arXiv subjects

Meicun Hou

Publications and source records attributed to Meicun Hou.

24 records · Page 2Linked to original sources

arXiv2019

A Trio of Massive Black Holes Caught in the Act of Merging

We report the discovery of SDSS J0849+1114 as the first known triple Type 2 Seyfert nucleus. It represents three active black holes that are identified from new spatially resolved optical slit spectroscopy using the Dual Imaging Spectrograph on the 3.5 m telescope at the Apache Point Observatory. We also present new complementary observations including the Hubble Space Telescope Wide Field Camera 3 U- and Y-band imaging, Chandra Advanced CCD Imaging Spectrometer S-array X-ray 0.5--8 keV imaging spectroscopy, and NSF Karl G. Jansky Very Large Array radio 9.0 GHz imaging in its most extended A configuration. These comprehensive multiwavelength observations, when combined together, strongly suggest that all three nuclei are active galactic nuclei. While they are now still at kiloparsec-scale separations, where the host-galaxy gravitational potential dominates, the black holes may evolve into a bound triple system in $\lesssim$2 Gyr. These triple merger systems may explain the overly massive stellar cores that have been observed in some elliptical galaxies such as M87, which are expected to be unique gravitational wave sources. Similar systems may be more common in the early universe, when galaxy mergers are thought to have been more frequent.

astro-ph.GA↗

arXiv2019

Active Galactic Nucleus Pairs from the Sloan Digital Sky Survey. III. Chandra X-ray Observations Unveil Obscured Double Nuclei

We present Chandra ACIS-S X-ray imaging spectroscopy for five dual active galactic nucleus (AGN) candidates. Our targets were drawn from a sample of 1286 [O III]-selected AGN pairs systematically selected from the Sloan Digital Sky Survey Seventh Data Release. Each of the targets contains two nuclei separated by $\sim$3-9 kpc in projection, both of which are optically classified as Type 2 (obscured) AGNs based on diagnostic ratios of the narrow emission lines. Combined with independent, empirical star formation rate estimates based on the host-galaxy stellar continua, the new Chandra X-ray observations allow us to evaluate the dual-AGN hypothesis for each merging system. We confirm two (SDSS J0907+5203 and SDSS J1544+0446) of the five targets as bona-fide dual AGNs. For the other three targets, the existing data are consistent with the dual-AGN scenario, but we cannot rule out the possibility of stellar/shock heating and/or one AGN ionizing both gaseous components in the merger. The average X-ray-to-[O III] luminosity ratio in our targets seems to be systematically smaller than that observed in single AGNs but is higher than that seen in dual AGNs selected from AGNs with double-peaked narrow emission lines. We suggest that the systematically smaller X-ray-to-[O III] luminosity ratio observed in dual AGNs than in single AGNs is due to a high nuclear gas column likely from strong merger-induced inflows. Unlike double-peaked-[O III]-selected dual AGNs, the new sample selected from resolved galaxy pairs are not subject to the orientation bias caused by the double-peak line-of-sight velocity splitting selection, which also contributes to lowering the X-ray-to-[O III] luminosity ratio.

astro-ph.GA↗

arXiv2019

Chandra Detection of Intra-cluster X-ray Sources in Fornax

Based on archival {\it Chandra} observations with a total exposure of 1.3 Ms, we study X-ray point sources in the Fornax cluster of galaxies, with the primary aim of searching for intra-cluster X-ray source populations. We detect 1177 point sources out to a projected radius of $\sim$30 arcmin ($\sim$180 kpc) from the cluster center and down to a limiting 0.5--8 keV luminosity of $\sim3\times10^{37}{\rm~erg~s^{-1}}$. We construct source surface density profile, after excluding sources associated with foreground stars, known globular clusters, ultra-compact dwarfs and galactic nuclei. From this profile we statistically identify $\sim$183 excess sources that are not associated with the bulk stellar content of the individual member galaxies of Fornax, nor with the cosmic X-ray background. Taking into account Poisson error and cosmic variance, the cumulative significance of this excess is at $\gtrsim 2\,σ$ level (with a maximum of 3.6\,$σ$) outside three effective radii of the central giant elliptical, NGC\,1399. The luminosity function of the excess sources is found to be significantly steeper than that of the GC-hosting sources (presumably low-mass X-ray binaries [LMXBs]), disfavoring the possibility that unidentified GCs are primarily responsible for the excess. We show that a large fraction of the excess can be related to the extended stellar halo of NGC\,1399 and/or the diffuse intra-cluster light, thus providing strong evidence for the presence of intra-cluster X-ray sources in Fornax, the second unambiguous case for a galaxy cluster after Virgo. Other possible origins of the excess, including supernova-kicked LMXBs and stripped nucleated dwarf galaxies are discussed.

