Modeling feedback from stars and black holes in galaxy mergers

TL;DR

This work introduces a flexible sub-resolution framework for embedding star formation feedback and supermassive black hole accretion into galaxy-scale simulations, enabling stable, gas-rich disks and realistic merger dynamics. By coupling a multiphase ISM model with Bondi-Hoyle accretion and isotropic thermal AGN feedback implemented in GADGET2, the authors demonstrate self-regulated BH growth and merger-driven gas flows. The results show that SN-driven pressurization stiffens the effective EOS, stabilizing disks at high gas fractions, while BH feedback can curtail central gas, modulate starbursts, and yield more spheroidal, red remnants post-merger. The approach highlights the coupled coevolution of galaxies and SMBHs and provides a versatile tool for exploring hierarchical galaxy formation with SMBH growth and feedback across cosmic time.

Abstract

We describe techniques for incorporating feedback from star formation and black hole accretion into simulations of isolated and merging galaxies. At present, the details of these processes cannot be resolved in simulations on galactic scales. Our basic approach therefore involves forming coarse-grained representations of the properties of the interstellar medium and black hole accretion starting from basic physical assumptions, so that the impact of these effects can be included on resolved scales. We illustrate our method using a multiphase description of star-forming gas. Feedback from star formation pressurises highly overdense gas, altering its effective equation of state. We show that this allows the construction of stable galaxy models with much larger gas fractions than possible in earlier numerical work. We extend the model by including a treatment of gas accretion onto central supermassive black holes in galaxies. Assuming thermal coupling of a small fraction of the bolometric luminosity of accreting black holes to the surrounding gas, we show how this feedback regulates the growth of black holes. In gas-rich mergers of galaxies, we observe a complex interplay between starbursts and central AGN activity when the tidal interaction triggers intense nuclear inflows of gas. Once an accreting supermassive black hole has grown to a critical size, feedback terminates its further growth, and expels gas from the central region in a powerful quasar-driven wind. Our simulation methodology is therefore able to address the coupled processes of gas dynamics, star formation, and black hole accretion during the formation of galaxies.

Figures (16)

• Figure 1: Density profiles of NFW and Hernquist model halos, matched to each other as described in the text. The halo has concentration $c=10$. The total mass of the Hernquist model is equal to he mass of the NFW profile within the virial radius $r_{200}$.
• Figure 2: Time evolution of a Hernquist dark matter halo with initial conditions constructed in the way described in this paper. The initially realised input profile is the thin red line. Because the approximation of a locally Gaussian velocity distribution is not exact, the central profile is not in perfect equilibrium in the beginning. As a result, the density in the centre fluctuates downwards on a short timescale of order the crossing time, and then relaxes to a slightly softer central profile. After time $t=0.08$ (in units of the dynamical time, $t_{\rm dyn}= r_{200}/v_{200}$, of the halo), the deviation is close to its maximum (dashed line). Already at $t=0.16$, however, a stable profile is reached, which then remains essentially invariant with time, as illustrated by the further output times that are overplotted (at times $t=0.24$, $0.32$, $0.48$, and $0.64$). The vertical dotted line marks the scale below which the force law is softened compared to the Newtonian value.
• Figure 3: Rotation curves of model galaxies with the following parameters: (a) $v_{200} = 160 \hbox{km s$^{-1}$}$, $c=9$, $m_{\rm d} = 0.041$, $m_{\rm b} = 0$, $f_{\rm gas} = 0.1$, $\lambda = 0.033$, $J_{\rm d} = 0.041$, $z_0 = 0.2$; (b) same as model (a) but with $m_{\rm b} = 0.01367$. The resulting disk scale lengths are $2.74\,h^{-1}{\rm kpc}$ and $2.46\,h^{-1}{\rm kpc}$, respectively.
• Figure 4: Effective equation of state for the star-forming gas (solid curve). We show the effective pressure measured in cgs units as a function of the gas density in hydrogen atoms per cubic centimetre. The vertical dotted line delineates the transition from an isothermal gas (dashed) at temperature $10^4\,{\rm K}$ to the regime governed by an effective pressure in our multiphase model. For the particular parameters chosen in this example, the transition density lies at $0.128\, {\rm cm^{-3}}$.
• Figure 5: Evolution of the gaseous disk in an isolated galaxy model with a bulge component. A fraction $f_{\rm gas} = 0.1$ of the disk mass is in gas. The remaining $90\%$ is in old stars. The panels show a face-on projection of the gas in the disk, and measure $30\,h^{-1}{\rm kpc}$ on a side. The colour-coding indicates both gas density (in terms of intensity) and local gas temperature (in terms of colour hue). Time in Gyrs is indicated in the upper left corner of each frame.