Day 717

Thursday.

1408.1097
Imfit: a fast, flexible new program for astronomical image fitting
Erwin

Fits to 2d surface-brightness functions, with the typical galaxy decompositions (Sersic, exponential, Gaussian) along with core-Sersic and broken-exponential profiles, elliptical rings, and 3 components which perform LoS integration through 3d luminosity density models of disks and rings seen at arbitrary inclinations.  Available minimization algorithms include Levenberg-Marquardt, Nelder-Mead simplex, and Differential Evolution, allowing trade-offs between speed and decreased sensitivity to local minima in the fit landscape.  Minimization can be done using the standard chi^2 statistic (using either data or model values to estimate per-pixel Gaussian errors, or else user-supplied error images) or the Cash statistic; the latter is particularly appropriate for cases of Poisson data in the low-count regime.  Show that fitting low-S/N galaxy images using chi^2 minimization can lead to significant biases in fitted parameter values, which are avoided if the Cash statistic is used; this is true even when Gaussian read noise is present.  [What about PSFs?  seems to be suited for fitting large, well resolved galaxies.]

1408.1099
The Atlas3D project - XXIV. The intrinsic shape distribution of early-type galaxies
Weijmans, et al

Use photometric and kinematic data from Atlas3D.  Based on their ellipticity measurements from SDSS DR7 and additional imaging, first invert the shape distribution of fast and slow rotators under the assumption of axisymmetry.  The so-obtained intrinsic shape distribution for the fast rotators can be described with a Gaussian with a mean flattening of q=0.25 and standard deviation sigma_q=0.14, and an additional tail towards rounder shapes.  The slow rotators are much rounder, and are well described with a Gaussian with mean q=0.63 and sigma_q=0.09 .  Then check that the realists are consistent when applying a different and independent method to obtain intrinsic shape distributions, by fitting the observed ellipticity distributions directly using Gaussian parameterizations for the intrinsic axis ratios.  Although both fast and slow rotators are identified as early-type galaxies in morphological studies, and in many previous shape studies are therefore grouped together, their shape distributions are significantly different, hinting at different formation scenarios.  The intrinsic shape distribution of the fast rotators shows similarities with the spiral galaxy population.  Including the observed kinematic misalignment in the intrinsic shape study shows that the fast rotators are predominantly axisymmetric, with only very little room for triaxiality.  For the slow rotators though there are very strong indications that they are (mildly) triaxial.