An empirical model to form and evolve galaxies in dark matter halos

Based on the star formation histories of galaxies in halos with different masses, we develop an empirical model to grow galaxies in dark matter halos. This model has very few ingredients, any of which can be associated with observational data and thus be efficiently assessed. By applying this model to a very high resolution cosmological N-body simulation, we predict a number of galaxy properties that are a very good match to relevant observational data. Namely, for both centrals and satellites, the galaxy stellar mass functions up to redshift z=4 and the conditional stellar mass functions in the local universe are in good agreement with observations. In addition, the two point correlation function is well predicted in the different stellar mass ranges explored by our model. Furthermore, after applying stellar population synthesis models to our stellar composition as a function of redshift, we find that the luminosity functions in the 0.1 u,0.19, 0.1r, 0.1i and 0.1z bands agree quite well with the SDSS observational results down to an absolute magnitude at about -17.0. The SDSS conditional luminosity function itself is predicted well. Finally, the cold gas is derived from the star formation rate to predict the HI gas mass within each mock galaxy. We find a remarkably good match to observed HI-to-stellar mass ratios. These features ensure that such galaxy/gas catalogs can be used to generate reliable mock redshift surveys.

The Halo Concentration and Mass Relation Traced by Satellite Galaxies

We study the relation between halo concentration and mass(c-M relation) using galaxy catalogs of the Seventh and Eighth Data Releases of the Sloan Digital Sky Survey(SDSS DR7 and DR8). Assuming that the satellite galaxies follow the distribution of dark matter, we derive the halo concentration by fitting the satellite radial profile with a Nararro Frank and White(NFW) format. The derived c-M relation covers a wide halo mass range from10^(11.6)to 10^(14.1)M_(⊙). We confirm the anti-correlation between the halo mass and concentration as predicted in cosmological simulations. Our results are in good agreement with those derived using galaxy dynamics and gravitational lensing for halos of 10^(11.6)– 10^(12.9)M_(⊙), while they are slightly lower for halos of 10^(12.9)– 10^(14.1)M_(⊙).This is because blue satellite galaxies are less concentrated, especially in the inner regions. Instead of using all satellite galaxies, red satellites could be better tracers of the underlying dark matter distribution in galaxy groups.

Comparison of Modeling SPARC spiral galaxies’rotation curves:halo models vs.MOND

The tension between luminous matter and dynamical matter has long been an interesting and controversial topic in the investigation of galaxies.This is particularly true when we study spiral galaxies for which we have high quality observations of rotation curves.The solutions to the tension are proposed in two different approaches,one is the dark matter hypothesis and the other is MOdified Newtonian Dynamics(MOND)theory.When we test the solutions by using observational data of rotation curves,the controversy arises when we apply them to both low surface brightness(LSB)galaxies and high surface brightness(HSB)galaxies.Usually one likes to use the rotation curves of LSB galaxies,since dark matter is needed or the Newtonian acceleration falls below the characteristic acceleration a0 in most regions of such galaxies,even near their centers.But for HSB galaxies,dark matter is needed or Newtonian acceleration falls below the characteristic acceleration a0 only in their outer regions so it is helpful to single out HSB galaxies from some large sample to test the solutions.To this end,we employ a sub-sample of the rotation curves consisting of45 non-bulgy HSB galaxies selected from the Spitzer Photometry and Accurate Rotation Curves(SPARC)database to test two dark halo models(NFW and Burkert)and MOND.We find that,among the three models,the core-dominated Burkert halo model(χ_(v)^(2)=1.00)provides a better description of the observed data than the NFW model(χ_(v)^(2)=1.44)or MOND model(χ_(v)^(2)=1.87).This is not consistent with the most recent numerical simulations,which tend to favor some cuspy density profiles for HSB galaxies.For MOND,when we take a0 as a free parameter,there is no obvious correlation between a0 and disk central surface brightness at 3.6μm of these HSB spiral galaxies,which is in line with the basic assumption of MOND that a0 should be a universal constant,but is surprisingly not consistent with the results when LSB galaxies are included.Furthermore,our fittings give a0 an average value of(0.74±0.45)×10^(-8)cm s^(-2),which only marginally supports the standard value of a0(1.21×10^(-8)cm s^(-2)).Since the standard value of a0 is strongly supported when both HSB and LSB galaxies are included in the large SPARC sample,we conclude that our slightly smaller value of a0 cannot be explained by the so called external field effect in MOND theory.

