arXiv subjects

Zhiyuan Li

Publications and source records attributed to Zhiyuan Li.

428 records · Page 24Linked to original sources

arXiv2012

Chandra observations of NGC4342, an optically faint, X-ray gas-rich early-type galaxy

Chandra X-ray observations of NGC4342, a low stellar mass (M_K=-22.79 mag) early-type galaxy, show luminous, diffuse X-ray emission originating from hot gas with temperature of kT~0.6 keV. The observed 0.5-2 keV band luminosity of the diffuse X-ray emission within the D_25 ellipse is L_0.5-2keV = 2.7 x 10^39 erg/s. The hot gas has a significantly broader distribution than the stellar light, and shows strong hydrodynamic disturbances with a sharp surface brightness edge to the northeast and a trailing tail. We identify the edge as a cold front and conclude that the distorted morphology of the hot gas is produced by ram pressure as NGC4342 moves through external gas. From the thermal pressure ratios inside and outside the cold front, we estimate the velocity of NGC4342 and find that it moves supersonically (M~2.6) towards the northeast. Outside the optical extent of the galaxy we detect ~17 bright (L_0.5-8keV > 3 x 10^37 erg/s) excess X-ray point sources. The excess sources are presumably low-mass X-ray binaries (LMXBs) located in metal-poor globular clusters (GCs) in the extended dark matter halo of NGC4342. Based on the number of excess sources and the average frequency of bright LMXBs in GCs, we estimate that NGC4342 may host roughly 850-1700 GCs. In good agreement with this, optical observations hint that NGC4342 may harbor 1200 +/- 500 GCs. This number corresponds to a GC specific frequency of S_N = 19.9 +/- 8.3, which is among the largest values observed in full-size galaxies.

astro-ph.HE↗

arXiv2012

Constraining Sterile Neutrino Warm Dark Matter with Chandra Observations of the Andromeda Galaxy

We use the Chandra unresolved X-ray emission spectrum from a 12'-28' (2.8-6.4 kpc) annular region of the Andromeda galaxy to constrain the radiative decay of sterile neutrino warm dark matter. By excising the most baryon-dominated, central 2.8 kpc of the galaxy, we reduce the uncertainties in our estimate of the dark matter mass within the field of view and improve the signal-to-noise ratio of prospective sterile neutrino decay signatures relative to hot gas and unresolved stellar emission. Our findings impose the most stringent limit on the sterile neutrino mass to date in the context of the Dodelson-Widrow model, m_s < 2.2 keV (95% C.L.). Our results also constrain alternative sterile neutrino production scenarios at very small active-sterile neutrino mixing angles.

astro-ph.CO↗

arXiv2011

Sections of Calabi-Yau threefolds with K3 fibration

We study sections of a Calabi-Yau threefold fibered over a curve by K3 surfaces. We show that there exist infinitely many isolated sections on certain K3 fibered Calabi-Yau threefolds and the subgroup of the Néron-Severi group generated by these sections is not finitely generated. This also gives examples of K3 surfaces over the function field F of a complex curve with Zariski dense F-rational points, whose geometric model is Calabi-Yau.

math.AG↗

arXiv2011

Dynamic S0 Galaxies II: the Role of Diffuse Hot Gas

Gas loss is thought to be important in SF quenching and morphological transition during the evolution of S0 galaxies. In high density environments, gas loss can be achieved via many external mechanisms. However, in relatively isolated environments, where these external mechanisms cannot be efficient, gas loss must be dominated by some internal processes. We perform Chandra analysis of hot gas in five nearby isolated S0 galaxies, based on quantitative subtraction of various stellar contributions. We find that all the galaxies studied in the present work are X-ray faint, with the hot gas luminosity LX<5% of the expected Ia SN energy injection rate. We further compare our results with those from relevant recent papers, in order to investigate the energy budget, cold-hot gas relation, and gas removal from S0 galaxies in isolated environments. We find that elliptical and S0 galaxies are not significantly different in LX at the low mass end (typically with LK<1e11Lsun). However, at the high mass end, S0 galaxies tend to have significantly lower LX than elliptical galaxies of the same stellar masses. We further discuss the potential relation of the diffuse X-ray emission with the cold gas content in S0 and elliptical galaxies. We find that LX/LK^2 tends to correlate positively with the total cold gas mass M(H2+HI) for gas-poor galaxies with M(H2+HI)<1e8Msun, while they anti-correlate with each other for gas-rich ones. This cold-hot gas relation can be explained in a scenario of galaxy evolution, with the leftover cold gas from the precursor star forming galaxy mainly removed by the long-lasting Ia SN feedback. The two different trends for gas-rich and -poor galaxies may be the results of the initial fast decreasing SN rate and the later fast decreasing mass-loading to hot gas, respectively.

