arXiv subjects

S. Mao

Publications and source records attributed to S. Mao.

At least 55 records · Page 3Linked to original sources

arXiv2010

Satellites in the field and lens galaxies: SDSS/COSMOS vs. SLACS/CLASS

The incidence of sub-galactic level substructures is an important quantity, as it is a generic prediction of high-resolution Cold Dark Matter (CDM) models which is susceptible to observational test. Confrontation of theory with observations is currently in an uncertain state. In particular, gravitational lens systems appear to show evidence for flux ratio anomalies, which are expected from CDM substructures although not necessarily in the same range of radius as observed. However, the current small samples of lenses suggest that the lens galaxies in these systems are unusually often accompanied by luminous galaxies. Here we investigate a large sample of unlensed elliptical galaxies from the COSMOS survey, and determine the fraction of objects with satellites, in excess of background counts, as a function of satellite brightness and separation from the primary object. We find that the incidence of luminous satellites within 20 kpc is typically a few tenths of one percent for satellites of a few tenths of the primary flux, comparable to what is observed for the wider but shallower SDSS survey. Although the environments of lenses in the SLACS survey are compatible with this observation, the CLASS radio survey lenses are significantly in excess of this.

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arXiv2009

Impacts of Source Properties on Strong Lensing by Rich Galaxy Clusters

We use a high-resolution $N$-body simulation to investigate the influence of background galaxy properties, including redshift, size, shape and clustering, on the efficiency of forming giant arcs by gravitational lensing of rich galaxy clusters. Two large sets of ray-tracing simulations are carried out for 10 massive clusters at two redshifts, i.e. $z_{\rm l} \sim 0.2$ and 0.3. The virial mass ($M_{\rm vir}$) of the simulated lens clusters at $z\sim0.2$ ranges from $6.8\times10^{14} h^{-1} {M_{\odot}}$ to $1.1\times 10^{15} h^{-1} M_{\odot}$. The information of background galaxies brighter than 25 magnitude in the $I$-band is taken from Cosmological Evolution Survey (COSMOS) imaging data. Around $1.7\times 10^5$ strong lensing realizations with these images as background galaxies have been performed for each set. We find that the efficiency for forming giant arcs for $z_{\rm l}=0.2$ clusters is broadly consistent with observations. The efficiency of producing giant arcs by rich clusters is weakly dependent on the source size and clustering. Our principal finding is that a small proportion ($\sim 1/3$) of galaxies with elongated shapes (e.g. ellipticity $ε=1-b/a>0.5$) can boost the number of giant arcs substantially. Compared with recent studies where a uniform ellipticity distribution from 0 to 0.5 is used for the sources, the adoption of directly observed shape distribution increases the number of giant arcs by a factor of $\sim2$. Our results indicate that it is necessary to account for source information and survey parameters (such as point-spread-function, seeing) to make correct predictions of giant arcs and further to constrain the cosmological parameters.(abridged)

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arXiv2009

Properties of Long Gamma-Ray Burst Host Galaxies in Cosmological Simulation

We use galaxy catalogues constructed by combining high-resolution N-body simulations with semi-analytic models of galaxy formation to study the properties of Long Gamma-Ray Burst (LGRB) host galaxies. We assume that LGRBs originate from the death of massive young stars and analyse how results are affected by different metallicity constraints on the progenitor stars. As expected, the host sample with no metallicity restriction on the progenitor stars provides a perfect tracer of the cosmic star formation history. When LGRBs are required to be generated by low-metallicity stars, they trace a decreasing fraction of the cosmic star formation rate at lower redshift, as a consequence of the global increase in metallicity. We study the properties of host galaxies up to high redshift (~9), finding that they typically have low-metallicity (Z<0.5 Z_sun) and that they are small (M<10^9 M_sun), bluer and younger than the average galaxy population, in agreement with observational data. They are also less clustered than typical L_* galaxies in the Universe, and their descendents are massive, red and reside in groups of galaxies with halo mass between 10^{13} M_sun to 10^{14} M_sun.

