---
abstract: 'We report the study of far-IR sizes of submillimeter galaxies (SMGs) in relation to their dust-obscured star formation rate (SFR) and active galactic nuclei (AGN) presence, determined using mid-IR photometry. We determined the millimeter-wave ($\lambda_{\rm obs}=1100\,\mu$m) sizes of 69 ALMA-identified SMGs, selected with $\geq10$$\sigma$ confidence on ALMA images ($F_{\rm 1100 \mu m}=1.7$–7.4mJy). We found that all the SMGs are located above an avoidance region in the millimeter size-flux plane, as expected by the Eddington limit for star formation. In order to understand what drives the different millimeter-wave sizes in SMGs, we investigated the relation between millimeter-wave size and AGN fraction for 25 of our SMGs at $z=1$–3. We found that the SMGs for which the mid-IR emission is dominated by star formation or AGN have extended millimeter-sizes, with respective median $R_{\rm c,e} = 1.6^{+0.34}_{-0.21}$ and 1.5$^{+0.93}_{-0.24}$kpc. Instead, the SMGs for which the mid-IR emission corresponds to star-forming/AGN composites have more compact millimeter-wave sizes, with median $R_{\rm c,e}=1.0^{+0.20}_{-0.20}$kpc. The relation between millimeter-wave size and AGN fraction suggests that this size may be related to the evolutionary stage of the SMG. The very compact sizes for composite star-forming/AGN systems could be explained by supermassive black holes growing rapidly during the SMG coalescing, star-formation phase.'
author:
- Soh Ikarashi
- 'KarinaI. Caputi'
- Kouji Ohta
- 'R.J. Ivison'
- 'ClaudiaD. P. Lagos'
- Laura Bisigello
- Bunyo Hatsukade
- Itziar Aretxaga
- 'JamesS. Dunlop'
- 'DavidH. Hughes'
- Daisuke Iono
- Takuma Izumi
- Nobunari Kashikawa
- Yusei Koyama
- Ryohei Kawabe
- Kotaro Kohno
- Kentaro Motohara
- Kouichiro Nakanishi
- Yoichi Tamura
- Hideki Umehata
- 'GrantW. Wilson'
- Kiyoto Yabe
- 'MinS. Yun'
title: 'Very compact millimeter sizes for composite star-forming/AGN submillimeter galaxies'
---

Introduction
============

The morphology and size of star-forming regions in submillimeter galaxies (SMGs) are important properties with which we can address the nature of their prodigious, dust-obscured star formation, and consequently the formation and evolution of the most massive galaxies. The Atacama Large Millimeter/submillimeter Array (ALMA) is enabling astronomers to image high-redshift SMGs with angular resolutions of $\lesssim0''$.3. Some ALMA studies have reported effective radii ($R_{\rm e}$) of $\sim0.3$–3kpc [e.g. @ika15; @sim15; @hod16]. These radii are small compared with what astronomers expected from studies of SMG sizes based on radio continuum and CO emission [e.g. @tac06; @big08; @ivi11]. These new results represent a new milestone in our understanding of star formation in SMGs, suggesting that these galaxies plausibly evolve to compact quiescent galaxies [e.g. @tof14; @ika15; @sim15].

As a next step, it would be useful to test the hypothesis that SMGs are connected to the formation of the most massive galaxies, being triggered by major mergers, and then evolving into compact quiescent galaxies via quenching in a QSO phase [e.g. @san88; @hop08; @tof14]. The compact submillimeter sizes of SMGs, including recent reports of the existence of subkilopersec-scale starburst cores [@ion16; @ika17; @ote17], suggests that the intense star-formation activity might be quenched by active galactic nuclei (AGN), as observed in some luminous QSOs [e.g. @mai12; @car16]. The link between SMGs and QSOs is still unclear, though. However, previous X-ray [e.g. @ale05; @wan13] and mid-IR [e.g. @ivi04; @cop10; @ser10] studies indicate that some SMGs do harbor AGN.

In this letter, we report a millimeter-wave size study of 69 ALMA-identified AzTEC SMGs. Firstly, we study the empirical relation between the ALMA continuum flux densities and the millimeter-wave sizes of SMGs. Secondly, we investigate the relationship between millimeter-wave sizes and the presence of AGN in SMGs at $z=1$–3, as determined from mid-IR data. We adopt throughout a cosmology with $H_{\rm 0}=70$kms$^{-1}$Mpc$^{-1}$, $\Omega_{\rm M}=0.3$, and $\Omega_{\rm \Lambda}=0.7$.

ALMA Observations and samples
=============================

The sample used in this paper comes from our ALMA 1100-$\mu$m continuum imaging survey of 144 bright AzTEC/ASTE sources with $F_{\rm 1100 \mu m,\,AzTEC}\geq 2.4$mJy in the Subaru/[*XMM-Newton*]{} Deep Field [SXDF; @fur08]. The SXDF survey was conducted in the ALMA Cycles 2 and 3 (2013.1.00781, 2015.1.00442.S: PI. Hatsukade; B.Hatsukadeetal.2017, in preparation).

