arXiv subjects

Shude Mao

Publications and source records attributed to Shude Mao.

303 records · Page 17Linked to original sources

arXiv1998

The Structure and Clustering of Lyman Break Galaxies

The number density and clustering properties of Lyman-break galaxies (LBGs) are consistent with them being the central galaxies of the most massive dark halos present at z~3. This conclusion holds in all currently popular hierarchical models for structure formation, and is almost independent of the global cosmological parameters. We examine whether the sizes, luminosities, kinematics and star-formation rates of LBGs are also consistent with this identification. Simple formation models tuned to give good fits to low redshift galaxies can predict the distribution of these quantities in the LBG population. The LBGs should be small (with typical half-light radii of 0.6-2 kpc/h), should inhabit haloes of moderately high circular velocity (180-290 km/s) but have low stellar velocity dispersions (70-120 km/s) and should have substantial star formation rates (15-100 Msun/yr). The numbers here refer to the predicted median values in the LBG sample of Adelberger et al. (1998); the first assumes an Omega=1 universe and the second a flat universe with Omega=0.3. For either cosmology these predictions are consistent with the current (rather limited) observational data. Following the work of Kennicutt (1998) we assume stars to form more rapidly in gas of higher surface density. This predicts that LBG samples should preferentially contain objects with low angular momentum, and so small size, for their mass. In contrast, samples of damped Lyman alpha systems (DLSs), should be biased towards objects with large angular momentum. Bright LBGs and DLSs may therefore form distinct populations, with very different sizes and star formation rates, LBGs being smaller and more metal-rich than DLSs of similar mass and redshift.

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arXiv1998

Observational constraints on disk galaxy formation

We use data from the literature to constrain theoretical models of galaxy formation. We show how to calculate the dimensionless spin parameter $λ$ of the halos of disk galaxies and we compare the distribution of $λ$ with that observed in cosmological $N$-body simulations. The agreement is excellent, which provides strong support for the hierarchical picture of galaxy formation. Assuming only that the radial surface density distribution of disks is exponential, we estimate crudely the maximum-disk mass-to-light ratio in the $I$-band and obtain $ \la 3.56h$, for a Hubble constant of $100h~\kms \mpc^{-1}$. We discuss this result and its limitations in relation to other independent determinations of $M/L$. We also define a dimensionless form of the Tully-Fisher relation, and use it to derive a value of the baryon fraction in disk galaxies. For galaxies with circular velocity $\vmax>100\kms$, the median value is $md = 0.086 (M/L/3.56h)$. Assuming that the gas fraction in galactic halos is at most as large as that in clusters, we also conclude that $ \la 2.48h^{-1/2}$.

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arXiv1998

The Origin of the Hubble Sequence for Spiral Galaxies

We suggest that the bulge-to-disc ratios of spiral galaxies are primarily determined by the angular momenta of their host haloes predicted in current hierarchical clustering models for structure formation. Gas with low specific angular momentum becomes self-gravitating and presumably forms stars before it can settle into a rotationally supported disc. We assume this part of the gas in a dark halo to form a bulge, while the rest is assumed to settle into a rotationally supported disc. With these assumptions the predicted bulge-to-disc ratios in mass and in size, and other correlations between the bulge and disc components can match current observational results. This model predicts the existence of a population of low-surface-brightness galaxies which are bulgeless. The model also predicts that the bulge component has many properties in common with the disc component, because both form through similar processes. In particular, many bulges should be supported (at least partially) by rotation.

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arXiv1998

Extended Source Effects in Astrometric Gravitational Microlensing

Extended source size effects have been detected in photometric monitoring of gravitational microlensing events. We study similar effects in the centroid motion of an extended source lensed by a point mass. We show that the centroid motion of a source with uniform surface brightness can be obtained analytically. For a source with circularly symmetric limb-darkening profile, the centroid motion can be expressed as a one-dimensional integral, which can be evaluated numerically. We found that when the impact parameter is comparable to the source radius, the centroid motion is significantly modified by the finite source size. In particular, when the impact parameter is smaller than the source radius, the trajectories become clover-leaf like. Such astrometric motions can be detected using space interferometers such as the Space Interferometry Mission. Such measurements offer exciting possibilities of determining stellar parameters such as stellar radius to excellent accuracy.