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arXiv2009

Effects of Dark Matter Substructures on Gravitational Lensing: Results from the Aquarius Simulations

We use high-resolution Aquarius simulations of Milky Way-sized haloes in the LCDM cosmology to study the effects of dark matter substructures on gravitational lensing. Each halo is resolved with ~ 10^8 particles (at a mass resolution ~ 10^3-4 M_sun/h) within its virial radius. Subhaloes with masses larger than 10^5 M_sun/h are well resolved, an improvement of at least two orders of magnitude over previous lensing studies. We incorporate a baryonic component modelled as a Hernquist profile and account for the response of the dark matter via adiabatic contraction. We focus on the "anomalous" flux ratio problem, in particular on the violation of the cusp-caustic relation due to substructures. We find that subhaloes with masses less than ~ 10^8 M_sun/h play an important role in causing flux anomalies; such low mass subhaloes have been unresolved in previous studies. There is large scatter in the predicted flux ratios between different haloes and between different projections of the same halo. In some cases, the frequency of predicted anomalous flux ratios is comparable to that observed for the radio lenses, although in most cases it is not. The probability for the simulations to reproduce the observed violations of the cusp lenses is about 0.001. We therefore conclude that the amount of substructure in the central regions of the Aquarius haloes is insufficient to explain the observed frequency of violations of the cusp-caustic relation. These conclusions are based purely on our dark matter simulations which ignore the effect of baryons on subhalo survivability.

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arXiv2008

Repeating microlensing events in the OGLE data

Microlensing events are usually selected among single-peaked non-repeating light curves in order to avoid confusion with variable stars. However, a microlensing event may exhibit a second microlensing brightening episode when the source or/and the lens is a binary system. A careful analysis of these repeating events provides an independent way to study the statistics of wide binary stars and to detect extrasolar planets. Previous theoretical studies predicted that 0.5 - 2 % of events should repeat due to wide binary lenses. We present a systematic search for such events in about 4000 light curves of microlensing candidates detected by the Optical Gravitational Lensing Experiment (OGLE) towards the Galactic Bulge from 1992 to 2007. The search reveals a total of 19 repeating candidates, with 6 clearly due to a wide binary lens. As a by-product we find that 64 events (~2% of the total OGLE-III sample) have been miss-classified as microlensing; these miss-classified events are mostly nova or other types of eruptive stars. The number and importance of repeating events will increase considerably when the next-generation wide-field microlensing experiments become fully operational in the future.

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arXiv2008

Luminous satellite galaxies in gravitational lenses

Substructures, expected in cold dark matter haloes, have been proposed to explain the anomalous flux ratios in gravitational lenses. About 25% of lenses in the Cosmic Lens All-Sky Survey (CLASS) appear to have luminous satellites within ~ 5 kpc/h of the main lensing galaxies, which are usually at redshift z ~ 0.2-1. In this work we use the Millennium Simulation combined with galaxy catalogues from semi-analytical techniques to study the predicted frequency of such satellites in simulated haloes. The fraction of haloes that host bright satellites within the (projected) central regions is similar for red and blue hosts and is found to increase as a function of host halo mass and redshift. Specifically, at z = 1, about 11% of galaxy-sized haloes (with masses between 10^{12} M_sun/h and 10^{13} M_sun/h) host bright satellite galaxies within a projected radius of 5 kpc/h. This fraction increases to about 17% (25%) if we consider bright (all) satellites of only group-sized haloes (with masses between 10^{13} M_sun/h and 10^{14} M_sun/h). These results are roughly consistent with the fraction (~ 25%) of CLASS lensing galaxies observed to host luminous satellites. At z = 0, only ~ 3% of galaxy-sized haloes host bright satellite galaxies. The fraction rises to ~ 6%, (10%) if we consider bright (all) satellites of only group-sized haloes at z = 0. However, most of the satellites found in the inner regions are `orphan' galaxies where the dark matter haloes have been completely stripped. Thus the agreement crucially depends on the true survival rate of these `orphan' galaxies. We also discuss the effects of numerical resolution and cosmologies on our results.

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arXiv2008

Towards A Census of Earth-mass Exo-planets with Gravitational Microlensing

Thirteen exo-planets have been discovered using the gravitational microlensing technique (out of which 7 have been published). These planets already demonstrate that super-Earths (with mass up to ~10 Earth masses) beyond the snow line are common and multiple planet systems are not rare. In this White Paper we introduce the basic concepts of the gravitational microlensing technique, summarise the current mode of discovery and outline future steps towards a complete census of planets including Earth-mass planets. In the near-term (over the next 5 years) we advocate a strategy of automated follow-up with existing and upgraded telescopes which will significantly increase the current planet detection efficiency. In the medium 5-10 year term, we envision an international network of wide-field 2m class telescopes to discover Earth-mass and free-floating exo-planets. In the long (10-15 year) term, we strongly advocate a space microlensing telescope which, when combined with Kepler, will provide a complete census of planets down to Earth mass at almost all separations. Such a survey could be undertaken as a science programme on Euclid, a dark energy probe with a wide-field imager which has been proposed to ESA's Cosmic Vision Programme.

