arXiv subjects

Shude Mao

Publications and source records attributed to Shude Mao.

At least 289 records · Page 16Linked to original sources

arXiv2005

Rotation in Gravitational Lenses

Gravitational lensing deflects light. A single lens deflector can only shear images, but cannot induce rotations. Multiple lens planes can induce rotations. Such rotations can be observed in quadruply imaged sources, and can be used to distinguish between two proposed solutions of the flux anomaly problem: substructures in lensing galaxies vs large scale structure. We predict the expected amount of rotation due to large scale structure in strong lensing systems, and show how this effect can be measured using ~ mas VLBI astrometry of quadruple lenses with extended source structures. The magnitude of rotation is around one degree. The biggest theoretical uncertainty is the power spectrum of dark matter on very small scales. This procedure can potentially be turned around to measure the dark matter power spectrum on very small scales. We list the predicted RMS rotation angles for several quadruple lenses with known lens and source redshifts.

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arXiv2005

The Physical Connections Among IR QSOs, PG QSOs and Narrow-Line Seyfert 1 Galaxies

We study the properties of infrared-selected QSOs (IR QSOs), optically-selected QSOs (PG QSOs) and Narrow Line Seyfert 1 galaxies (NLS1s). We compare their properties from the infrared to the optical and examine various correlations among the black hole mass, accretion rate, star formation rate and optical and infrared luminosities. We find that the infrared excess in IR QSOs is mostly in the far infrared, and their infrared spectral indices suggest that the excess emission is from low temperature dust heated by starbursts rather than AGNs. The infrared excess is therefore a useful criterion to separate the relative contributions of starbursts and AGNs. We further find a tight correlation between the star formation rate and the accretion rate of central AGNs for IR QSOs. The ratio of the star formation rate and the accretion rate is about several hundred for IR QSOs, but decreases with the central black hole mass. This shows that the tight correlation between the stellar mass and the central black hole mass is preserved in massive starbursts during violent mergers. We suggest that the higher Eddington ratios of NLS1s and IR QSOs imply that they are in the early stage of evolution toward classical Seyfert 1's and QSOs, respectively.

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arXiv2004

Chandra X-ray Analysis of Galaxy Cluster A168

We present Chandra X-ray observations of galaxy cluster A168 (z=0.045). Two X-ray peaks with a projected distance of 676 kpc are found to be located close to two dominant galaxies, respectively. Both peaks are significantly offset from the peak of the number density distribution of galaxies. This suggests that A168 consists of two subclusters, a northern subcluster (A168N) and a southern subcluster (A168S). Further X-ray imaging analysis reveals that (1) the X-ray isophotes surrounding the two X-ray peaks are heavily distorted, (2) an elongated and ontinuous filament connects the two X-ray peaks. These suggest that strong interactions have occurred between the two subclusters. Spectral analysis shows that A168 has a mean temperature of 2.53 +/- 0.09 keV and a mean metallicity of 0.31 +/- 0.04 Z_{solar}. The metallicity is roughly a constant across the cluster but the temperature shows some systematic variations. Most X-ray, optical and radio properties of A168 are consistent with it being an off-axis merger several Gyrs after a core passage, although detailed numerical simulations are required to see whether the observed properties, in particular the significant offset between the optical and X-ray centers, can be reproduced in such a scenario.

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arXiv2004

Galaxy formation in pre-processed dark halos

Recent N-body simulations show that the formation of a present-day, galaxy sized dark matter halo in the CDM cosmogony in general consists of an early fast collapse phase, during which the potential associated with a halo is established, followed by a slow accretion phase, during which mass is added rather gently in the outer region. Here we outline a scenario in which the fast collapse phase is accompanied with rapid formation of cold clouds and with starbursts that can eject a large amount of gas from the halo center.The loss of orbital energy of the cold clouds to the dark matter and the ejection of gas from halo center by starburst can significantly reduce the halo concentration. The outflow from the starburst can also heat the gas in the protogalaxy region. Subsequent formation of galaxies in the slow accretion regime is therefore in halos that have been pre-processed by these processes and may have properties different from that given by N-body simulations. This scenario can help to solve several outstanding problems in the standard Lambda CDM model of galaxy formation without compromising its success in allowing structure formation at high redshift. The predicted rotation curves are quite flat,alleviating the discrepancy of the Tully-Fisher relation predicted in the Lambda CDM model with observations. The flattened galaxy halos allow accreted minihalos to survive in their central regions longer, which may be helpful in producing the flux anomalies observed in some gravitational lensing systems. The preheating by the early starbursts effectively reduces the amount of gas that can be accreted into galaxy halos, which may explain why the baryon fraction in a spiral galaxy is in general much lower than the universal baryon fraction, f_B~0.16, in the standard Lambda CDM model.