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arXiv1998

An X-ray Luminous, Dwarf Seyfert Companion of Mrk 273

We report the discovery of the brightest X-ray source hosted by a faint (M_B=-16) dwarf galaxy in the immediate vicinity of the ultraluminous IRAS merging galaxy Mrk 273. The dwarf galaxy, 1.3 arcmin away from Mrk 273, is at the tip of a faint northeast plume of Mrk 273. Its spectrum exhibits strong [OIII], Halpha, [NII] emission lines, which establish the redshift of the dwarf galaxy, $z=0.0376$, the same as that of Mrk 273. The emission line ratios are typical of Seyfert galaxies. The X-ray emission is consistent with a point-like source coincident with the center of the dwarf galaxy. The intrinsic X-ray luminosity, 6.3x10^{41} ergs, in the 0.1--2.4 keV energy range, is about seven times larger than the B band luminosity. The X-ray spectrum of the source can be fit with a power-law. All the evidence is consistent with the source being a Seyfert galaxy. It is mysterious why out of $\sim 10$ faint objects in the same field only one is detected by ROSAT and its ratio of soft X-ray to optical luminosity is as high as those for BL Lac objects and few active galactic nuclei (AGNs). If there is a population of such dwarf AGNs hidden as companions of major merger galaxies (such as Mrk 273), they may contribute to the luminosity function of AGNs and the cosmic X-ray background at the faint end.

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arXiv1997

The Evolution of Galactic Disks

We use recent observations of high-redshift galaxies to study the evolution of galactic disks over the redshift range 0<z<1. The data are inconsistent with models in which disks were already assembled at z=1 and have evolved only in luminosity since that time. Assuming that disk properties change with redshift as powers of 1+z and analysing the observations assuming an Einstein-de Sitter universe, we find that for given rotation speed, disk scalelength decreases with z as ~(1+z)^{-1}, total B-band mass-to-light ratio decreases with z as ~(1+z)^{-1}, and disk luminosity (again in B) depends only weakly on z. These scalings are consistent with current data on the evolution of disk galaxy abundance as a function of size and luminosity. Both the scalings and the abundance evolution are close to the predictions of hierarchical models for galaxy formation. If different cosmogonies are compared, the observed evolution in disk-size and disk abundance favours a flat low-density universe over an Einstein-de Sitter universe.

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arXiv1997

Global stability and the mass-to-light ratio of galactic disks

We examine the global stability of an exponential stellar disk embedded in a dark matter halo and constrain the mass-to-light ratio of observed disks in the I-band, Y_I. Assuming only that the radial surface density distribution of disks is exponential, we derive an analytic upper limit: Y_I <~ 2.4h, for a Hubble constant of 100h~\kms\mpc^{-1}. Using N-body simulations we derive a stability criterion significantly different from that of previous authors. Using this criterion we argue that, almost independent of the concentration of the halo, Y_I <~ 1.9h. We discuss this result in relation to other independent determinations of Y_I, its limitations and its implications for theories of disk formation and barred galaxies.

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arXiv1997

Evidence for substructure in lens galaxies?

We discuss whether one should expect that multiply imaged QSOs can be understood with `simple' lens models which contain a handful of parameters. Whereas for many lens systems such simple mass models yield a remarkably good description of the observed properties, there are some systems which are notoriously difficult to understand quantitatively. We argue that at least in one case (B 1422+231) these difficulties are not due to a `wrong' parametrization of the lens model, but that the discrepancy between observed and model-predicted flux ratios are due to substructure in the lens. Similar to microlensing for optical fluxes, such substructure can distort also the radio flux ratios predicted by `simple' mass models, in particular for highly magnified images, without appreciably changing image positions. Substructure also does not change the time delay significantly and therefore has little effect on the determination of the Hubble constant using time delays. We quantify these statements with several simple scenarios for substructure, and propose a strategy to model lens systems in which substructure is suspected.