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arXiv2008

Mid-Infrared Spectroscopic Properties of Ultra-Luminous Infrared Quasars

We analyse mid-infrared (MIR) spectroscopic properties for 19 ultra-luminous infrared quasars (IR QSOs) in the local universe based on the spectra from the Infrared Spectrograph on board the Spitzer Space Telescope. The MIR properties of IR QSOs are compared with those of optically-selected Palomar-Green QSOs (PG QSOs) and ultra-luminous infrared galaxies (ULIRGs). The average MIR spectral features from ~ 5 to 30um, including the spectral slopes, 6.2um PAH emission strengths and [NeII] 12.81um luminosities of IR QSOs, differ from those of PG QSOs. In contrast, IR QSOs and ULIRGs have comparable PAH and [NeII] luminosities. These results are consistent with IR QSOs being at a transitional stage from ULIRGs to classical QSOs. We also find that the colour index alpha(30, 15) is a good indicator of the relative contribution of starbursts to AGNs for all QSOs. Correlations between the [NeII] 12.81um and PAH 6.2um luminosities and those between the [NeII], PAH with 60um luminosities for ULIRGs and IR QSOs indicate that both [NeII] and PAH luminosities are approximate star formation rate indicators for IR QSOs and starburst-dominated galaxies; the scatters are, however, quite large (~ 0.7 to 0.8 dex). Finally the correlation between the EW(PAH 6.2um) and outflow velocities suggests that star formation activities are suppressed by feedback from AGNs and/or supernovae.

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arXiv2007

The first direct detection of a gravitational micro-lens toward the Galactic bulge

We present a direct detection of the gravitational lens that caused the microlensing event MACHO-95-BLG-37. This is the first fully resolved microlensing system involving a source in the Galactic bulge, and the second such system in general. The lens and source are clearly resolved in images taken with the High Resolution Channel of the Advanced Camera for Surveys on board the Hubble Space Telescope (HST) ~9 years after the microlensing event. The presently available data are not sufficient for the final, unambiguous identification of the gravitational lens and the microlensed source. While the light curve models combined with the high resolution photometry for individual objects indicate that the source is red and the lens is blue, the color-magnitude diagram for the line of sight and the observed proper motions strongly support the opposite case. The first scenario points to a metal-poor lens with mass M = ~0.6 M_Sun at the distance D_l = ~4 kpc. In the second scenario the lens could be a main-sequence star with M = 0.8 - 0.9 M_Sun about half-way to the Galactic bulge or in the foreground disk, depending on the extinction.

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arXiv2007

Properties of Wide-separation Lensed Quasars by Clusters of Galaxies in the SDSS

We use high-resolution N-body numerical simulations to study the number of predicted large-separation multiply-imaged systems produced by clusters of galaxies in the SDSS photometric and spectroscopic quasar samples. We incorporate the condensation of baryons at the centre of clusters by (artificially) adding a brightest central galaxy (BCG) as a truncated isothermal sphere. We make predictions in two flat cosmological models: a LCDM model with a matter density $Ω_0=0.3$, and $σ_8=0.9$ (LCDM0), and a model favoured by the WMAP three-year data with $Ω_0=0.238$, and $σ_8=0.74$ (WMAP3). We found that the predicted multiply-imaged quasars with separation >10" is about 6.2 and 2.6 for the SDSS photometric (with an effective area 8000 deg$^2$) and spectroscopic (with an effective area 5000 deg$^2$) quasar samples respectively in the LCDM0 model; the predicted numbers of large-separation lensed quasars agree well with the observations. These numbers are reduced by a factor of 7 or more in the WMAP3 model, and are consistent with data at < 8% level. The predicted cluster lens redshift peaks around redshift 0.5, and 90% are between 0.3 and 1. We find that the BCG creates a central circular region, comparable to the Einstein ring of the BCG, where the central image disappears in the usual three-image and five-image configurations. If we include four image systems as an extreme case of five-image systems (with an infinitely demagnified central image), we find that 68% of the central images are fainter by a factor of 100 than the brightest image, and about 80% are within 1.5"of the BCG.