astro-ph.GA↗

arXiv2018

Deep Chandra observations of X-ray point sources in M87

We present a study of X-ray source populations in M87, the cD galaxy of the Virgo cluster, using 12 archival {\it Chandra} observations with a total exposure of $\sim$680 ks spanning about a decade. A total of 346 point-like sources are detected down to a limiting 0.5--8 keV luminosity of $4\times10^{37}{\rm~erg~s^{-1}}$ and out to a galactocentric radius of $\sim$40 kpc. We cross-correlate the X-ray sources with published catalogs of globular clusters (GCs), derived from the ACS Virgo Cluster Survey and the Next Generation Virgo Cluster Survey. This results in 122 matches, making it one of the largest samples of GC-hosting X-ray sources in an external galaxy. These sources, most likely low-mass X-ray binaries (LMXBs), correspond to $\sim$5\% of all known GCs within the {\it Chandra} field-of-view. Conversely, $\sim$50\% of the detected X-ray sources are found in a GC. Moreover, red (metal-rich) GCs are $\sim$2.2 times more likely to host an X-ray source than blue (metal-poor) GCs. We also examine 76 currently known ultra-compact dwarf galaxies around M87, but find no significant X-ray counterparts. After statistically accounting for the cosmic X-ray background, we identify $\sim$110 field-LMXBs. The GC-LMXBs and field-LMXBs differ in their luminosity function and radial distribution, which indicates that the latter cannot be primarily originated from GCs. Using another set of deep {\it Chandra} observations toward $\sim$100 kpc northwest of the M87 center, we statistically constrain the abundance of field-LMXBs in the stellar halo, which is consistent with that found in the central region. We also identify 40 variable X-ray sources, among which one source is likely a black hole binary residing in a GC.

astro-ph.HE↗

arXiv2017

Chandra Detection of Intra-cluster X-ray sources in Virgo

We present a survey of X-ray point sources in the nearest and dynamically young galaxy cluster, Virgo, using archival Chandra observations that sample the vicinity of 80 early-type member galaxies. The X-ray source populations at the outskirt of these galaxies are of particular interest. We detect a total of 1046 point sources (excluding galactic nuclei) out to a projected galactocentric radius of $\sim$40 kpc and down to a limiting 0.5-8 keV luminosity of $\sim$$2\times10^{38}{\rm~erg~s^{-1}}$. Based on the cumulative spatial and flux distributions of these sources, we statistically identify $\sim$120 excess sources that are not associated with the main stellar content of the individual galaxies, nor with the cosmic X-ray background. This excess is significant at a 3.5 $σ$ level, when Poisson error and cosmic variance are taken into account. On the other hand, no significant excess sources are found at the outskirt of a control sample of field galaxies, suggesting that at least some fraction of the excess sources around the Virgo galaxies are truly intra-cluster X-ray sources. Assisted with ground-based and HST optical imaging of Virgo, we discuss the origins of these intra-cluster X-ray sources, in terms of supernova-kicked low-mass X-ray binaries (LMXBs), globular clusters, LMXBs associated with the diffuse intra-cluster light, stripped nucleated dwarf galaxies and free-floating massive black holes.

astro-ph.GA↗

arXiv2016

Chandra Detection of X-ray Emission from Ultra-compact Dwarf Galaxies and Extended Star Clusters

We have conducted a systematic study of X-ray emission from ultra-compact dwarf (UCD) galaxies and extended star clusters (ESCs), based on archival {\sl Chandra} observations. Among a sample of 511 UCDs and ESCs complied from the literature, 17 X-ray counterparts with 0.5-8 keV luminosities above $\sim$$5\times10^{36} {\rm~erg~s^{-1}}$ are identified, which are distributed in eight early-type host galaxies. To facilitate comparison, we also identify X-ray counterparts of 360 globular clusters (GCs) distributed in four of the eight galaxies. The X-ray properties of the UCDs and ESCs are found to be broadly similar to those of the GCs. The incidence rate of X-ray-detected UCDs and ESCs, $(3.3\pm0.8)$\%, while lower than that of the X-ray-detected GCs [($7.0\pm0.4)$\%], is substantially higher than expected from the field populations of external galaxies. A stacking analysis of the individually undetected UCDs/ESCs further reveals significant X-ray signals, which corresponds to an equivalent 0.5-8 keV luminosity of $\sim$$4\times10^{35} {\rm~erg~s^{-1}}$ per source. Taken together, these provide strong evidence that the X-ray emission from UCDs and ESCs is dominated by low-mass X-ray binaries having formed from stellar dynamical interactions, consistent with the stellar populations in these dense systems being predominantly old. For the most massive UCDs, there remains the possibility that a putative central massive black hole gives rise to the observed X-ray emission.

astro-ph.GA↗