The ALMA observations in Cycle 2 were carried out with the array configurations C34-5 and C34-7, with 37–38 working 12-m antennas covering up to a $uv$ distance of $\sim 1500$k$\lambda$. In Cycle 3, the observations were executed in array configuration C40-4, covering up to a $uv$ distance of $\sim 1000$k$\lambda$. On-source integration times per source in each cycle were 0.6min. The typical synthesized beam size for our ALMA continuum images is $\sim 0.''30 \times 0.''23$ ($\rm PA \sim 56^{\circ}$), after combining the Cycle 2 and 3 data. The average r.m.s. noise level is 120$\mu$Jybeam$^{-1}$. The images were generated with Briggs weighting, using a robust parameter of 0.3.

The ALMA continuum maps yielded 70 ALMA-identified AzTEC SMGs (hereafter ASXDF SMGs) with $S_{\rm peak}/N\geq 10$ detections, suitable for reliable ALMA millimeter-wave size measurements [e.g. @ika15]. We removed one lensed SMG [ASXDF1100.001; @ika11], leaving 69 SMGs. ALMA fluxes were re-measured in tapered ALMA images with a synthesized beam of $\sim0{''}.6$, which is larger than the measured mm-wave sizes of SMGs in this paper, using the IMFIT task in CASA.

For 51 ASXDF SMGs, we obtained well-constrained photometric redshifts, with a median error $\delta z= 0.13\pm0.02$, based on the individual 1-$\sigma$ errors estimated by [*Le Phare*]{} [e.g. @ilb06] in spectral energy distribution (SED) model fitting using the $B$, $V$, $Rc$, $i'$, $z'$, $J$, $H$, $Ks$, 3.6 and 4.5$\mu$m data (S.Ikarashi et al. 2017, in preparation). The remaining SMGs lie outside the coverage of the optical/near-IR images, or have individual 1-$\sigma$ errors of $>1$. Photometric and spectroscopic redshifts from the literature are listed in Table \[tbl-1\].

ALMA millimeter-wave source size measurements
=============================================

We measured millimeter-wave sizes as circularized effective radii ($R_{\rm c,e}$) for the 69 ASXDF SMGs with ALMA visibility data, in the same manner as @ika15. We used $uv$-distance versus amplitude plots (hereafter $uv$-amp plots) for our measurements. Although the ALMA data cover $uv$ distances up to $\sim 1500$k$\lambda$, we used only data at $\leq 500$k$\lambda$, which corresponds to a scale of $\sim0.''2$. Adopting this cutoff for the longest $uv$ distance is the equivalent of smoothing with a larger size kernel in the image plane. We aim to mitigate the effects of possible clumpy structures in the size measurements and to measure $R_{\rm c,e}$ robustly. For the sources detected with $\geq10\sigma$ in the ALMA Cycle-2 images alone, we measured their sizes using only Cycle-2 data, to avoid effects due to any systematic absolute flux calibration offsets between our Cycle 2 and 3 data [^1]. We measured sizes by fitting a Gaussian model to the observed data in the $uv$-amp plots. When we measure the size, the other sources ($\geq5\sigma$) in each ALMA image were removed from the visibility data based on simple source properties derived by IMFIT task.

In order to estimate possible systematics in the size measurements, we injected mock sources into ALMA noise visibility images, generated from the actual ALMA data as in @ika15. Briefly we injected a symmetric Gaussian component for a range of source sizes and flux densities that cover the putative parameter range of our ASXDF sources with uniform probability. As tested in @ika15, our method can measure circularized effective radii correctly even if a source has an asymmetric Gaussian profile. We corrected our raw measured sizes based on the results of the simulations for the data used in this paper. As a result, we obtained ALMA millimeter-wave sizes of 0$''.08$–0$''.68$ (FWHM) for the 69 ASXDF SMGs. Note that ASXDF1100.009.1 has two distinct millimeter-wave components with a separation of $\sim$0$''$.6, sharing a host galaxy at $z_{\rm spec}=0.9$.

Relation between millimeter sizes and fluxes
============================================

Fig. \[fig:sizeflux\] (left panel) shows all 69 ASXDF SMGs in an ALMA 1100-$\mu$m vs. millimeter-wave size plot. Additionally, we plot 13 ALMA-identified, fainter SMGs at $z\gtrsim 3$ from @ika15. ASXDF SMGs are absent from the top-left and the bottom-right corners of this plot. The expected source selection limit for $\geq10\sigma$ continuum detection based on simple Gaussian models explains the absence of SMGs in the top-left corner. The bottom-right corner, instead, is free from any such selection biases, so the absence of SMGs requires an explanation.

The absence of SMGs in the bottom-right corner of Fig.  \[fig:sizeflux\] can be interpreted as the influence of Eddington-limited star formation [@mur05]. According to @you08, which reported pioneering studies of maximum star formation in bright SMGs, a maximum star-formation rate is given by $$SFR_{\rm max} = 480\sigma^2_{400}D_{\rm kpc}\,\kappa^{-1}_{100} M_{\odot} yr^{-1},$$ where $D_{\rm kpc}$ is the characteristic physical scale of the starburst region in kpc, $\sigma_{\rm 400}$ is the line-of-sight gas velocity dispersion in units of 400kms$^{-1}$, and $\kappa_{\rm 100}$ is the dust opacity in units of 100cm$^2$g$^{-1}$. Here we adopt a Chabrier initial mass function [@cha03]; $\kappa_{\rm 100}=1$, as in @you08; and a median gas velocity dispersion of 510kms$^{-1}$ from CO line observations of SCUBA SMGs [@bot13]. We also adopt 2$\times$ FWHM or 4$\times R_{\rm c,e}$, which is expected to include 94% of the total light, as $D_{\rm kpc}$. The derived $SFR_{\rm max}$ was corrected with this factor of 0.94.