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arXiv1997

The Formation of Galactic Disks

We study the population of galactic disks expected in current hierarchical clustering models for structure formation. A rotationally supported disk with exponential surface density profile is assumed to form with a mass and angular momentum which are fixed fractions of those of its surrounding dark halo. We assume that haloes respond adiabatically to disk formation, and that only stable disks can correspond to real systems. With these assumptions the predicted population can match both present-day disks and the damped Lyman alpha absorbers in QSO spectra. Good agreement is found provided: (i) the masses of disks are a few percent of those of their haloes; (ii) the specific angular momenta of disks are similar to those of their haloes; (iii) present-day disks were assembled recently (at z<1). In particular, the observed scatter in the size-rotation velocity plane is reproduced, as is the slope and scatter of the Tully-Fisher relation. The zero-point of the TF relation is matched for a stellar mass-to-light ratio of 1 to 2 h in the I-band, consistent with observational values derived from disk dynamics. High redshift disks are predicted to be small and dense, and could plausibly merge together to form the observed population of elliptical galaxies. In many (but not all) currently popular cosmogonies, disks with rotation velocities exceeding 200 km/s can account for a third or more of the observed damped Lyman alpha systems at z=2.5. Half of the lines-of-sight to such systems are predicted to intersect the absorber at r>3kpc/h and about 10% at r>10kpc/h. The cross-section for absorption is strongly weighted towards disks with large angular momentum and so large size for their mass. The galaxy population associated with damped absorbers should thus be biased towards low surface brightness systems.

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arXiv1997

Probing the Structure of Lensing Galaxies with Quadruple Lenses: The Effect of the ``External'' Shear

We study a general elliptical potential of the form $ψ(x^2+y^2/q^2)~ (0<q\le 1)$ plus an additional shear (with an arbitrary direction) as models for the observed quadruple lenses. It is shown that a minimum additional shear is needed even just to reproduce the observed positions alone. We also obtain the dependence of the axial ratio, $q$, on the orientation of the major axis of potential. A general relation also exists between the shear, the position angle and axial ratio of the lensing galaxy. The relation shows a generic degeneracy in modelling quadruple lenses. In particular, it shows that only the ratio of the ellipticity, $ε\equiv (1-q^2)/(1+q^2)$, to the magnitude of shear, $γ$ can be determined. All these results are valid regardless of the radial profile of the potential. Our formalism applies when the galaxy position is observed, which is the case for seven of the eight known quadruple lenses. Application to these seven cases reveals two quadruple lenses CLASS 1608+656 and HST 12531-2914, requiring highly significant shear with magnitude $\approx 0.2$. For HST 12531-2914, there must be a misalignment between the major axis of light and the major axis of potential (mass). We conclude that detailed modelling of quadruple lenses can yield valuable quantitative information about the shape of lensing galaxies and their dark matter halos.

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arXiv1996

On the Microlensing Optical Depth of the Galactic Bar

The microlensing probability (optical depth $τ$) toward the Galactic center carries information about the mass distribution of the Galactic bulge/bar, so can be used to constrain the very uncertain shape parameters of the bar. We find $tau$ depends on the bar mass, radial profile, angle, axis scale lengths and boxyness by a few simple analytical formulae, which shows: (1) $τ$ is proportional to the mass of the bar, $M$. (2) $τ$ falls along the minor axis with a strong gradient. (3) An oblate bulge can have more optical depth than a triaxial bar if the bar angle $α>45$ degress. (4) $τ$ is the largest if the angle $α$ and the axis ratio $y_0/x_0$ conspires so that $y_0/x_0=\tan α$. (5) At a fixed field on the minor axis but away from the center, boxy bars with a flat density profile tend to give a larger optical depth than ellipsoidal bars with a steep profile. (6) Main sequence sources should have a significantly lower (20-50\% lower) optical depth than red clump giants if main sequence stars are not observed as deep as the bright clump giants. An application to four COBE-constrained models (Dwek et al. 1994) shows most models produce optical depth $2σ$ lower than MACHO and OGLE observed values even with both a massive bar $2.8\times 10^{10} M_\odot$ and a full disk. The high $τ$ argues for a massive ($> 2\times 10^{10}M_\odot$) boxy bar with $y_0/x0\approx \tanα$ and $α<20$ deg and with a flat radial profile up to corotation.

