arXiv subjects

S. Mao

Publications and source records attributed to S. Mao.

90 records · Page 5Linked to original sources

arXiv2001

Chandra Observations of MRK 273: Unveiling the Central AGN and the Extended Hot Gas Halo

We report X-ray observations of the field containing the ultraluminous IRAS galaxy Mrk~273 Using the ACIS-S3 instrument on board Chandra. The high resolution X-ray image, for the first time, reveals a compact hard X-ray nucleus in Mrk~273. Its X-ray energy distribution is well described by a heavily obscured power-law spectrum plus a narrow $\Feka$ emission line at 6.4 keV. The neutral hydrogen column density is about $4\times10^{23}\cm^{-2}$, implying an absorption -corrected X-ray luminosity (0.1--10 keV) for the nucleus of $\Lx\approx 6.5\times 10^{43} \ergs$. There are also bright soft X-ray clumps and diffuse soft X-ray emissions surrounding the central hard X-ray nucleus within the $10\arcsec$ of the nuclear region. Its spectrum can be fitted by a MEKAL thermal model with temperature of about 0.8 keV and high metallicity ($Z\sim 1.5Z_\odot$) plus emission lines from $α$ elements and ions. Further outside the central region, the Chandra observations reveal a very extended hot gas halo with a projected diameter of about $108 \kpc\times 68 \kpc$ and soft X-ray luminosity of $\Lx\approx 1.9\times 10^{41} \ergs$. The temperature of the hot gas is about 0.62 keV with a low metallicity ($Z \sim 0.1 Z_\odot$). We discuss the nature of the AGN in Mrk~273 and the implications of our results on the origin of X-ray halos in elliptical galaxies. We also discuss the properties of Mrk~273x, a background AGN in the Mrk~273 field. The AGN has an X-ray luminosity of $\Lx \approx 2.43\times 10^{44}\ergs$ in the 0.5-10 keV band. Its X-ray properties resemble those of Seyfert 1 galaxies while its optical properties are similar to Seyfert 2 galaxies. Such mixed classifications may be a challenge for the unification scheme of AGNs.

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arXiv2001

Parallax Microlensing Events in the OGLE II Database Toward the Galactic Bulge

We present a systematic search for parallax microlensing events among a total of 512 microlensing candidates in the OGLE II database for the 1997-1999 seasons. We fit each microlensing candidate with both the standard microlensing model and also a parallax model that accounts for the Earth's motion around the Sun. We then search for the parallax signature by comparing the chi^2 of the standard and parallax models. For the events which show a significant improvement, we further use the `duration' of the event and the signal-to-noise ratio as criteria to separate true parallax events from other noisy microlensing events. We have discovered one convincing new candidate, sc33_4505, and seven other marginal cases. The convincing candidate (sc33_4505) is caused by a slow-moving, and likely low-mass, object, similar to other known parallax events. We found that irregular sampling and gaps between observing seasons hamper the recovery of parallax events. We have also searched for long-duration events that do not show parallax signatures. The lack of parallax effects in a microlensing event puts a lower-limit on the Einstein radius projected onto the observer plane, which in turn imposes a lower limit on the lens mass divided by the relative lens-source parallax. Most of the constraints are however quite weak.

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arXiv2001

Modelling The First Probable Two Plane Lens System B2114+022: Reproducing Two Compact Radio Cores A, D

We test possible lensing scenarios of the JVAS system B2114+022, in which two galaxies at different redshifts (``G1'' at $z_1 = 0.3157$ and ``G2'' at $z_2 = 0.5883$) are found within 2 arcseconds of quadruple radio sources. For our investigation, we use possible lensing constraints derived from a wealth of data on the radio sources obtained with VLA, MERLIN, VLBA and EVN as well as HST imaging data on the two galaxies, which are presented in Augusto et al. In the present study, we focus on reproducing the widest separated, observationally similar radio components A and D as lensed images. We first treat G2 (which is the more distant one from the geometric centre) as a shear term, and then consider two plane lensing explicitly including G2's potential at the $z_2$ plane as the first case of two plane lens modelling. Our modelling results not only support the hypothesis that the system includes gravitationally lensed images of a higher redshift extragalactic object, but they also show that the explicit inclusion of G2's potential at the second lens plane is necessary in order to fit the data with astrophysically plausible galaxy parameters. Finally, we illustrate a natural consequence of a two-plane lens system, namely the prediction of distortion as well as shift and stretching of G2's isophotes by G1's potential, which can in principle be measured by subtracting out G1's light distribution in a high S/N and good angular resolution image, especially a multi-colour one.

