Chandra Survey of Nearby Galaxies: A Significant Population of Candidate   Central Black Holes in Late-type Galaxies

Rui She; Luis C. Ho; Hua Feng

arXiv:1706.01201·astro-ph.GA·July 26, 2017

Chandra Survey of Nearby Galaxies: A Significant Population of Candidate Central Black Holes in Late-type Galaxies

Rui She, Luis C. Ho, Hua Feng

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TL;DR

This study uses Chandra X-ray data to identify candidate active galactic nuclei in nearby galaxies, revealing a significant population of potential low-mass central black holes especially in late-type, bulgeless galaxies.

Contribution

It provides the first large-scale X-ray survey of late-type galaxies, estimating a black hole occupation fraction of over 21% in these often bulgeless systems.

Findings

01

Identified 314 candidate AGNs in 719 nearby galaxies.

02

Detected 51 X-ray cores in H II nuclei, with high likelihood of hosting black holes.

03

Estimated a black hole occupation fraction of >21% in late-type galaxies.

Abstract

Based on the Chandra data archive as of March 2016, we have identified 314 candidate active galactic nuclei in 719 galaxies located closer than 50 Mpc, among them late-type (Hubble types Sc and later) galaxies that previously had been classified from optical observations as containing star-forming (H II) nuclei. These late-type galaxies comprise a valuable subsample to search for low-mass (<~ 10^6 solar masses) central black holes. For the sample as a whole, the overall dependence of the fraction of active nuclei on galaxy type and nuclear spectral classification is consistent with previous results based on optical surveys. We detect 51 X-ray cores among the 163 H II nuclei and estimate that, very conservatively, ~74% of them with luminosities above 10^38 erg/s are not contaminated by X-ray binaries; the fraction increases to ~92% for X-ray cores with a luminosity of 10^39 erg/s or…

Tables2

Table 1. Table 1 AGN numbers and detection rates as a function of Hubble type

Type	All Galaxies			Unbarred Galaxies			Barred Galaxies
	$N_{G}$	$N_{A}$	Rate (%)	$N_{G}$	$N_{A}$	Rate (%)	$N_{G}$	$N_{A}$	Rate (%)
E	142	80	56 ${}_{- 6}^{+ 6}$	$\dots$	$\dots$	$\dots$	$\dots$	$\dots$	$\dots$
S0	126	70	55 ${}_{- 7}^{+ 7}$	37	26	69 ${}_{- 12}^{+ 11}$	51	27	52 ${}_{- 11}^{+ 11}$
S0/a-Sab	71	47	65 ${}_{- 9}^{+ 8}$	22	17	75 ${}_{- 15}^{+ 12}$	38	24	62 ${}_{- 12}^{+ 12}$
Sb-Sbc	95	53	55 ${}_{- 8}^{+ 8}$	29	16	54 ${}_{- 14}^{+ 14}$	60	35	58 ${}_{- 10}^{+ 10}$
Sc-Scd	100	39	39 ${}_{- 7}^{+ 8}$	34	10	30 ${}_{- 11}^{+ 13}$	58	29	50 ${}_{- 10}^{+ 10}$
Sd-Sdm	52	10	20 ${}_{- 8}^{+ 9}$	11	4	38 ${}_{- 20}^{+ 22}$	37	6	17 ${}_{- 8}^{+ 10}$
Sm-Im	60	5	9 ${}_{- 5}^{+ 6}$	4	2	50 ${}_{- 31}^{+ 31}$	40	2	7 ${}_{- 5}^{+ 7}$
I0	6	4	62 ${}_{- 28}^{+ 24}$	$\dots$	$\dots$	$\dots$	$\dots$	$\dots$	$\dots$
pec	6	1	25 ${}_{- 19}^{+ 27}$	$\dots$	$\dots$	$\dots$	$\dots$	$\dots$	$\dots$
Unknown	61	5	9 ${}_{- 5}^{+ 6}$	$\dots$	$\dots$	$\dots$	$\dots$	$\dots$	$\dots$
All	719	314	43 ${}_{- 3}^{+ 3}$	137	75	54 ${}_{- 6}^{+ 6}$	284	123	43 ${}_{- 4}^{+ 4}$

