Weipeng Lin

A Fitting Formula for the Merger Timescale of Galaxies in Hierarchical Clustering

We study galaxy mergers using a high-resolution cosmological hydro/N-body simulation with star formation and compare the measured merger timescales with theoretical predictions based on the Chandrasekhar formula. In contrast to Navarro et al., our numerical results indicate that the commonly used equation for the merger timescale given by Lacey and Cole systematically underestimates the merger timescales for minor mergers and overestimates those for major mergers. This behavior is partly explained by the poor performance of their expression for the Coulomb logarithm, ln (mpri/msat) . The two alternative forms ln (1 + mpri/msat) and ½ln[1 + (mpri/msat)2] for the Coulomb logarithm can account for the mass dependence of merger timescale successfully, but both of them underestimate the merger timescale by a factor 2. Since ln (1 + mpri/msat) represents the mass dependence slightly better, we adopt this expression for the Coulomb logarithm. Furthermore, we find that the dependence of the merger timescale on the circularity parameter is much weaker than the widely adopted power law 0.78, whereas 0.940.60 + 0.60 provides a good match to the data. Based on these findings, we present an accurate and convenient fitting formula for the merger timescale of galaxies in cold dark matter models.

THE SPIN AND ORIENTATION OF DARK MATTER HALOS WITHIN COSMIC FILAMENTS

Clusters, filaments, sheets, and voids are the building blocks of the cosmic web. Forming dark matter halos respond to these different large-scale environments, and this in turn affects the properties of galaxies hosted by the halos. It is therefore important to understand the systematic correlations of halo properties with the morphology of the cosmic web, as this informs both about galaxy formation physics and possible systematics of weak lensing studies. In this study, we present and compare two distinct algorithms for finding cosmic filaments and sheets, a task which is far less well established than the identification of dark matter halos or voids. One method is based on the smoothed dark matter density field and the other uses the halo distributions directly. We apply both techniques to one high-resolution N-body simulation and reconstruct the filamentary/sheet like network of the dark matter density field. We focus on investigating the properties of the dark matter halos inside these structures, in particular, on the directions of their spins and the orientation of their shapes with respect to the directions of the filaments and sheets. We find that both the spin and the major axes of filament halos with masses 1013 h –1 M ☉ are preferentially aligned with the direction of the filaments. The spins and major axes of halos in sheets tend to lie parallel to the sheets. There is an opposite mass dependence of the alignment strength for the spin (negative) and major (positive) axes, i.e. with increasing halo mass the major axis tends to be more strongly aligned with the direction of the filament, whereas the alignment between halo spin and filament becomes weaker with increasing halo mass. The alignment strength as a function of the distance to the most massive node halo indicates that there is a transit large-scale environment impact: from the two-dimensional collapse phase of the filament to the three-dimensional collapse phase of the cluster/node halo at small separation. Overall, the two algorithms for filament/sheet identification investigated here agree well with each other. The method based on halos alone can be easily adapted for use with observational data sets.

The influence of baryons on the clustering of matter and weak-lensing surveys

Future weak-lensing measurements of cosmic shear will reach such high accuracy that second-order effects in weak-lensing modeling, such as the influence of baryons on structure formation, become important. We use a controlled set of high-resolution cosmological simulations to quantify this effect by comparing pure N-body dark matter runs with corresponding hydrodynamic simulations, carried out both in nonradiative form and in dissipative form with cooling and star formation. In both hydrodynamic simulations, the clustering of the gas is suppressed while that of dark matter is boosted at scales k > 1 h Mpc(-1). Despite this counterbalance between dark matter and gas, the clustering of the total matter is suppressed by up to 1% at 1 h Mpc(-1) less than or similar to k less than or similar to 10 h Mpc(-1), while for k approximate to 20 h Mpc(-1) it is boosted, up to 2% in the nonradiative run and 10% in the run with star formation. The stellar mass formed in the latter is highly biased relative to the dark matter in the pure N-body simulation. Using our power spectrum measurements to predict the effect of baryons on the weak-lensing signal at scales corresponding to multipole moments 100 < l < 10,000, we find that baryons may change the lensing power spectrum by less than 0.5% at l < 1000, but by 1% to 10% at 1000 < l < 10,000. The size of the effect exceeds the predicted accuracy of future lensing power spectrum measurements and will likely be detected. Precise determinations of cosmological parameters with weak lensing, and studies of small-scale fluctuations and clustering, therefore rely on properly including baryonic physics.