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arXiv2001

Statistical properties of ultraluminous IRAS galaxies from an HST imaging survey

We perform photometric measurements on a large HST snapshot imaging survey sample of 97 ultraluminous infrared galaxies (ULIRGs). We select putative nuclei from bright clumps in all the sample targets, mainly based on a quantitative criterion of I-band luminosity as well as the global and local morphological information. All the sources are then classified into three categories with multiple, double and single nucleus/nuclei. The resultant fractions of multiple, double and single nucleus/nuclei ULIRGs are 18%, 39% and 43%, respectively. This supports the multiple merger scenario as a possible origin of ULIRGs, in addition to the commonly-accepted pair merger model. Further statistical studies indicate that the AGN fraction increases from multiple (36%) to double (65%) and then to single (80%) nucleus/nuclei ULIRGs. For the single nucleus category, there is a high luminosity tail in the luminosity distribution, which corresponds to a Seyfert 1/QSO excess. This indicates that active galactic nuclei tend to appear at final merging stage. For multiple/double nuclei galaxies, we also find a high fraction of very close nucleus pairs (e.g., 3/4 for those separated by less than 5 kpc). This strengthens the conclusion that systems at late merging phase preferentially host ULIRGs.

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arXiv2001

Optical Gravitational Lensing Experiment. OGLE 2000-BUL-43: A Spectacular Ongoing Parallax Microlensing Event. Difference Image Analysis

We present the photometry and theoretical models for a Galactic bulge microlensing event OGLE-2000-BUL-43. The event is very bright with I=13.54 mag, and has a very long time scale, t_E=156 days. The long time scale and its light curve deviation from the standard shape strongly suggest that it may be affected by the parallax effect. We show that OGLE-2000-BUL-43 is the first discovered microlensing event, in which the parallax distortion is observed over a period of 2 years. Difference Image Analysis (DIA) using the PSF matching algorithm of Alard & Lupton enabled photometry accurate to 0.5%. All photometry obtained with DIA is available electronically. Our analysis indicates that the viewing condition from a location near Jupiter will be optimal and can lead to magnifications ~ 50 around January 31, 2001. These features offer a great promise for resolving the source (a K giant) and breaking the degeneracy between the lens parameters including the mass of the lens, if the event is observed with the imaging camera on the Cassini space probe.

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arXiv2000

A New Quadruple Gravitational Lens System: CLASS B0128+437

High resolution MERLIN observations of a newly-discovered four-image gravitational lens system, B0128+437, are presented. The system was found after a careful re-analysis of the entire CLASS dataset. The MERLIN observations resolve four components in a characteristic quadruple-image configuration; the maximum image separation is 542 mas and the total flux density is 48 mJy at 5 GHz. A best-fit lens model with a singular isothermal ellipsoid results in large errors in the image positions. A significantly improved fit is obtained after the addition of a shear component, suggesting that the lensing system is more complex and may consist of multiple deflectors. The integrated radio spectrum of the background source indicates that it is a GigaHertz-Peaked Spectrum (GPS) source. It may therefore be possible to resolve structure within the radio images with deep VLBI observations and thus better constrain the lensing mass distribution.