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arXiv2007

An anomaly detector with immediate feedback to hunt for planets of Earth mass and below by microlensing

(abridged) The discovery of OGLE 2005-BLG-390Lb, the first cool rocky/icy exoplanet, impressively demonstrated the sensitivity of the microlensing technique to extra-solar planets below 10 M_earth. A planet of 1 M_earth in the same spot would have provided a detectable deviation with an amplitude of ~ 3 % and a duration of ~ 12 h. An early detection of a deviation could trigger higher-cadence sampling which would have allowed the discovery of an Earth-mass planet in this case. Here, we describe the implementation of an automated anomaly detector, embedded into the eSTAR system, that profits from immediate feedback provided by the robotic telescopes that form the RoboNet-1.0 network. It went into operation for the 2007 microlensing observing season. As part of our discussion about an optimal strategy for planet detection, we shed some new light on whether concentrating on highly-magnified events is promising and planets in the 'resonant' angular separation equal to the angular Einstein radius are revealed most easily. Given that sub-Neptune mass planets can be considered being common around the host stars probed by microlensing (preferentially M- and K-dwarfs), the higher number of events that can be monitored with a network of 2m telescopes and the increased detection efficiency for planets below 5 M_earth arising from an optimized strategy gives a common effort of current microlensing campaigns a fair chance to detect an Earth-mass planet (from the ground) ahead of the COROT or Kepler missions. The detection limit of gravitational microlensing extends even below 0.1 M_earth, but such planets are not very likely to be detected from current campaigns. However, these will be within the reach of high-cadence monitoring with a network of wide-field telescopes or a space-based telescope.

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arXiv2007

The Growth of Black Holes and Their Host Spheroids in (Sub)mm-loud QSOs at High Redshift

We study the growth of black holes and stellar population in spheroids at high redshift using several (sub)mm-loud QSO samples. Applying the same criteria established in an earlier work, we find that, similar to IR QSOs at low redshift, the far-infrared emission of these (sub)mm-loud QSOs mainly originates from dust heated by starbursts. By combining low-z IR QSOs and high-z (sub)mm-loud QSOs, we find a trend that the star formation rate ($\Mstardot$) increases with the accretion rate ($\Mdot$). We compare the values of $\Mstardot/\Mdot$ for submm emitting galaxies (SMGs), far-infrared ultraluminous/hyperluminous QSOs and typical QSOs, and construct a likely evolution scenario for these objects. The (sub)mm-loud QSO transition phase has both high $\Mdot$ and $\Mstardot$ and hence is important for establishing the correlation between the masses of black holes and spheroids.

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arXiv2006

The influence of baryons on the mass distribution of dark matter halos

Using a set of high-resolution N-body/SPH cosmological simulations with identical initial conditions but run with different numerical setups, we investigate the influence of baryonic matter on the mass distribution of dark halos when radiative cooling is NOT included. We compare the concentration parameters of about 400 massive halos with virial mass from $10^{13}$ \Msun to $7.1 \times 10^{14}$ \Msun. We find that the concentration parameters for the total mass and dark matter distributions in non radiative simulations are on average larger by ~3% and 10% than those in a pure dark matter simulation. Our results indicate that the total mass density profile is little affected by a hot gas component in the simulations. After carefully excluding the effects of resolutions and spurious two-body heating between dark matter and gas particles, we conclude that the increase of the dark matter concentration parameters is due to interactions between baryons and dark matter. We demonstrate this with the aid of idealized simulations of two-body mergers. The results of individual halos simulated with different mass resolutions show that the gas profiles of densities, temperature and entropy are subjects of mass resolution of SPH particles. In particular, we find that in the inner parts of halos, as the SPH resolution increases the gas density becomes higher but both the entropy and temperature decrease.

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arXiv2006

Smoothing Algorithms and High-order Singularities in Gravitational Lensing

We propose a new smoothing method for obtaining surface densities from discrete particle positions from numerical simulations. This is an essential step for many applications in gravitational lensing. This method is based on the ``scatter'' interpretation of the discrete density field in the Smoothed Particle Hydrodynamics. We use Monte Carlo simulations of uniform density fields and one isothermal ellipsoid to empirically derive the noise properties, and best smoothing parameters (such as the number of nearest neighbors used). A cluster from high-resolution simulations is then used to assess the reality of high-order singularities such as swallowtails and butterflies in caustics, which are important for the interpretation of substructures in gravitational lenses. We also compare our method with the Delaunay tesselation field estimator using the galaxy studied by Bradac et al. (2004), and find good agreements. We show that higher order singularities are not only connected with bound subhaloes but also with the satellite streams. However, the presence of high-order singularities are sensitive to not only the fluctuation amplitude of the surface density, but also the detailed form of the underlying smooth lensing potential (such as ellipticity and external shear).

