arXiv subjects

Xiaobo Dong

Publications and source records attributed to Xiaobo Dong.

At least 19 recordsLinked to original sources

arXiv2026

WaveDM.jl: An Adaptable Simulation Framework for Dynamics of Baryonic and Wave Dark Matter on Galaxy Scales

We present WaveDM.jl, an open-source Julia package for high-performance simulations of wave dark matter dynamics on galaxy scales, with a design philosophy centered on extensibility and adaptability. The code solves the time-dependent Schrodinger--Poisson equation (SPE) using a pseudo-spectral Fourier method. The spectral solver is tightly integrated with N-body gravitational force solvers, enabling simultaneous evolution of wave dark matter and baryonic components in galaxy-scale simulations. WaveDM.jl unifies shared-memory, distributed-memory, and GPU execution within a multi-level parallelization framework, enabling the same computational workflow to scale from single-node to multi-node computing environments without requiring major code changes. To further facilitate user-friendly galaxy-scale simulations, the package provides a dedicated toolbox that integrates flexible initial condition generators, trajectory lookback, tidal force calculations, and real-time visualization. Beyond astrophysical applications, the code's modular architecture and general nonlinear Schrodinger framework enable cross-disciplinary studies such as nonlinear optics and cold-atom physics. The code is open source and available on https://github.com/JuliaAstroSim/WaveDM.jl.

astro-ph.GA↗

arXiv2025

A Promise for the JWST era: Massive black holes directly collapsed from wave dark matter haloes, and Star formation in and around their accretion flows

There are several puzzling phenomena from recent JWST observations, which seem to push the standard ΛCDM cosmology over the edge. All those puzzles can be understood in a coherent way if we assume that first massive black holes (MBHs, the "heavy eggs") formed by directly collapsing from wave CDM haloes, which can be even earlier than the formation of first galaxies (the "chickens"). We elucidate two false obstacles that have been believed to prevent wave CDM haloes from collapsing into black holes (namely "maximum stable mass" and "bosenova") and point out that general-relativistic instabilities (e.g., the non-axisymmetric instability numerically found in spinning scalar-field boson stars) could serve as the concrete mechanisms for direct-collapse black holes (DCBHs) born from wave dark matter. Once the MBHs formed, star formation bursts in and around the accretion flows, characteristic of a special mode in compact regions of high gravitational accelerations.

astro-ph.GA↗

arXiv2024

A high-velocity star recently ejected by an intermediate-mass black hole in M15

The existence of intermediate-mass black holes (IMBHs) is crucial for understanding various astrophysical phenomena, yet their existence remains elusive, except for the LIGO-Virgo detection. We report the discovery of a high-velocity star J0731+3717, whose backward trajectory about 21 Myr ago intersects that of globular cluster M15 within the cluster tidal radius. Both its metallicity [Fe/H] and its alpha-to-iron abundance ratio [$α$/Fe] are consistent with those of M15. Furthermore, its location falls right on the fiducial sequence of the cluster M15 on the color-absolute magnitude diagram, suggesting similar ages. These support that J0731+3717 is originally associated with M15 at a confidence level of "seven nines". We find that such a high-velocity star ($V_{\rm ej} = 548^{+6}_{-5}$ km s$^{-1}$) was most likely tidally ejected from as close as one astronomical unit to the center of M15, confirming an IMBH ($\ge 100 M_{\odot}$ with a credibility of 98%) as the exclusive nature of the central unseen mass proposed previously.

astro-ph.GA↗

arXiv2020

Discovery of A candidate Hypervelocity star originated from the Sagittarius Dwarf Spheroidal galaxy

In this letter, we report the discovery of an intriguing HVS (J1443+1453) candidate that is probably from the Sagittarius Dwarf Spheroidal galaxy (Sgr dSph). The star is an old and very metal-poor low-mass main-sequence turn-off star (age $\sim14.0$ Gyr and [Fe/H] $= -2.23$ dex) and has a total velocity of $559.01^{+135.07}_{-87.40}$ km s$^{-1}$ in the Galactic rest-frame and a heliocentric distance of $2.90^{+0.72}_{-0.48}$ kpc. The velocity of J1443+1453 is larger than the escape speed at its position, suggesting it a promising HVS candidate. By reconstructing its trajectory in the Galactic potential, we find that the orbit of J1443+1453 intersects closely with that of the Sgr dSph $37.8^{+4.6}_{-6.0}$ Myr ago, when the latter has its latest pericentric passage through the Milky Way. The encounter occurs at a distance $2.42^{+1.80}_{-0.77}$ kpc from the centre of Sgr dSph, smaller than the size of the Sgr dSph. Chemical properties of this star are also consistent with those of one Sgr dSph associated globular cluster or of the Sgr stream member stars. Our finding suggests that J1443+1453 is an HVS either tidally stripped from the Sgr dSph or ejected from the Sgr dSph by the gravitational slingshot effect, requiring a (central) massive/intermediate-mass black hole or a (central) massive primordial black hole in the Sgr dSph.