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arXiv2003

Limits on the evolution of galaxies from the statistics of gravitational lenses

We use gravitational lenses from the Cosmic Lens All-Sky Survey (CLASS) to constrain the evolution of galaxies since redshift $z \sim 1$ in the current $\LCDM$ cosmology. This constraint is unique as it is based on a mass-selected lens sample of galaxies. Our method of statistical analysis is the same as in Chae (2003). We parametrise the early-type number density evolution in the form of $(1+z)^{ν_n}$ and the velocity dispersion as $(1+z)^{ν_v}$. We find that $ν_n=-0.11^{+0.82}_{-0.89}$ ($1σ$) if we assume $ν_v =0$, implying that the number density of early-type galaxies is within 50% to 164% of the present-day value at redshift $z=1$. Allowing the velocity dispersion to evolve, we find that $ν_v=-0.4^{+0.5}_{-0.4}$ ($1σ$), indicating that the velocity dispersion must be within 57% and 107% of the present-day value at $z=1$. These results are consistent with the early formation and passive evolution of early-type galaxies. More stringent limits from lensing can be obtained from future large lens surveys and by using very high-redshift quasars ($z \ga 5$) such as those found from the Sloan Digital Sky Survey.

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arXiv2003

An analytic model for the galactic winds and mass outflows

Galactic winds and mass outflows are observed both in nearby starburst galaxies and in high-redshift star-forming galaxies. In this paper we develop a simple analytic model to understand the observed superwind phenomenon. Our model is built upon the model of McKee & Ostriker (1977) for the interstellar medium. It allows one to predict how properties of a superwind, such as wind velocity and mass outflow rate, are related to properties of its star-forming host galaxy, such as size, gas density and star formation rate. The model predicts a threshold of star formation rate density for the generation of observable galactic winds. Galaxies with more concentrated star formation produce superwinds with higher velocities. The predicted mass outflow rates are comparable to (or slightly larger than) the corresponding star formation rates. We apply our model to both local starburst galaxies and high-redshift Lyman break galaxies, and find its predictions to be in good agreement with current observations. Our model is simple, and so can be easily incorporated into numerical simulations and semi-analytical models of galaxy formation.

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arXiv2002

Constraining the Galactic Bar Parameters with Red Clump Giants

We show that the small intrinsic spread in luminosities of red clump giants can be used to constrain the differences in the streaming motions of Galatic bulge stars on the near side and those on the far side. We propose two methods to select two samples with one preferentially on the near side and the other on the far side. In the first method, we divide red clump giants into a bright sample and a faint one; stars in the bright sample will be on average more on the near side and vice versa. The second method relies on the fact that lensed bulge stars lie preferentially on the far side due to the enhanced lensing probability by the stars on the near side and in the disk. If the radial streaming motion is ~50km/s, we find the difference in the average radial velocity between the bright and faint samples can reach ~33km/s while the corresponding difference is about ~10km/s between the lensed stars and all observed stars. The difference in the average proper motion between the bright and faint samples is about ~1.6mas/yr if there is a tangential streaming motion of 100km/s; the corresponding shift between the lensed stars and all observed stars is ~1mas/yr. To observe the shifts in the radial velocity and proper motion, roughly one hundred microlensing events, and/or bright/faint red clump giants, need to be observed either spectroscopically or astrometrically. The spectroscopic observations can be performed efficiently using multi-object spectrographs already available. The proper motion signature of microlensed objects can be studied using ground-based telescopes and HST. These observations will provide strong constraints on the Galactic bar parameters (abridged).

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arXiv2001

Optical Gravitational Lensing Experiment. OGLE-1999-BUL-32: the Longest Ever Microlensing Event -- Evidence for a Stellar Mass Black Hole?

We describe the discovery of the longest microlensing event ever observed, OGLE-1999-BUL-32, also independently identified by the MACHO collaboration as MACHO-99-BLG-22. This unique event has an Einstein radius crossing time of 641 days. The high quality data obtained with difference image analysis shows a small but significant parallax signature. This parallax effect allows one to determine the Einstein radius projected onto the observer plane as rE^hat ~ 29.2AU. The transverse velocity projected onto the observer plane is about 79km/s. We argue that the lens is likely to be have a mass of at least a few solar masses, i.e., it could be a stellar black hole. The black hole hypothesis can be tested using the astrometric microlensing signature with the soon-to-be installed Advanced Camera for Surveys on board the Hubble Space Telescope. Deep X-ray and radio images may also be useful for revealing the nature of the object.

