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

Simulations of disk galaxies with cosmic ray driven galactic winds

We present results from high-resolution hydrodynamic simulations of isolated SMC- and Milky Way-sized galaxies that include a model for feedback from galactic cosmic rays (CRs). We find that CRs are naturally able to drive winds with mass loading factors of up to ~10 in dwarf systems. The scaling of the mass loading factor with circular velocity between the two simulated systems is consistent with \propto v_c^{1-2} required to reproduce the faint end of the galaxy luminosity function. In addition, simulations with CR feedback reproduce both the normalization and the slope of the observed trend of wind velocity with galaxy circular velocity. We find that winds in simulations with CR feedback exhibit qualitatively different properties compared to SN driven winds, where most of the acceleration happens violently in situ near star forming sites. In contrast, the CR-driven winds are accelerated gently by the large-scale pressure gradient established by CRs diffusing from the star-forming galaxy disk out into the halo. The CR-driven winds also exhibit much cooler temperatures and, in the SMC-sized system, warm (T~10^4 K) gas dominates the outflow. The prevalence of warm gas in such outflows may provide a clue as to the origin of ubiquitous warm gas in the gaseous halos of galaxies detected via absorption lines in quasar spectra.Comment: ApJL accepted. Replaced with accepted version. Minor revision in response to referee comment

Towards a complete accounting of energy and momentum from stellar feedback in galaxy formation simulations

Stellar feedback plays a key role in galaxy formation by regulating star formation, driving interstellar turbulence and generating galactic scale outflows. Although modern simulations of galaxy formation can resolve scales of 10-100 pc, star formation and feedback operate on smaller, "subgrid" scales. Great care should therefore be taken in order to properly account for the effect of feedback on global galaxy evolution. We investigate the momentum and energy budget of feedback during different stages of stellar evolution, and study its impact on the interstellar medium using simulations of local star forming regions and galactic disks at the resolution affordable in modern cosmological zoom-in simulations. In particular, we present a novel subgrid model for the momentum injection due to radiation pressure and stellar winds from massive stars during early, pre-supernova evolutionary stages of young star clusters. Early injection of momentum acts to clear out dense gas in star forming regions, hence limiting star formation. The reduced gas density mitigates radiative losses of thermal feedback energy from subsequent supernova explosions, leading to an increased overall efficiency of stellar feedback. The detailed impact of stellar feedback depends sensitively on the implementation and choice of parameters. Somewhat encouragingly, we find that implementations in which feedback is efficient lead to approximate self-regulation of global star formation efficiency. We compare simulation results using our feedback implementation to other phenomenological feedback methods, where thermal feedback energy is allowed to dissipate over time scales longer than the formal gas cooling time. We find that simulations with maximal momentum injection suppress star formation to a similar degree as is found in simulations adopting adiabatic thermal feedback.Comment: ApJ submitted. For a high-resolution version of the paper, see http://kicp.uchicago.edu/~agertz

El proceso kafkiano a la ciencia y la razón de p. K. Feyerabend

Cinquenes Jornades de Foment de la Investigació de la FCHS (Any 1999-2000