The Milky Way Tomography with SDSS. I. Stellar Number Density Distribution

Abstract

Using the photometric parallax method we estimate the distances to ∼48 million stars detected by the Sloan Digital Sky Survey (SDSS) and map their three-dimensional number density distribution in the Galaxy. The currently available data sample the distance range from 100 pc to 20 kpc and cover 6,500 deg 2 of sky, mostly at high galactic latitudes (|b| > 25). These stellar number density maps allow an investigation of the Galactic structure with no a priori assumptions about the functional form of its components. The data show strong evidence for a Galaxy consisting of an oblate halo, a disk component, and a number of localized overdensities. The number density distribution of stars as traced by M dwarfs in the Solar neighborhood (D < 2 kpc) is well fit by two exponential disks (the thin and thick disk) with scale heights and lengths, bias-corrected for an assumed 35% binary fraction, of H 1 = 300 pc and L 1 = 2600 pc, and H 2 = 900 pc and L 2 = 3600 pc, and local thick-tothin disk density normalization ρ thick (R ⊙ )/ρ thin (R ⊙ ) = 12%. We use the stars near main-sequence turnoff to measure the shape of the Galactic halo. We find a strong preference for oblate halo models, with best-fit axis ratio c/a = 0.64, ρ H ∝ r −2.8 power-law profile, and the local halo-to-thin disk normalization of 0.5%. Based on a series of Monte-Carlo simulations, we estimate the errors of derived model parameters not to be larger than ∼ 20% for the disk scales and ∼ 10% for the density normalization, with largest contributions to error coming from the uncertainty in calibration of the photometric parallax relation and poorly constrained binary fraction. While generally consistent with the above model, the measured density distribution shows a number of statistically significant localized deviations. In addition to known features, such as the Monoceros stream, we detect two overdensities in the thick disk region at cylindrical galactocentric radii and heights (R, Z) ∼ (6.5, 1.5) kpc and (R, Z) ∼ (9.5, 0.8) kpc, and a remarkable density enhancement in the halo covering over a thousand square degrees of sky towards the constellation of Virgo, at distances of ∼6-20 kpc. Compared to counts in a region symmetric with respect to the l = 0 • line and with the same Galactic latitude, the Virgo overdensity is responsible for a factor of 2 number density excess, and may be a nearby tidal stream or a low-surface brightness dwarf galaxy merging with the Milky Way. The u − g color distribution of stars associated with it implies metallicity lower than that of thick disk stars, and consistent with the halo metallicity distribution. After removal of the resolved overdensities, the remaining data are consistent with a smooth density distribution; we detect no evidence of further unresolved clumpy substructure at scales ranging from ∼ 50 pc in the disk, to ∼ 1 − 2 kpc in the halo.

Evaluating galactic habitability using high-resolution cosmological simulations of galaxy formation

“…As we reach the present day, the GHZ becomes more uniform, but the simulations indicate that such uniformity is a recent phenomenon. Note that our vertical density distributions are quite similar to those shown by Jurić et al (2008), suggesting that these galaxies follow exponential profiles both radially Figure 2: The physical properties of the M33 galaxy. We plot the local average stellar metallicity (top left), the local average star formation rate (top right) and the number density of stars (bottom left) at three instances in the simulation, and the age-metallicity relation for all stars in the simulation (bottom right) at z = 0.…”

Separation of stellar populations by an evolving bar: implications for the bulge of the Milky Way

“…These two discs are therefore not to be thought of as the thin and thick discs of a galaxy such as the Milky Way. Indeed both discs are considerably thinner than the Milky Way's thick disc and closer to the thin disc (Jurić et al 2008), which accounts for our terminology 'thinner' and 'thicker' rather than 'thin' and 'thick'. As before, σR 2 declines exponentially, from σR0 = 90 km s −1 for the thicker disc.…”

Tidal features of classical Milky Way satellites in a Λ cold dark matter universe

“…The enclosed stellar mass within this region is M⋆ ∼ 7.0 × 10 4 M⊙. For comparison, the surface brightness of the Virgo Overdensity discovered by SDSS is Σr ∼ 32.5 mag arcsec −2 (Jurić et al 2008), and the surface brightness of the Sagittarius dwarf Northern stream is ΣV ∼ 31 mag arcsec −2 (Martínez- Delgado et al 2004). The stellar overdensity in Eridanus-Phoenix recently discovered by DES has Σr ∼ 32.8 mag arcsec −2 (Li et al 2016).…”