Searching the Parameters of Dark Matter Halos on the Basis of Dwarf Galaxies’ Dynamics

Article devoted to searching the parameters of dark matter halos on the base of dwarf galaxies’ dynamics (Messier 32 and Leo I). For doing this, we propose the new approach founded on construction the coupled elliptical trajectory for a probe body in the gravitational fields of Newtonian potential and potential of dark matter’s halo. This allows more accuracy estimate its central density for the Navarro-Frenk-White profile and free parameter for the Einasto profile . Our result is in good correlation with results of other authors that are got by different numerical methods.

The fundamental plane relation of early-type galaxies:environmental dependence

Using a sample of 70 793 early-type galaxies from SDSS DR7, we study the environmental dependence of the fundamental plane relation. With the help of the galaxy group catalog based on SDSS DR7, we calculate the fundamental planes in different dark matter halo mass bins for both central and satellite galaxies. We find the environmental dependence of the fundamental plane coefficients is similar in the g, r, i and z bands. The environmental dependence for central and satellite galaxies is significantly different. Although the fundamental plane coefficients for centrals vary systematically with the halo mass, those of satellites are similar in different halo mass bins. The discrepancy between centrals and satellites is significant in small halos, but negligible in the largest halo mass bins. These results remain the same when we only keep red galaxies, or galaxies with b/a 〉 0.6, or galaxies in a specific radius range in the sample. After the correction for the sky background, the results are still similar. We suggest that the different environmental effects of the halo mass on centrals and satellites may arise from their different quenching processes.

Comparisons of Jet Properties between GeV Radio Galaxies and Blazars

We compile a sample of spectral energy distributions （SEDs） of 12 GeV radio galaxies （RGs）, including eight FR I RGs and four FR II RGs. These SEDs can be represented with the one-zone leptonic model. No significant unification, as expected in the unification model, is found for the derived jet parameters between FR I RGs and BL Lacertae objects （BL Lacs） and between FR II RGs and flat spectrum radio quasars （FSRQs）. However, on average FR I RGs have a larger ＇Tb （break Lorentz factor of electrons） and lower B （magnetic field strength） than FR II RGs, analogous to the differences be- tween BL Lacs and FSRQs. The derived Doppler factors （~） of RGs are on average smaller than those of blazars, which is consistent with the unification model such that RGs are the misaligned parent pop- ulations of blazars with smaller tS. On the basis of jet parameters from SED fits, we calculate their jet powers and the powers carded by each component, and compare their jet compositions and radiation efficiencies with blazars. Most of the RG jets may be dominated by particles, like BL Lacs, not FSRQs. However, the jets of RGs with higher radiation efficiencies tend to have higher jet magnetization. A strong anticorrelation between synchrotron peak frequency and jet power is observed for GeV RGs and blazars in both the observer and co-moving frames, indicating that the ＂sequence＂ behavior among blazars, together with the GeV RGs, may be intrinsically dominated by jet power.

