arXiv subjects

Shude Mao

Publications and source records attributed to Shude Mao.

At least 253 records · Page 14Linked to original sources

arXiv2013

Biases in physical parameter estimates through differential lensing magnification

We study the lensing magnification effect on background galaxies. Differential magnification due to different magnifications of different source regions of a galaxy will change the lensed composite spectra. The derived properties of the background galaxies are therefore biased. For simplicity, we model galaxies as a superposition of an axis-symmetric bulge and a face-on disk in order to study the differential magnification effect on the composite spectra. We find that some properties derived from the spectra (e.g., velocity dispersion, star formation rate and metallicity) are modified. Depending on the relative positions of the source and the lens, the inferred results can be either over- or under-estimates of the true values. In general, for an extended source at strong lensing regions with high magnifications, the inferred physical parameters (e.g. metallicity) can be strongly biased. Therefore detailed lens modelling is necessary to obtain the true properties of the lensed galaxies.

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arXiv2013

Gravitational lensing effects on sub-millimetre galaxy counts

We study the effects on the number counts of sub-millimetre galaxies due to gravitational lensing. We explore the effects on the magnification cross section due to halo density profiles, ellipticity and cosmological parameter (the power-spectrum normalisation $σ_8$). We show that the ellipticity does not strongly affect the magnification cross section in gravitational lensing while the halo radial profiles do. Since the baryonic cooling effect is stronger in galaxies than clusters, galactic haloes are more concentrated. In light of this, a new scenario of two halo population model is explored where galaxies are modeled as a singular isothermal sphere profile and clusters as a Navarro, Frenk and White (NFW) profile. We find the transition mass between the two has modest effects on the lensing probability. The cosmological parameter $σ_8$ alters the abundance of haloes and therefore affects our results. Compared with other methods, our model is simpler and more realistic. The conclusions of previous works is confirm that gravitational lensing is a natural explanation for the number count excess at the bright end.

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arXiv2012

Made-to-measure galaxy models - III Modelling with Milky Way observations

We demonstrate how the Syer & Tremaine made-to-measure method of stellar dynamical modelling can be adapted to model a rotating galactic bar. We validate our made-to-measure changes using observations constructed from the existing Shen et al. (2010) N-body model of the Milky Way bar, together with kinematic observations of the Milky Way bulge and bar taken by the Bulge Radial Velocity Assay (BRAVA). Our results for a combined determination of the bar angle and bar pattern speed (~30 degrees and ~40 km/s/kpc) are consistent with those determined by the N-body model. Whilst the made-to-measure techniques we have developed are applied using a particular N-body model and observational data set, they are in fact general and could be applied to other Milky Way modelling scenarios utilising different N-body models and data sets. Additionally, we use the exercise as a vehicle for illustrating how N-body and made-to-measure methods might be combined into a more effective method.

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arXiv2012

Determining neutron star masses with weak microlensing

The masses of stars including stellar remnants are almost exclusively known from binary systems. In this work, we study gravitational microlensing of faint background galaxies by isolated neutron stars (pulsars). We show that the resulting surface brightness distortions can be used to determine the masses of neutron star. Due to different evolutionary histories, isolated neutron stars may have different masses from those in binary systems, and thus provide unique insight into their equation of states under extreme physical conditions. We search for existing pulsar catalogs and find one promising pair of a nearby pulsar and a background galaxy. This method will become more practical for the next generation optical and radio surveys and telescopes.

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arXiv2012

A New Model for the Milky Way Bar

We use Schwarzschild's orbit-superposition technique to construct self-consistent models of the Galactic bar. Using $χ^2$ minimisation, we find that the best-fit Galactic bar model has a pattern speed $Ω_{\rm p}=60 \rm{km s^{-1} kpc^{-1}}$, disk mass $\rm{M_{\rm d}=1.0\times10^{11}M_{\odot}}$ and bar angle $θ_{\rm bar}=20^{\circ}$ for an adopted bar mass $\rm{M_{\rm bar}=2\times10^{10}M_{\odot}}$. The model can reproduce not only the three-dimensional and projected density distributions but also velocity and velocity dispersion data from the BRAVA survey. We also predict the proper motions in the range $l=[-12^{\circ},12^{\circ}]$, $b=[-10^{\circ},10^{\circ}]$, which appear to be higher than observations in the longitudinal direction. The model is stable within a timescale of 0.5 Gyr, but appears to deviate from steady-state on longer timescales. Our model can be further tested by future observations such as those from GAIA.