Table 2. Table 4 Properties of AGN Candidates in H ii Nuclei

Name	Hubble Type	$\log$ $M_{BH}$	$\log$ $L_{H α}$	Model	$Γ$	$N_{H}$	$\log$ $L_{X}$
		( $M_{☉}$ )	(erg s^-1)			( $10^{22}$ cm^-2)	(erg s^-1)
IC 342	$SAB (rs) cd$	6.07 ${}^{+ 0.32}_{- 0.38}$	38.11	PL+VME	1.92 ${}^{+ 0.21}_{- 0.25}$	$\dots$	38.38 ${}^{+ 0.05}_{- 0.05}$
M 61	$SAB (rs) bc$	6.35 ${}^{+ 0.08}_{- 0.08}$	39.93	PL+ME	1.53 ${}^{+ 0.46}_{- 0.71}$	$\dots$	38.91 ${}^{+ 0.37}_{- 0.32}$
M 101	$SAB (rs) cd$	3.48 ${}^{+ 0.71}_{- 1.04}$	39.14	PL	1.88 ${}^{+ 0.20}_{- 0.20}$	$\dots$	38.07 ${}^{+ 0.14}_{- 0.16}$
NGC 45	$SA (s) dm$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 1.60	37.22 ${}^{+ 0.54}_{- 0.67}$
NGC 891	$SA (s) b ?$	6.04 ${}^{+ 0.29}_{- 0.34}$	38.85L	$\dots$	$\dots$	9.04 ${}^{+ 17.31}_{- 2.89}$	38.38 ${}^{+ 0.67}_{- 0.37}$
NGC 922	$SB (s) cd pec$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 0.24	39.02 ${}^{+ 0.28}_{- 0.40}$
NGC 925	$SAB (s) d$	6.00 ${}^{+ n a n}_{- n a n}$	38.36	$\dots$	$\dots$	$<$ 0.45	38.75 ${}^{+ 0.35}_{- 0.46}$
NGC 1073	$SB (rs) c$	3.60 ${}^{+ 0.68}_{- 0.98}$	38.36	$\dots$	$\dots$	0.57 ${}^{+ 0.83}_{- 0.54}$	39.27 ${}^{+ 0.40}_{- 0.37}$
NGC 1370	$cD ?$	5.96 ${}^{+ 0.08}_{- 0.08}$	$\dots$	$\dots$	$\dots$	$<$ 0.52	38.73 ${}^{+ 0.40}_{- 0.52}$
NGC 1493	$SB (r) cd$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 0.32	38.41 ${}^{+ 0.26}_{- 0.36}$
NGC 1603	$E ?$	7.03 ${}^{+ 0.10}_{- 0.10}$	$\dots$	$\dots$	$\dots$	$<$ 0.13	38.99 ${}^{+ 0.30}_{- 0.43}$
NGC 1637	$SAB (rs) c$	$\dots$	$\dots$	PL	3.01	$\dots$	38.27 ${}^{+ 0.23}_{- 0.31}$
NGC 1808	$(R) SAB (s) a$	7.64 ${}^{+ 0.14}_{- 0.15}$	$\dots$	PL+VME	0.74 ${}^{+ 0.31}_{- 0.39}$	$\dots$	39.50 ${}^{+ 0.10}_{- 0.10}$
NGC 2276	$SAB (rs) c$	6.34 ${}^{+ 0.24}_{- 0.27}$	40.83	$\dots$	$\dots$	$<$ 0.02	38.87 ${}^{+ 0.26}_{- 0.36}$
NGC 2500	$SB (rs) d$	5.04 ${}^{+ 0.87}_{- 1.43}$	38.00	$\dots$	$\dots$	$<$ 0.83	38.17 ${}^{+ 0.61}_{- 0.40}$
NGC 2748	$SAbc$	6.32 ${}^{+ 0.21}_{- 0.23}$	39.99	$\dots$	$\dots$	$<$ 0.35	38.32 ${}^{+ 0.30}_{- 0.43}$