Luminosity Function of the Cluster of Galaxies Abell 566

We investigate the Luminosity Function （LF） of the cluster of galaxies Abell 566. The photometric data of 15 intermediate-bands are obtained from the Beijing-Arizona- Taiwan-Connecticut （BATC） photometric sky survey. For each of the 15 wavebands, the LF of cluster galaxies is well modelled by the Schechter function, with characteristic luminosities from -18.0 to -21.9 magnitude, from the α- to the p-band. Morphological dependence of the LF is investigated by separating the cluster members into ＇red＇ and ＇blue＇ subsamples. It is clear that late type galaxies have a steeper shape of LF than the early type galaxies. We also divided the sample galaxies by their local environment. It was found that galaxies in the sparser region have steeper shape of LF than galaxies in the denser region. Combining the results of morphological and environmental dependence of LFs, we show that Abell 566 is a well relaxed cluster with positive evidence of galaxy interaction and merger, and excess number of bright early type galaxies located in its denser region.

A new estimation of manganese distribution for local dwarf spheroidal galaxies

The distribution of abundance for iron-peak elements in dwarf spheroidal galaxies （dSphs） is important for galaxy evolution and supernova （SN） nucleosynthesis. Nowadays, manganese （Mn） is one of the most observed iron-peak elements in local dSphs. Studies of its distributions allow us to derive and understand the evolution history of these dSphs. We improve a phenomenological model by a two-curve model including a new initial condition, that includes detailed calculations of SN explosion rates and yields. We compare the results with the observed Mn distribution data for three dSphs： Fornax, Sculpture and Sextans. We find that the model can describe the observed Fe and Mn distributions well simultaneously for the three dSphs. The results also indicate that the initial conditions should be determined by the low metallicity sam- ples in the beginning time of the galaxies and the previous assumption of metellicity-dependant Mn yield of SNIa is not needed when a wide mass range of core-collapse SNe is included. Our method is applicable to the chemical evolution of other iron-peak elements in dSphs and can be modified to provide more detailed processes for the evolution of dSphs.

Are Seyfert 2 Galaxies without Polarized Broad Emission Lines More Obscured?

New XMM-Newton data of seven Seyfert 2 galaxies with optical spectropolarimetric observations are presented. An analysis of the 0.5 - 10 keV spectra shows that four Seyfert 2 galaxies with polarized broad lines （PBLs） are absorbed by NH 〈 1024 cm^-2, while two of three Seyfert 2 galaxies without PBLs show evidence of Compton-thick obscuration, supporting the conclusion that Seyfert 2 galaxies without PBLs are more obscured than those with PBLs, Adding the measured obscuration indicators （NH, T ratio, and Fe Kα line EW） of six luminous AGNs to our previous sample improves the significance level of the difference in absorption from 92.3% to 96.3% for NH, 99.1% to 99.4% for T ratio, and 95.3% to 97.4% for Fe Kα line EW. The present results support and enhance the suggestions that the absence of PBLs in Seyfert 2 galaxies can be explained by larger viewing angles of the line of sight to the putative dusty toms, which leads to the obscuration of the broad-line scattering screen, as expected in the unification model.

Lensing clusters of galaxies in the SDSS-Ⅲ

We identify new strong lensing clusters of galaxies from the Sloan Digital Sky Survey Ⅲ （SDSS DR8） by visually inspecting color images of a large sample of clusters of galaxies. We find 68 new clusters showing giant arcs in addition to 30 known lensing systems. Among 68 cases, 13 clusters are ＂almost certain＂ lensing systems with tangential giant arcs, 22 clusters are ＂probable＂ and 31 clusters are ＂pos- sible＂ lensing systems. We also find two exotic systems with blue rings. The giant arcs have angular separations of 2.0jj - 25.7j~ from the bright central galaxies. We note that the rich clusters are more likely to be lensing systems and the separations between the arcs and the central galaxies increase with cluster richness.