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arXiv2006

The giant arc statistics in the three year WMAP cosmological model

We use high-resolution $N$-body simulations to investigate the optical depth of giant arcs with length-to-width ratio larger than 7.5 and 10 in the `standard' $\LCDM$ model with $σ_8=0.9$ and $Ω_{\rm m,0}=0.3$ and a model based on three-year Wilkinson Microwave Anisotropy Probe (WMAP) data. We find that, in dark-matter only simulations, the lensing probability in the three-year WMAP model (with $σ_8=0.74$ and $Ω_{\rm m,0}=0.238)$ decreases by a factor of $\sim 6$ compared with that in the `standard' $\LCDM$ model. The effects of baryonic cooling, star formation and feedbacks are uncertain, but we argue that baryons will only increase the the lensing cross-section by a moderate factor, $\sim 2$. We conclude that the low central value of $σ_8$ and $Ω_{\rm m,0}$ preferred by the WMAP three-year data may be too low to be compatible with observations if conventional assumptions of the background source population are correct.

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arXiv2006

The Optical Gravitational Lensing Experiment. Variable Baseline Microlensing Events in the Galactic Bulge

We present the first systematic search for microlensing events with variability in their baselines using data from the third phase of the Optical Gravitational Lensing Experiment (OGLE-III). A total of 137 candidates (88 new) was discovered toward the Galactic bulge. Among these, 21 have periodic oscillations in their baselines, 111 are irregular variables and 5 are potential long period detached eclipsing binaries. This is about 10% of the total number of constant baseline events. They are hence quite common and can be regarded as a new type of exotic events, which allow the determination of extra parameters of the events. We show that microlensing of variable stars may allow us to break the degeneracy between the blending parameter and magnification. We note that in some cases variability hidden in the baseline due to strong blending may be revealed in highly magnified events and resemble other exotic microlensing behavior, including planetary deviation. A new system (VEWS) for detecting ongoing variable baseline microlensing events is presented.

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arXiv2006

Isophotal Shapes of Elliptical/S0 Galaxies from the Sloan Digital Sky Survey

We revisit the shapes of isophotes for elliptical (E) and lenticular (S0) galaxies by studying 847 nearby early-type galaxies selected from the Sloan Digital Sky Survey Data Release 4 with velocity dispersions above 200 km/s. The IRAF task {\tt ellipse} was used to derive the deviations of the isophotes from pure ellipses (Fourier coefficients a_3/a and a_4/a), position angles and ellipticities as a function of radius. We show the statistical distributions of the a_3/a and a_4/a parameters as a function of velocity dispersion, magnitude, and colour. The a_4/a parameter is correlated with the ellipticity and absolute Petrosian magnitude of galaxies. No significant correlation was found between the a_4/a parameter with colour and velocity dispersion. A cross-correlation between the FIRST survey and the SDSS data reveals a lack of powerful radio emitters in discy E/S0s, as previously found by Bender et al. We also find that boxy E/S0s favor denser environments while discy E/S0s favor more isolated environments. The median values of changes in the ellipticity and position angle between one and one and a half Petrosian half-light radii in the isophotes are about -0.023 and 1.61 degree respectively. The average change in the position angle is much larger, about 4.12 degree, due to an extended tail. The change in ellipticity is weakly correlated with the ellipticity itself, with an increasing ellipticity for galaxies with higher ellipticity as the radius increases. The isophote parameters for the 847 galaxies are available online.

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arXiv2006

Luminous Infrared Galaxies in the Local Universe

We study the morphology and star formation properties of 159 local luminous infrared galaxy (LIRG) using multi-color images from Data Release 2 (DR2) of the Sloan Digital Sky Survey (SDSS). The LIRGs are selected from a cross-correlation analysis between the IRAS survey and SDSS. They are all brighter than 15.9 mag in the r-band and below redshift ~ 0.1, and so can be reliably classified morphologically. We find that the fractions of interacting/merging and spiral galaxies are ~ 48% and ~ 40% respectively. Our results complement and confirm the decline (increase) in the fraction of spiral (interacting/merging) galaxies from z ~1 to z ~ 0.1, as found by Melbourne, Koo & Le Floc'h (2005). About 75% of spiral galaxies in the local LIRGs are barred, indicating that bars may play an important role in triggering star formation rates > 20 M_{sun}/yr in the local universe. Compared with high redshift LIRGs, local LIRGs have lower specific star formation rates, smaller cold gas fractions and a narrower range of stellar masses. Local LIRGs appear to be either merging galaxies forming intermediate mass ellipticals or spiral galaxies undergoing high star formation activities regulated by bars.

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