NGC 2782	$SAB (rs) a pec$	8.11 ${}^{+ 0.11}_{- 0.12}$	41.51	PABS*PL+ME+G	0.55 ${}^{+ 0.42}_{- 0.43}$	54.27 ${}^{+ 122.70}_{- 37.43}$	41.05 ${}^{+ 0.66}_{- 0.37}$
NGC 2798	$SB (s) a pec$	7.01 ${}^{+ 0.48}_{- 0.60}$	$\dots$	PL+ME	2.62 ${}^{+ 0.25}_{- 0.25}$	$\dots$	39.56 ${}^{+ 0.12}_{- 0.15}$
NGC 2993	$Sa pec$	$\dots$	$\dots$	PL+ME	1.37 ${}^{+ 0.33}_{- 0.28}$	$<$ 0.14	40.03 ${}^{+ 0.11}_{- 0.12}$
NGC 3077	$I0 pec$	4.20 ${}^{+ 0.87}_{- 1.43}$	38.93	PL	1.69 ${}^{+ 0.66}_{- 0.57}$	1.42 ${}^{+ 0.92}_{- 0.68}$	38.22 ${}^{+ 0.08}_{- 0.09}$
NGC 3184	$SAB (rs) cd$	4.85 ${}^{+ 0.42}_{- 0.52}$	39.72	$\dots$	$\dots$	$<$ 0.54	37.94 ${}^{+ 0.39}_{- 0.26}$
NGC 3198	$SB (rs) c$	5.00 ${}^{+ 0.39}_{- 0.47}$	39.62	$\dots$	$\dots$	$<$ 0.95	38.76 ${}^{+ 0.34}_{- 0.27}$
NGC 3310	$SAB (r) bc pec$	6.35 ${}^{+ 0.03}_{- 0.03}$	40.53	PL	1.41 ${}^{+ 0.16}_{- 0.15}$	0.44 ${}^{+ 0.10}_{- 0.09}$	40.25 ${}^{+ 0.04}_{- 0.04}$
NGC 3367	$SB (rs) c$	5.64 ${}^{+ 0.34}_{- 0.41}$	41.34	PL+ME	0.98 ${}^{+ 0.16}_{- 0.15}$	$\dots$	40.71 ${}^{+ 0.06}_{- 0.06}$
NGC 3521	$SAB (rs) bc$	7.35 ${}^{+ 0.12}_{- 0.13}$	39.10b	PL	1.92 ${}^{+ 0.65}_{- 0.48}$	$<$ 0.64	38.92 ${}^{+ 0.13}_{- 0.15}$
NGC 3665	${SA0}^{0} (s)$	8.69 ${}^{+ 0.09}_{- 0.09}$	39.75b	$\dots$	$\dots$	$<$ 0.75	39.70 ${}^{+ 0.37}_{- 0.24}$
NGC 3877	$SA (s) c ?$	6.41 ${}^{+ 0.23}_{- 0.26}$	40.29	$\dots$	$\dots$	$<$ 0.11	37.99 ${}^{+ 0.29}_{- 0.40}$
NGC 4038	$SB (s) m pec$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 0.06	38.22 ${}^{+ 0.28}_{- 0.38}$
NGC 4102	$SAB (s) b ?$	8.00 ${}^{+ 0.12}_{- 0.13}$	41.50	PL+ME+G	1.52 ${}^{+ 0.50}_{- 0.49}$	$\dots$	40.47 ${}^{+ 0.19}_{- 0.24}$
NGC 4136	$SAB (r) c$	4.58 ${}^{+ 0.46}_{- 0.58}$	38.30	$\dots$	$\dots$	1.01 ${}^{+ 1.27}_{- 0.81}$	38.14 ${}^{+ 0.49}_{- 0.42}$
NGC 4194	$IBm pec$	6.83 ${}^{+ 0.49}_{- 0.62}$	$\dots$	PL+VME	1.44 ${}^{+ 0.42}_{- 0.42}$	$<$ 0.66	40.32 ${}^{+ 0.09}_{- 0.10}$
NGC 4217	$Sb$	6.54 ${}^{+ 0.11}_{- 0.11}$	38.84	$\dots$	$\dots$	2.59 ${}^{+ 2.39}_{- 1.47}$	38.45 ${}^{+ 0.52}_{- 0.55}$
NGC 4490	$SB (s) d pec$	4.95 ${}^{+ 0.41}_{- 0.50}$	38.83	PL	2.40 ${}^{+ 0.18}_{- 0.17}$	0.92 ${}^{+ 0.13}_{- 0.11}$	39.63 ${}^{+ 0.04}_{- 0.04}$