astro-ph.CO↗

arXiv2011

X-ray emission from the Sombrero galaxy: a galactic-scale outflow

Based on new and archival Chandra observations of the Sombrero galaxy (M 104), we study the diffuse X-ray emission in and around its massive stellar bulge. The 2-6 keV unresolved emission from the bulge region closely follows the K-band star light and most likely arises from unresolved stellar sources. At lower energies, however, the unresolved emission reaches a galactocentric radius of at least 23 kpc, significantly beyond the extent of the starlight, clearly indicating the presence of diffuse hot gas. We isolate the emission of the gas by properly accounting for the emission from unresolved stellar sources, predominantly cataclysmic variables and coronally active binaries, whose quasi-universal X-ray emissivity is recently established. We find a gas temperature of ~0.6 keV with little variation across the field of view, except for a lower temperature of ~0.3 keV along the stellar disk. We measure a total intrinsic 0.3-2 keV luminosity of ~2e39 erg/s, which is comparable to the prediction by the latest galaxy formation models for disk galaxies as massive as Sombrero. However, such numerical models do not fully account for internal feedback processes, such as nuclear feedback and stellar feedback, against accretion from the intergalactic medium. On the other hand, we find no evidence for either the nucleus or the very modest star-forming activities in the disk to be a dominant heating source for the diffuse gas. We also show that neither the expected energy released by Type Ia supernovae nor the expected mass returned by evolved stars is recovered by observations. We argue that in Sombrero a galactic-scale subsonic outflow of hot gas continuously removes much of the "missing" energy and mass input from the bulge region. The observed density and temperature distributions of such an outflow, however, continues to pose challenges to theoretical studies.

astro-ph.CO↗

arXiv2011

The Murmur of The Hidden Monster: Chandra's Decadal View of The Super-massive Black Hole in M31

The Andromeda galaxy (M31) hosts a central super-massive black hole (SMBH), known as M31$^\ast$, which is remarkable for its mass ($\sim$$10^8{\rm~M_\odot}$) and extreme radiative quiescence. Over the past decade, the Chandra X-ray observatory has pointed to the center of M31 $\sim$100 times and accumulated a total exposure of $\sim$900 ks. Based on these observations, we present an X-ray study of a highly variable source that we associate with M31$^\ast$ based on positional coincidence. We find that M31$^\ast$ remained in a quiescent state from late 1999 to 2005, exhibiting an average 0.5-8 keV luminosity $\lesssim$$10^{36}{\rm~ergs~s^{-1}}$, or only $\sim$$10^{-10}$ of its Eddington luminosity. We report the discovery of an outburst that occurred on January 6, 2006, during which M31$^\ast$ radiated at $\sim$$4.3\times10^{37}{\rm~ergs~s^{-1}}$. After the outburst, M31$^\ast$ entered a more active state that apparently lasts to the present, which is characterized by frequent flux variability around an average luminosity of $\sim$$4.8\times10^{36}{\rm~ergs~s^{-1}}$. These flux variations are similar to the X-ray flares found in the SMBH of our Galaxy (Sgr A$^\ast$), making M31$^\ast$ the second SMBH known to exhibit recurrent flares. Future coordinated X-ray/radio observations will provide useful constraints on the physical origin of the flaring emission and help rule out a possible stellar origin of the X-ray source.

astro-ph.HE↗

arXiv2010

X-ray emission from the Sombrero galaxy: discrete sources

We present a study of discrete X-ray sources in and around the bulge-dominated, massive Sa galaxy, Sombrero (M104), based on new and archival Chandra observations with a total exposure of ~200 ks. With a detection limit of L_X = 1E37 erg/s and a field of view covering a galactocentric radius of ~30 kpc (11.5 arcminute), 383 sources are detected. Cross-correlation with Spitler et al.'s catalogue of Sombrero globular clusters (GCs) identified from HST/ACS observations reveals 41 X-rays sources in GCs, presumably low-mass X-ray binaries (LMXBs). We quantify the differential luminosity functions (LFs) for both the detected GC and field LMXBs, whose power-low indices (~1.1 for the GC-LF and ~1.6 for field-LF) are consistent with previous studies for elliptical galaxies. With precise sky positions of the GCs without a detected X-ray source, we further quantify, through a fluctuation analysis, the GC LF at fainter luminosities down to 1E35 erg/s. The derived index rules out a faint-end slope flatter than 1.1 at a 2 sigma significance, contrary to recent findings in several elliptical galaxies and the bulge of M31. On the other hand, the 2-6 keV unresolved emission places a tight constraint on the field LF, implying a flattened index of ~1.0 below 1E37 erg/s. We also detect 101 sources in the halo of Sombrero. The presence of these sources cannot be interpreted as galactic LMXBs whose spatial distribution empirically follows the starlight. Their number is also higher than the expected number of cosmic AGNs (52+/-11 [1 sigma]) whose surface density is constrained by deep X-ray surveys. We suggest that either the cosmic X-ray background is unusually high in the direction of Sombrero, or a distinct population of X-ray sources is present in the halo of Sombrero.

