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Register a new userPhotometric Calibration of the $[alpha/$Fe] Element: II. Calibration with SDSS Photometry
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We present the calibration of the [$alpha/$Fe] element in terms of ultra-violet excess for 465 dwarf stars with spectral type F0-K2. We used a single calibration, fitted to a third degree polynomial with a square of the correlation coefficient 0.74 and standard deviation 0.05 mag, for all stars due to their small colour range, $0.1<(g-r)_0leq 0.6$ mag, and high frequency in the blueward of the spectrum which minimize the guillotine effect. Our calibration provides [$alpha/$Fe] elements in the range $(-0.05, 0.35]$ dex. We applied the procedure to a high-latitude field, $85^circ leq b leq 90^circ$ with size 78 deg$^2$ and we could estimate the [$alpha/$Fe] elements of 23,414 dwarf stars which occupy a Galactic region up to a vertical distance of $z=9$ kpc. We could detect a small positive gradient, $d[alpha/{rm Fe}]/dz=+0.032 pm0.002$ dex kpc$^{-1}$, for the range $0<z<5$ kpc, while the distribution of the [$alpha/$Fe] element is flat for further $z$ distances.
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We used the updated [Fe/H] abundances of 168 F-G type dwarfs and calibrated them to a third order polynomial in terms of reduced ultraviolet excess, $delta_{0.41}$ defined with $ugr$ data in the SDSS. We estimated the $M_g$ absolute magnitudes for the same stars via the re-reduced Hipparcos parallaxes and calibrated the absolute magnitude offsets, $Delta M_g$, relative to the intrinsic sequence of Hyades to a third order polynomial in terms of $delta_{0.41}$. The ranges of the calibrations are $-2<$[Fe/H]$leq$0.3 dex and $4<M_gleq6$ mag. The mean of the residuals and the corresponding standard deviation for the metallicity calibration are 0 and 0.137 mag; while, for the absolute magnitude calibration they are 0 and 0.179 mag, respectively. We applied our procedures to 23,414 dwarf stars in the Galactic field with the Galactic coordinates $85^{circ}leq bleq90^{circ}$, $0^{circ}leq lleq360^{circ}$ and size 78 deg$^{2}$. We estimated absolute magnitude $M_g$ dependent vertical metallicity gradients as a function of vertical distance $Z$. The gradients are deep in the range of $0<Zleq5$ kpc, while they are very small positive numbers beyond $Z=5$ kpc. All dwarfs with $5<M_gleq6$ mag are thin-disc stars and their distribution shows a mode at $(g-r)_0approx 0.38$ mag, while the absolute magnitudes $4<M_gleq5$ are dominated by thick disc and halo stars, i.e. the apparently bright ones ($g_0leq18$ mag) are thick-disc stars with a mode at $(g-r)_0sim0.38$ mag, while the halo population is significant in the faint stars ($g_0>18$ mag).
We present metallicity and photometric parallax calibrations for the F and G type dwarfs with photometric, astrometric and spectroscopic data. The sample consists of 168 dwarf stars covering the colour, iron abundance and absolute magnitude intervals $0.30<(B-V)_0<0.68$ mag, $-2.0<[Fe/H]<0.4$ dex and $3.4<M_V<6.0$ mag, respectively. The means and standard deviations of the metallicity and absolute magnitude residuals are small, i.e. $langleDelta[Fe/H]_{res}rangle=0$ and $sigma=0.134$ dex, and $langleDelta (M_V)_{res}rangle=0$ and $sigma=0.174$ mag, respectively, which indicate accurate metallicity and photometric parallax estimations.