astro-ph.GA↗

arXiv2018

Discovery of An Active Intermediate-Mass Black Hole Candidate in the Barred Bulgeless Galaxy NGC 3319

We report the discovery of an active intermediate-mass black hole (IMBH) candidate in the center of nearby barred bulgeless galaxy $\rm NGC~3319$. The point X-ray source revealed by archival Chandra and XMM-Newton observations is spatially coincident with the optical and UV galactic nuclei from Hubble Space Telescope observations. The spectral energy distribution derived from the unresolved X-ray and UV-optical flux is comparable with active galactic nuclei (AGNs) rather than ultra-luminous X-ray sources, although its bolometric luminosity is only $3.6\times10^{40}~\rm erg~s^{-1}$. Assuming an Eddington ratio range between 0.001 and 1, the black hole mass ($M_\rm{BH}$) will be located at $3\times10^2-3\times10^5~M_{\odot}$, placing it in the so-called IMBH regime and could be the one of the lowest reported so far. Estimates from other approaches (e.g., fundamental plane, X-ray variability) also suggest $M_\rm{BH}\lesssim10^5~M_{\odot}$.Similar to other BHs in bulgeless galaxies, the discovered IMBH resides in a nuclear star cluster with mass of $\sim6\times10^6~M_{\odot}$. The detection of such a low-mass BH offers us an ideal chance to study the formation and early growth of SMBH seeds, which may result from the bar-driven inflow in late-type galaxies with a prominent bar such as $\rm NGC~3319$.

astro-ph.GA↗

arXiv2016

Differences in Halo-Scale Environments between Type 1 and Type 2 AGNs at Low Redshift

Using low-redshift (z<0.09) samples of AGNs, normal galaxies and groups of galaxies selected from the Sloan Digital Sky Survey (SDSS), we study the environments of type 1 and type 2 AGNs both on small and large scales. Comparisons are made for galaxy samples matched in redshift, $r$-band luminosity, [OIII] luminosity, and also the position in groups (central or satellite). We find that type 2 AGNs and normal galaxies reside in similar environments. Type 1 and type 2 AGNs have similar clustering properties on large scales ($\gtrsim1$Mpc), but at scales smaller than 100 kpc, type 2s have significant more neighbors than type 1s ($3.09\pm0.69$ times more for central AGNs at $\lesssim30$kpc). These results suggest that type 1 and type 2 AGNs are hosted by halos of similar masses, as is also seen directly from the mass distributions of their host groups ($\sim10^{12}h^{-1} M_{\odot}$ for centrals and $\sim10^{13}h^{-1} M_{\odot}$ for satellites). Type~2s have significantly more satellites around them, and the distribution of their satellites is also more centrally concentrated. The host galaxies of both types of AGNs have similar optical properties, but their infrared colors are significantly different. Our results suggest that the simple unified model based solely on torus orientation is not sufficient, but that galaxy interactions in dark matter halos must have played an important role in the formation of the dust structure that obscures AGNs.

astro-ph.GA↗

arXiv2015

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope Quasar Survey: Quasar Properties from First Data Release

We present preliminary results of the quasar survey in Large Sky Area Multi- Object Fiber Spectroscopic Telescope (LAMOST) first data release (DR1), which includes pilot survey and the first year regular survey. There are 3921 quasars identified with reliability, among which 1180 are new quasars discovered in the survey. These quasars are at low to median redshifts, with highest z of 4.83. We compile emission line measurements around the Hα, Hβ, Mg II, and C IV regions for the new quasars. The continuum luminosities are inferred from SDSS photo- metric data with model fitting as the spectra in DR1 are non-flux-calibrated. We also compile the virial black hole mass estimates, and flags indicating the selec- tion methods, broad absorption line quasars. The catalog and spectra for these quasars are available online. 28% of the 3921 quasars are selected with optical- infrared colours independently, indicating that the method is quite promising in completeness of quasar survey. LAMOST DR1 and the on-going quasar survey will provide valuable data in the studies of quasars.