In order to plot the relation between SFR and physical scale described by Equation 1 on Fig. \[fig:sizeflux\] (the left panel), we assume a fixed redshift $z=2$. The conversion factors from ALMA fluxes to SFRs were derived by bootstrapping given a dust temperature ($T_d$) distribution for lensed 1.3mm-selected galaxies [@wei13] and an SED library with $T_d$ information compiled in @swi14. For these assumptions, we obtain a possible range of Eddington-limited star formation rates.

For a more direct comparison of the millimeter fluxes and sizes of SMGs with Eddington-limited star formation, we re-plot 51 of the 69 SMGs at $z=0.7$–6.8 with optical/near-IR photometric or spectroscopic redshifts on the SFR–physical size plane (Fig. \[fig:sizeflux\], right panel). The SFRs are derived from $F_{\rm 1100 \mu m}$, given the range of possible dust temperatures $T_d$ and SEDs noted above. We assume that the AGN contribution to the submillimeter flux is negligible [see references in @ros12]. In order to visualize the coverage of the size-SFR plane produced by the large SFR uncertainties (due to the unknown dust SED temperatures), we show the full SFR probability density distribution (rather than a single value) for each SMG. The results in both panels of Fig. \[fig:sizeflux\] show that the SMGs avoid the SFR region around the Eddington limit, suggesting that the minimum possible millimeter-wave sizes for bright SMGs are given by the Eddington limited star formation.

The empirical relation between flux and size can explain the apparent discrepancy between the reported (sub)millimeter-wave (median) sizes of $0.''20^{+0.''03}_{-0.''05}$ by @ika15 and $0.''3\pm0.''04$ by @sim15. Given $F_{\rm 870 \mu m}/F_{\rm 1100 \mu m}=2$ for conversion of the observed fluxes, @sim15 covered $F_{\rm 1100 \mu m} \gtrsim 2.5$mJy. In this regime, our ASXDF SMGs have a median size of$0.''31^{+0.''03}_{-0.''03}$.

\[fig:sizeflux\]

Relation between millimeter sizes and AGN {#sec:agn}
=========================================

We present our studies of the connection between the millimeter-wave sizes and AGN in SMGs, based on a mid-IR AGN diagnostic. We consider 25 ALMA-identified SMGs with $1<z_{\rm phot\,or\,spec}<3$, which are detected in all IRAC and MIPS 24$\mu$m images. All SMGs here have redshift information and a single component at $\sim$0$''$.2 resolution. More than 15 out of the 25 are located above $4\times$ the main sequence at $z\sim2$ in the stellar mass vs. SFR plane (Fig. \[fig:masssfr\]), indicating that the majority of the sample are starbursts [@bis18]. Note that among the 29 SMGs with $z=1$–3, four are not considered in our analysis: two SMGs are not detected at 24$\mu$m and the other two are blended in the IRAC maps.

Mid-IR AGN diagnostic
---------------------

A 4.5$\mu$m/8$\mu$m/24$\mu$m color-color plot has often been used as an AGN diagnostic for high-redshift, dusty infrared-luminous galaxies, such as SMGs and DOGs at $z\sim2$ [e.g. @ivi04; @ivi07; @pop08a; @pop08b]. We refer the reader to @kir15, who presented a detailed study of mid-IR SED evolution versus AGN fraction for high-$z$ galaxies. Empirical SED templates (top left panel in Fig. \[fig:sizeagn\]) suggest that high-redshift galaxies dominated by star formation or AGN in mid-IR light can be segregated from each other in the mid-IR color-color plane. The position of our 25 SMGs in this color-color plot shows that some of them do not follow either the model tracks for star-formation-dominated or AGN-dominated galaxies.

We generated the expected mid-IR colors of galaxies that are a composite of SF and AGN by combining SEDs of SF and AGN with various SF/AGN ratios. This ‘toy’ color prediction reproduces the colors of ‘composite SMGs’ which are likely to be dominated by neither an AGN nor a starburst in the mid-IR (top right panel in Fig. \[fig:sizeagn\]).

We divided the 25 SMGs into four sub-groups based on their 4.5/8/24-$\mu$m colors: star-forming, composite, AGN-dominant and ‘no class’. The criteria are:

-   $F_{\rm 8 \mu m}/F_{\rm 4.5 \mu m}<1.15$ $\bigwedge$ $F_{\rm 24 \mu m}/F_{\rm 8 \mu m}\geq 5$ (star-forming)

-   $F_{\rm 8 \mu m}/F_{\rm 4.5 \mu m}\geq1.15$ $\bigwedge$ $F_{\rm 24 \mu m}/F_{\rm 8 \mu m}\geq 5$ (composite)

-   $F_{\rm 8 \mu m}/F_{\rm 4.5 \mu m}\geq1.50$ $\bigwedge$ $F_{\rm 24 \mu m}/F_{\rm 8 \mu m}< 5$ (AGN)

-   $F_{\rm 8 \mu m}/F_{\rm 4.5 \mu m}<1.50$ $\bigwedge$ $F_{\rm 24 \mu m}/F_{\rm 8 \mu m}< 5$ (no class).

