuracy.

We employed standard emission-line ratios of [NII]$\lambda$6584/H$\alpha$ and
[OIII]$\lambda$5007/H$\beta$. These ratios are commonly represented in line
ratio diagrams known as "BPT diagrams" (Baldwin et al., 1981). An elevated
value of [NII]/H$\alpha$ (>0.6) is specifically for classifying the presence
of AGN activity, while [OIII]$\lambda$5007/H$\beta$ helps distinguish between
low-ionization nuclear emission-line regions (LINERs; Heckman 1980 and
Seyfert-type objects, which LINERs have [OIII]$\lambda$5007/H$\beta$ < 3).
Kauffmann et al. (2003) proposed two demarcation lines (see Fig.7) on the BPT
diagram to separate AGNs, star-forming galaxies, and AGN-star-forming
composites. The upper line represents the theoretical upper limit for
starburst galaxy emission-line ratios based on Kewley et al. (2001). Galaxies
positioned above this line in the BPT diagram are expected to exhibit a
significant contribution from an AGN to their emission-line flux. The second
line, which corresponds to typical star-forming galaxies, is a modified
version of the Kewley et al. (2001) line proposed by (Kauffmann et al., 2003).
The optical classification of galaxies lying between these two lines is called
the AGN-starburst composite. Composite galaxies are anticipated to have
approximately 41% of their [OIII]$\lambda$5007 luminosity density and 11 % of
their H$\alpha$ luminosity density attributed to an AGN (Brinchmann et al.,
2004).

To estimate the gas-phase metallicity for three galaxies out of six, the
location of those galaxies on the BPT diagram based on Kauffmann et al., 2003
are in the AGN part. In this case, we consider the AGN gas-phase metallicity
relation based on strong emission lines ([OIII]$\lambda$5007, H$\alpha$,
H$\beta$, and [NII]$\lambda$6584), according to (Storchi-Bergmann et al.,
1998) and also we consider the correction in the gas-phase metallicity
derivation according to the electron density effects on the mentioned
calibration relation based on [SII]$\lambda$6716 and [SII]$\lambda$6731 ratio.
We presented the BPT diagrams (see Fig. 7), emphasizing the reliance on the
BPT diagnostic diagram for gas-phase metallicity estimation. We also reported
the strong gas-phase metallicity values for these galaxies.

For the remaining galaxies, however, difficulties were encountered in
estimating gas-phase metallicity due to the lack of detectable strong emission
lines. We tested the different approaches such as multiple component fits,
high-degree polynomial fits, and boundary constraints on both stellar velocity
and gas velocity. Nonetheless, none of these methods made the emission lines
detectable. Moreover, we acknowledged the challenges associated with detecting
certain emission lines ([OIII]$\lambda$4363 or
[OII]$\lambda$$\lambda$7320,7330) for the metallicity estimation. Applying the
Kauffmann et al. (2003) criterion would classify all galaxies as AGNs, while
Kewley et al. (2001) would classify two galaxies as SF and one as an AGN. The
gas-phase metallicity values are 8.48, 8.43, and 8.47 for A1367-1, RXCJ0751-1,
and RXCJ2214-2, respectively based on AGNs metallicity calibration relation.
To be cautious enough, if we consider the combination of two fits on the BPT
diagram, there is no straightforward gas-phase metallicity estimation in the
AGN-starburst composite region of the BPT diagram. Moreover, we estimated the
gas-phase metallicity based on the metallicity calibration of star-forming
galaxies (Curti et al., 2016) for two galaxies, while considering the fit on
the BPT diagram established by Kewley et al. (2001). The results including
errors of 0.1 remained in sub solar region.

Figure 7: BPT diagram for our three X-ray detected dwarf galaxies in SDSS. The
red solid line is the edge of the composite ionization region from Kewley et
al. (2001). The blue dotted line shows the edge of the star formation
photoionization region, derived by Kauffmann et al. (2003).