astro-ph.GA↗

arXiv2013

Anomalously Steep Reddening Law in Quasars: An Exceptional Example Observed in IRAS14026+4341

A fraction of the heavily reddened quasars require a reddening curve which is even steeper than that of the Small Magellanic Cloud. In this paper, we thoroughly characterize the anomalously steep reddening law in quasars, via an exceptional example observed in IRAS 14026+4341. By comparing the observed spectrum to the quasar composite spectrum, we derive a reddening curve in the rest-frame wavelength range of 1200 Å--10000 Å. It is featured with a steep rise at wavelengths shorter than 3000 Å, but no significant reddening at longer wavelengths. The absence of dust reddening in optical continuum is confirmed by the normal broad-line Balmer decrement (the H$α$/H$β$ ratio) in IRAS 14026+4341. The anomalous reddening curve can be satisfactorily reproduced by a dust model containing silicate grains in a power-law size distribution, $dn(a)/da \propto a^{-1.4}$, truncated at a maximum size $a_{max}=70 {\rm nm}$. The unusual size distribution may be caused by the destruction of large "stardust" grains by quasar activities or a different dust formation mechanism (i.e., the in situ formation of dust grains in quasar outflows). It is also possible that the analogies of the dust grains observed toward the Galactic center is responsible for the steep reddening curve. In addition, we find that IRAS 14026+4341 is a weak emission-line quasar (i.e., PHL 1811 analogies) with heavy dust reddening and blueshifted broad absorption lines.

astro-ph.CO↗

arXiv2011

Dust reddening in star-forming galaxies

We present empirical relations between the global dust reddening and other physical galaxy properties including the Halpha luminosity, Halpha surface brightness, metallicity and axial ratio for star-forming disc galaxies. The study is based on a large sample of ~22 000 well-defined star-forming galaxies selected from the Sloan Digital Sky Survey (SDSS). The reddening parameterized by color excess E(B-V) is derived from the Balmer decrement. Besides the dependency of reddening on Halpha luminosity / surface brightness and gas phase metallicity, it is also correlated with the galaxy inclination, in the sense that edge-on galaxies are more attenuated than face-on galaxies at a give intrinsic luminosity. In light of these correlations, we present the empirical formulae of E(B-V) as a function of these galaxy properties, with a scatter of only 0.07 mag. The empirical relation can be reproduced if most dust attenuation to the HII region is due to diffuse interstellar dust distributing in a disc thicker than that of HII regions. The empirical formulae can be incorporated into semi-analytical models of galaxy formation and evolution to estimate the dust reddening and enable comparison with observations more practically.

astro-ph.CO↗

arXiv2011

Coexistence of Gravitationally Bound and Radiation Driven CIV Emission Line Regions in Active Galactic Nuclei

There are mutually contradictory views in the literature of the kinematics and structure of high-ionization line (e.g. CIV) emitting regions in active galactic nuclei (AGNs). Two kinds of broad emission line region (BELR) models have been proposed, outflow and gravitationally bound BELR, which are supported respectively by blueshift of the CIV line and reverberation mapping observations. To reconcile these two apparently different models, we present a detailed comparison study between the CIV and MgII lines using a sample of AGNs selected from the Sloan Digital Sky Survey. We find that the kinematics of the CIV region is different from that of MgII, which is thought to be controlled by gravity. A strong correlation is found between the blueshift and asymmetry of the CIV profile and the Eddington ratio. This provides strong observational support for the postulation that the outflow is driven by radiation pressure. In particular, we find robust evidence that the CIV line region is largely dominated by outflow at high Eddington ratios, while it is primarily gravitationally bounded at low Eddington ratios. Our results indicate that these two emitting regions coexist in most of AGNs. The emission strength from these two gases varies smoothly with Eddington ratio in opposite ways. This explanation naturally reconciles the apparently contradictory views proposed in previous studies. Finally, candidate models are discussed which can account for both, the enhancement of outflow emission and suppression of normal BEL, in AGN with high Eddington ratios.

astro-ph.CO↗

arXiv2011

What controls the Fe II strength in active galactic nuclei?

