Day 419

Wednesday.


1304.6080
Segue 2: the least massive galaxy
Kirby, Boylan-Kolchin, Gohen, Geha, Bullock, Kaplinghat

A galaxy with only 900 Msun.  Spectroscopy of 25 members.  Velocity dispersion too small to be measured; upper limit is 2.2 km/s (90% CL).  Mass within 3d HLR (46 pc) is m_1/2 < 1.5e5 Msun [I assume this includes DM, since it's based on velocity dispersion].  Identify Segue 2 as a galaxy rather than a star cluster, based on the wide dispersion in [Fe/H] (-2.8 to -1.4) among the member stars.  The stars' [alpha/Fe] ratios decline with increasing [Fe/H], indicating that Segue 2 retained SNIa ejecta despite its presently small mass and that SF lasted for at least 100 Myr.  The mean metallicity <[Fe/H]>=-2.26pm0.13 (about the same as Ursa Minor galaxy, 330 times more luminous than Segue 2), is higher than expected from the luminosity-metallicity relation defined by more luminous dwarf galaxy satellites of the MW.  Segue 2 may be the barest remnant of a tidally stripped, Ursa Minor-sized galaxy.  If so, it is the best example of an ultra-faint dwarf galaxy that came to be ultra-faint through tidal stripping.  Alternative.y, Segue 2 could have been born in a very low-mass DM subhalo (v_max < 10km/s), below the atomic hydrogen cooling limit.

1304.6083
The impact of bars on disk breaks as probed by S4G imaging
Munoz-Mateos, et al

Spitzer Survey of Stellar Structure in Galaxies (S4G).  Analyze radial distribution of old stars in a sample of 218 nearby face-on disks, in 3.6 um.  Disks with a broken or down-bending profile; find that, on average, disks with a genuine single exponential profile have a scale-length and a central surface brightness which are intermediate to those of the inner and outer components of a down-bending disk with the same total stellar mass.  In the case of barred galaxies, the ratio between the break and the bar radii (Rbr/Rbar) depends strongly on the total stellar mass of the galaxy.  For galaxies more massive than 1e10 Msun, the distribution is bimodal, peakng at Rbr/Rbar~2 and ~3.5.  THe first peak, which is the most populated one, is linked to the Outer LIndblad Resonance of the bar, whereas the second one is consistent with a dynamical coupling between the bar and the spiral pattern.  For galaxies below 1e10 Msun, breaks are found up to ~10 Rbar, but we show that they could still be caused by resonatnces given the rising nature of rotation curves in these low-mass disks.  While not ruling out star formation thresholds, results imply that radial stellar migration induced by non-axysummetric features can be responsible not only for those breaks at 2 Rbar, but also for many of those found at larger radii.  [I didn't get it]

1304.6084
Asymmetric streaming motion in the galactic bulge X-shaped structure revealed by the OGLE-III proper motions
Poleski, .. Gould

As the title says.