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arXiv2000

The Influence of Central Black Holes on Gravitational Lenses

Recent observations indicate that many if not all galaxies host massive central black holes. In this paper we explore the influence of black holes on the lensing properties. We model the lens as an isothermal ellipsoid with a finite core radius plus a central black hole. We show that the presence of the black hole substantially changes the critical curves and caustics. If the black hole mass is above a critical value, then it will completely suppress the central images for all source positions. Realistic central black holes likely have masses below this critical value. Even in such sub-critical cases, the black hole can suppress the central image when the source is inside a zone of influence, which depends on the core radius and black hole mass. In the sub-critical cases, an additional image may be created by the black hole in some regions, which for some radio lenses may be detectable with high-resolution and large dynamic-range VLBI maps. The presence of central black holes 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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arXiv2000

Analytic Time Delays and H_0 Estimates for Gravitational Lenses

We study gravitational lens time delays for a general family of lensing potentials, which includes the popular singular isothermal elliptical potential and singular isothermal elliptical density distribution but allows general angular structure. Using a novel approach, we show that the time delay can be cast in a very simple form, depending only on the observed image positions. Including an external shear changes the time delay proportional to the shear strength, and varying the radial profile of the potential changes the time delay approximately linearly. These analytic results can be used to obtain simple estimates of the time delay and the Hubble constant in observed gravitational lenses. The naive estimates for four of five time delay lenses show surprising agreement with each other and with local measurements of H_0; the complicated Q 0957+561 system is the only outlier. The agreement suggests that it is reasonable to use simple isothermal lens models to infer H_0, although it is still important to check this conclusion by examining detailed models and by measuring more lensing time delays.

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arXiv2000

Gravitational Lenses With More Than Four Images: I. Classification of Caustics

We study the problem of gravitational lensing by an isothermal elliptical density galaxy in the presence of a tidal perturbation. When the perturbation is fairly strong and oriented near the galaxy's minor axis, the lens can produce image configurations with six or even eight highly magnified images lying approximately on a circle. We classify the caustic structures in the model and identify the range of models that can produce such lenses. Sextuple and octuple lenses are likely to be rare because they require special lens configurations, but a full calculation of the likelihood will have to include both the existence of lenses with multiple lens galaxies and the strong magnification bias that affects sextuple and octuple lenses. At optical wavelengths these lenses would probably appear as partial or complete Einstein rings, but at radio wavelengths the individual images could probably be resolved.

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arXiv1999

The Nature of the Diffuse Clumps and the X-ray Companion of Mrk 273

We present an optical spectrum for Mrk 273x, the X-ray source 1.3' to the northeast of Mrk 273. The new spectrum indicates that the object is at a much higher redshift (0.458) than the value (0.0376) previously reported. All the detected emission lines show properties of Seyfert 2 galaxies. Mrk 273x has one of the highest X-ray and radio luminosities L_x ~ 1.1 x 10^{44} erg/s, L_{1.37 GHz} ~ 2.0 x 10^{40} erg/s among Seyfert 2 galaxies, yet it has a low neutral hydrogen column density, N_H ~ 4.4 x 10^{20}/cm^2. These properties seem difficult to explain in the unified scheme of active galactic nuclei. The spectrum previously used to identify the redshift for Mrk 273x turns out to be for diffuse clumps in the northeast tail/plume about 20'' (~20 kpc in projected distance) from the nuclear region of Mrk 273. A new spectrum for this region was obtained; this spectrum is essentially the same as the previous one. These observations indicate that these clumps have the same redshift as Mrk 273 and are physically associated with the merger. The spectrum is dominated by strong emission from the [O III] doublet with Seyfert 2-like line ratios, which indicate that these northeast clumps are probably excited by the shock plus precursor mechanism during the merging process. This mechanism may be operating in other ultraluminous IRAS galaxies as well. (abridged)