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arXiv2012

Internal kinematics of groups of galaxies in the Sloan Digital Sky Survey data release 7

We present measurements of the velocity dispersion profile (VDP) for galaxy groups in the final data release of the Sloan Digital Sky Survey (SDSS). For groups of given mass we estimate the redshift-space cross-correlation function (CCF) with respect to a reference galaxy sample, xi(r_p, pi), the projected CCF, w_p(r_p), and the real-space CCF, xi(r). The VDP is then extracted from the redshift distortion in xi(r_p, pi), by comparing xi(r_p, pi) with xi(r). We find that the velocity dispersion (VD) within virial radius (R_200) shows a roughly flat profile, with a slight increase at radii below ~0.3 R_200 for high mass systems. The average VD within the virial radius, sigma_v, is a strongly increasing function of central galaxy mass. We apply the same methodology to N-body simulations with the concordance Lambda cold dark matter cosmology but different values of the density fluctuation parameter sigma_8, and we compare the results to the SDSS results. We show that the sigma_v-M_* relation from the data provides stringent constraints on both sigma_8 and sigma_ms, the dispersion in log M_* of central galaxies at fixed halo mass. Our best-fitting model suggests sigma_8 = 0.86 +/- 0.03 and sigma_ms = 0.16 +/- 0.03. The slightly higher value of sigma_8 compared to the WMAP7 result might be due to a smaller matter density parameter assumed in our simulations. Our VD measurements also provide a direct measure of the dark matter halo mass for central galaxies of different luminosities and masses, in good agreement with the results obtained by Mandelbaum et al. (2006) from stacking the gravitational lensing signals of the SDSS galaxies.

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arXiv2012

Astrophysical Applications of Gravitational Microlensing

Since the first discovery of microlensing events nearly two decades ago, gravitational microlensing has accumulated tens of TBytes of data and developed into a powerful astrophysical technique with diverse applications. The review starts with a theoretical overview of the field and then proceeds to discuss the scientific highlights. (1) Microlensing observations toward the Magellanic Clouds rule out the Milky Way halo being dominated by MAssive Compact Halo Objects (MACHOs). This confirms most dark matter is non-baryonic, consistent with other observations. (2) Microlensing has discovered about 20 extrasolar planets (16 published), including the first two Jupiter-Saturn like systems and the only "cold Neptunes" yet detected. They probe a different part of the parameter space and will likely provide the most stringent test of core accretion theory of planet formation. (3) Microlensing provides a unique way to measure the mass of isolated stars, including brown dwarfs to normal stars. Half a dozen or so stellar mass black hole candidates have also been proposed. (4) High-resolution, target-of-opportunity spectra of highly-magnified dwarf stars provide intriguing "age" determinations which may either hint at enhanced helium enrichment or unusual bulge formation theories. (5) Microlensing also measured limb-darkening profiles for close to ten giant stars, which challenges stellar atmosphere models. (6) Data from surveys also provide strong constraints on the geometry and kinematics of the Milky Way bar (through proper motions); the latter indicates predictions from current models appear to be too anisotropic compared with observations. The future of microlensing is bright given the new capabilities of current surveys and forthcoming new telescope networks from the ground and from space. Some open issues in the field are identified and briefly discussed.

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arXiv2012

On the soft X-ray emission of M82

We present a spatial analysis of the soft X-ray and Hα emissions from the outflow of the starburst galaxy M82. We find that the two emissions are tightly correlated on various scales. The O VII triplet of M82, as resolved by X-ray grating observations of XMM-Newton, is dominated by the forbidden line, inconsistent with the thermal prediction. The O VII triplet also shows some spatial variations. We discuss three possible explanations for the observed O VII triplet, including the charge exchange at interfaces between the hot outflow and neutral cool gas, a collisional non-equilibrium-ionization recombining plasma, and resonance scattering.

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arXiv2012

Star formation activities in early-type brightest cluster galaxies

We identify a total of 120 early-type Brightest Cluster Galaxies (BCGs) at 0.1<z<0.4 in two recent large cluster catalogues selected from the Sloan Digital Sky Survey (SDSS). They are selected with strong emission lines in their optical spectra, with both Hα and [O II]λ3727 line emission, which indicates significant ongoing star formation. They constitute about ~ 0.5% of the largest, optically-selected, low-redshift BCG sample, and the fraction is a strong function of cluster richness. Their star formation history can be well described by a recent minor and short starburst superimposed on an old stellar component, with the recent episode of star formation contributing on average only less than 1 percent of the total stellar mass. We show that the more massive star-forming BCGs in richer clusters tend to have higher star formation rate (SFR) and specific SFR (SFR per unit galaxy stellar mass). We also compare their statistical properties with a control sample selected from X-ray luminous clusters, and show that the fraction of star-forming BCGs in X-ray luminous clusters is almost one order of magnitude larger than that in optically-selected clusters. BCGs with star formation in cooling flow clusters usually have very flat optical spectra and show the most active star formation, which may be connected with cooling flows.

