Testing a simple recipe for estimating galaxy masses from minimal observational data

Abstract

The accuracy and robustness of a simple method to estimate the total mass profile of a galaxy is tested using a sample of 65 cosmological zoom-simulations of individual galaxies. The method only requires information on the optical surface brightness and the projected velocity dispersion profiles and therefore can be applied even in case of poor observational data. In the simulated sample massive galaxies ($\sigma \simeq 200-400$ $\kms$) at redshift $z=0$ have almost isothermal rotation curves for broad range of radii (RMS $\simeq 5%$ for the circular speed deviations from a constant value over $0.5R_{\rm eff} < r < 3R_{\rm eff}$). For such galaxies the method recovers the unbiased value of the circular speed. The sample averaged deviation from the true circular speed is less than $\sim 1%$ with the scatter of $\simeq 5-8%$ (RMS) up to $R \simeq 5R_{\rm eff}$. Circular speed estimates of massive non-rotating simulated galaxies at higher redshifts ($z=1$ and $z=2$) are also almost unbiased and with the same scatter. For the least massive galaxies in the sample ($\sigma < 150$ $\kms$) at $z=0$ the RMS deviation is $\simeq 7-9%$ and the mean deviation is biased low by about $1-2%$. We also derive the circular velocity profile from the hydrostatic equilibrium (HE) equation for hot gas in the simulated galaxies. The accuracy of this estimate is about RMS $\simeq 4-5%$ for massive objects ($M > 6.5\times 10^{12} M_\odot$) and the HE estimate is biased low by $\simeq 3-4%$, which can be traced to the presence of gas motions. This implies that the simple mass estimate can be used to determine the mass of observed massive elliptical galaxies to an accuracy of $5-8 %$ and can be very useful for galaxy surveys.