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arXiv1999

An HST Surface Photometric Study of Ultraluminous Infrared Galaxies

We study the surface photometry for 13 single-nucleus ultraluminous IRAS galaxies (ULIRGs), selected from a parent sample of 58 galaxies obtained in a Hubble Space Telescope snapshot survey. We find that these galaxies can be classified into three classes according to their surface photometry. The surface brightness profiles of the four objects in the first class are well fitted by the r^{1/4} law. The isophotes are all disky at R < 1 h^{-1}kpc, consistent with the molecular disks/rings found in nearby ULIRGs from CO observations. Each of the four galaxies in the second class has an inner r^{1/4} component plus an outer extension. Remarkably all these four galaxies are Seyfert 1 galaxies with luminosities in the quasar regime and with relatively narrow permitted and strong FeII emission lines. The remaining five galaxies fall into the third class; these objects have surface brightness profiles that deviate to various degrees from the r^{1/4} law; indeed, one is fitted perfectly by an exponential law. We also present new spectroscopic observations for some of these galaxies. We establish the redshift of IR09427+1929 to be 0.284, instead of 0.149 as adopted in the literature. These observations firmly support the idea that merging of disk galaxies produce ellipticals. Our data also strongly suggest that the formation of QSOs may be an integral part of elliptical galaxy formation (at low redshifts).

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arXiv1999

The Host Galaxy of GRB 990123

We present deep images of the field of gamma-ray burst (GRB) 990123 obtained in a broad-band UV/visible bandpass with the Hubble Space Telescope, and deep near-infrared images obtained with the Keck-I 10-m telescope. Both the HST and Keck images show that the optical transient (OT) is clearly offset by 0.6 arcsec from an extended object, presumably the host galaxy. This galaxy is the most likely source of the metallic-line absorption at z = 1.6004 seen in the spectrum of the OT. With magnitudes V_{C} ~ 24.6 +/- 0.2 and K = 21.65 +/- 0.30 mag this corresponds to an L ~ 0.7 L_* galaxy, assuming that it is located at z = 1.6. The estimated unobscured star formation rate is SFR ~ 6 M_sun/yr, which is not unusually high for normal galaxies at comparable redshifts. The strength of the observed metallic absorption lines is suggestive of a relatively high metallicity of the gas, and thus of a chemically evolved system which may be associated with a massive galaxy. It is also indicative of a high column density of the gas, typical of damped Ly-alpha systems at high redshifts. We conclude that this is the host galaxy of GRB 990123. No other obvious galaxies are detected within the same projected radius from the OT. There is thus no evidence for strong gravitational lensing magnification of this burst, and some alternative explanation for its remarkable energetics may be required. The observed offset of the OT from the center of its apparent host galaxy, 5.5 +/- 0.9 proper kpc (projected) in the galaxy's rest-frame, both refutes the possibility that GRBs are related to galactic nuclear activity and supports models of GRBs which involve the death and/or merger of massive stars. Further, the HST image suggests an intimate connection of GRB 990123 and a star-forming region.

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arXiv1998

Detecting Luminous Gravitational Microlenses Using Spectroscopy

We propose a new method to detect the gravitational lenses in the ongoing microlensing experiments using medium and high resolution spectroscopy (resolution > 6000). Since the radial velocity of the lens and lensed source typically differs by ~100km/s, the spectral lines from the lens and source will be shifted relative to each other by (1-2)A in the optical. We simulate realistic composite spectra assuming different spectral types for the lens and source and study the lens detectability as a function of the signal-to-noise ratio, spectral resolution and lens-to-source light ratio. We show that it is possible to measure the difference in radial velocity from an unequivocal signature in the difference of cross- and auto-correlation functions calculated from two spectra obtained at different magnifications. If the lens is brighter than 10% of the unmagnified source we find that a spectral resolution of ~6000 and a signal-to-noise of 50 (at magnification maximum) are sufficient to determine the relative radial velocity of the lens. At resolution ~40000, the spectral resolution of high resolution spectrographs of 8-10m class telescopes, the lens could even be detected at a brightness of ~3% of the source. Radial velocities higher than 50km/s can be measured with an accuracy of a few km/s. Practical difficulties and observation strategies are also discussed.

