Category Archives: Uncategorized

Mergers with large satellites bring in little friends

Posted on by

Ever since Richard D’Souza and I learned that M31 suffered a large merger a few Gyr ago, we have been very curious about the impacts that this merger could have had on the satellite population. It is clear that the LMC has brought in a number of satellites, and so it’s reasonable to expect that other large mergers (e.g., the Milky Way’s merger with Gaia-Enceladus, or M31’s large merger) might have brought in satellites. If mergers do bring in satellites, are their properties distinctive? This may help us to understand:

  • star formation histories of satellites
  • their satellite orbits, angular momentum, planes
  • their radial distributions

We wanted to build intuition for how these events proceed using high-resolution N-body models – the ELVIS suite of simulations.

In brief, we find that massive accretions (like the LMC merging with the Milky Way, or M31’s large merger) should deliver significant numbers of satellites to galaxies. The fraction varies depending on merger history, spanning 10%-70% of all satellites. These accretions temporarily enhance the number of satellites close to the main Milky Way-like galaxy, can lead to escaping galaxies(!), and often impart angular momentum to a satellite system.

The most important effect that mergers have on satellites is to clump their accretion time distributions. These satellites are typically accreted within a Gyr of the massive satellite, leading to a short-lived surge of satellite accretion. This signature can be observed! By using star formation shut-off time as a proxy for infall time in the Milky Way and M31, we measure a clear signature of satellite infall at the time of the three most important mergers in the Local Group – the mergers of Gaia-Enceladus and the LMC with the Milky Way, and M31’s large merger several Gyr ago. 

This work is published in the Monthly Notices of the Royal Astronomical Society, and was presented at the Summer 2021 AAS meeting. Sky and Telescope picked up the story, and did an excellent write-up, linked here.

  • Illustration of a massive accretion (blue) bringing in both bound satellites (purple) and satellites that it gravitationally influences (yellow).

Mapping the efficiency of AGN feedback in galaxies

Posted on by

The M31 galaxy, an example of a partially quiescent galaxy, where we argue that feedback from AGN incompletely suppresses star formation.

AGN feedback efficiency is a gradual function of supermassive black hole mass and stellar mass. This empirical mapping of the effects of AGN feedback from Bryan Terrazas and collaborators (ApJ, in press) both tests galaxy formation models and highlights a population of galaxies with partially suppressed star formation – galaxies like M31 or M81 that we term partially quiescent.

One of the ‘grand challenges’ of galaxy evolution is to better understand the drivers of a quiescence – a lack of star formation in galaxies. Early cosmological simulations predicted that all galaxies in the centers of their dark matter halos (‘central’ galaxies) should have star formation, governed by the cooling rate of gas in halos and regulated by feedback from massive stars and supernovae. Yet, many galaxies – typically those with high central stellar densities, high velocity dispersion, often high stellar mass, and large supermassive black holes – lack star formation. Many physical processes are possibly relevant – e.g., changes in cooling efficiency of halos as a function of mass, changes in star formation efficiency as a function of gravitational potential, the injection of large amounts of energy from supermassive black holes (AGN feedback).

In an earlier paper, we used a sample of central galaxies with directly measured black hole masses to show that quiescent galaxies have a much larger supermassive black hole than star-forming galaxies of the same stellar mass, in excellent accord with galaxy formation models where AGN feedback is responsible for suppressing star formation in galaxies.

Projections of the sSFR–MBH–M∗ data cube with our sample of 91 central galaxies with directly measured black hole masses, stellar masses and specific star formation rates denoted by colored points. (a) sSFR as a function of M∗. The gray data points indicate a sample of local galaxies to show the star forming main sequence. The dashed line indicates the boundary below which the sample is no longer complete; (b) sSFR as a function of MBH; (c) MBH as a function of M∗. Color gradients indicate the values for the axis not shown. The lines at the bottom right of (a) and (c) indicate the errors on M∗. Figure 1 of Terrazas et al. 2017.

In this paper, we asked a crucial follow-up question – is the shut-off of star formation all or nothing, or is partial suppression of star formation possible? We find that star formation rate is a continuous function of supermassive black hole mass. Galaxies with low-mass supermassive black holes (like our Milky Way) have typical star formation rates, galaxies with intermediate mass black holes (like M31 or M81) have somewhat suppressed star formation, whereas galaxies with huge supermassive black holes (like M87) have dramatically suppressed star formation. We call galaxies with intermediate black hole mass partially quiescent (in other works these have been dubbed ‘green valley’ galaxies, as they often have colors intermediate between the blue star forming galaxies and red quiescent galaxies). These galaxies are not uncommon, and given that many of them (like M31 or M81) have long-lived stable star forming disks, we suggest that these are in a near-equilibrium state in which star formation is incompletely suppressed by AGN feedback. Interestingly, no galaxy formation model correctly reproduces the observed trends in detail, offering a new and important test of their AGN feedback implementations.