The model colors (top, Fig. \[fig:sizeagn\]) indicate that the SMGs categorized as ‘no class’ could be in the star-forming or composite classes. Due to this ambiguity, we consider the ‘no class’ separately.

Note that, In our diagnostic, the star-forming class and AGN dominant class are defined first. We choose $F_{\rm 8.0 \mu m}/F_{\rm 4.5 \mu m}=$1.15 as criterion for separation, as this ensures that all galaxies that satisfy neither an AGN criteria by @don12 nor another criteria by @ste05 also lie on the star-forming region of the colour-colour diagram. The predicted 24$\mu$m/8$\mu$m color evolution with redshift, as derived by public empirical mid-IR SED templates for high-$z$ star-forming galaxies, composite galaxies, and AGN dominant galaxies [@kir15], are shown along with our sample SMGs (bottom left, Fig. \[fig:sizeagn\]). For these templates, the respective mid-IR AGN fractions of each sample are $<$20, 20–80, and $\geq$80%. In this plot we averaged the public SEDs in each AGN class, after scaling all fluxes at $\lambda_{\rm rest}=8$$\mu$m. The predictions based on the Kirkpatrick et al. SED templates suggest that our criteria for 24$\mu$m/8$\mu$m color can work to select an AGN-dominant class, and show that our composite type is expected to have typically AGN fractions of around $\sim$50%, consistently with our ’toy’ models.

Results
-------

In the millimeter-wave physical size vs. SFR plot (bottom right panel in Fig. \[fig:sizeagn\]), all SMGs with composite mid-IR components are evidently more compact and located closer to the Eddington limit than the other SMGs with star-forming dominant or AGN dominant mid-IR components.

The respective median $R_{\rm c,e}$ for the SMGs classified as star-forming dominant, composites, and AGN dominant are 1.6$^{+0.34}_{-0.21}$, 1.0$^{+0.20}_{-0.20}$, and 1.5$^{+0.93}_{-0.24}$kpc. The size difference between the SMGs with composite and star-forming mid-IR components, and the difference between the SMGs with composite and AGN-dominant mid-IR components are real, with a significance level of $>99$%, according to the Kolmogorov-Smirnov test. This indicates that the composite type galaxies are characterized by more compact star-forming regions than those of the star-forming or AGN-dominant types.

We also explored the relation between size and stellar mass in our sample and found that the size differences are not a consequence of different stellar masses. Composite SMGs are the most compact of the three types, even at fixed stellar mass.

None of our ALMA-identified AzTEC SMGs are detected in the existing [*XMM-Newton*]{} X-ray maps [@ued08], probably because these maps are too shallow. Nevertheless, we can compare our results with the sizes derived for the host galaxies of five high-$z$, X-ray-selected AGN ($L_{\rm 2-8keV}=10^{42.1-43.6}$ergs$^{-1}$) by @har16. These authors reported a size distribution for their AGN hosts similar to the SMG sizes in @sim15. The most X-ray luminous source in their sample (with $L_{\rm 2-8keV}=10^{43.6}$ergs$^{-1}$) has an extended size, and the remaining four ($L_{\rm 2-8keV}=10^{42.1-43.4}$ergs$^{-1}$) have compact sizes, which are comparable to those of our composite type here (Fig. \[fig:sizeagn\], bottom right).

AGN growth during a very compact star-forming phase?
----------------------------------------------------

The very compact millimeter-wave sizes of the SMGs with composite mid-IR components suggest that a central supermassive black hole could be growing in a compact and coalescing star-forming phase, which is consistent with the predictions of @spr05 for galaxy major mergers. The extended millimeter-wave sizes of the SMGs of the star-forming dominant class can be explained by a mid-stage merger as seen in, e.g., VV114 [@sai15]. Actually ASXDF1100.055.1 with the star-forming dominant class shows merger-like near-IR morphology (Fig.\[fig:hst\]). Instead, the extended sizes of the SMGs with the AGN-dominant class are puzzling. In line with the evolutionary scenarios of, e.g., @san88 [@hop08; @tof14] that SMGs evolve into QSOs, these extended sizes may be explained by positive AGN feedback by a growing supermassive black hole in the phase of star-formation quenching, as it is suggested by simulations for luminous AGN/QSOs [e.g. @ish12; @zub13] and considered for some luminous QSOs [e.g. @car16]. In fact, ASXDF1100.057.1 with the AGN dominant class has a QSO-like near-IR morphology (Fig.\[fig:hst\]). However, no significant near-IR morphological difference between AGN-host and non-AGN-host galaxies, that are not submillimeter selected, is reported [e.g. @koc12]. The extended submillimeter sizes in our SMGs may come from the nature of their host galaxies.

