arXiv subjects

Yong-Heng Zhao

Publications and source records attributed to Yong-Heng Zhao.

At least 37 records · Page 2Linked to original sources

arXiv2017

Spectral classification and composites of galaxies in LAMOST DR4

We study the classification and composite spectra of galaxy in the fourth data release (DR4) of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). We select 40,182 spectra of galaxies from LAMOST DR4, which have photometric in- formation but no spectroscopic observations in the Sloan Digital Sky Survey(SDSS). These newly observed spectra are re-calibrated and classified into six classes, i.e. pas- sive, Hα-weak, star-forming, composite, LINER and Seyfert using the line intensity (H\b{eta}, [OIII]λ5007, Hα and [NII]λ6585). We also study the correlation between spectral classes and morphological types through three parameters: concentration index, (u - r) color, and D4000n index. We calculate composite spectra of high signal-to-noise ra- tio(S/N) for six spectral classes, and using these composites we pick out some features that can differentiate the classes effectively, including H\b{eta}, Fe5015, HγA, HK, and Mg2 band etc. In addition, we compare our composite spectra with the SDSS ones and analyse their difference. A galaxy catalogue of 40,182 newly observed spectra (36,601 targets) and the composite spectra of the six classes are available online.

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arXiv2017

The LAMOST Complete Spectroscopic Survey of Pointing Area (LaCoSSPAr) in the Southern Galactic Cap I. The Spectroscopic Redshift Catalog

We present a spectroscopic redshift catalog from the LAMOST Complete Spectroscopic Survey of Pointing Area (LaCoSSPAr) in the Southern Galactic Cap (SGC), which is designed to observe all sources (Galactic and extra-galactic) by using repeating observations with a limiting magnitude of $r=18.1~mag$ in two $20~deg^2$ fields. The project is mainly focusing on the completeness of LAMOST ExtraGAlactic Surveys (LEGAS) in the SGC, the deficiencies of source selection methods and the basic performance parameters of LAMOST telescope. In both fields, more than 95% of galaxies have been observed. A post-processing has been applied to LAMOST 1D spectrum to remove the majority of remaining sky background residuals. More than 10,000 spectra have been visually inspected to measure the redshift by using combinations of different emission/absorption features with uncertainty of $σ_{z}/(1+z)<0.001$. In total, there are 1528 redshifts (623 absorption and 905 emission line galaxies) in Field A and 1570 redshifts (569 absorption and 1001 emission line galaxies) in Field B have been measured. The results show that it is possible to derive redshift from low SNR galaxies with our post-processing and visual inspection. Our analysis also indicates that up to 1/4 of the input targets for a typical extra-galactic spectroscopic survey might be unreliable. The multi-wavelength data analysis shows that the majority of mid-infrared-detected absorption (91.3%) and emission line galaxies (93.3%) can be well separated by an empirical criterion of $W2-W3=2.4$. Meanwhile, a fainter sequence paralleled to the main population of galaxies has been witnessed both in $M_r$/$W2-W3$ and $M_*$/$W2-W3$ diagrams, which could be the population of luminous dwarf galaxies but contaminated by the edge-on/highly inclined galaxies ($\sim30\%$).

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arXiv2017

Sky Subtraction for LAMOST

Sky subtraction is the key technique in data reduction of multi-fiber spectra. Knowledge of the related instrument character is necessary to determine the method adopted in sky subtraction. In this study, we described the sky subtraction method designed for LAMOST(Large sky Area Multi-Object fiber Spectroscopic Telescope) survey. The method has been intergrated into LAMOST 2D Pipeline v2.6 and applied to data of LAMOST DR3 and later. For LAMOST, sky emission line calibration is used to alleviate the position-dependent (thus time-dependent) ~4% fiber throughput uncertainty and the small wavelength instability (0.1Å) during observation. PCA (Principal Component Analysis) sky subtraction further reduces 25% of the sky line residual of the OH lines in the red part of the LAMOST spectra after the mater sky spectrum, which is derived from a B-spline fit of 20 sky fibers in each spectrograph, is adjusted by sky emission line and subtracted from each fiber. Further analysis shows that our wavelength calibration accuracy is about 4.5km/s, and the average sky subtraction residuals are about 3% for sky emission lines and 3% for continuum region. The relative sky subtraction residuals vary with the moon light background brightness, could reach as low as 1.5% for the sky emission line regions in the dark night. Tests on the F stars of both similar sky emission line strength and similar object continuum intensity show that the sky emission line residual of LAMOST is smaller than those of SDSS survey.

