What you need to know about SPH

What you need to know about SPH

by | Nov 11, 2011 | Daily Paper Summaries

Paper title: Smoothed Particle Hydrodynamics: Things I wish my mother taught me Author: D. J. Price Author’s Affiliation: Monash Centre for Astrophysics (MoCA), School of Mathematical Sciences, Monash University, Vic 3800, AustraliaThe title of this article caught my eye on the arXiv for two reasons: rarely do authors mention their mother in a paper, and like the author I also wished that my mother (or anyone) had taught me about smoothed particle hydrodynamics. The author soon clarifies that actually it was his PhD supervisor, not his mother, who he wishes had taught him the important details related to simulations, so in fact the rest of the paper pays no further attention to family ties. But, the subject of smoothed particle hydrodynamics (SPH) still fascinates me, and seems to be extremely relevant to current astronomy, so I will summarize the topic here. I should also make the disclaimer that I have never used SPH, so what I present is merely what I have learned from reading the paper.Fundamentals: SPH is a computational method used to model flows, and it is used in a variety of subjects including astrophysics and oceanography. It begins with a known distribution of point-mass particles, and computes density such that density is independent of the following particle characteristics: (1) the absolute positions of the particles, (2) arbitrary rotations, and (3) histories of the particles. Summing up the density comprises what the author calls the fundamental axiom of SPH, as it use discrete particles of fixed mass to describe fluid properties. The resolution of the method (or the smallest step-size that one can take) depends on the...
Bars Rejuvenating Bulges?

Bars Rejuvenating Bulges?

by | Nov 10, 2011 | Daily Paper Summaries

Paper title: Bars rejuvenating bulges? Evidence from stellar population analysis Authors: Paula Coelho and Dimitri A. Gadotti First author’s affiliation:Núcleo de Astrofísica Teórica, Universidade Cruzeiro do Sul, São Paulo, Brasil SummaryAs you may recall from Nathan Sanders’ April post, some spiral galaxies have central bulges with high concentrations of stars. Coelho and Gadotti examine a sample of 575 of these galaxies to investigate whether the presence or absence of a bar (a bar-shaped overdensity of stars) influences the rate of star formation in the central bulge. Astronomers expect to see a higher star formation rate in the bulges of barred galaxies because bars can transport gas from the outer regions of the galaxy into the center and supply fuel for growing stars. Previous detections of star-formation indicators (such as enhanced Hα emission) have indicated that the current star formation rates are higher in barred spirals than in unbarred spirals, but Coelho and Gadotti take the alternative approach of determining the ages of the stellar populations in the bulges. They find that the bulges of barred galaxies are systematically younger than the bulges of unbarred galaxies, which is consistent with the expectation that bars should promote star formation. The Galaxy SampleIn a previous paper, Gadotti derived stellar masses, bulge stellar masses, bar properties, and other parameters for a sample of face-on galaxies observed by Sloan Digital Sky Survey (SDSS). All of the galaxies had stellar masses above 10^10 solar masses and redshifts between 0.02 and 0.07. Selecting face-on galaxies reduced the effect of dust and simplified the process of identifying bars and bulges in the galaxies. In this paper, Coelho...
Simulating the Milky Way’s stellar halo

Simulating the Milky Way’s stellar halo

by | Nov 9, 2011 | Daily Paper Summaries

The Milky Way’s stellar halo – a roughly spherical distribution of stars surrounding our spiral galaxy – is a valuable tool for probing the early evolution of our galaxy. The stellar halo contains some of the oldest stars in our galaxy, whose properties reflect that of the environment in which they formed. This paper focuses on using cosmological simulations of galaxy formation to match the observed structure and kinematics (how the stars move) of stars in Milky Way’s halo.