[ l c c c c c c c c c ]{} ID & R.A. & Dec. &SNR& $F_{\rm 1100 \mu m}$ & $z_{\rm photo}$ & SFR & mm-wave size & mm-wave size & AGN\
& (J2000) & (J2000) & & (mJy) & & ($M_{\odot}$yr$^{-1}$) & (FWHM; arcsec) & ($R_{\rm c,e}$; kpc) & (mid-IR)\
ASXDF1100.002.1 & 2:17:30.63 & -4:59:36.8 & 15 & 4.81$\pm$0.43 & 3.3$^{+0.07}_{-0.87}$ & 990$^{+720}_{-340}$ & 0.42$^{+0.06}_{-0.02}$ & 1.6$^{+0.2}_{-0.1}$ &\
ASXDF1100.004.1 & 2:18:05.65 & -5:10:49.7 & 14 & 4.39$\pm$0.56 & 3.5$^{+0.35}_{-0.16}$ & 880$^{+420}_{-290}$ & 0.40$^{+0.06}_{-0.04}$ & 1.5$^{+0.2}_{-0.1}$ &\
ASXDF1100.005.1 & 2:17:30.45 & -5:19:22.5 & 25 & 7.24$\pm$0.45 & 0.7$^{+0.01}_{-0.01}$ & 1200$^{+990}_{-420}$ & 0.34$^{+0.04}_{-0.02}$ & 1.2$^{+0.1}_{-0.1}$ &\
ASXDF1100.006.1 & 2:17:27.32 & -5:06:42.8 & 10 & 5.11$\pm$0.50 & 4.5$^{+0.18}_{-0.15}$ & 930$^{+340}_{-330}$ & 0.68$^{+0.06}_{-0.06}$ & 2.2$^{+0.2}_{-0.2}$ &\
ASXDF1100.007.1 & 2:18:03.01 & -5:28:42.0 & 20 & 6.26$\pm$0.53 & 3.2$^{+0.28}_{-0.22}$ & 1300$^{+930}_{-450}$ & 0.32$^{+0.04}_{-0.02}$ & 1.2$^{+0.1}_{-0.1}$ &\
ASXDF1100.008.1 & 2:16:47.93 & -5:01:29.9 & 12 & 6.45$\pm$0.59 & 2.2$^{+0.02}_{-0.08}$ & 1500$^{+950}_{-460}$ & 0.62$^{+0.06}_{-0.06}$ & 2.6$^{+0.2}_{-0.2}$ & AGN\
ASXDF1100.009.1A & 2:17:42.11 & -4:56:27.6 & 19 & 4.68$\pm$0.40 &(0.5)$^a$ & 550$^{+430}_{-190}$ & 0.30$^{+0.02}_{-0.04}$ & 0.9$^{+0.1}_{-0.1}$ &\
ASXDF1100.009.1B & 2:17:42.16 & -4:56:28.5 & 11 & 1.16$\pm$0.12 &(0.5)$^a$ & 140$^{+110}_{-50}$ & 0.10$^{+0.08}_{-0.06}$ & 0.6$^{+0.5}_{-0.4}$ &\
ASXDF1100.011.1 & 2:17:50.59 & -5:30:59.2 & 13 & 4.22$\pm$0.41 & 5.5$^{+0.08}_{-0.63}$ & 730$^{+440}_{-260}$ & 0.38$^{+0.04}_{-0.04}$ & 1.1$^{+0.1}_{-0.1}$ &\
ASXDF1100.014.1$^{\dagger}$ & 2:17:29.77 & -5:03:18.6 & 11 & 3.12$\pm$0.17 & 2.2$^{+0.04}_{-0.03}$ & 690$^{+270}_{-210}$ & 0.50$^{+0.06}_{-0.08}$ & 2.1$^{+0.2}_{-0.3}$ & SF\
ASXDF1100.016.1 & 2:16:41.11 & -5:03:51.4 & 19 & 4.79$\pm$0.35 & 5.0$^{+0.54}_{-0.06}$ & 850$^{+390}_{-240}$ & 0.24$^{+0.02}_{-0.04}$ & 0.8$^{+0.1}_{-0.1}$ &\
ASXDF1100.018.1 & 2:18:13.83 & -4:57:43.5 & 14 & 3.47$\pm$0.32 & 1.7$^{+0.09}_{-0.02}$ & 850$^{+650}_{-280}$ & 0.26$^{+0.04}_{-0.04}$ & 1.1$^{+0.2}_{-0.2}$ & NO\
ASXDF1100.020.1$^{\bullet}$ & 2:18:23.73 & -5:11:38.5 & 13 & 4.94$\pm$0.43 & 2.7$^{+0.01}_{-0.01}$ & 1100$^{+460}_{-380}$ & 0.30$^{+0.04}_{-0.02}$ & 1.2$^{+0.2}_{-0.1}$ &\
ASXDF1100.021.1 & 2:18:16.49 & -4:55:08.8 & 16 & 4.03$\pm$0.28 & 2.3$^{+0.03}_{-0.04}$ & 920$^{+720}_{-310}$ & 0.28$^{+0.02}_{-0.04}$ & 1.1$^{+0.1}_{-0.2}$ & COM\
ASXDF1100.022.1 & 2:18:42.68 & -4:59:32.1 & 15 & 3.09$\pm$0.31 & 2.3$^{+0.01}_{-0.06}$ & 710$^{+550}_{-240}$ & 0.20$^{+0.04}_{-0.04}$ & 0.8$^{+0.2}_{-0.2}$ & COM\