astro-ph.CO↗

arXiv2010

The circumnuclear environment in M31

Studies of galactic circumnuclear environments is important to our understanding of the feeding and feedback of the central super-massive black hole and in turn the global evolution of the host galaxy. We present an observational overview of the circumnuclear environment in M31 and a tentative understanding of its regulation. Notes on selected open issues, as well as on a comparison with the Galactic Center and other extragalactic circumnuclear environments, are also presented.

astro-ph.CO↗

arXiv2010

X-ray spectroscopy of the hot has in the M31 bulge

We present an X-ray spectroscopic study of the nuclear region of the M31 bulge, based on observations of the Xmm-Newton Reflection Grating Spectrometers. The obtained high-resolution grating spectra clearly show individual emission lines of highly-ionized iron and oxygen, which unambiguously confirm the presence of diffuse hot gas in the bulge, as indicated from previous X-ray CCD imaging studies. We model the spectra with detailed Monte-Carlo simulations, which provide a robust spectroscopic estimate of the hot gas temperature $\sim0.29$ keV and the O/Fe ratio $\sim0.3$ solar. The results indicate that iron ejecta of type Ia supernovae are partly-mixed with the hot gas. The observed spectra show an intensity excess at the OVII triplet, which most likely represents emission from charge exchanges at the interface between the hot gas and a known cool gas spiral in the same nuclear region.

astro-ph.GA↗

arXiv2009

Dynamic S0 Galaxies: a Case Study of NGC 5866

S0 galaxies are often thought to be passively evolved from spirals after star formation is quenched. To explore what is occurring in S0s, we present a multi-wavelength study of NGC5866--an isolated nearby edge-on S0. This study shows strong evidence for dynamic activities in the interstellar medium, which are most likely driven by supernova explosions in the galactic disk and bulge. We utilize Chandra, HST, and Spitzer data as well as ground-based observations to characterize the content, structure, and physical state of the medium and its interplay with stars in NGC5866. A cold gas disk is detected with an exponential scale height of 100pc. Numerous off-disk dusty spurs are clearly present: prominent ones can extend as far as 300pc from the galactic plane and are probably produced by individual SNe, whereas faint filaments can have ~ kpc scale and are likely produced by SNe collectively in disk/bulge. We also detect substantial amounts of diffuse Ha- and Pa-emitting gas with a comparable scale height as the cold gas. We find that the heating of the dust and warm ionized gas cannot be explained by the radiation from evolved stars alone, strongly indicating the presence of young stars in the disk at a low formation rate of ~ 0.05Msun/yr. We reveal the presence of diffuse X-ray-emitting hot gas, which extends as far as 3.5kpc from the disk and can be heated easily by Type Ia SNe in the bulge. However, the temperature of this gas is ~ 0.2keV, substantially lower than what might be expected from the mass-loss of evolved stars and Type Ia SNe heating alone, indicating mass loading from cool gas is important. The total masses of the cold, warm and hot gases are ~ 5*10^8, 10^4 and 3*10^7Msun. The relative richness of the gases, undergoing circulations between disk/halo, is perhaps a result of its relative isolation.

astro-ph.CO↗

arXiv2009

M31* and its circumnuclear environment

We present a multiwavelength investigation of the circumnuclear environment of M31. Based on Chandra/ACIS data, we tightly constrain the X-ray luminosity of M31*, the central supermassive black hole of the galaxy, to be L (0.3-7 keV)<= 1.2x10^{36}erg/s, approximately 10^{-10} of the Eddington luminosity. From the diffuse X-ray emission, we characterize the circumnuclear hot gas with a temperature of ~0.3 keV and a density of ~0.1 cm^{-3}. In the absence of an active SMBH and recent star formation, the most likely heating source for the hot gas is Type Ia SNe. The presence of cooler, dusty gas residing in a nuclear spiral has long been known in terms of optical line emission and extinction. We further reveal the infrared emission of the nuclear spiral and evaluate the relative importance of various possible ionizing sources. We show evidence for interaction between the nuclear spiral and the hot gas, probably via thermal evaporation. This mechanism lends natural understandings to 1) the inactivity of M31*, in spite of a probably continuous supply of gas from outer disk regions, and 2) the launch of a bulge outflow of hot gas, primarily mass-loaded from the circumnuclear regions. One particular prediction of such a scenario is the presence of gas with intermediate temperatures arising from the conductive interfaces. The FUSE observations do show strong OVI$λ$1032 and 1038 absorption lines against the bulge starlight, but the effective OVI column density (~4x10^{14} cm^{-2}), may be attributed to foreground gas located in the bulge and/or the highly inclined disk of M31. Our study strongly argues that stellar feedback, particularly in the form of energy release from SNe Ia, may play an important role in regulating the evolution of SMBHs and the interstellar medium in galactic bulges.