Correlation between excitation index and Eddington ratio in radio galaxies

We use a sample of 111 radio galaxies with redshift z 〈 0.3 to investigate their nuclear properties. The black hole masses of the sources in this sample are estimated with the velocity dispersion/luminosity of the galaxies, or the width of the broad-lines. We find that the excitation index, the relative intensity of low and high excitation lines, is correlated with the Eddington ratio for this sample. The size of the narrow-line region （NLR） was found to vary with ionizing luminosity as RNLR ∝ Lion^0.25 （Liu et al. 2013）. Using this empirical relation, we find that the correlation between the excitation index and the Eddington ratio can be reproduced by photoionization models. We adopt two sets of spectral energy distributions （SEDs）, with or without a big blue bump in ultraviolet as the ionizing continuum, and infer that the modeled correlation between the excitation index and the Eddington ratio is insensitive to the applied SED. This means that the difference between high excitation galaxies and low excitation galaxies is not caused by the different accretion modes in these sources. Instead, it may be caused by the size of the NLR.

Multiwavelength study of nearly face-on low surface brightness disk galaxies

We study the ages of a large sample （1802） of nearly face-on disk low surface brightness galaxies （LSBGs） using the evolutionary population synthesis （EPS） model PEGASE with an exponentially decreasing star formation rate to fit their mul- tiwavelength spectral energy distributions （SEDs） from far-ultraviolet （FUV） to nearinfrared （NIR）. The derived ages of LSBGs are 1-5 Gyr for most of the sample no matter if constant or varying dust extinction is adopted, which are similar to most of the previous studies on smaller samples. This means that these LSBGs formed the majority of their stars quite recently. However, a small part of the sample （~2%-3%） has larger ages of 5-8 Gyr, meaning their major star forming process may have occurred earlier. At the same time, a large sample （5886） of high surface brightness galaxies （HSBGs） are selected and studied using the same method for comparisons. The de- rived ages are 1-5 Gyr for most of the sample （97%） as well. These results probably mean that these LSBGs have not much different star formation histories from their HSBGs counterparts. However, we should notice that the HSBGs are generally about 0.2 Gyr younger, which could mean that the HSBGs may have undergone more recent star forming activities than the LSBGs.

Modeling the Newtonian dynamics for rotation curve analysis of thin-disk galaxies

We present an efficient, robust computational method for modeling the Newtonian dynamics for rotation curve analysis of thin-disk galaxies. With appropriate mathematical treatments, the apparent numerical difficulties associated with singularities in computing elliptic integrals are completely removed. Using a boundary element discretization procedure, the governing equations are transformed into a linear algebra matrix equation that can be solved by straightforward Gauss elimination in one step without further iterations. The numerical code implemented according to our algorithm can accurately determine the surface mass density distribution in a disk galaxy from a measured rotation curve （or vice versa）. For a disk galaxy with a typical flat rotation curve, our modeling results show that the surface mass density monotonically decreases from the galactic center toward the periphery, according to Newtonian dynamics. In a large portion of the galaxy, the surface mass density follows an approximately exponential law of decay with respect to the galactic radial coordinate. Yet the radial scale length for the surface mass density seems to be generally larger than that of the measured brightness distribution, suggesting an increasing mass-tolight ratio with the radial distance in a disk galaxy. In a nondimensionalized form, our mathematical system contains a dimensionless parameter which we call the ＂galactic rotation number＂ that represents the gross ratio of centrifugal force and gravitational force. The value of this galactic rotation number is determined as part of the numerial solution. Through a systematic computational analysis, we have illustrated that the galactic rotation number remains within 4-10% of 1.70 for a wide variety of rotation curves. This implies that the total mass in a disk galaxy is proportional to V02 Rg, with V0 denoting the characteristic rotation velocity （such as the ＂flat＂ value in a typical ro- tation curve） and Rg the radius of the galactic disk. The predicted total galactic mass of the Milky Way is in good agreement with the star-count data.