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arXiv2001

Predicting the Second Caustic Crossing in Binary Microlensing Events

We fit binary lens models to the data covering the initial part of real microlensing events in an attempt to predict the time of the second caustic crossing. We use approximations during the initial search through the parameter space for light curves that roughly match the observed ones. Exact methods for calculating the lens magnification of an extended source are used when we refine our best initial models. Our calculations show that the reliable prediction of the second crossing can only be made very late, when the light curve has risen appreciably after the minimum between the two caustic-crossings. The best observational strategy is therefore to sample as frequently as possible once the light curve starts to rise after the minimum.

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arXiv2001

The Host Haloes of Lyman Break Galaxies and Sub-millimeter Sources

We use the observed sizes and star formation rates of Lyman-break galaxies (LBGs) to understand their properties expected in the hierarchical scenario of galaxy formation. The observed size distribution constrains the masses of the host haloes of LBGs from below, while the observed star-formation distribution constrains the masses from above. Assuming a flat CDM model with a cosmological constant ($Ω_Λ=0.7$), we find that consistency with observations requires the circular velocities of LBG host haloes to be in the range 100--$300 {\rm km s^{-1}}$. The predicted comoving correlation length of these objects is $\sim 3 h^{-1}{\rm Mpc}$, and the predicted velocity dispersion of their stellar contents is typically $70 {\rm km s^{-1}}$. The same prescription applied to larger haloes in the CDM cosmogony predicts the existence of galaxies with star formation rates $\sim 1000 {\rm M_\odot yr^{-1}}$ at redshift $z\sim 3$. We explore the possibility of identifying these galaxies to be the bright sub-millimeter (sub-mm) sources detected by SCUBA. The model predicts that the host haloes of these sub-mm sources are massive. These galaxies are predicted to be strongly correlated, with a comoving correlation length of $\sim 7 h^{-1}{\rm Mpc}$. The descendants of the bright sub-mm sources should reside in clusters of galaxies at the present time, and it is likely that these objects are the progenitors of giant ellipticals. The model predicts the existence of a relatively bright and red population of galaxies at $z\sim 3$, which may be observed in the K-band.

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arXiv2000

Gravitational Microlensing of Gamma-Ray Burst Afterglows by Single and Binary Stars

We calculate the magnification light curves due to stellar microlensing of gamma-ray burst (GRB) afterglows. A GRB source appears on the sky as a thin ring which expands faster than the speed of light and is maximally magnified as it crosses the lens caustics. While a single star lens produces a single peak in the magnification light curve, binary star lenses may produce multiple peaks. The shape of the magnification light curve provides invaluable information on the surface brightness distribution of the afterglow photosphere on sub micro-arcsecond scales. We find that all afterglows are likely to show variability at the level of a few percent about a year following the explosion, due to stars which are separated by tens of Einstein radii from their line-of-sight.

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arXiv2000

Tully-Fisher Relation and its Implications for Halo Density Profile and Self-interacting Dark Matter

We show that the Tully-Fisher relation observed for spiral galaxies can be explained in the current scenario of galaxy formation without invoking subtle assumptions, provided that galactic-sized dark haloes have shallow, core-like central profiles, with a core radius proportional to halo circular velocity. In such a system, both disk and halo contribute significantly to the maximum rotation of the disk, and the interaction between the disk and halo components acts to reduce the scatter in the Tully-Fisher relation. With model parameters chosen in plausible ranges, the model can well accommodate the zero-point, slope, scatter of the observed Tully-Fisher relation, as well as the large range of disk sizes. The model predicts that LSB disks obey a Tully-Fisher relation similar to that of normal disks, if disk mass-to-light ratio is properly taken into account. The halo profile required by the Tully-Fisher relation is as shallow as that required by the rotation curves of faint disks, but much shallower than that predicted by conventional CDM models. Our results cannot be explained by some of the recent proposals for resolving the conflict between conventional CDM models and the rotation-curve shapes of faint galaxies. If dark matter self-interaction (either scattering or annihilation) is responsible for the shallow profile, the observed Tully-Fisher relation requires the interaction cross section σ_X to satisfy <σ_{X}|v|>/m_{X}~10^{-16} cm^3/s/GeV.

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arXiv1999

Gravitational Microlensing: Past, Present and Future

Ongoing microlensing surveys have already yielded more than five hundred microlensing events, most of which have been identified in real-time. In this review I present the basic theory and observational status of these surveys. I highlight the discoveries made so far - these include constraints on the dark matter content of the Galaxy, the structure and mass function of stars in the Galactic bulge and applications on stellar atmospheres and populations. I also briefly discuss the scientific returns (such as planet detection) from real-time photometric and spectroscopic follow-up surveys. With improved software algorithms and more powerful instrumentations, the number of microlensing events will be greatly increased in the future; the Space Interferometry Mission, on the other hand, provides the possibility to break the lens mass and distance degeneracy. I conclude that microlensing is developing into an exciting technique with diverse applications.