ASXDF1100.023.2 & 2:18:20.40 & -5:31:43.2 & 10 & 2.17$\pm$0.27 & 2.5$^{+0.10}_{-0.12}$ & 480$^{+350}_{-160}$ & 0.16$^{+0.10}_{-0.06}$ & 0.6$^{+0.4}_{-0.2}$ &\
ASXDF1100.025.2$^{\dagger}$ & 2:17:32.59 & -4:50:26.4 & 13 & 2.34$\pm$0.12 & 3.4$^{+0.16}_{-0.07}$ & 470$^{+320}_{-150}$ & 0.34$^{+0.06}_{-0.04}$ & 1.3$^{+0.2}_{-0.1}$ &\
ASXDF1100.029.1$^{\dagger}$ & 2:17:20.80 & -4:49:49.5 & 11 & 2.67$\pm$0.21 & 2.8$^{+0.16}_{-0.17}$ & 570$^{+360}_{-180}$ & 0.46$^{+0.08}_{-0.10}$ & 1.8$^{+0.3}_{-0.4}$ & AGN\
ASXDF1100.031.1$^{\dagger}$ & 2:17:37.24 & -4:47:53.0 & 13 & 2.09$\pm$0.15 & 2.5$^{+0.18}_{-0.12}$ & 480$^{+380}_{-170}$ & 0.28$^{+0.04}_{-0.06}$ & 1.1$^{+0.2}_{-0.2}$ & COM\
ASXDF1100.033.1 & 2:18:03.56 & -4:55:27.3 & 15 & 4.86$\pm$0.33 & (2.6)$^c$ & 1100$^{+860}_{-350}$ & 0.34$^{+0.04}_{-0.02}$ & 1.4$^{+0.2}_{-0.1}$ & COM\
ASXDF1100.034.1 & 2:17:59.32 & -5:05:04.6 & 11 & 2.84$\pm$0.32 & (1.6)$^b$& 680$^{+640}_{-220}$ & 0.16$^{+0.08}_{-0.06}$ & 0.7$^{+0.3}_{-0.3}$ &\
ASXDF1100.035.1$^{\dagger,\bullet}$ & 2:17:35.37 & -5:28:37.3 & 12 & 2.09$\pm$0.12 & 2.7$^{+0.07}_{-0.11}$ & 450$^{+360}_{-150}$ & 0.52$^{+0.08}_{-0.08}$ & 2.1$^{+0.3}_{-0.3}$ &\
ASXDF1100.041.1 & 2:17:53.87 & -5:26:35.7 & 10 & 2.91$\pm$0.29 & 0.8$^{+0.00}_{-0.00}$ & 520$^{+260}_{-180}$ & 0.42$^{+0.06}_{-0.10}$ & 1.6$^{+0.2}_{-0.4}$ &\
ASXDF1100.042.1 & 2:18:38.29 & -5:03:18.3 & 12 & 3.26$\pm$0.40 & 3.2$^{+0.02}_{-0.01}$ & 680$^{+440}_{-240}$ & 0.42$^{+0.04}_{-0.06}$ & 1.6$^{+0.1}_{-0.2}$ &\
ASXDF1100.044.1 & 2:17:45.85 & -5:00:56.7 & 12 & 1.93$\pm$0.26 & 6.8$^{+0.20}_{-0.72}$ & 330$^{+210}_{-84}$ & 0.09$^{+0.07}_{-0.05}$ & 0.2$^{+0.2}_{-0.1}$ &\
ASXDF1100.046.1 & 2:17:13.34 & -4:58:57.4 & 16 & 4.00$\pm$0.32 & 3.5$^{+0.01}_{-0.10}$ & 810$^{+620}_{-280}$ & 0.28$^{+0.04}_{-0.04}$ & 1.0$^{+0.1}_{-0.1}$ &\
ASXDF1100.047.1$^{\dagger}$ & 2:17:56.73 & -4:52:39.0 & 11 & 2.25$\pm$0.17 & 2.2$^{+0.01}_{-0.02}$ & 500$^{+400}_{-160}$ & 0.40$^{+0.08}_{-0.06}$ & 1.6$^{+0.3}_{-0.2}$ & SF\
ASXDF1100.048.1$^{\dagger}$ & 2:17:46.16 & -4:47:47.2 & 14 & 2.55$\pm$0.11 & 2.5$^{+0.21}_{-0.12}$ & 570$^{+460}_{-200}$ & 0.40$^{+0.06}_{-0.04}$ & 1.6$^{+0.2}_{-0.2}$ & NO\
ASXDF1100.050.1$^{\star}$ & 2:18:22.30 & -5:07:37.0 & 11 & 3.32$\pm$0.40 & 3.0$^{+0.15}_{-0.15}$ & 700$^{+360}_{-240}$ & 0.24$^{+0.08}_{-0.08}$ & 0.9$^{+0.3}_{-0.3}$ &\
ASXDF1100.051.1$^{\dagger}$ & 2:18:23.96 & -5:32:07.8 & 12 & 2.63$\pm$0.23 & 0.7$^{+0.00}_{-0.04}$ & 430$^{+270}_{-150}$ & 0.08$^{+0.06}_{-0.04}$ & 0.3$^{+0.2}_{-0.1}$ &\
ASXDF1100.051.2$^{\dagger}$ & 2:18:24.59 & -5:31:48.5 & 11 & 2.88$\pm$0.23 & 4.7$^{+0.24}_{-0.15}$ & 520$^{+270}_{-160}$ & 0.30$^{+0.10}_{-0.06}$ & 1.0$^{+0.3}_{-0.2}$ &\
ASXDF1100.052.1$^{\dagger}$ & 2:17:33.17 & -5:01:54.5 & 11 & 2.05$\pm$0.14 & 2.8$^{+0.25}_{-0.65}$ & 440$^{+340}_{-150}$ & 0.34$^{+0.04}_{-0.06}$ & 1.3$^{+0.2}_{-0.2}$ & AGN\