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arXiv2016

LAMOST Spectrograph Response Curves: Stability and Application to flux calibration

The task of flux calibration for LAMOST (Large sky Area Multi-Object Spectroscopic Telescope) spectra is difficult due to many factors. For example, the lack of standard stars, flat fielding for large field of view, and variation of reddening between different stars especially at low galactic latitudes etc. Poor selection, bad spectral quality, or extinction uncertainty of standard stars not only might induce errors to the calculated spectral response curve (SRC), but also might lead to failures in producing final 1D spectra. In this paper, we inspected spectra with Galactic latitude |b|>=60 degree and reliable stellar parameters, determined through the LAMOST Stellar Parameter Pipeline (LASP), to study the stability of the spectrograph. To guarantee the selected stars had been observed by each fiber, we selected 37,931 high quality exposures of 29,000 stars from LAMOST DR2, and more than 7 exposures for each fiber. We calculated the SRCs for each fiber for each exposure, and calculated the statistics of SRCs for spectrographs with both the fiber variations and time variations. The result shows that the average response curve of each spectrograph (henceforth ASPSRC) is relatively stable with statistical errors <= 10%. From the comparison between each ASPSRC and the SRCs for the same spectrograph obtained by 2D pipeline, we find that the ASPSRCs are good enough to use for the calibration. The ASPSRCs have been applied to spectra which were abandoned by LAMOST 2D pipeline due to the lack of standard stars, increasing the number of LAMOST spectra by 52,181 in DR2. Comparing those same targets with SDSS, the relative flux differences between SDSS spectra and that of LAMOST spectra with the ASPSRC method are less than 10%, which underlines that the ASPSRC method is feasible for LAMOST flux calibration.

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arXiv2016

The MZ relation for local star-forming galaxies

We investigate the evolution of the mass-metallicity (MZ) relation with a large sample of 53,444 star-forming galaxies (SFGs) at $0.04 41.0$ and log$(L_{\rm O\ III})>39.7$, we find that metallicity evolution is shown well, and that SFR evolution still is shown well under the latter luminosity threshold, but the evolution is not observed under the former one; (4) the evolution of the MZ relation seems to disappear at about $\rm log(M_{*}/M_\odot)>10.0$ after applying the luminosity threshold of log$(L_{\rm H α})>41.0$ or log$(L_{\rm O\ III})>39.7$; (5) we find $α=0.09$ and $α=0.07$ in the equation ($μ={\rm log}M_{*}-α\rm log(SFR)$) for log$(L_{\rm H α})>41.0$ and log$(L_{\rm O\ III})>39.7$ samples, respectively, and these imply that the evolution of the MZ relation may have a weaker dependence on SFR in our sample.