We used a large, homogeneous sample of 4178 z <= 0.8 Seyfert 1 galaxies and QSOs selected from the Sloan Digital Sky Survey to investigate the strength of Fe II emission and its correlation with other emission lines and physical parameters of active galactic nuclei. We find that the strongest correlations of almost all the emission-line intensity ratios and equivalent widths (EWs) are with the Eddington ratio (L/L_{Edd}), rather than with the continuum luminosity at 5100Å (L_{5100}) or black hole mass (M_{BH}); the only exception is the EW of ultraviolet Fe II emission, which does not correlate at all with broad-line width, L_{5100}, M_{BH}, or L/L_{Edd}. By contrast, the intensity ratios of both the ultraviolet and optical Fe II emission to Mg II λ2800 correlate quite strongly with L/L_{Edd}. Interestingly, among all the emission lines in the near-UV and optical, the EW of narrow optical Fe II emission has the strongest correlation with L/L_{Edd}. We suggest that the variation of the emission-line strength in active galaxies is regulated by L/L_{Edd} because it governs the global distribution of the column density of the clouds gravitationally bound in the line-emitting region, as well as its overall gas supply. The systematic dependence on L/L_{Edd} must be corrected when using the FeII/MgII intensity ratio as a measure of the Fe/Mg abundance ratio to study the history of chemical evolution in QSO environments.

astro-ph.GA↗

arXiv2010

On the scattering enhancement of Nv1240 emission line of quasi-stellar objects

The Nv emission line of active galactic nuclei shows peculiar behavior in the line--continuum correlation, which may be indicative of an extra line component in addition to that from the normal broad emission line region. In this paper, we investigate possible contribution to the Nv emission via resonant scattering of both continuum and Ly alpha in a broad absorption line (BAL) outflow, by performing the Sobolev Monte Carlo simulations. The contribution is dependent on the covering factor, optical depth and velocity profile of the outflow, as well as the equivalent width (EW) of Ly alpha. Adopting model parameters constrained by observations, we find that the measured Nv EW in the spectra of non-BAL quasi-stellar objects (QSOs) could have been enhanced by a factor of 1.82~2.73 on average, while there is only moderate absorption of Ly alpha along the BAL outflow direction. Our model can produce a relatively narrow scattering line profile. About 80% of the total scattered flux falls within the central +-4500km/s. We find that the resonant scattering can produce a prominent polarized emission line around Nv. Both the broad excess emission and the unusually large polarized flux observed around Nv in BAL QSOs are considered as strong evidence for the scattering enhancement. Future spectropolarimetric observations and spectroscopic monitoring of luminous QSOs may offer crucial tests for this interpretation, and provide useful information on the physical and geometrical properties of QSO outflows. We argue that the scattering offers a promising and robust process for producing the peculiar behavior of Nv emission compared to the other processes proposed previously.

astro-ph.CO↗

arXiv2009

Optical identification of radio-loud active galactic nuclei in the ROSAT-Green-Bank sample with SDSS spectroscopy

Results are presented of extended and refined optical identification of 181 radio/X-ray sources in the RASS-Green Bank (RGB) catalog which have been spectroscopically observed in the Sloan digital sky survey (SDSS) DR5. For 72 sources, the identifications for are presented for the first time. It is confirmed that the majority of strong radio/X-ray emitters are radio-loud active galactic nuclei (AGNs), particularly blazars. Taking advantage of the high spectral quality and resolution and our refined spectral modeling, we are able to disentangle narrow line radio galaxies (NLRGs), as vaguely termed in most previous identification work, into Seyfert II galaxies and LINERs, based on the standard emission line diagnostics. The NLRGs in the RGB sample, mostly belonging to `weak line radio galaxies', are found have optical spectra consistent predominantly with LINERs, and only a small fraction with Seyfert II galaxies. A small number of LINERs have radio power as high as $10^{23}-10^{26}$ \whz at 5 GHz, being among the strongest radio emitting LINERs known so far. Two sources are identified with radio-loud narrow line Seyfert 1 galaxies (NLS1s), a class of rare objects. The presence is also confirmed of flat-spectrum radio quasars with the radio--optical--X-ray effective spectral indices similar to those of High-energy peaked BL Lac (HBL), albeit it is yet a debate as to whether this is the case for their actual spectral energy distributions.