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arXiv1996

Mass Determination With Gravitational Microlensing

We present a simple toy model of the distribution of objects responsible for gravitational microlensing. We use Monte Carlo simulations to demonstrate how difficult it is to determine the parameters of the lens mass distribution on the basis of the observed distribution of event time scales. A robust determination requires $ \sim 100 $ events, or more, even if the geometry of lens distribution, and the lens kinematics are known.

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arXiv1995

Cosmological Constant and Statistical Lensing of Giant Arcs

Using a singular isothermal sphere model for the matter distribution of foreground clusters of galaxies, we study the statistics of giant arcs in flat cosmologies with and without a cosmological constant. We find that the relative number of arcs predicted within $z=1$ in a universe with $Ω_0=0.3$ and $λ_0 \equiv Λ/(3H_0^2)=0.7$ is a factor of $\sim2$ larger than the one in the Einstein-de Sitter universe ($Ω_0=1, λ_0=0$). For a luminosity-dependent evolution model of the number density of background galaxies that accounts for the over-density of faint blue galaxies at $z_s\approx0.4$, the Einstein- de Sitter cosmological model predicts that about $5\%$ of clusters of galaxies with an X-ray luminosity $L_x > 2 \times 10^{44}$ erg s$^{-1}$ should have giant arcs with length-to-width ratio larger than 10. This is a factor of $\sim4$ lower than the observed fraction in the gravitational lensing survey of distant X-ray selected EMSS clusters of galaxies, indicating that the matter distribution of clusters of galaxies deviates significantly from simple isothermal spheres or/and the presence of a significant cosmological constant. It is profitable to further study the constraint on the cosmological constant from giant arcs using more realistic cluster models.

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arXiv1994

Evidence from Gravitational Lensing for a Non-Thermal Pressure Support in the Cluster of Galaxies A2218

The central mass distribution of clusters of galaxies can be inferred from gravitationally lensed arcs with known redshifts. For the Abell cluster 2218, this method yields a core mass which is larger by a factor of $2.5 \pm 0.5$ than the value deduced from X-ray observations, under the assumptions that the gas is supported by thermal pressure and that the cluster is spherical. We show that a non-thermal pressure support is the most plausible explanation for this discrepancy. Such a pressure can be naturally provided by strong turbulence and equipartition magnetic fields ($\sim 50μ$G) that are tangled on small spatial scales (<10 kpc). The turbulent and magnetic pressures do not affect the measured Sunyaev-Zel'dovich effect for this cluster. Intracluster magnetic fields with a comparable magnitude ($\sim 10^{1-2} μ$G) have already been detected by Faraday rotation in other clusters. If generic, a small-scale equipartition magnetic field should affect the structure of cooling flows and must be included in X-ray determinations of cluster masses.

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arXiv1993

On the Bimodal Distribution of Gamma-Ray Bursts

Kouveliotou et al. (1993) recently confirmed that gamma-ray bursts are bimodal in duration. In this paper we compute the statistical properties of the short ($\le 2$~s) and long ($>2$~s) bursts using a method of analysis that makes no assumption regarding the location of the bursts, whether in the Galaxy or at a cosmological distance. We find the 64 ms channel on BATSE to be more sensitive to short bursts and the 1024 ms channel is more sensitive to long bursts. We show that all the currently available data are consistent with the simple hypothesis that both short and long bursts have the same spatial distribution and that within each population the sources are standard candles. The rate of short bursts is $\sim 0.4$ of the rate of long bursts. Although the durations of short and long gamma-ray bursts span several orders of magnitude and the total energy of a typical short burst is smaller than that of a typical long burst by a factor of $\sim 20$, surprisingly the peak luminosities of the two kinds of bursts are equal to within a factor of $\sim 2$.

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