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arXiv2015

The stellar kinematics in the solar neighborhood from LAMOST data

We use about 200,000 FGK type main-sequence stars from the LAMOST DR1 data to map the local stellar kinematics. With the velocity de-projection technique, we are able to derive the averaged 3 dimensional velocity and velocity ellipsoids using only the line-of-sight velocity for the stars with various effective temperatures within $100 < |z| < 500$ pc. Using the mean velocities of the cool stars, we derive the solar motion of ($U_{\!\odot}$, $V_{\!\odot}$, $W_{\!\odot}$)=(9.58$\pm2.39$, 10.52$\pm1.96$, 7.01$\pm1.67$)$km\,s^{-1}$ with respect to the local standard of rest. Moreover, we find that the stars with ${T}_{\rm eff}>6000$K show a net asymmetric motion of $\sim3 km\,s^{-1}$ in $\langle W\rangle$ compared to the stars with ${T}_{\rm eff}<6000$K. And their azimuthal velocity increases when $|z|$ increases. This peculiar motion in the warmer stars is likely because they are young and not completely relaxed, although other reasons, such as the resonance induced by the central rotating bar or the spiral structures, and the perturbation of the merging dwarf galaxies, can not be ruled out. The derived velocity dispersions and cross terms for the data are approximately consistent with previous studies. We also find that the vertical gradients of $σ_{U}$ and $σ_V$ are larger than that of $σ_W$ . And the vertical gradient of $σ_U$ shows clear correlation with ${T}_{\rm eff}$, while the other two do not. Finally, our sample shows vertex deviation of about 11$^\circ$, at $300 < |z| < 500$pc, but roughly zero at $100 < |z| < 300$pc.

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arXiv2015

19 low mass hyper-velocity star candidates from the first data release of LAMOST survey

Hyper-velocity stars are believed to be ejected out from the Galactic center through dynamical interactions between (binary) stars and the central massive black hole(s). In this paper, we report 19 low mass F/G/K type hyper-velocity star candidates from over one mil- lion stars of the first data release of the LAMOST general survey. We determine the unbound probability for each candidate using a Monte-Carlo simulation by assuming a non-Gaussian proper-motion error distribution, Gaussian heliocentric distance and radial velocity error dis- tributions. The simulation results show that all the candidates have unbound possibilities over 50% as expected, and one of them may even exceed escape velocity with over 90% probabili- ty. In addition, we compare the metallicities of our candidates with the metallicity distribution functions of the Galactic bulge, disk, halo and globular cluster, and conclude that the Galactic bulge or disk is likely the birth place for our candidates.

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arXiv2014

Data-mining Based Expert Platform for the Spectral Inspection

We propose and preliminarily implement a data-mining based platform to assist experts to inspect the increasing amount of spectra with low signal to noise ratio (SNR) generated by large sky surveys. The platform includes three layers: data-mining layer, data-node layer and expert layer. It is similar to the GalaxyZoo project and VO-compatible. The preliminary experiment suggests that this platform can play an effective role in managing the spectra and assisting the experts to inspect a large number of spectra with low SNR.

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arXiv2014

Measure fiber position errors from spectra data

Precise fiber positioning is crucial to a wide field, multi-fiber spectroscopic survey like LAMOST. Nowadays, most position error measurements are based on CCD photographic and imaging processing techniques. Those methods only work for measuring errors orthogonal to the telescope optical axis, while there also lies errors parallel to the telescope optical axis, like defocusing, and error caused by the existing deviation angle between optical axes of a fiber and the telescope. Directly measuring two latter types of position errors is difficult for individual fiber, especially during observation. Possible sources of fiber position errors are discussed in brief for LAMOST. By constructing a model of magnitude loss due to the fiber position error for a point source, we propose an indirect method to calculate the total and systematic position errors for each individual fiber from spectra data. Restrictions and applications of this method are also discussed.

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arXiv2014

A Search for Double-peaked narrow emission line Galaxies and AGNs in the LAMOST DR1

LAMOST has released more than two million spectra, which provide the opportunity to search for double-peaked narrow emission line (NEL) galaxies and AGNs. The double-peaked narrow-line profiles can be well modeled by two velocity components, respectively blueshifted and redshifted with respect to the systemic recession velocity. This paper presents 20 double-peaked NEL galaxies and AGNs found from LAMOST DR1 using a search method based on multi-gaussian fit of the narrow emission lines. Among them, 10 have already been published by other authors, either listed as genuine double-peaked NEL objects or as asymmetric NEL objects, the remaining 10 being first discoveries. We discuss some possible origins for double-peaked narrow-line features, as interaction between jet and narrow line regions, interaction with companion galaxies and black hole binaries. Spatially resolved optical imaging and/or follow-up observations in other spectral bands are needed to further discuss the physical mechanisms at work.