NGC 4526	${SAB0}^{0} ? (s)$	8.45 ${}^{+ 0.09}_{- 0.09}$	39.61	PL	1.10 ${}^{+ 0.31}_{- 0.16}$	$<$ 0.13	39.33 ${}^{+ 0.10}_{- 0.12}$
NGC 4559	$SAB (rs) cd$	5.14 ${}^{+ 0.36}_{- 0.43}$	38.58	$\dots$	$\dots$	$<$ 0.59	38.23 ${}^{+ 0.47}_{- 0.27}$
NGC 4561	$SB (rs) dm$	$\dots$	$\dots$	PL	1.45	$\dots$	39.56 ${}^{+ 0.33}_{- 0.18}$
NGC 4654	$SAB (rs) cd$	5.07 ${}^{+ 0.39}_{- 0.47}$	39.97	$\dots$	$\dots$	$<$ 1.49	39.44 ${}^{+ 0.48}_{- 0.44}$
NGC 4666	$SABc ?$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 1.09	39.04 ${}^{+ 0.49}_{- 0.38}$
NGC 4670	$SB0 / a (s) pec ?$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 0.76	39.28 ${}^{+ 0.38}_{- 0.27}$
NGC 4900	$SB (rs) c$	5.54 ${}^{+ 0.32}_{- 0.38}$	39.48L	$\dots$	$\dots$	$<$ 0.62	38.52 ${}^{+ 0.55}_{- 0.28}$
NGC 5102	${SA0}^{-}$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 0.68	36.56 ${}^{+ 0.49}_{- 0.30}$
NGC 5248	$SAB (rs) bc$	6.73 ${}^{+ 0.20}_{- 0.22}$	39.76	$\dots$	$\dots$	$<$ 0.77	38.15 ${}^{+ 0.53}_{- 0.54}$
NGC 5253	$pec$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 0.01	36.90 ${}^{+ 0.25}_{- 0.33}$
NGC 5483	$SA (s) c$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 1.14	39.03 ${}^{+ 0.42}_{- 0.33}$
NGC 5775	$SBc ?$	6.75 ${}^{+ 0.16}_{- 0.17}$	39.54	$\dots$	$\dots$	$<$ 1.00	38.62 ${}^{+ 0.39}_{- 0.32}$
NGC 6764	$SB (s) bc$	$\dots$	$\dots$	PL+ME	1.82 ${}^{+ 0.31}_{- 0.36}$	$\dots$	40.05 ${}^{+ 0.08}_{- 0.10}$
NGC 6946	$SAB (rs) cd$	5.43 ${}^{+ 0.35}_{- 0.42}$	40.14	PL	2.23 ${}^{+ 0.30}_{- 0.27}$	0.58 ${}^{+ 0.19}_{- 0.17}$	38.83 ${}^{+ 0.06}_{- 0.07}$
NGC 7320	$SA (s) d$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 0.86	38.40 ${}^{+ 0.35}_{- 0.26}$
NGC 7714	$SB (s) b ? pec$	7.14 ${}^{+ 0.36}_{- 0.42}$	$\dots$	PL	1.62 ${}^{+ 0.12}_{- 0.12}$	0.16 ${}^{+ 0.04}_{- 0.04}$	40.55 ${}^{+ 0.04}_{- 0.04}$
UGC 5720	$Im pec ?$	$\dots$	$\dots$	$\dots$	$\dots$	$<$ 0.01	39.14 ${}^{+ 0.24}_{- 0.32}$
UGCA 166	$\dots$	$\dots$	$\dots$	PL	1.72 ${}^{+ 0.19}_{- 0.18}$	$\dots$	39.34 ${}^{+ 0.11}_{- 0.12}$