ASXDF1100.055.1$^{\dagger}$ & 2:17:20.03 & -5:13:05.8 & 13 & 2.54$\pm$0.15 & 2.1$^{+0.02}_{-0.24}$ & 570$^{+290}_{-180}$ & 0.34$^{+0.06}_{-0.06}$ & 1.4$^{+0.2}_{-0.2}$ & SF\
ASXDF1100.057.1 & 2:17:32.41 & -5:12:50.9 & 12 & 3.54$\pm$0.38 & 1.9$^{+0.04}_{-0.11}$ & 820$^{+360}_{-260}$ & 0.34$^{+0.04}_{-0.06}$ & 1.4$^{+0.2}_{-0.3}$ & AGN\
ASXDF1100.076.1 & 2:16:41.04 & -5:01:12.5 & 13 & 4.13$\pm$0.55 & 4.8$^{+0.13}_{-0.41}$ & 750$^{+550}_{-230}$ & 0.34$^{+0.04}_{-0.06}$ & 1.1$^{+0.1}_{-0.2}$ &\
ASXDF1100.077.1$^{\dagger}$ & 2:18:11.00 & -4:49:51.9 & 12 & 1.69$\pm$0.20 & 4.1$^{+0.02}_{-0.12}$ & 320$^{+190}_{-110}$ & 0.22$^{+0.08}_{-0.08}$ & 0.8$^{+0.3}_{-0.3}$ &\
ASXDF1100.089.1 & 2:18:10.64 & -5:34:53.6 & 21 & 4.73$\pm$0.30 & 5.4$^{+0.11}_{-0.09}$ & 830$^{+600}_{-200}$ & 0.24$^{+0.04}_{-0.02}$ & 0.7$^{+0.1}_{-0.1}$ &\
ASXDF1100.095.1$^{\dagger}$ & 2:17:12.97 & -5:14:12.2 & 10 & 1.91$\pm$0.19 & 2.2$^{+0.11}_{-0.08}$ & 440$^{+320}_{-150}$ & 0.32$^{+0.08}_{-0.08}$ & 1.3$^{+0.3}_{-0.3}$ & AGN\
ASXDF1100.100.1 & 2:17:53.25 & -4:49:51.5 & 13 & 2.84$\pm$0.29 & 2.2$^{+0.16}_{-0.08}$ & 670$^{+550}_{-210}$ & 0.24$^{+0.04}_{-0.04}$ & 1.0$^{+0.2}_{-0.2}$ & COM\
ASXDF1100.105.1 & 2:18:02.86 & -5:00:31.6 & 13 & 2.86$\pm$0.30 & (1.1)$^b$ & 630$^{+460}_{-220}$ & 0.24$^{+0.06}_{-0.08}$ & 1.0$^{+0.2}_{-0.3}$ & COM\
ASXDF1100.107.1$^{\dagger}$ & 2:18:07.85 & -5:25:49.3 & 11 & 1.67$\pm$0.16 & 4.6$^{+0.18}_{-0.86}$ & 310$^{+190}_{-80}$ & 0.34$^{+0.06}_{-0.06}$ & 1.1$^{+0.2}_{-0.2}$ &\
ASXDF1100.115.1 & 2:16:59.42 & -5:10:55.8 & 12 & 4.23$\pm$0.33 & (0.6)$^a$ & 600$^{+500}_{-220}$ & 0.50$^{+0.06}_{-0.06}$ & 1.7$^{+0.2}_{-0.2}$ &\
ASXDF1100.134.1 & 2:17:54.80 & -5:23:23.8 & 15 & 3.27$\pm$0.27 & 2.5$^{+0.16}_{-0.05}$ & 740$^{+500}_{-260}$ & 0.24$^{+0.06}_{-0.04}$ & 1.0$^{+0.2}_{-0.2}$ & COM\
ASXDF1100.156.1 & 2:16:38.33 & -5:01:21.5 & 11 & 3.33$\pm$0.31 & 1.8$^{+0.04}_{-0.10}$ & 810$^{+630}_{-260}$ & 0.34$^{+0.06}_{-0.06}$ & 1.4$^{+0.3}_{-0.3}$ & SF\
ASXDF1100.188.1$^{\dagger,\star}$ & 2:16:41.94 & -5:07:04.3 & 10 & 2.42$\pm$0.18 & 2.6$^{+0.28}_{-0.20}$ & 530$^{+450}_{-180}$ & 0.22$^{+0.10}_{-0.08}$ & 0.9$^{+0.4}_{-0.3}$ &\
ASXDF1100.203.1$^{\dagger}$ & 2:18:23.15 & -5:27:02.0 & 11 & 1.90$\pm$0.12 & 2.5$^{+0.03}_{-0.15}$ & 440$^{+330}_{-150}$ & 0.34$^{+0.10}_{-0.10}$ & 1.4$^{+0.4}_{-0.4}$ & NO\
ASXDF1100.227.1 & 2:17:44.27 & -5:20:08.6 & 24 & 7.42$\pm$0.57 & 3.7$^{+0.35}_{-0.14}$ & 1400$^{+760}_{-510}$ & 0.34$^{+0.02}_{-0.02}$ & 1.2$^{+0.1}_{-0.1}$ &\
ASXDF1100.228.1 & 2:18:09.66 & -5:18:43.1 & 12 & 3.11$\pm$0.34 & 1.9$^{+0.05}_{-0.14}$ & 740$^{+610}_{-240}$ & 0.38$^{+0.06}_{-0.06}$ & 1.6$^{+0.3}_{-0.2}$ & SF\