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arXiv2013

The properties of cross-correlation and spectra of the low-mass X-ray binary 4U 1608-52

With {\it RXTE} data, we analyzed the cross-correlation function between the soft and hard X-rays of the transient atoll source 4U 1608-52. We found anti-correlations in three outbursts occurred in 1998, 2002 and 2010, and significant time lags of several hundreds of seconds in the latter two outbursts. Our results show no correlation between the soft and hard X-rays in the island state, and a dominated positive correlation in the lower banana state. Anti-correlations are presented at the upper banana state for the outburst of 2010 and at the lower left banana states for the other two outbursts. So far for atoll sources the cross-correlation has been studied statistically only for 4U 1735-44, where anti-correlations showed up in the upper banana state. Here our investigation upon 4U 1608-52 provides a similar result in its 2010 outburst. In addition, we notice that the luminosities in the upper banana of 1998 and 2002 outbursts are about 1.5 times that of 2010 outburst whose luminosity in the upper banana is close to that of 4U 1735-44. The results suggest that the states in color-color diagram of a source could be correlated with the luminosity of the source. A further spectral analysis shows that, during the 2010 outburst, although an anti-correlation presents at the highest fluxes, the contemporary spectrum is not the softest one along the outburst evolution. This suggests that the observed anti-correlation may be relevant to the transition between the hard and soft states, which is consistent with the previous results on 4U 1735-44 and several black hole X-ray binaries that anti-correlations are observed during the transition states.

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arXiv2013

DA white dwarfs observed in LAMOST pilot survey

A total of $\sim640,000$ objects from LAMOST pilot survey have been publicly released. In this work, we present a catalog of DA white dwarfs from the entire pilot survey. We outline a new algorithm for the selection of white dwarfs by fitting Sérsic profiles to the Balmer H$β$, H$γ$ and H$δ$ lines of the spectra, and calculating the equivalent width of the CaII K line. 2964 candidates are selected by constraining the fitting parameters and the equivalent width of CaII K line. All the spectra of candidates are visually inspected. We identify 230 (59 of them are already in Villanova and SDSS WD catalog) DA white dwarfs, 20 of which are DA white dwarfs with non-degenerate companions. In addition, 128 candidates are classified as DA white dwarf/subdwarfs, which means the classifications are ambiguous. The result is consistent with the expected DA white dwarf number estimated based on the LEGUE target selection algorithm.

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arXiv2013

Moon night sky brightness simulation for Xinglong station

With a sky brightness monitor in Xinglong station of National Astronomical Observatories of China (NAOC), we collected data from 22 dark clear nights and 90 lunar nights. We first measured the sky brightness variation with time in dark nights, found a clear correlation between the sky brightness and human activity. Then with a modified sky brightness model of moon night and data from moon night, we derived the typical value for several important parameters in the model. With these results, we calculated the sky brightness distribution under a given moon condition for Xinglong station. Furthermore, we simulated the moon night sky brightness distribution in a 5 degree field of view telescope (such as LAMOST). These simulations will be helpful to determine the magnitude limit, exposure time as well as the survey design for LAMOST at lunar night.