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Full text

Chandra Survey of Nearby Galaxies: A Significant Population of Candidate Central Black Holes in Late-type Galaxies

Rui She11affiliation: Department of Engineering Physics and Center for Astrophysics, Tsinghua University, Beijing 100084, China , Luis C. Ho22affiliation: Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100087, China 33affiliation: Department of Astronomy, School of Physics, Peking University, Beijing 100087, China , and Hua Feng11affiliation: Department of Engineering Physics and Center for Astrophysics, Tsinghua University, Beijing 100084, China

Abstract

Based on the Chandra data archive as of March 2016, we have identified 314 candidate active galactic nuclei in 719 galaxies located closer than 50 Mpc, among them late-type (Hubble types Sc and later) galaxies that previously had been classified from optical observations as containing star-forming (H ii) nuclei. These late-type galaxies comprise a valuable subsample to search for low-mass ( $\lesssim 10^{6}$ $M_{\sun}$ ) central black holes. For the sample as a whole, the overall dependence of the fraction of active nuclei on galaxy type and nuclear spectral classification is consistent with previous results based on optical surveys. We detect 51 X-ray cores among the 163 H ii nuclei and estimate that, very conservatively, $\sim$ 74% of them with luminosities above $10^{38}$ erg s*-1* are not contaminated by X-ray binaries; the fraction increases to $\sim$ 92% for X-ray cores with a luminosity of $10^{39}$ erg s*-1* or higher. This allows us to estimate a black hole occupation fraction of $\gtrsim 21$ % in these late-type, many bulgeless, galaxies.

Subject headings:

galaxies: active — galaxies: nuclei — galaxies: Seyfert — X-rays: galaxies

1. Introduction

Nearby galaxies offer us the unique laboratories to search for and study weak active galactic nuclei (AGNs), including those emanating from low-mass central black holes ( $M_{\rm BH}<10^{6}$ $M_{\sun}$ ). Although their formation mechanism is still poorly constrained, low-mass black holes may have played a key role in forming supermassive black holes (SMBHs) in the early universe. The early manifestation of SMBHs at high redshifts (Mortlock et al., 2011; Wu et al., 2015) requires the growth of these giants to start with “seed” black holes that are significantly more massive than stellar-mass black holes of typical mass 10 $M_{\sun}$ (Volonteri, 2010). Some of the seed black holes may have survived until today and appear as low-mass black holes lurking at the centers of low-mass (late-type spiral and dwarf) galaxies (Volonteri et al., 2008; Greene, 2012; Reines & Comastri, 2016). Present-day low-mass galaxies, to the extent that they have remained relatively unevolved since their formation, are the optimal sites to search for such relic seed black holes. Finding these low-mass black holes, knowing their occupation fraction in low-mass galaxies, and measuring their mass function may help constrain their formation mechanism and distinguish between the two competing models: whether they are Population III remnants (Heger et al., 2003) or due to direct collapse (Haehnelt & Rees, 1993).