ASXDF1100.229.1 & 2:18:18.84 & -4:50:29.9 & 11 & 3.60$\pm$0.36 & 2.3$^{+0.05}_{-0.11}$ & 820$^{+620}_{-270}$ & 0.26$^{+0.06}_{-0.08}$ & 1.1$^{+0.2}_{-0.3}$ & COM\
ASXDF1100.235.1 & 2:17:36.00 & -5:20:34.4 & 13 & 4.64$\pm$0.40 & 2.3$^{+0.04}_{-0.14}$ & 1100$^{+820}_{-370}$ & 0.26$^{+0.06}_{-0.04}$ & 1.1$^{+0.2}_{-0.2}$ & COM\
ASXDF1100.236.1$^{\dagger}$ & 2:17:21.54 & -5:19:07.7 & 11 & 1.65$\pm$0.14 & 2.4$^{+0.02}_{-0.02}$ & 370$^{+250}_{-120}$ & 0.15$^{+0.09}_{-0.09}$ & 0.6$^{+0.4}_{-0.4}$ & COM\
ASXDF1100.247.1$^{\dagger}$ & 2:16:33.85 & -5:02:42.7 & 11 & 1.87$\pm$0.18 & 2.6$^{+0.11}_{-0.14}$ & 410$^{+260}_{-140}$ & 0.24$^{+0.08}_{-0.10}$ & 1.0$^{+0.3}_{-0.4}$ & COM\
ASXDF1100.003.1$^{\dagger}$ & 2:16:44.48 & -5:02:21.6 &15 &2.85$\pm$0.13 && &0.36$^{+0.04}_{-0.04}$ &&\
ASXDF1100.010.1 & 2:17:39.79 & -5:29:19.2 &24 &5.94$\pm$0.37 && &0.28$^{+0.02}_{-0.02}$ &&\
ASXDF1100.026.1$^{\dagger}$ & 2:17:42.55 & -5:29:00.3 &11 &1.69$\pm$0.17 && & 0.18$^{+0.06}_{-0.12}$&&\
ASXDF1100.040.1 & 2:17:55.24 & -5:06:45.1 &15 &3.14$\pm$0.35 && &0.20$^{+0.06}_{-0.04}$&&\
ASXDF1100.053.1 & 2:16:48.20 & -4:58:59.6 &10 &4.02$\pm$0.51 && &0.42$^{+0.06}_{-0.06}$ &&\
ASXDF1100.054.1 & 2:17:15.41 & -4:57:55.6 &11 &4.12$\pm$0.38 && &0.38$^{+0.06}_{-0.06}$ &&\
ASXDF1100.068.1 & 2:17:42.17 & -5:25:46.8 &12 &3.24$\pm$0.30 && &0.24$^{+0.04}_{-0.06}$&&\
ASXDF1100.070.1$^{\dagger}$ & 2:18:46.15 & -5:04:12.5 &12 &2.17$\pm$0.13 && &0.30$^{+0.04}_{-0.06}$&&\
ASXDF1100.074.1 & 2:18:33.31 & -4:58:07.0 &10 &2.77$\pm$0.33 && &0.32$^{+0.06}_{-0.06}$ &&\
ASXDF1100.097.1 & 2:18:18.54 & -5:34:34.7 &11 &2.53$\pm$0.26 && &0.20$^{+0.08}_{-0.06}$ & &\
ASXDF1100.097.2$^{\dagger}$ & 2:18:17.61 & -5:34:27.9 &10 &2.14$\pm$0.26 && &0.32$^{+0.08}_{-0.10}$& &\
ASXDF1100.133.1 & 2:18:05.51 & -5:35:46.5 &11 &2.25$\pm$0.26 && &0.08$^{+0.08}_{-0.04}$ & &\
ASXDF1100.161.1$^{\dagger}$ & 2:18:13.76 & -5:37:27.3 &12 &2.68$\pm$0.20 && &0.44$^{+0.06}_{-0.06}$ & &\
ASXDF1100.168.1 & 2:18:04.37 & -5:34:03.5 &11 &1.79$\pm$0.21 && &0.16$^{+0.08}_{-0.06}$& &\
ASXDF1100.213.1$^{\dagger}$ & 2:18:44.02 & -5:35:31.3 &12 &2.90$\pm$0.28 && &0.16$^{+0.08}_{-0.08}$& &\
ASXDF1100.231.1 & 2:17:59.65 & -4:46:49.8 &12 &2.88$\pm$0.36 && &0.28$^{+0.08}_{-0.08}$ &&\
ASXDF1100.243.1$^{\dagger}$ & 2:16:50.43 & -5:10:16.2 &10 &2.09$\pm$0.20 && &0.37$^{+0.09}_{-0.11}$&&\
ASXDF1100.252.1 & 2:17:05.65 & -5:15:04.9 &12 &2.62$\pm$0.25 && &0.24$^{+0.06}_{-0.08}$ &&\
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[^1]: Comparisons of the fluxes of ASXDF sources in our Cycle-1, 2 and 3 data indicated that the fluxes in the Cycle-3 data are systematically $\sim$20% smaller. Therefore, we corrected the primary flux calibration for this effect.
