ﻻ يوجد ملخص باللغة العربية
We present evidence for a metal-poor, [Fe/H]$sim-1.4$ $sigma$=0.2 dex, stellar halo component detectable at radii from 10 kpc to 70 kpc, in our nearest giant spiral neighbor, the Andromeda galaxy. This metal-poor sample underlies the recently-discovered extended rotating component, and has no detected metallicity gradient. This discovery uses a large sample of 9861 radial velocities of Red Giant Branch (RGB) stars obtained with the Keck-II telescope and DEIMOS spectrograph, with 827 stars with robust radial velocity measurements isolated kinematically to lie in the halo component primarily by windowing out the extended rotating component which dominates the photometric profile of Andromeda out to $<$50 kpc (de-projected). The stars lie in 54 spectroscopic fields spread over an 8 square degree region, and are expected to fairly sample the halo to a radius of $sim$70 kpc. The halo sample shows no significant evidence for rotation. Fitting a simple model in which the velocity dispersion of the component decreases with radius, we find a central velocity dispersion of $152kms$ decreasing by $-0.90kms/kpc$. By fitting a cosmologically-motivated NFW halo model to the halo stars we constrain the virial mass of M31 to be greater than $9.0 times 10^{11} msun$ with 99% confidence. The properties of this halo component are very similar to that found in our Milky Way, revealing that these roughly equal mass galaxies may have led similar accretion and evolutionary paths in the early Universe.
We have analyzed new HST/ACS and HST/WFC3 imaging in F475W and F814W of two previously-unobserved fields along the M31 minor axis to confirm our previous constraints on the shape of M31s inner stellar halo. Both of these new datasets reach a depth of
We present spectroscopic observations of red giant branch (RGB) stars over a large expanse in the halo of the Andromeda spiral galaxy (M31), acquired with the DEIMOS instrument on the Keck II 10-m telescope. Using a combination of five photometric/sp
Many clues about the galaxy assembly process lurk in the faint outer regions of galaxies. The low surface brightnesses of these parts pose a significant challenge for studies of diffuse light, and few robust constraints on galaxy formation models hav
We present the resolved stellar populations in the inner and outer halo of the nearby lenticular galaxy NGC~3115. Using deep HST observations, we analyze stars two magnitudes fainter than the tip of the red giant branch (TRGB). We study three fields
The Sloan Digital Sky Survey has revealed an overdensity of luminous red giant stars ~ 3 degrees (40 projected kpc) to the northeast of M31, which we have called Andromeda NE. The line-of-sight distance to Andromeda NE is within approximately 50 kpc