Faint Streams Research Articles

Tracing Kinematic and Chemical Properties of Sagittarius Stream by K-Giants, M-Giants, and BHB stars

Abstract We characterize the kinematic and chemical properties of ∼3000 Sagittarius (Sgr) stream stars, including K-giants, M-giants, and blue horizontal branch stars (BHBs), selected from SEGUE-2, Large Sky Area Multi-Object Fibre Spectroscopic Telescope, and Sloan Digital Sky Survey separately in Integrals-of-Motion space. The orbit of the Sgr stream is quite clear from the velocity vector in the X–Z plane. Stars traced by K-giants and M-giants show that the apogalacticon of the trailing steam is ∼100 kpc. The metallicity distributions of Sgr K-giants, M-giants, and BHBs indicate that the M-giants are on average the most metal-rich population, followed by K-giants and BHBs. All of the K-giants, M-giants, and BHBs indicate that the trailing arm is on average more metal-rich than the leading arm, and the K-giants show that the Sgr debris is the most metal-poor part. The α-abundance of Sgr stars exhibits a similar trend with the Galactic halo stars at lower metallicity ([Fe/H] <∼ −1.0 dex), and then evolve down to lower [α/Fe] than disk stars at higher metallicity, which is close to the evolution pattern of the α-element of Milky Way dwarf galaxies. We find that V Y and metallicity of K-giants have gradients along the direction of the line of sight from the Galactic center in the X–Z plane, and the K-giants show that V Y increases with metallicity at [Fe/H] >∼ −1.5 dex. After dividing the Sgr stream into bright and faint streams according to their locations in equatorial coordinates, the K-giants and BHBs show that the bright and faint streams present different V Y and metallicities, the bright stream is on average higher in V Y and metallicity than the faint stream.

The star formation history of the Sagittarius stream

We present the first detailed quantitative study of the stellar populations of the Sagittarius (Sgr) streams within the Stripe 82 region, using photometric and spectroscopic observations from the Sloan Digital Sky Survey (SDSS). The star formation history (SFH) is determined separately for the bright and faint Sgr streams, to establish whether both components consist of a similar stellar population mix or have a distinct origin. Best fit SFH solutions are characterised by a well-defined, tight sequence in age-metallicity space, indicating that star formation occurred within a well-mixed, homogeneously enriched medium. Star formation rates dropped sharply at an age of ~5-7 Gyr, possibly related to the accretion of Sgr by the MW. Finally, the Sgr sequence displays a change of slope in age-metallicity space at an age between 11-13 Gyr consistent with the Sgr alpha-element knee, indicating that supernovae type Ia started contributing to the abundance pattern ~1-3 Gyr after the start of star formation. Results for both streams are consistent with being drawn from the parent Sgr population mix, but at different epochs. The SFH of the bright stream starts from old, metal-poor populations and extends to a metallicity of [Fe/H]~-0.7, with peaks at ~7 and 11 Gyr. The faint SFH samples the older, more metal-poor part of the Sgr sequence, with a peak at ancient ages and stars mostly with [Fe/H]<-1.3 and age>9 Gyr. Therefore, we argue in favour of a scenario where the faint stream consists of material stripped i) earlier, and ii) from the outskirts of the Sgr dwarf.

SUBSTRUCTURE IN THE STELLAR HALOS OF THE AQUARIUS SIMULATIONS

We characterize substructure in the simulated stellar halos of Cooper et al. (2010) which were formed by the disruption of satellite galaxies within the cosmological N-body simulations of galactic halos of the Aquarius Project. These stellar halos exhibit a wealth of tidal features: broad overdensities and very narrow faint streams akin to those observed around the Milky Way. The substructures are distributed anisotropically on the sky, a characteristic that should become apparent in the next generation of photometric surveys. The normalized RMS of the density of stars on the sky appears to be systematically larger for our halos compared to the value estimated for the Milky Way from main sequence turn-off stars in the Sloan Digital Sky Survey. We show that this is likely to be due in part to contamination by faint QSOs and redder main sequence stars, and might suggest that ~10% of the Milky Way halo stars have formed in-situ.

The Chimney

THE CHIMNEY / Dan Stryk (after Thoreau) To build this. To build this lonely way to heaven. The white sand gathered, grey rocks dug from the shadowed littoral, the loon's cry wavering like a hollow face, my face so long extinguished, in the violent branch-crossed red of evening sun. I tow my dug-out slowly through the chill muck near the shore— generations of small things have trampled or slow-sifted to this slime my coarse toes knead. To build this, I have given half my life. It spires not yet halfway up, a white-grey roughness beckoning the falling sun, the heaven's leaving heat—like a last dull finger of despair. Yet I am not alone. The hut, firm, rough-logged, now surrounds me like a shell. Still, winter's nearing, thin ice, here and there, has slowed the water's riffling pulse. But the screech-owl, frozen sculpture in the trees, will leap to truth—swift rising in its secret heat—when once again I near. It flies away, dark pinions spread, the rabbit leaping from its hunched repose. And the chimney, not yet finished, 36 · The Missouri Review purls its faint stream from these hands. Gloom of a cold season, once again the wait for spring: the oozing mud— bronze glitter from the railroad bank, the slow grey spiral lifework of a man. Dan Stryk The Missouri Review · 37 ...

3C 129 close-up

High-resolution VlA observations of the head-tail radio galaxy 3C 129 have shown thin, faint streams connecting the nucleus to the larger tails. The multiple bends and wiggles in the source are easily described in terms of precessing beams, as shown in a companion Letter by Icke. As in NGC 1265, the jets can be mildly supersonic or mildly relativistic. We point out the major features of these curved streams and their importance for general models of extragalactic jets.