119237976
A large sample of low surface brightness disc galaxies from the SDSS – II. Metallicities in surface brightness bins
We study the spectroscopic properties of a large sample of low surface brightness galaxies (LSBGs) [with B-band central surface brightness μ0(B) > 22 mag arcsec −2 ] selected from the Sloan Digital Sky Survey Data Release 4 (SDSS-DR4) main galaxy sample. A large sample of disc-dominated high surface brightness galaxies [HSBGs, with μ0(B) < 22 mag arcsec −2 ] are also selected for comparison simultaneously. To study them in more details, these sample galaxies are further divided into four subgroups according to μ0(B) (in units of mag arcsec −2 ): very low surface brightness galaxies (vLSBGs) (24.5–22.75), intermediate low surface brightness galaxies (iLSBGs) (22.75–22.0), intermediate high surface brightness galaxies (iHSBGs) (22.0–21.25) and very high surface brightness galaxies (vHSBGs) (<21.25). The diagnostic diagram from spectral emission-line ratios shows that the active galactic nucleus fractions of all the four subgroups are small (<9 per cent). The 21 032 star-forming galaxies with good-quality spectroscopic observations are further selected for studying their dust extinction, strong-line ratios, metallicities and stellar mass–metallicity relations. The vLSBGs have lower extinction values and have less metal-rich and massive galaxies than the other subgroups. The oxygen abundances of our LSBGs are not as low as those of the H II regions in LSBGs studied in literature, which could be because our samples are more luminous, and because of the different metallicity calibrations used. We find a correlation between 12 + log(O/H) and μ0(B) for vLSBGs, iLSBGs and iHSBGs but show that this could be a result of correlation between μ0(B) and stellar mass and the well-known mass–metallicity relation. This large sample shows that LSBGs span a wide range in metallicity and stellar mass, and they lie nearly on the stellar mass versus metallicity and N/O versus O/H relations of normal galaxies. This suggests that LSBGs and HSBGs have not had dramatically different star formation and chemical enrichment histories.
119256368
The properties of a large sample of low surface brightness galaxies from SDSS
A large sample of low surface brightness (LSB) disk galaxies is selected from SDSS with B-band central surface brightness mu_0(B) from 22 to 24.5 mag arcsec^(-2). Some of their properties are studied, such as magnitudes, surface brightness, scalelengths, colors, metallicities, stellar populations, stellar masses and multiwavelength SEDs from UV to IR etc. These properties of LSB galaxies have been compared with those of the galaxies with higher surface brightnesses. Then we check the variations of these properties following surface brightness.
118442181
GRB 070518: a gamma-ray burst with optically dim luminosity
We present our optical observations of the GRB 070518 afterglow obtained at the 0.8-m Tsinghua University-National Astronomical Observatory of China Telescope (TNT) at Xinglong Observatory. Our follow-up observations were performed from 512 s after the burst trigger. With the upper limit of redshift ~0.7, GRB 070518 is found to be an optically dim burst. The spectra indices β ox of optical to X-ray are slightly larger than 0.5, which implies that the burst might be a dark burst. The extinction A V of the host galaxy is 3.2 mag, inferred from the X-ray hydrogen column density with Galactic extinction law, and 0.3 mag with the Small Magellanic Cloud extinction law. Also, it is similar to three other low-redshift optically dim bursts, which belong to X-ray rich gamma-ray bursts or X-ray flashes and have mid-term duration (T 90 < 10, except for GRB 070419A, T 90 £= 116 s). Moreover, its R-band afterglow flux is well fitted by a single power law with an index of 0.87. The optical afterglow and the X-ray afterglow in the normal segment might have the same mechanism, as they are consistent with the prediction of the classical external shock model. Besides, GRB 070518 agrees with the Amati relation under reasonable assumptions. The Ghirlanda relation is also tested with the burst.
16379990
A high peculiarity rate for Type Ia SNe
We have compiled a sample of 90 SNe Ia from 1997 to 1999 (up to SN 1999da) and studied the peculiarity rate of SN 1991T, SN 1991bg and SN 1986G-like objects. A Monte Carlo code was written to study the observational biases involved in the evaluation of the intrinsic peculiarity rate of SNe Ia. We find that SNe Ia have a high peculiarity rate (>30%) and a flat luminosity function.
18113816
GRB follow-up observations in the East-Asian region
In 2004, we established a Japan-Taiwan-China collaboration for GRB study in the East-Asian region. This serves as a valuable addition to the world-wide optical and infrared follow-up network, because the East-Asia region would otherwise be blank. We have been carrying out imaging and spectroscopic follow-up observations at Lulin (Taiwan), Kiso (Japan), WIDGET (Japan) and Xinglong (China). From Xinglong and Kiso, we can locate candidates and obtain early time spectra for afterglows. While WIDGET provides early time observations before the burst, the high-time resolution for multi-band light curves can be obtained at Lulin. With the data from these sites, we can obtain detailed information about the light curve and redshift of GRBs, which are important to understand the mechanism of the afterglows. Up to March 2005, ten follow-up observations have been provided by this East-Asia cooperation. Two optical afterglows were detected, GRB 040924 and GRB 041006. The results of the two detected afterglows are reported in this article.