Alignment between galaxies and large-scale structure

Based on the Sloan Digital Sky Survey DR6 （SDSS） and the ＇Millennium Simulation （MS）, we investigate the alignment between galaxies and large-scale structure. For this purpose, we develop two new statistical tools, namely the alignment correlation function and the cos（20）-statistic. The former is a two-dimensional extension of the traditional two-point correlation function and the latter is related to the ellipticity correlation function used for cosmic shear measurements. Both are based on the cross correlation between a sample of galaxies with orientations and a reference sample which represents the large-scale structure. We apply the new statistics to the SDSS galaxy catalog. The alignment correlation function reveals an overabundance of reference galaxies along the major axes of red, luminous （L 〉 ～L＊） galaxies out to projected separations of 60 h-lMpc. The signal increases with central galaxy luminosity. No alignment signal is detected for blue galaxies. The cos（2θ）-statistic yields very similar results. Starting from a MS semi-analytic galaxy catalog, we assign an orientation to each red, luminous and central galaxy, based on that of the central region of the host halo （with size similar to that of the stellar galaxy）. As an alternative, we use the orientation of the host halo itself. We find a mean projected misalignment between a halo and its central region of -25°. The misalignment decreases slightly with increasing luminosity of the central galaxy. Using the orientations and luminosities of the semi-analytic galaxies, we repeat our alignment analysis on mock surveys of the MS. Agreement with the SDSS results is good if the central orientations are used. Predictions using the halo orientations as proxies for cen- tral galaxy orientations overestimate the observed alignment by more than a factor of 2. Finally, the large volume of the MS allows us to generate a two-dimensional map of the alignment correlation function, which shows the reference galaxy distribution to be flat- tened parallel to the orientations of red luminous galaxies with axis ratios of -0.5 and ,-0.75 for halo and central orientations, respectively. These ratios are almost independent of scale out to 60 h^-1 Mpc.

Stellar Mass Function of Lyman Break Galaxies： Theoretical Perspective

Adopting the observational distributions of star formation rates and half-light radii of Lyman break galaxies （LBGs） in the rest frame UV, we investigate empirically the predicted stellar mass function for LBGs. It is found that a peak exists at mass around 1.5 × 10^10M⊙ for their stellar mass function and there have been significant amount of LBGs with stellar masses higher than 10^11M⊙ at z ～ 3. There should be two families for LBG population observed by the Infrared Array Camera （IRAC） （which is a key instrument in the infrared space telescope Spitzer）, one is red and the other is blue, which are consistent with recent works. Moreover, the predicted comoving correlation lengths for LBGs with different stellar masses, i.e., with different 8 μm magnitudes, are also discussed. The red LBGs should be clustered in space more strongly than the blue LBGs. These predictions can and will be tested by the observations of IRAC, and will provide strong constraints on the current hierarchical galaxy formation model.

Morphology and structure of BzK-selected galaxies at z ～ 2 in the CANDELS-COSMOS field

Utilizing a BzK-selecfion technique, we obtain 14 550 star-forming galaxies （sBzKs） and 1763 passive galaxies （pBzKs） at z - 2 from the K-selected （KAB 〈 22.5） catalog in the COSMOS/UltraVISTA field. The differential number counts of sBzKs and pBzKs are consistent with the results from the literature. Compared to the observed results, semi-analytic models of galaxy formation and evolution provide too few （many） galaxies at the high （low） mass end. Moreover, we find that the star formation rate and stellar mass of sBzKs follow the relation of the main sequence. Based on HST/Wide Field Camera 3 F160W imaging, we find a wide range of morphological diversities for sBzKs, from diffuse to early-type spiral structures, with relatively high M20, large size and low G, while pBzKs have elliptical-like compact morphologies with lower M20, smaller size and higher G, indicating a more concentrated and symmetric spatial extent of stellar population distribution in pBzKs than sBzKs. Furthermore, the sizes of pBzKs （sBzKs） at z - 2 are on average two to three （one to two） times smaller than those of local early-type （late-type） galaxies with similar stellar mass. Our findings imply that the two classes have different evolution models and mass assembly histories.