According to the scaling relations between black hole mass and host galaxy properties (Kormendy & Ho, 2013), the best place to unveil low-mass black holes is in late-type bulgeless galaxies. Optical and X-ray searches have been conducted in the past. Systematic searches for low-mass black holes using the Sloan Digital Sky Survey uncovered several hundred new objects residing in low-mass galaxies (Greene & Ho, 2004, 2007; Dong et al., 2012; Reines et al., 2013), but the statistical completeness of these optical searches are difficult to quantify (Greene & Ho, 2009), as AGN samples selected by optical emission lines tend to have black holes radiating at moderate to high fractions of their Eddington limits. On the contrary, X-ray observations, especially of high resolution and high sensitivity afforded by the Chandra Advanced CCD Imaging Spectrometer (ACIS; Weisskopf et al., 2002), can reveal very weak nuclear activity in galaxies, even with brief exposures ( $\sim$ ks; e.g., Ho et al., 2001; Gallo et al., 2008; Miller et al., 2012). Desroches & Ho (2009) performed the first dedicated search for low-mass black holes with Chandra and detected 17 candidates from 64 late-type spirals. Later on, cross-matching nearby dwarf galaxies with Chandra and XMM-Newton catalogs of X-ray point-like sources increased the sample size to $\sim$ 93 (Lemons et al., 2015; Pardo et al., 2016; Nucita et al., 2017). Using stacked Chandra images, Mezcua et al. (2016) revealed a population of low-mass black holes that are too faint to be detected individually in dwarf galaxies. The hard X-ray NuSTAR observations of Chen et al. (2017), albeit of a small sample, also suggested a non-negligible fraction of accreting low-mass central black holes that may have been missed in optical surveys.

Here we report the search of low-mass black holes from an even larger sample of nearby late-type galaxies. We have recently conducted an X-ray survey of a large sample of nearby galaxies. The main goals of this survey, described in She et al. (2017, hereafter Paper I), are to systematically search for and study AGNs in all galaxies within 50 Mpc contained in the Chandra archives observed with ACIS. The sample contains 719 galaxies, of which 314 are identified as AGN candidates by the cross-correlation of X-ray point-like sources and the near-infrared or optical stellar nuclei of these galaxies. The technical details of the survey and the X-ray properties of the AGN candidates are presented in Paper I. With 2 $-$ 10 keV luminosities less than 1042 erg s*-1* for most objects, these sources occupy the regime of low-luminosity AGNs (for a review, see Ho, 2008)

This paper focuses on the identification of a population of previously hidden AGNs in the centers of late-type, mostly bulgeless galaxies previously classified as H ii nuclei in optical surveys. Most of them have very low luminosities and are mostly likely associated with low-mass black holes. Unlikely their optically identified counterparts (e.g., Greene & Ho, 2004, 2007; Dong et al., 2012; Reines et al., 2013), this X-ray–selected population, having significantly lower accretion rates, is much more numerous, allowing us to establish a new, firmer estimate of the occupation fraction of low-mass black holes in the nearby universe. For completeness, the paper also addresses the overall demographics of AGNs in the full sample of nearby galaxies, which covers a broad range of Hubble types.

Section 2 gives a brief introduction of the sample and measurements. Section 3 presents the detection rates of X-ray AGN candidates in different kinds of galaxies and their possible dependence on the presence of a large-scale bar. Black hole masses and Eddington ratios are estimated in Section 4. Section 5 highlights the properties of X-ray AGN candidates associated with H ii nuclei. We summarize our results in Section 7.

2. Sample and Data Analysis

Paper I gives a detailed description of the sample and data reduction. Here we just give a brief summary of the most important aspects. The sample is assembled based on the full list of Chandra imaging observations, with both ACIS-S and ACIS-I, as of March 2016. All Chandra-observed galaxies with a distance of less than 50 Mpc were included in our sample, resulting in a total of 719 galaxies.