36184018
CCD Photometry of Candidate RR Lyrae Stars in the Sloan Digital Sky Survey
We present unfiltered CCD photometry of a sample of 71 candidate RR Lyrae stars during 2003 and 2004, out of 148 candidate stars that were selected from Sloan Digital Sky Survey (SDSS) commissioning data by Ivezic and coworkers. We obtain light curves of 69 candidate stars and present a catalog of their positions, amplitudes, periods, types, and mean magnitudes. We confirm that the 69 stars are true RR Lyrae variables based on periods and light curves, for the first time for 41 of them. We identify 57 type ab, 11 type c, and 1 type d (double-mode) in our sample. The double-mode RR Lyrae star shows properties similar to globular cluster stars and are unlike field stars of the Galactic halo in the Petersen diagram. We also find one RR Lyrae star that is related to Pal 5 and recover another. In our sample, only two candidate stars do not have a reasonable light curve. We conclude that the ratio of contamination in our sample is less than ~8%, which shows that the criteria used by Ivezic and coworkers to select RR Lyrae stars from the SDSS two-epoch data are reliable.
119402342
ROSAC: Studying the clustering properties of X-ray selected AGNs
We present the first ROSAC results of an AGN clustering analysis. This study comprises a sample of 200 AGNs, 75% of which being at low redshifts z<0.5, in the Ursa Major constellation. The spatial 2-point-correlation function (SCF) as well as the minimal spanning tree (MST) technique were applied. Some evidence for clustering is found in the SCF, although with low significance. Using the MST technique, we could find two AGN groups. This result is preliminary and the exact significance will be tested with careful simulations.
123336656
The importance of pion and extended Brueckner–Hartree–Fock theory
Abstract The pion is important in nuclear physics. The fact that the pion is a pseudoscalar particle makes it very difficult to treat explicitly in the nuclear many-body problem. Matrix elements of the tensor component in the pion exchange interaction vanish in the spin-saturated Hartree–Fock (HF) state. Hence, we are forced to include 2 p − 2 h excitations from the HF state in the model space and use the variational principle to get an extended differential equation for single particle states in the HF state. The structure of this differential equation resembles to that of the Brueckner–Hartree–Fock (BHF) theory and we name the new many-body framework as extended BHF (EBHF) theory. We discuss the similarity and difference between the BHF and EBHF theories. We apply the EBHF theory to nuclear matter and study the equation of state (EOS) and the occupation probability of high momentum states.
17626951
Time expansion chamber system for characterization of TWIST low energy muon beams
A low-mass time expansion chamber (TEC) has been developed to measure distributions of position and angle of the TRIUMF low-energy surface muon beam used for the TRIUMF Weak Interaction Symmetry Test (TWIST) experiment. The experiment is a high-precision measurement of muon decay and is dominated by systematic uncertainties, including the stability, reproducibility, and characterization of the beam. The distributions measured by two TEC modules are one essential ingredient of an accurate simulation of TWIST. The uncertainties, which are extracted through comparisons of data and simulation, must be known to assess potential systematic uncertainties of the TWIST results. The design criteria, construction, alignment, calibration, and operation of the TEC system are discussed, including experiences from initial beam studies. A brief description of the use of TEC data in the TWIST simulation is also included.
119755058
Relativistic Hartree-Fock model with short range correlation in nuclear matter
We introduce the form factor and short range correlation in the relativistic Hartree-Fock approximation (RHFU) to study the properties of nuclear matter. We treat the short range correlation with the unitary correlation operator method (UCOM). In this framework, we can perfectly reproduce the equation of state of pure neutron matter obtained by the relativistic Brueckner-Hartree-Fock theory with the Bonn potential. We apply the RHFU model to symmetric nuclear matter. The contribution of ρ meson is cut down largely by the form factor of the size in the realistic nucleon-nucleon interaction. The short range correlation has a significant effect on the Fock energies of pion and ρ meson exchange interaction.
13913450
Ten Overdensities and an Arc Structure in the Galactic Halo
Aims. We use Sloan Digital Sky Survey (SDSS) DR5 photometric data to explore the structure of the Galaxy halo and hope to discover new companions or substructures which may reveal the forming history of the halo. Methods. We study overdensities in a sky area where � = 120 ◦ � 270 ◦ and � = 25 ◦ � 70 ◦ and pick out ten overdensity areas by comparing color-magnitude diagrams with known globular clusters or dwarf spheroidal galaxies. An automatic template matching method is developed to fit the color-magnitude diagram for ea ch overdensities with a suitable stellar evolution isochro nic line so that distance module and metalicity are roughly estimated. The performance test of the method indicates that our estimates on distance is acceptable. Results. The 10 overdensities and three known objects, UMa I, UMa II and Willman 1 are found to be connected with a long arc substructure starts from � 8kpc above the disk and spread to � 150kpc into north galactic cap. Evidence also shows a relationship between metalicities and distance modules of the overdensities. Conclusions. C7, one of the 10 overdensities is most likely a dwarf spheroidal galaxy according to radial density profile study and its absolute magnitude. Other overdensties are not classifi ed for poor information. The arc structure may either be a long tidal stream or be an intrinsic substructure of the halo. Follow-up study need to be done to clarify this problem.