Long term optical variability of bright X-ray point sources in elliptical galaxies

We present long term optical variability studies of bright X-ray sources in four nearby elliptical galaxies with the Chandra Advanced CCD Imaging Spectrometer array （ACIS-S） and observations from the Hubble Space Telescope （HST） Advanced Camera for Surveys. Out of the 46 bright （X-ray counts 〉 60） sources that are in the common field of view of the Chandra and HST observations, 34 of them have potential optical counterparts, while the rest of them are optically dark. After taking into account systematic errors, estimated using optical sources in the field as a reference, we find that four of the X-ray sources （three in NGC 1399 and one in NGC 1427） have variable optical counterparts at a high level of significance. The X-ray luminosities of these sources are ～10^38 erg S^-1 and are also variable on similar time scales. The optical variability implies that the optical emission is asso- ciated with the X-ray source itself rather than being the integrated light from a host globular cluster. For one source, the change in optical magnitude is 〉 0.3, which is one of the highest reported for this class of X-ray sources and this suggests that the optical variability is induced by the X-ray activity. However, the optically variable sources in NGC 1399 have been reported to have blue colors （g - z 〉 1）. All four sources have been detected in the infrared （IR） by Spitzer as point sources, and their ratios of 5.8 to 3.6 μm flux are 〉 0.63, indicating that their IR spectra are like those of Active Galactic Nuclei （AGNs）. While spectroscopic confirmation is required, it is likely that all four sources are background AGNs. We find none of the X-ray sources having opticalfiR colors different from AGNs to be optically variable.

Stellar ages and metallicities of nearby elliptical galaxies

Stellar ages and metallicities are crucial for understanding the formation and evolution of elliptical galaxies. However, due to the age-metallicity degeneracy, it is hard to measure these two parameters accurately with broad-band photometry. In this paper, we observed high-resolution spectra for a sample of 20 nearby elliptical galaxies （EGs） with the NAOC 2.16 m telescope, and determined stellar ages and metallicities by using the empirical population synthesis and Lick/IDS index system methods. We found that stellar ages from these two methods are consistent with each other for purely old EGs; however, stellar metallicities show a zeropoint offset of 0.5 Z⊙. Our results confirm that stellar populations in low-density environment galaxies are more diverse compared to their high-density counterparts. We also investigated the element abundance-galaxy mass relation for nearby elliptical galaxies.

The properties of a large volume-limited sample of face-on low surface brightness disk galaxies

We select a large volume-limited sample of low surface brightness galax- ies （LSBGs, 2021） to investigate in detail their statistical properties and their dif- ferences from high surface brightness galaxies （HSBGs, 3639）. The distributions of stellar masses of LSBGs and HSBGs are nearly the same and they have the same me- dian values. Thus this volume-limited sample has good completeness and is further removed from the effect of stellar masses on their other properties when we compare LSBGs to HSBGs. We found that LSBGs tend to have lower stellar metallicities and lower effective dust attenuations, indicating that they have lower dust than HSBGs. The LSBGs have relatively higher stellar mass-to-light ratios, higher gas fractions, lower star forming rates （SFRs）, and lower specific SFRs than HSBGs. Moreover, with the decreasing surface brightness, gas fraction increases, but the SFRs and spe- cific SFRs decrease rapidly for the sample galaxies. This could mean that the star formation histories between LSBGs and HSBGs are different, and HSBGs may have stronger star forming activities than LSBGs.

CO observations towards a sample of nearby galaxies

We have simultaneously observed 12CO, laCO and clSo （J = 1 - 0） rotational transitions in the centers of a sample of 58 nearby spiral galaxies using the 13.7-m millimeter-wave telescope administered by Purple Mountain Observatory. Forty-two galaxies were detected in 13CO emission, but there was a null detection for C180 emission with a cr upper limit of 2 mK. The central beam ratios, R, of 12CO and 13CO range mostly from 5 to 13, with an average value of 8.1 ±4.2, which is slightly lower than previous estimates for normal galaxies. Clear correlations are found between ^12CO and ^13CO luminosities. An average X factor of 1.44± 0.84 × 10^20 cm^-2 （K km s-l）-1 is slightly lower than that in the Milky Way.