The Hubble types of the galaxies come from the NASA/IPAC Extragalactic Database (NED),111http://ned.ipac.caltech.edu which are taken largely from the Third Reference Catalogue of Bright Galaxies (RC3; de Vaucouleurs et al. 1991). A total of 285 galaxies have a large-scale bar structure (with Hubble type SAB or SB). Optical nuclear spectral classifications are available for 249 galaxies from the Palomar spectroscopic survey of bright, northern galaxies (Ho et al., 1995, 1997c). Nuclear spectral classifications for another 44 galaxies are found in the catalog of Véron-Cetty & Véron (2010). For the 125 galaxies whose spectral classifications are not included in these two catalogs, we performed our own classification using optical emission-line ratios. Methods of spectral fitting and classification can be found in Paper I. To summarize: 418 out of 719 galaxies in our sample have an optical nuclear spectral classification (for a review see Ho, 2008), including 59 Seyferts, 66 low-ionization nuclear emission-line regions (LINERs; Heckman, 1980), 41 transition objects (emission-line nuclei with [O i] strengths intermediate between those of H ii nuclei and LINERs; see Ho et al. 1993), 163 H ii nuclei, and 89 absorption-line nuclei (those without emission lines).

Point-like X-ray sources are detected using CIAO task wavdetect, and then AGN candidates are identified by detection of an X-ray core astrometrically coincident with the near-infrared/optical position of the galaxy nucleus, taking various errors into account. This results in 314 X-ray AGN candidates, of which 228 are located in the sky plane less than 1 $\arcsec$ from the near-infrared/optical nucleus. X-ray spectral fitting is conducted for AGN candidates with enough photons; otherwise, hardness ratios are obtained to characterize the shape of the spectrum. The X-ray luminosity in the 2 $-$ 10 keV band of each AGN candidate is derived from spectral fitting if available, or from count rates otherwise. For 250 objects, Eddington ratios are calculated based on black holes masses derived from the $M_{\rm BH}-\sigma$ relation222The $M_{\rm BH}-\sigma$ relation we used in Paper I is $\log[M_{\rm BH}/(10^{9}~{}M_{\sun})]=-(0.68\pm 0.05)+(5.20\pm 0.37)\log[\sigma/(200~{}{\rm km~{}s^{-1}})]$ . (Kormendy & Ho, 2013, as supplemented in Paper I).

3. Demographics of AGNs

The numbers of galaxies and AGN candidates as a function of Hubble type are shown in Table 3 and in Figure 1 (left), which also displays the X-ray core detection rate with a 90% error range for each type. For early-type galaxies (E $-$ Sbc), the X-ray core detection rate is as high as $\sim$ 60% (250/434), whereas for late-type galaxies (Sc $-$ Im), the X-ray core detection rate drops to $\sim$ 25% (54/212). For galaxies with a Hubble type of Sm $-$ Im, at the extreme end of the late-type sequence, the fraction is less than 10%. The X-ray core detection rate in all galaxies is 43.7%. These AGN detection rates are broadly consistent with those derived from optical spectroscopic observations (Ho et al., 1997d; Ho, 2008), which reported AGN fractions of 50% $-$ 70% in early-type galaxies and $\sim$ 15% in late-type systems. There appears to be a slight overabundance of X-ray AGN candidates among Sc $-$ Sdm galaxies (see Table 3) compared to optical AGN detection rate found in the Palomar survey. X-ray observations are more sensitive to weak AGNs than optical observations. The results in this work largely confirm the findings of former X-ray investigations (Ho, 2008, 2009; Desroches & Ho, 2009; Zhang et al., 2009; Grier et al., 2011). Compared to previous similar X-ray studies, this work targets a much larger sample of galaxies and utilizes high-quality, high-resolution Chandra data, which were uniformly analyzed. We also perform a more comprehensive analysis on position errors, which is critical to evaluate whether any given compact X-ray source is likely to be associated with the nucleus of the galaxy.

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