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arXiv1999

An Ongoing OGLE Parallax Microlensing Event Toward Carina

We study the first microlensing event toward the Carina spiral arm discovered by the OGLE collaboration. We demonstrate that this long duration event exhibits strong parallax signatures. Additional information from the parallax effect allows us to determine the lens transverse velocity projected onto the Sun-source line to be ~145 km/s We also estimate the optical depth, event rate and duration distribution for microlensing toward Carina. We show that this event is broadly consistent with these predictions.

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arXiv1999

On the Magnfication Relations in Quadruple Lenses: A Moment Approach

We present a new method of studying quadruple lenses in elliptical power-law potentials parameterized by $ψ(x,y) \propto (x^2+y^2/q^2)^{β/2}/β(0 \leq β< 2)$. For this potential, the moments of the four image positions weighted by signed magnifications (magnification times parity) have very simple properties. In particular, we find that the zeroth moment -- the sum of four signed magnifications satisfies $\simeq 2/(2-β)$; the relation is exact for $β=0$ (point-lens) and $β=1$ (isothermal potential), independent of the axial ratio. Similar relations can be derived when a shear is present along the major or minor axes. These relations, however, do not hold well for the closely-related elliptical density distributions. For a singular isothermal elliptical density distribution without shear, the sum of signed magnifications for quadruple lenses is $\approx 2.8$, again nearly independent of the ellipticity. For the same distribution with shear, the total signed magnification is around 2-3 for most cases, but can be significantly different for some combinations of the axial ratio and shear where more than four images can appear.

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arXiv1998

Ultraluminous IRAS galaxy 10026+4347

We study optical and X-ray properties of the ultraluminous IRAS galaxy 10026+4347. This galaxy is a narrow-line QSO with very strong FeII emission. Three optical spectra were taken over two years. The full widths at half maximum (FWHMs) of the emission lines are constant whereas the third spectrum seems to show a continuum change. Intermediate-band photometry also shows a small (0.1 mag) but significant decrease in flux. HST WFPC2 images suggest that this object is a post-merger galaxy. The source is X-ray luminous (L_x ~ 10^45 erg/s) with a very soft X-ray spectrum (photon index ~ 3.2) The X-ray luminosity exhibits variabilities of a factor of ~8 over four years and a factor of two within two days. During these X-ray flux changes, the X-ray spectral shapes are consistent with no variation. All the optical and X-ray properties resemble those of narrow-line Seyfert 1 galaxies, except that the FWHM of Hbeta is about 2500 km/s, larger than that for most NLS1s. We discuss the implications of our results on models of NLS1s.

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arXiv1998

The Formation and Evolution of Disk Galaxies

We review some of our recent progress in modelling the formation of disk galaxies in the framework of hierarchical structure formation. Our model is not only consistent with the local observations, but also provides a good description of disk evolution out to redshift z~1. We use this model to interpret recent observational results on Lyman-break galaxies (LBGs) at z~3. Assuming that these galaxies are associated with the most massive halos and adopting an empirical law for their star formation rates, we find that many properties of the LBG population, including their correlations, sizes and kinematics can be accommodated in the model.

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arXiv1998

The Nature of the Host Galaxies for Gamma-Ray Bursts

It has been suggested recently that the rate of gamma-ray bursts (GRBs) is proportional to the star formation rate in the universe. In this paper, we study the nature of GRB hosts expected in this scenario. We improve upon previous studies by incorporating a luminosity function for the GRBs, as required by observations. This model provides a good match to the observed number counts of GRBs as a function of peak-count rate. The model predicts that the host galaxies have their redshift distribution peaked around z~1, and about 15 percent have z>2.5. This high-redshift fraction have the same properties as the star-forming galaxies recently discovered by the Lyman-break technique. At z<~1, many of the GRBs may be hosted by faint blue galaxies. Using a photometric redshift sample of galaxies from the Hubble Deep Field, we find that the host galaxies have magnitudes in the range from 21.5 to 28 in the I-band, and about 90 percent of them have semi-major axis smaller than 1.3 arcsec. Assuming isotropic emission, the typical peak-luminosity and total energy of GRBs are in an Einstein-de Sitter universe with H_0=100km/s/Mpc. We also discuss further observational tests of this scenario.

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