We present improved methods for using stars found in astronomical exposures to calibrate both star and galaxy colors as well as to adjust the instrument flat field. By developing a spectroscopic model for the SDSS stellar locus in color-color space, synthesizing an expected stellar locus, and simultaneously solving for all unknown zeropoints when fitting to the instrumental locus, we increase the calibration accuracy of stellar locus matching. We also use a new combined technique to estimate improved flat-field models for the Subaru SuprimeCam camera, forming `star flats based on the magnitudes of stars observed in multiple positions or through comparison with available SDSS magnitudes. These techniques yield galaxy magnitudes with reliable color calibration (< 0.01 - 0.02 mag accuracy) that enable us to estimate photometric redshift probability distributions without spectroscopic training samples. We test the accuracy of our photometric redshifts using spectroscopic redshifts z_s for ~5000 galaxies in 27 cluster fields with at least five bands of photometry, as well as galaxies in the COSMOS field, finding sigma((z_p - z_s)/(1 + z_s)) ~ 0.03 for the most probable redshift z_p. We show that the full posterior probability distributions for the redshifts of galaxies with five-band photometry exhibit good agreement with redshifts estimated from thirty-band photometry in the COSMOS field. The growth of shear with increasing distance behind each galaxy cluster shows the expected redshift-distance relation for a flat Lambda-CDM cosmology. Photometric redshifts and calibrated colors are used in subsequent papers to measure the masses of 51 galaxy clusters from their weak gravitational shear. We make our Python code for stellar locus matching available at http://big-macs-calibrate.googlecode.com; the code requires only a catalog and filter functions.
We apply the spectroscopy-based stellar-color regression (SCR) method to perform an accurate photometric re-calibration of the second data release from the SkyMapper Southern Survey (SMSS DR2). From comparison with a sample of over 200,000 dwarf stars with stellar atmospheric parameters taken from GALAH+ DR3 and with accurate, homogeneous photometry from $Gaia$ DR2, zero-point offsets are detected in the original photometric catalog of SMSS DR2, in particular for the gravity- and metallicity-sensitive $uv$ bands. For $uv$ bands, the zero-point offsets are close to zero at very low extinction, and then steadily increase with $E (B - V)$, reaching as large as 0.174 and 0.134 mag respectively, at $E (B - V) sim 0.5$ mag. These offsets largely arise from the adopted dust term in the transformations used by SMSS DR2 to construct photometric calibrators from the ATLAS reference catalog. For the $gr$ bands, the zero-point offsets exhibit negligible variations with SFD $E(B - V )$, due to their tiny coefficients on the dust term in the transformation. Our study also reveals small, but significant, spatial variations of the zero-point offsets in all $uvgr$ bands. External checks using Stromgren photometry, WD loci and the SDSS Stripe 82 standard-star catalog independently confirm the zero-points found by our revised SCR method.
We improve the identification and isolation of individual stellar populations in the Galactic halo based on an updated set of empirically calibrated stellar isochrones in the Sloan Digital Sky Survey (SDSS) and Pan-STARRS 1 (PS1) photometric systems. Along the Galactic prime meridian ($l=0^{circ}$ and $180^{circ}$), where proper motions and parallaxes from Gaia DR2 can be used to compute rotational velocities of stars in the rest frame of the Milky Way, we use the observed double color-magnitude sequences of stars having large transverse motions, which are attributed to groups of stars in the metal-poor halo and the thick disk with halo-like kinematics, respectively. The Gaia sequences directly constrain color-magnitude relations of model colors, and help to improve our previous calibration using Galactic star clusters. Based on these updated sets of stellar isochrones, we confirm earlier results on the presence of distinct groups of stars in the metallicity versus rotational-velocity plane, and find that the distribution of the most metal-poor ([Fe/H] $<-2$) stars in our sample can be modeled using two separate groups on prograde and retrograde orbits, respectively. At $4$-$6$ kpc from the Galactic plane, we find approximately equal proportions of the Splashed Disk, and the metal-rich ($langle {rm [Fe/H]} ranglesim-1.6$) and metal-poor ($langle {rm [Fe/H]} ranglesim-2.5$) halos on prograde orbits. The Gaia-Sausage-Enceladus, the metal-weak thick disk, and the retrograde halo structure(s) ($langle {rm [Fe/H]} ranglesim-2.2$) constitute approximately $10%$ of the rest of the stellar populations at these distances.
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