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arXiv2012

On the Effects of Line-of-Sight Structures on Lensing Flux-ratio Anomalies in a LCDM Universe

The flux-ratio anomalies observed in multiply-lensed quasar images are most plausibly explained as the result of perturbing structures superposed on the underlying smooth matter distribution of the primary lens. The cold dark matter cosmological model predicts that a large number of substructures should survive inside larger halos but, surprisingly, this population alone has been shown to be insufficient to explain the observed distribution of the flux ratios of quasar's multiple images. Other halos (and their own subhalos) projected along the line of sight to the primary lens have been considered as additional source of perturbation. In this work, we use ray tracing through the Millennium II simulation to investigate the importance of projection effects due to halos and subhalos of mass m>1E8 Msun/h and extend our analysis to lower masses, m>1E6 Msun/h, using Monte-Carlo halo distributions. We find that the magnitude of the violation depends strongly on the density profile and concentration of the intervening halos, but clustering plays only a minor role. For a typical lensing geometry (lens at redshift 0.6 and source at redshift 2), background haloes (behind the main lens) are more likely to cause a violation than foreground halos. We conclude that line-of-sight structures can be as important as intrinsic substructures in causing flux-ratio anomalies. The combined effect of perturbing structures within the lens and along the line of sight in the LCDM universe results in a cusp-violation probability of 20-30%. This alleviates the discrepancy between models and current data, but a larger observational sample is required for a stronger test of the theory.

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arXiv2012

Made-to-measure galaxy models - II Elliptical and Lenticular Galaxies

We take a sample of 24 elliptical and lenticular galaxies previously analysed by the SAURON project using three-integral dynamical models created with Schwarzschild's method, and re-analyse them using the made-to-measure (M2M) method of dynamical modelling. We obtain good agreement between the two methods in determining the dynamical mass-to-light (M/L) ratios for the galaxies with over 80% of ratios differing by < 10% and over 95% differing by < 20%. We show that (M/L)_M2M is approximately equal to (M/L)_Sch. For the global velocity dispersion anisotropy parameter delta, we find similar values but with fewer of the made-to-measure models tangentially anisotropic by comparison with their SAURON Schwarzschild counterparts. Our investigation is the largest comparative application of the made-to-measure method to date.

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arXiv2011

Measurement of halo properties with weak lensing shear and flexion

We constrain properties of cluster haloes by performing likelihood analysis using lensing shear and flexion data. We test our analysis using two mock cluster haloes: an isothermal ellipsoid (SIE) model and a more realistic elliptical Navarro-Frenk-White (eNFW) model. For both haloes, we find that flexion is more sensitive to the halo ellipticity than shear. The introduction of flexion information significantly improves the constraints on halo ellipticity, orientation and mass. We also point out that there is a degeneracy between the mass and the ellipticity of SIE models in the lensing signal.

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arXiv2011

Charge Exchange X-ray Emission of Nearby Star-forming Galaxies

Properties of hot gas outflows from galaxies are generally measured from associated X-ray line emission assuming that it represents atomic transitions in thermally excited hot gas. X-ray line emission, however, can also arise from the charge exchange between highly ionised ions and neutral species. The Kα triplet of He-like ions can be used as a powerful diagnostic, because the charge exchange X-ray emission (CXE) favours the inter-combination and forbidden lines, while the thermal emission favours the resonance line. We analyse the OVII triplet of a sample of nine nearby star-forming galaxies observed by the XMM-Newton reflection grating spectrometers. For most galaxies, the forbidden lines are comparable to or stronger than the resonance lines, which is in contrast to the thermal prediction. For NGC 253, M51, M83, M61, NGC 4631, and the Antennae galaxy, the observed line ratios are consistent with the ratio of the CXE; for M94 and NGC 2903, the observed ratios indicate multiple origins; for M82, different regions show different line ratios, also indicating multiple origins. We discuss other possible mechanisms that can produce a relatively strong forbidden line, such as a collisional non-equilibrium-ionization recombining/ionizing plasma, which are not favoured. These results suggest that the CXE may be a common process and contribute a significant fraction of the soft X-ray line emission for galaxies with massive star formation.

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arXiv2011

Lensing by a singular isothermal sphere and a black hole

Most galaxies host central supermassive black holes. As two galaxies merge, the black holes also merge. The final single black hole may suffer a kick due to asymmetric gravitational radiation and may not be at the centre of the galaxy; off-centre black holes may also be produced by other means such as sustained acceleration due to asymmetric jet power. We model the main galaxy as a singular isothermal sphere and the black hole as an off-centre point lens, and study the critical curves and caustics using complex notation. We identify the critical parameters that govern the transitions in the topology of critical curves, caustics and pseudo-caustics, and find the number of images can be two, three, four and five. We show examples of image configurations, including cases where three highly de-magnified images are found close to the centre. The perturbation on the image magnification due to the black hole scales linearly with its mass in the off-centre case, and quadratically when the black hole is at the centre. Such images are difficult to observe unless high-contrast and high-resolution imaging facilities (e.g., the Square Kilometer Array in the radio) become available.