astro-ph.GA↗

arXiv2007

Surface plasmon polaritons assisted diffraction in periodic subwavelength holes of metal films with reduced interplane coupling

Metal films grown on Si wafer perforated with a periodic array of subwavelength holes have been fabricated and anomalous enhanced transmission in the mid-infrared regime has been observed. High order transmission peaks up to Si(2,2) are clearly revealed due to the large dielectric constant contrast of the dielectrics at the opposite interfaces. Si(1,1) peak splits at oblique incidence both in TE and TM polarization, which confirms that anomalous enhanced transmission is a surface plasmon polaritons (SPPs) assisted diffraction phenomenon. Theoretical transmission spectra agree excellently with the experimental results and confirm the role of SPPs diffraction by the lattice.

cond-mat.mes-hall↗

arXiv2007

Chandra and XMM-Newton Detection of Large-scale Diffuse X-ray Emission from the Sombrero Galaxy

We present an X-ray study of the massive edge-on Sa galaxy, Sombrero (M~104; NGC 4594), based on XMM-Newton and Chandra observations. A list of 62 XMM-Newton and 175 Chandra discrete X-ray sources is provided. At energies above 2 keV, the source-subtracted X-ray emission is distributed similarly as the stellar K-band light and is primarily due to the residual emission from discrete sources. At lower energies, however, a substantial fraction of the source-subtracted emission arises from diffuse hot gas extending to ~20 kpc from the galactic center. The observed diffuse X-ray emission from the galaxy shows a steep spectrum that can be characterized by an optically-thin thermal plasma with temperatures of ~0.6-0.7 keV, varying little with radius. The diffuse emission has a total luminosity of ~3e39 erg/s in the 0.2-2 keV energy range. This luminosity is significantly smaller than the prediction by current numerical simulations for galaxies as massive as Sombrero. We argue that the stellar feedback likely plays an essential role in regulating the physical properties of hot gas. Indeed, the observed diffuse X-ray luminosity of Sombrero accounts for at most a few percent of the expected mechanical energy input from Type Ia supernovae. The inferred gas mass and metal content are also substantially less than those expected from stellar ejecta. We speculate that a galactic bulge wind, powered primarily by Type Ia supernovae, has removed much of the 'missing' energy and metal-enriched gas from regions revealed by the X-ray observations.

astro-ph↗

arXiv2006

An XMM-Newton Observation of the Massive Edge-on Sb Galaxy NGC 2613

We present an XMM-Newton observation of the massive edge-on Sb galaxy NGC 2613. We discover that this galaxy contains a deeply embedded active nucleus with a 0.3-10 keV luminosity of 3.3x10^40 erg/s and a line-of-sight absorption column of 1.2x10^23 cm^-2. Within the 25 mag/arcsec^2 optical B-band isophote of the galaxy, we detect an additional 4 sources with an accumulated luminosity of 4.3x10^39 erg/s. The bulk of the unresolved X-ray emission spatially follows the near-infrared (NIR) K-band surface brightness distribution; the luminosity ratio L_X/L_K ~ 8x10^-4 is consistent with that inferred from galactic discrete sources. This X-ray-NIR association and the compatibility of the X-ray spectral fit with the expected spectrum of a population of discrete sources suggest that low-mass X-ray binaries (LMXBs) are the most likely emitters of the unresolved emission in the disk region. The remaining unresolved emission is primarily due to extraplanar hot gas. The luminosity of this gas is at least a factor of 10 less than that predicted by recent simulations of intergalactic gas accretion by such a massive galaxy with a circular rotation speed V_c ~ 304 km/s^2 (Toft et al. 2002). Instead, we find that the extraplanar hot gas most likely represents discrete extensions away from the disk, including two ``bubble-like'' features on either side of the nucleus. These extensions appear to correlate with radio continuum emission and, energetically, can be easily explained by outflows from the galactic disk.

astro-ph↗