On the interplay between star formation and feedback in galaxy formation simulations

We investigate the star formation-feedback cycle in cosmological galaxy formation simulations, focusing on progenitors of Milky Way (MW)-sized galaxies. We find that in order to reproduce key properties of the MW progenitors, such as semi-empirically derived star formation histories and the shape of rotation curves, our implementation of star formation and stellar feedback requires 1) a combination of local early momentum feedback via radiation pressure and stellar winds and subsequent efficient supernovae feedback, and 2) efficacy of feedback that results in self-regulation of the global star formation rate on kiloparsec scales. We show that such feedback-driven self-regulation is achieved globally for a local star formation efficiency per free fall time of $\epsilon_{\rm ff}\approx 10\%$. Although this value is larger that the $\epsilon_{\rm ff}\sim 1\%$ value usually inferred from the Kennicutt-Schmidt (KS) relation, we show that it is consistent with direct observational estimates of $\epsilon_{\rm ff}$ in molecular clouds. Moreover, we show that simulations with local efficiency of $\epsilon_{\rm ff}\approx 10\%$ reproduce the global observed KS relation. Such simulations also reproduce the cosmic star formation history of the Milky Way sized galaxies and satisfy a number of other observational constraints. Conversely, we find that simulations that a priori assume an inefficient mode of star formation, instead of achieving it via stellar feedback regulation, fail to produce sufficiently vigorous outflows and do not reproduce observations. This illustrates the importance of understanding the complex interplay between star formation and feedback and the detailed processes that contribute to the feedback-regulated formation of galaxies.Comment: 20 pages, 13 figures, accepted for publication in Ap

Observing the circumgalactic medium of simulated galaxies through synthetic absorption spectra

We explore the multiphase structure of the circumgalactic medium (CGM) probed by synthetic spectra through a cosmological zoom-in galaxy formation simulation. We employ a Bayesian method for modelling a combination of absorption lines to derive physical properties of absorbers with a formal treatment of detections, including saturated systems, and non-detections in a uniform manner. We find that in the lines of sight passing through localized density structures, absorption lines of low, intermediate and high ions are present in the spectrum and overlap in velocity space. Low, intermediate and high ions can be combined to derive the mass-weighted properties of a density-varying peak, although the ions are not co-spatial within the structure. By contrast, lines of sight that go through the hot halo only exhibit detectable HI and high ions. In such lines of sight, the absorption lines are typically broad due to the complex velocity fields across the entire halo. We show that the derived gas density, temperature, and metallicity match closely the corresponding HI mass-weighted averages along the LOS. We also show that when the data quality allows, our Bayesian technique allows one to recover the underlying physical properties of LOS by incorporating both detections and non-detections. It is especially useful to include non-detections, of species such as NV or NeVIII, when the number of detections of strong absorbers, such as HI and OVI, is smaller than the number of model parameters (density, temperature, and metallicity).Comment: Accepted for publication in MNRA