On the assembly history of dark matter haloes

We study the mass assembly history (MAH) of dark matter haloes. We compare MAHs obtained using (i) merger trees constructed with the extended Press-Schechter (EPS) formalism, (ii) numerical simulations and (iii) the Lagrangian perturbation code PINOCCHIO. We show that the PINOCCHIO MAHs are in excellent agreement with those obtained using numerical simulations, while the EPS formalism predicts MAHs that occur too late. PINOCCHIO, which is much less CPU intensive than N-body simulation, can be run on a simple personal computer, and does not require any labour intensive post-simulation analysis, therefore provides a unique and powerful tool to investigate the growth history of dark matter haloes. Using a suite of 55 PINOCCHIO simulations, with 256(3) particles each, we study the MAHs of 12 924 cold dark matter (CDM) haloes in a Lambda CDM concordance cosmology. This is by far the largest set of haloes used for any such analysis. For each MAH we derive four different formation redshifts, which characterize different epochs during the assembly history of a dark matter halo. We show that haloes less massive than the characteristic non-linear mass scale establish their potential wells much before they acquire most of their mass. The time when a halo reaches its maximum virial velocity roughly divides its mass assembly into two phases, a fast-accretion phase which is dominated by major mergers, and a slow-accretion phase dominated by minor mergers. Each halo experiences about 3 +/- 2 major mergers since its main progenitor had a mass equal to 1 per cent of the final halo mass. This major merger statistic is found to be virtually independent of halo mass. However, the average redshift at which these major mergers occur is strongly mass dependent, with more massive haloes experiencing their major mergers later.

Ring Structure and Warp of NGC 5907: Interaction with Dwarf Galaxies

The edge-on, nearby spiral galaxy NGC 5907 has long been used as the prototype of a “noninteracting” warped galaxy. We report here the discovery of two interactions with companion dwarf galaxies that substantially change this picture. First, a faint ring structure is discovered around this galaxy that is likely due to the tidal disruption of a companion dwarf spheroidal galaxy. The ring is elliptical in shape with the center of NGC 5907 close to one of the ring’s foci. This suggests that the ring material is in orbit around NGC 5907. No gaseous component to the ring has been detected either with deep Ha images or in Very Large Array H i 21 cm line maps. The visible material in the ring has an integrated luminosity •10 8 L,, and its brightest part has a color . R 2 I » 0.9 All of these properties are consistent with the ring being a tidally disrupted dwarf spheroidal galaxy. Second, we find that NGC 5907 has a dwarf companion galaxy, PGC 54419, which is projected to be only 36.9 kpc from the center of NGC 5907, close in radial velocity ( km s 21 ) to the giant spiral galaxy. This dwarf DV 5 45 is seen at the tip of the H i warp and in the direction of the warp. Hence, NGC 5907 can no longer be considered noninteracting but is obviously interacting with its dwarf companions much as the Milky Way interacts with its dwarf galaxies. These results, coupled with the finding by others that dwarf galaxies tend to be found around giant galaxies, suggest that tidal interaction with companions, even if containing a mere 1% of the mass of the parent galaxy, might be sufficient to excite the warps found in the disks of many large spiral galaxies. Subject headings: galaxies: individual (NGC 5907, PGC 054419) — galaxies: interactions — galaxies: photometry — galaxies: spiral — radio lines: galaxies

Deep Intermediate-Band Surface Photometry of NGC 5907

Intrigued by the initial report of an extended luminosity distribution perpendicular to the disk of the edge-on Sc galaxy NGC 5907, we have obtained very deep exposures of this galaxy with a Schmidt telescope, large-format CCD, and intermediate-band filters centered at 6660 A and 8020 A. These two filters, part of a 15-filter set, are custom designed to avoid the brightest (and most variable) night skylines. As a result, our images are able to go deeper with lower sky noise than those taken with broadband filters at similar effective wavelengths: e.g., 0.6 e- arcsec-2 s-1 for our observations versus 7.4 e- arcsec-2 s-1 for the R-band measures of Morrison et al. In our assessment of both random and systematic errors, we show that the flux level where the errors of observation reach 1 mag arcsec-2 are 29.00 mag arcsec-2 in the 6660 A image (corresponding to 28.7 in the R band) and 27.4 mag arcsec-2 in the 8020 A image (essentially on the I-band system). In a previous paper we have shown that NGC 5907 has a luminous ring around it, most plausibly caused by the tidal disruption of a dwarf spheroidal galaxy by the much more massive spiral. Here we show that, for values fainter than 27 R mag arcsec-2, the surface brightness around NGC 5907 is strongly asymmetric, being mostly brighter on the northwest (ring) side of the galactic midplane. This asymmetry rules out a halo as the cause of the faint surface brightness we see. We find this asymmetry is likely an artifact resulting from a combination of ring light and residual surface brightness at faint levels from stars that our star-masking procedure cannot completely eliminate. The possible existence of an optical face-on warp in NGC 5907, suggested by our Very Large Array H I observations, is too confused with foreground star contamination to be independently studied. Good agreement with the surface photometry of NGC 5907 by other observers leads us to conclude that their data are similarly affected at faint levels by ring light and the residual effects of star masking procedures. Inspection of published images confirm this to be the case. Thus, we conclude that NGC 5907 does not have a faint extended halo.