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arXiv1997

Real-Time Spectroscopy of Gravitational Microlensing Events - Probing the Evolution of the Galactic Bulge

We describe a real-time spectroscopic program to observe bright gravitational microlensing events toward the Galactic Bulge. The program is carried out using the NTT at ESO. We present the preliminary analysis of the microlensing events we have observed thus far. We demonstrate that these data provide detailed information on the intrinsic properties of the source stars; radial velocities, atmospheric parameters, distances and abundances. A high priority objective of this campaign is to obtain spectra of highly magnified cool dwarfs and subgiants in order to investigate their chemical composition and by implication, the formation and evolution of the bulge. The data will also allow us to investigate the nature of the sources (and the lenses) which in turn may aid the interpretation of the observed optical depth towards the Bulge.

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arXiv1997

Properties of Point Mass Lenses On A Regular Polygon and The Problem of Maximum Number of Images

We study the critical curves, caustics, and multiple imaging due to one of the simplest many-body gravitational lens configurations: equal-mass point masses on the vertices of a regular polygon. Some examples of the critical curves and caustics are also displayed. We pose the problem of determining the maximum number of lensed images due to regular-polygon and general point mass configurations. Our numerical simulations suggest a maximum that is linear, rather than quadratic, in the number of point masses.

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arXiv1997

On the Physical Connections between Galaxies of Different Types

Galaxies can be classified in two broad sequences which are likely to reflect their formation mechanism. The `main sequence', consisting of spirals, irregulars and all dwarf galaxies, is probably produced by gas settling within dark matter haloes. We show that the sizes and surface densities along this sequence are primarily determined by the distributions of the angular momentum and formation time of dark haloes. They are well reproduced by current cosmogonies provided that galaxies form late, at $z \la 2$. In this scenario, dwarf ellipticals were small `disks' at $z\sim 1$ and become `ellipticals' after they fall into cluster environments. The strong clustering of dwarf ellipticals is then a natural by-product of the merging and transformation process. The number of dwarf galaxies predicted in a cluster such as Virgo is in good agreement with the observed number. On the other hand, the `giant branch', consisting of giant ellipticals and bulges, is probably produced by the merging of disk galaxies. Based on the observed phase-space densities of galaxies, we show that the main bodies of all giant ellipticals can be produced by dissipationless mergers of high-redshift disks. However, high-redshift disks, although denser than present-day ones, are still not compact enough to produce the high central phase space density of some low-luminosity ellipticals. Dissipation must have occurred in the central parts of these galaxies during the merger which formed them.

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arXiv1995

Object DUO 2: A New Binary Lens Candidate

We present the light curve of an unusual variable object, DUO 2, detected during the search for microlensing events by the DUO project. The star remained stable for more than 150 days before it brightened by more than two magnitudes in 6 days in the B and R bands. The light curves are achromatic during the variability. We consider possible explanations of the photometric behavior, with particular emphasis on the binary lens interpretation of the event. The masses of the lenses are quite small, with the companion possibly in the range of a brown dwarf or even a few times of Jupiter. We report evidence of blending of the source by a companion through the first detection of shift in the light centroid among all the microlensing experiments. This shift sets a lower limit of $0.3^{\prime\prime}$ on the separation between the stars. The best lens model obtained requires moderate blending, which was what motivated us to check the centroid shift that was subsequently found. The best lens model predicts a separation of $1^{\prime\prime}$ between the two blended stars. This prediction was recently tested using two CCD images taken under good seeing conditions. Both images show two components. Their separation and position angle are in good agreement with our model.

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arXiv1994

The Optical Gravitational Lensing Experiment. OGLE #7: Binary Microlens or a New Unusual Variable?

We present the light curve of an unusual variable object, OGLE #7, detected during the OGLE search for microlensing events. After one season of being in a low, normal state, the star brightened by more than 2~mag with a characteristic double-maximum shape, and returned to normal brightness after 60 days. We consider possible explanations of the photometric behavior of OGLE #7. The binary microlens model seems to be the most likely explanation -- it reproduces well the observed light curve and explains the observed colors of OGLE #7. The characteristic time scale of the OGLE #7 event, $t_E$, is equal to 80 days, the longest observed to date. The binary microlens model predicts that the spectrum of the star should be composite, with roughly 50% of its light in the $I$-band coming from a non-lensed source.

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