SEGUE: A Spectroscopic Survey of 240,000 stars with g=14-20

TL;DR

SEGUE introduces a large-scale spectroscopic survey within SDSS-II to map the Milky Way's kinematic and chemical structure across the disk and halo. By obtaining ~240,000 spectra at $R \sim 1800$ over 3900–9000 Å and pairing them with 3500 deg^2 of ugriz imaging, SEGUE delivers uniform radial velocities and atmospheric parameters ([$M/H$], log g, $T_{\rm eff}$) via the SSPP, enabling robust analyses of Galactic substructure and population gradients. The paper details the target-category taxonomy, selection algorithms, plate design, data processing, and public data release (DR7), highlighting the rich science potential for halo streams, thin/thick disk transitions, and low-metallicity populations. It also notes follow-up SEGUE-2 and provisions for data access and reproducible queries, underscoring the survey’s lasting impact on Galactic archaeology and public astronomy software pipelines.

Abstract

The SEGUE survey obtained 240,000 moderate resolution (R = 1800) spectra from 3900 - 9000 Angstroms of fainter Milky Way stars (14.0 < g < 20.3) of a wide variety of spectral types, both main sequence and evolved objects, with the goal of studying the kinematics and populations of our Galaxy and its halo. The spectra are clustered in 212 regions spaced over three-quarters of the sky. Radial velocity accuracies for stars are 4 km/s at g < 18, degrading to 15 km/s at g = 20. For stars with S/N > 10 per resolution element, stellar atmospheric parameters are estimated, including metallicity, surface gravity, and effective temperature. SEGUE obtained 3500 square degrees of additional ugriz imaging (primarily at low Galactic latitudes) providing precise multi-color photometry (g,r,i = 2%), (u,z = 3%) and astrometry (0.1 arcsec) for spectroscopic target selection. The stellar spectra, imaging data, and derived parameter catalogs for this survey are publicly available as part of SDSS Data Release 7 (DR7).

Figures (17)

• Figure 1: Upper panel: the SEGUE Survey footprint in the Equatorial coordinates from $360^\circ$ to $0^\circ$ (left to right) and $-26^\circ$ to $90^\circ$ (bottom to top). Selected stripes are labeled with their stripe number. The SDSS North Galactic Cap stripes are numbered from 9 to 44. Southern SDSS stripes are numbered 76, 82 and 86. SEGUE fills in Southern stripes 72 and 79. SEGUE's constant Galactic longitude stripes are numbered with $stripe = 1000 + 2l$ where $l$ is the Galactic longitude. Each SEGUE plate pointing (usually representing a pair of 640 hole plates), is indicated with a blue circle. Lower panel: the SEGUE Survey footprint in $(l,b)$ in Aitoff projection, centered on the Galactic anticenter. The line marking the Southern limit of the telescope observing site $\delta = -20^\circ$ is indicated in magenta. Red and green filled areas represent South and North SDSS and SEGUE strips respectively.
• Figure 2: top panel shows the histogram of differences in spectroscopic pipeline measurements of RV for two separate observations of the same star on different survey SEGUE plates for 2200 stars with $r \sim 18$ and S/N $> 10$. The quoted sigma is divided by $\sqrt{2}$ to estimate the measurement error for a single observation. The red curve is the best fit Gaussian, it has a mean offset of $< 0.3 \rm ~km~s^{-1}$ and a $\sigma/\sqrt{2} = 4.4 ~\rm km~s^{-1}$. Lower panel: RV accuracy by S/N and $(g-r)_0$ color. The color code is magenta: $(g-r)_0 < 0.1$ (BHB/BS/A), blue: $0.1 < (g-r)_0 < 0.3$ (F), green: $0.3 < (g-r)_0 < 0.48$ (F/G), cyan: $0.48 < (g-r)_0 < 0.55$ (G, orange: $(0.55 < (g-r)_0 < 0.75$ (G/K) and red: $0.75 < (g-r)_0 < 1.0$ (K).
• Figure 3: target selection categories in SEGUE. Top panel: $(g-r)_0,(u-g)_0$ color-color diagram showing different SEGUE target categories in different colors/symbols. Note the 'Low Metal' category hugs the blue side (in $(u-g)_0$) of the stellar locus, and a substantial fraction of F stars with redder $(u-g)_0$ and $0.2 < (g-r)_0 < 0.48$ are not targeted, except by the F/G category. Middle panel: the same as above, except categories which use redder $(i-z)_0,(r-i)_0$ colors are highlighted. Note the L dwarf candidate at $(i-z,r-i)_0 = (1.72,1.9)$. The proper motion selected extreme M subdwarf candidates are shown as open blue circles. Lower panel: a $(g-i)_0, H_g$ selection diagram for categories which use USNO-B proper motion in their selection. $H_g = g + 5\rm log_{10}(\mu/1000)+5$ where $\mu$ is the proper motion in $mas\>yr^{-1}$. Note the cool WD candidates (high proper motion) as magenta circles and the K giant candidates (consistent with 0 proper motion).
• Figure 4: top: a sample SEGUE DA (hydrogen) WD. middle: a candidate sdO star showing a characteristic hot spectrum. lower: a candidate sdB star, which has colors similar to the WD.
• Figure 5: sample SEGUE cool WD where CIA due to $H_2$ has removed flux redward of 6000Å, leaving a spectrum with very unusual colors.