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arXiv2013

The LAMOST Survey of Background Quasars in the Vicinity of the Andromeda and Triangulum Galaxies -- II. Results from the Commissioning Observations and the Pilot Surveys

We present new quasars discovered in the vicinity of the Andromeda and Triangulum galaxies with the LAMOST during the 2010 and 2011 observational seasons. Quasar candidates are selected based on the available SDSS, KPNO 4 m telescope, XSTPS optical, and WISE near infrared photometric data. We present 509 new quasars discovered in a stripe of ~135 sq. deg from M31 to M33 along the Giant Stellar Stream in the 2011 pilot survey datasets, and also 17 new quasars discovered in an area of ~100 sq. deg that covers the central region and the southeastern halo of M31 in the 2010 commissioning datasets. These 526 new quasars have i magnitudes ranging from 15.5 to 20.0, redshifts from 0.1 to 3.2. They represent a significant increase of the number of identified quasars in the vicinity of M31 and M33. There are now 26, 62 and 139 known quasars in this region of the sky with i magnitudes brighter than 17.0, 17.5 and 18.0 respectively, of which 5, 20 and 75 are newly-discovered. These bright quasars provide an invaluable collection with which to probe the kinematics and chemistry of the ISM/IGM in the Local Group of galaxies. A total of 93 quasars are now known with locations within 2.5 deg of M31, of which 73 are newly discovered. Tens of quasars are now known to be located behind the Giant Stellar Stream, and hundreds behind the extended halo and its associated substructures of M31. The much enlarged sample of known quasars in the vicinity of M31 and M33 can potentially be utilized to construct a perfect astrometric reference frame to measure the minute PMs of M31 and M33, along with the PMs of substructures associated with the Local Group of galaxies. Those PMs are some of the most fundamental properties of the Local Group.

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arXiv2013

Evolution of cross-correlation and time lag of 4U 1735-44 along the branches

We analyze the cross-correlation function between the soft and hard X-rays of atoll source 4U 1735-44 with RXTE data, and find the anti-correlated soft and hard time lags of about hecto-second. On the island state, the observations do not show any obvious correlations, and most observations of banana branch show positive correlation. However, the anti-correlations are detected on the upper banana branch. These results are different from those of Z sources (Cyg X-2, GX 5-1), where the anti-correlation is detected in the low luminosity states, then the lag timescales of both this atoll and Z sources are found to be similar, at the magnitude of several tens to hundreds of seconds. As a comparison, it is noted that the anti-correlated lags of thousand-second have been reported from the several black hole candidates in their intermediate states. Finally, we compare the correspondent results of atoll source 4U 1735-44 with those observed in black hole candidates and Z sources, and the possible origins of the anti-correlated time lags are discussed.

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arXiv2012

Spectroscopic Study of Globular Clusters in the Halo of M31 with Xinglong 2.16m Telescope II: Dynamics, Metallicity and Age

(Abridged) We performed the spectroscopic observations of 11 confirmed GCs in M31 with the Xinglong 2.16m telescope and we mainly focus on the fits method and the metallicity gradient for the M31 GC sample. We analyzed and discussed more about the dynamics, metallicity and age, and their distributions as well as the relationships between these parameters. Eight more confirmed GCs in the halo of M31 were observed, most of which lack the spectroscopic information before. These star clusters are located far from the galactic center at a projected radius of ~14 to ~117 kpc. The Lick absorption-line indices and the radial velocities have been measured and ages, metallicities [Fe/H] and alpha-element [alpha/Fe] have also been fitted by comparing the observed spectral feature indices and Thomas et al.SSP model. Our results show that most of the star clusters of our sample are older than 10 Gyr except B290 ~5.5 Gyr, and most of them are metal-poor with the metallicity [Fe/H]<-1, suggesting that these clusters were born at the early stage of the galaxy's formation. We find that the metallicity gradient for the outer halo clusters with r_p>25 kpc may not exist with a slope of -0.005+-0.005 dex kpc^-1. We also find that the metallicity is not a function of age for the GCs with age < 7 Gyr while for the old GCs with age >7 Gyr there seems to be a trend that the older ones have lower metallicity. Besides, We plot metallicity distributions with the largest sample of M31 GCs so far and it shows the bimodality is not significant and the number of the metal-poor and metal-rich groups becomes comparable. The spatial distributions shows that the metal-rich group is more centrally concentrated while the metal-poor group is occupy a more extended halo and the young population is centrally concentrated while the old population is more extended spatially to the outer halo.