THE DEPENDENCE OF THE OCCUPATION OF GALAXIES ON THE HALO FORMATION TIME

We study the dependence of the galaxy contents within halos on the halo formation time using two galaxy formation models, one being a semianalytic model utilizing the halo assembly history from a high-resolution N-body simulation and the other being a smoothed particle hydrodynamics simulation including radiative cooling, star formation, and energy feedback from galactic winds. We confirm the finding by Gao et al. that at fixed mass, the clustering of halos depends on the halo formation time, especially for low-mass halos. This age dependence of halo clustering makes it desirable to study the correlation between the occupation of galaxies within halos and the halo age. We find that, in halos of fixed mass, the number of satellite galaxies has a strong dependence on halo age, with fewer satellites in older halos. The youngest one-third of the halos can have an order of magnitude more satellites than the oldest one-third. For central galaxies, in halos that form earlier, they tend to have more stars and thus appear to be more luminous, and the dependence of their luminosity on halo age is not as strong as that of stellar mass. The results can be understood through the star formation history in halos and the merging of satellites onto central galaxies. The age dependence of the galaxy contents within halos would constitute an important ingredient in a more accurate halo-based model of galaxy clustering.

Spatially resolved spectrophotometry of M81: Age, metallicity, and reddening maps

In this paper we present a multicolor photometric study of the nearby spiral galaxy M81, using images obtained with the Beijing Astronomical Observatory 60/90 cm Schmidt telescope in 13 intermediate-band filters from 3800 to 10000 A. The observations cover the whole area of M81, with a total integration of 51 hr from 1995 to 1997 February. This provides a multicolor map of M81 in pixels of 17 × 17. Using theoretical stellar population synthesis models, we demonstrate that some BATC (Beijing-Arizona-Taiwan-Connecticut Multicolor Sky Survey) colors and color indices can be used to disentangle the age and metallicity effect. We compare in detail the observed properties of M81 with the predictions from population synthesis models and quantify the relative chemical abundance, age, and reddening distributions for different components of M81. We find that the metallicity of M81 is about Z = 0.03, with no significant difference over the whole galaxy. In contrast, an age gradient is found between stellar populations of the central regions and of the bulge and disk regions of M81: the stellar population in its central regions is older than 8 Gyr, while the disk stars are considerably younger (~2 Gyr). We also give the reddening distribution in M81. Some dust lanes are found in the galaxy bulge region, and the reddening in the outer disk is higher than that in the central regions.

Intermediate-band surface photometry of the edge-on galaxy NGC 4565

We present a deep, 42.79 hr image of the nearby, edge-on galaxy NGC 4565 in the Beijing-Arizona-Taipei-Connecticut 6660 Angstrom band using the large-format CCD system on the 0.6 m Schmidt telescope at the Xinglong Station of the National Astronomical Observatories of China. Following the procedures previously developed by our team for the analysis of deep images of galaxies, we obtain a final image that is calibrated to an accuracy of 0.02 mag in zero point and for which we can measure galaxy surface brightness to an accuracy of 0.25 mag at a surface brightness of 27.5 mag arcsec(-2) at 6660 Angstrom, corresponding to a distance of 22 kpc from the center of the disk. The integrated magnitude of NGC 4565 in our filter is m(6660) = 8.99 (=R magnitude of 9.1) to a surface brightness of 28 mag arcsec(-2). We analyze the faint outer parts of this galaxy using a two-dimensional model comprised of three components: an exponential thin disk, an exponential thick disk, and a power-law halo. Combined with a need to provide a cutoff radius for the disk, a total of 12 parameters are included in our model. We determine the best values of our model parameters via 10,000 random initial values, 3700 of which converge to final values. We then plot the chi(2) for each converged fit versus parameter value for each of the 12 parameters. The thin-disk and thick-disk parameters that we determine here are consistent with those of previous studies of this galaxy. However, our very deep image permits a better determination of the power-law fit to the halo, constraining this power law to be between r(-3.2) and r(-4.0), with a best-fit value of r(-3.88). We find the axis ratio of the halo to be 0.44 and its core radius to be 14.4 kpc ( for an adopted distance of 14.5 Mpc). We also agree with others that the bulge of NGC 4565 is fitted well by an exponential luminosity distribution with a scale height similar to that found for the thin disk.

Calibration of the BATC Survey: Methodology and Accuracy

ABSTRACT We describe in detail the extinction correction procedures used for the Beijing‐Arizona‐Taiwan‐Connecticut Sky Survey (BATC Survey). The survey covers the spectral range 3200–9900 A by utilizing a set of 15 intermediate‐band filters. These filters are specifically designed to exclude most of the bright and variable night‐sky emission lines. We also present extinction coefficients for the filter passbands for typical photometric nights at the Xinglong Observing Station, Beijing Astronomical Observatory (where the observations of the survey are being carried out). Time‐dependent, low‐amplitude (∼1%), nightly extinction variation has been observed. Such variation is demonstrably independent of filter bandpass and air mass, with amplitudes ranging from ∼0.01 to ∼0.03 mag. The variation is plausibly caused by slowly varying (at ∼1%) atmospheric extinction, possibly related to changes in air pressure/temperature/humidity that occur during the night. An iterative fitting scheme has been developed to tak...