Where do galaxies end? Comparing measurement techniques of hydrodynamic-simulation galaxies’ integrated properties

Abstract

Using the suite of high-resolution zoom re-simulations of individual haloes by Martig et al., and the large-scale simulation MassiveBlack-II, we examine the differences in measured galaxy properties from techniques with various aperture definitions of where galaxies end. We perform techniques popular in the literature and present a new technique of our own, where the aperture radius is based on the baryonic mass profiles of simulated (sub)haloes. For the average Milky-Way-mass system, we find the two most popular techniques in the literature return differences of order 30 per cent for stellar mass, a factor of 3 for gas mass, 40 per cent for star formation rate, and factors of several for gas accretion and ejection rates. Individual cases can show variations greater than this, with the severity dependent on the concentration of a given system. The average difference in integrated properties for a more general galaxy population are not as striking, but are still significant for stellar and gas mass, especially for optical-limit apertures. The large differences that can occur are problematic for comparing results from various publications. We stress the importance of both defining and justifying a technique choice and discourage using popular apertures that use an exact fraction of the virial radius, due to the unignorable variation in galaxy-to-(sub)halo size. Finally, we note that technique choice does not greatly affect simulated galaxies from lying within the scatter of observed scaling relations, but it can alter the derived best-fit slope for the Kennicutt-Schmidt relation.

The BlueTides simulation: first galaxies and reionization

“…Star formation is implemented based on the multi-phase star formation model in Springel & Hernquist (2003), and incorporating several effects following Vogelsberger et al (2013). Gas is allowed to cool both radiatively following Katz, Weinberg & Hernquist (1996) and via metal cooling.…”

“…In imaging surveys, objects are identified by selecting peaks in a 2-dimensional image, and the total luminosity depends on an aperture radius. (we refer the readers to Stevens et al 2014, and references herein) The canonical implementation of such an algorithm is Source Extractor (Bertin & Arnouts 1996); We use SEP, a reimplementation of source extractor into python (Barbary & contributors 2014).…”

Physical drivers of galaxies’ cold-gas content: exploring environmental and evolutionary effects with Dark Sage

“…Our results suggest the direct stripping of cold gas in galaxies should have a large impact on their H i content (which we reaffirm in Section 5), but that the current implementation in Dark Sage is systematically too efficient at removing H i. Potential solutions could include: (i) non-instantaneous stripping of the cold gas within the annuli, (ii) a more physically strict criterion for when cold gas stripping is allowed (rather than the current case that m cold +m * > m hot ), (iii) adding inclination effects to equation (2), or (iv) the observed H i masses include gas that the model would not consider 'part of the galaxy' (which is hard to define, even when one has full three-dimensional information like in a hydrodynamic simulation -see Stevens et al 2014). However, each of these is unlikely to be satisfactory by itself, as this will have consequences for the relative H i fractions of satellites in different environments (as the strength of stripping is dependent on local environment), which, as we show in Section 5, is an area the model performs well in.…”

How to get cool in the heat: comparing analytic models of hot, cold, and cooling gas in haloes and galaxies with EAGLE

“…where the sums are over all star particles associated with the main subhalo within the 'BaryMP' galactic radius defined by Stevens et al (2014, where the cumulative mass profile of the stars and cold gas reaches a constant gradient), and j z and v z are the specific angular momentum and velocity components along the galaxy's rotation axis, respectively, as measured in the galaxy's centre-ofmomentum frame. 2 In the bottom panels of Fig.…”