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arXiv2011

A new synthetic library of the Near-Infrared CaII triplet indices. I.Index Definition, Calibration and Relations with stellar atmospheric parameters

Adopting the SPECTRUM package, we have synthesized a set of 2,890 Near-InfraRed (NIR) synthetic spectra with a resolution and wavelength sampling similar to the SDSS and the forthcoming LAMOST spectra. During the synthesis, we have applied the `New grids of ATLAS9 Model Atmosphere' to provide a grid of local thermodynamic equilibrium (LTE) model atmospheres. This synthetic stellar library is composed of 1,350 solor scaled abundance (SSA) and 1,530 non-solar scaled abundance (NSSA) spectra, grounding on which we have defined a new set of NIR CaII triplet indices and an index CaT as the sum of the three. Then, these defined indices have been automatically measured on the synthetic spectra and calibrated with the indices computed on the observational spectra from the INDO-U.S. stellar library. In order to check the effect of alpha-element enhancement on the so-defined CaII indices, we have compared indices measured on the SSA spectra with those on the NSSA ones at the same terns of stellar parameters (Teff, log g, [Fe/H]), and luckily, little influences of alpha-element enhancement has been found. Furthermore, comparisons of synthetic indices with the observational ones from the INDO-U.S. spectra, the SDSS-DR7, and DR8 spectroscopic survey have been presented respectively for dwarfs and giants in detail. Finally, a new synthetic library of NIR CaII indices has been founded for deeper studies on the NIR waveband of stellar spectra, and is particularly appropriate for the SDSS and the forthcoming LAMOST stellar spectra. Later on, we have regressed the strength of the CaT index as a function of stellar parameters for both dwarfs and giants after a series of experimental investigations into relations of the indices with stellar parameters. Ultimately, a supplemental experimentation has been carried out to show that spectral noises do have effects on our set of NIR CaII indices.

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arXiv2011

Spectroscopic Study of Globular Clusters in the Halo of M31 with Xinglong 2.16m Telescope

We present the spectroscopic observations for 11 confirmed globular clusters of M31 with the OMR spectrograph on 2.16m telescope at Xinglong site of National Astronomical Observatories, Chinese Academy of Sciences. Nine of our sample clusters are located in the halo of M31 and the most remote one is out to a projected radius of 78.75 kpc from the galactic center. For all our sample clusters, we measured the Lick absorption-line indices and the radial velocities. It is noted that most GCs of our sample are distinct from the HI rotation curve of M31 galaxy, especially for B514, MCGC5, H12 and B517, suggesting that most of our sample clusters do not have kinematic association with the star forming young disk of the galaxy. We fitted the absorption line indices with the updated stellar population model Thomas et al. (2010) with two different tracks of Cassisi and Padova, separately, by applying the $χ^2-$minimization method. The fitting results show that all our sample clusters are older than 10 Gyr, and metal-poor ($-0.91 \le $ [Fe/H] $\le -2.38$ dex). After merging the spectroscopic metallicity of our work with the previously published ones, we extended the cluster sample out to a projected radius of 117 kpc from the galaxy's center. We found the metallicity gradient for all the confirmed clusters exists with a slope of $-0.028\pm0.001$ dex kpc$^{-1}$. However, the slope turns to be $-0.018\pm0.001$ dex kpc$^{-1}$ for all the halo clusters, which is much shallower. If we only consider the outer halo clusters with $r_{\rm p}>25$ kpc, the slope becomes $-0.010\pm0.002$ dex kpc$^{-1}$ and if one cluster G001 is excluded from the outer halo sample, the slope is $-0.004\pm0.002$ dex kpc$^{-1}$. Thus we conclude that metallicity gradient for M31 outer halo clusters is not significant, which agrees well with the previous findings.

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