astro-ph.HE↗

arXiv2009

Metal-Enriched Outflows in the Ultra-Luminous infrared Quasar Q1321+058

Quasar outflows may play important role in the evolution of its host galaxy and central black hole. In this paper, we present a detailed study of multiple outflows in the obscured ultra-luminous infrared quasar Q1321+058. The outflows reveal themselves in the complex optical and UV emission line spectrum, with a broad component blueshifted by 1650 km/s and a narrow component by 360 km/s, respectively.The higher velocity component shows ever strong N III] and strong Si III], in addition to strong [O III]5007 and [Ne III]3869 emission lines, suggesting an overabundance of N and Si relative to C. The abundance pattern is consistent a fast chemical enriching process associated with a recent starburst. The outflow extends to several tens to hundred parsecs from the quasar, and covers only a very small sky. We find that the outflow with line emitting gas is energetically insufficient to remove the ISM of the host galaxy. The velocity range and the column density suggest that the outflow might be part of the low ionization broad absorption line region as seen in a small class of quasars. The optical and UV continuum is starlight-dominated and can be modelled with a young-aged (1 Myr) plus an intermediate-aged (~0.5-1 Gyr) stellar populations, suggesting a fast building of the stellar mass in the host galaxy, consistent with the starburst-type metal abundances inferred from the high velocity outflow spectrum. The broad band spectral energy distribution shows that it is an obscured quasar with its bulk emission in the middle infrared. The star formation rate, independently estimated from UV, far-infrared, and emission line luminosity, is much lower than that is required for the co-evolution of the black hole and its host spheroid.

astro-ph.CO↗

arXiv2009

The origin and physical mechanism of the ensemble Baldwin effect

We have conducted a systematic investigation of the origin and underlying physics of the line--line and line--continuum correlations of AGNs, particularly the Baldwin effect. Based on the homogeneous sample of Seyfert 1s and QSOs in the SDSS DR4, we find the origin of all the emission-line regularities is Eddington ratio (L/Ledd). The essential physics is that L/Ledd regulates the distributions of the properties (particularly column density) of the clouds bound in the line-emitting region.

astro-ph.GA↗

arXiv2009

A Study of Active Galactic Nuclei in Low Surface Brightness Galaxies with Sloan Digital Sky Survey Spectroscopy

Active galactic nuclei (AGN) in low surface brightness galaxies (LSBGs) have received little attention in previous studies. In this paper, we present detailed spectral analysis of 194 LSBGs from the Impey et al. (1996) APM LSBG sample which have been observed spectroscopically by the Sloan Digital Sky Survey Data Release 5 (SDSS DR5). Our elaborate spectral analysis enables us to carry out, for the first time, reliable spectral classification of nuclear activities in LSBGs based on the standard emission line diagnostic diagrams in a rigorous way. Star-forming galaxies are common, as found in about 52% LSBGs. We find, contrary to some of the previous claims, that the fraction of galaxies containing an AGN is significantly lower than that found in nearby normal galaxies of high surface brightness. This is qualitatively in line with the finding of Impey et al. (2001). This result holds true even within each morphological type from Sa to Sc. LSBGs having larger central stellar velocity dispersions, or larger physical sizes, tend to have a higher chance to harbor an AGN. For three AGNs with broad emission lines, the black hole masses estimated from the emission lines are broadly consistent with the well known M-$σ_\ast$ relation established for normal galaxies and AGNs.

astro-ph.GA↗

arXiv2009

Exploring the Partially-obscured BLR and Partially-covered NLR

We have conducted a series of investigations on the geometry of the reddening material in AGNs, which have important implications to the AGN unification and SMBH demography. According to our statistics of partially obscured quasars, we found that SMBHs in partially obscured type/phase (i.e., intermediate type) are at least as abundant as normal quasars in the local Universe; the reddening material in most objects are located in between the BLR and NLR. According to our comparison of narrow lines in type 1 and 2 AGNs, we found that for high-ionization or high-critical density narrow lines (e.g. OIII 5007, Balmer lines and FeII) a significant fraction of the emission arises from the inner dense part of the NLR; this inner NLR is located very close to the central engine and thus can be covered by the torus.

astro-ph.GA↗

arXiv2008

The Correlation between X-ray and UV Properties of BAL QSOs

We compile a large sample of broad absorption lines (BAL) quasars with X-ray observations from the \xmm archive data and Sloan Digital Sky Survey Data Release 5. The sample consists of 41 BAL QSOs. Among 26 BAL quasars detected in X-ray, spectral analysis is possible for twelve objects. X-ray absorption is detected in all of them. Complementary to that of \citet{gall06} (thereafter G06), our sample spans wide ranges of both BALnicity Index (BI) and maximum outflow velocity (\vmax). Combining our sample with G06's, we find very significant correlations between the intrinsic X-ray weakness with both BALnicity Index (BI) and the maximum velocity of absorption trough. We do not confirm the previous claimed correlation between absorption column density and broad absorption line parameters. We tentatively interpret this as that X-ray absorption is necessary to the production of the BAL outflow, but the properties of the outflow are largely determined by intrinsic SED of the quasars.

astro-ph↗