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arXiv2011

Effects of supermassive binary black holes on gravitational lenses

Recent observations indicate that many if not all galaxies host massive central black holes (BHs). In this paper we explore the influence of supermassive binary black holes (SMBBHs) on their actions as gravitational lenses. When lenses are modelled as singular isothermal ellipsoids, binary black holes change the critical curves and caustics differently as a function of distance. Each black hole can in principle create at least one additional image, which, if observed, provides evidence of black holes. By studying how SMBBHs affect the cumulative distribution of magnification for images created by black holes, we find that the cross section for at least one such additional image to have a magnification larger than $10^{-5}$ is comparable to the cross section for producing multiple-images in singular isothermal lenses. Such additional images may be detectable with high-resolution and large dynamic range maps of multiply-imaged systems from future facilities, such as the SKA. The probability of detecting at least one image (two images) with magnification above $10^{-3}$ is $\sim 0.2 \fBH$ ($\sim 0.05 \fBH$) in a multiply-imaged lens system, where $\fBH$ is the fraction of galaxies housing binary black holes. We also study the effects of SMBBHs on the core images when galaxies have shallower central density profiles (modelled as non-singular isothermal ellipsoids). We find that the cross section of the usually faint core images is further suppressed by SMBBHs. Thus their presence should also be taken into account when one constrains the core radius from the lack of central images in gravitational lenses.

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arXiv2011

Rapidly Rotating Lenses: Repeating features in the lightcurves of short period binary microlenses

Microlensing is most sensitive to binary lenses with relatively large orbital separations, and as such, typical binary microlensing events show little or no orbital motion during the event. However, despite the strength of binary microlensing features falling off rapidly as the lens separation decreases, we show that it is possible to detect repeating features in the lightcurve of binary microlenses that complete several orbits during the microlensing event. We investigate the lightcurve features of such Rapidly Rotating Lens (RRL) events. We derive analytical limits on the range of parameters where these effects are detectable, and confirm these numerically. Using a population synthesis Galactic model we estimate the RRL event rate for a ground-based and space-based microlensing survey to be 0.32fb and 7.8fb events per year respectively, assuming year-round monitoring and where fb is the binary fraction. We detail how RRL event parameters can be quickly estimated from their lightcurves, and suggest a method to model RRL events using timing measurements of lightcurve features. Modelling RRL lightcurves will yield the lens orbital period and possibly measurements of all orbital elements including the inclination and eccentricity. Measurement of the period from the lightcurve allows a mass-distance relation to be defined, which when combined with a measurement of microlens parallax or finite source effects, can yield a mass measurement to a two-fold degeneracy. With sub-percent accuracy photometry it is possible to detect planetary companions, but the likelihood of this is very small.

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arXiv2011

Estimate of halo ellipticity as a function of radius with flexions

The cold dark matter theory predicts triaxial dark matter haloes. The radial distribution of halo ellipticity depends on baryonic processes and the nature of dark matter particles (collisionless or collisional). Here we show that we can use lensing flexion ratios to measure the halo ellipticity as a function of radius. We introduce a weight function and study the relationship between the first and second order statistics of flexion ratios, both of which can be used to reduce the bias in the estimate of ellipticity. we perform numerical tests for our method, and demonstrate that it can reduce the bias and determine the halo ellipticity as a function of radius. We also point out that the minimum mean flexion ratio can be used to trace the centres of galaxy clusters.

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arXiv2011

Charge Exchange X-ray Emission of M82: Kα triplets of O VII, Ne IX, and Mg XI

Starburst galaxies are primary feedback sources of mechanical energy and metals, which are generally measured from associated X-ray emission lines assuming that they are from the thermal emission of the outflowing hot gas. Such line emission, however, can also arise from the charge exchange X-ray emission (CXE) between highly ionized ions and neutral species. To understand the feedback of energy and metals, it is crucial to determine the origin of the X-ray emission lines and to distinguish the contributions from the CXE and the thermal emission. The origin of the lines can be diagnosed by the Kα triplets of He-like ions, because the CXE favors the inter-combination and forbidden lines, while the thermal emission favors the resonance line. We analyze the triplets of O VII, Ne IX, and Mg XI observed in the XMM- Newton reflection grating spectra of the starburst galaxy M82. The flux contribution of the CXE is 90%, 50%, and 30% to the O VII, Ne IX, and Mg XI triplet, respectively. Averaged over all the three triplets, the contribution of the CXE is \sim 50% of the total observed triplet flux. To correctly understand the hot outflow of starburst galaxies, it is necessary to include the CXE. Based on the measured CXE contributions to the O VII, Ne IX, and Mg XI triplets, we estimate the relative abundances of O, Ne, and Mg of the outflow and find they are similar to the solar ratios.

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