Abstract
In order to investigate the role of the host halo in quenching satellite galaxies, we have characterized a single Milky Way-like host galaxy from the FIRE simulations from z = 0–1.76 by quantifying the gas density of the host halo environment with respect to distance from the host and galactocentric latitude. The gas density decreases with increasing distance from the host according to a broken power law. At earlier times (2–10 Gyr ago), the density in the inner regions of the host halo was enhanced relative to z = 0. Thus, earlier infalling satellites experienced more ram-pressure and were more efficiently quenched compared to later infalling satellites. We also find that in the inner halo (<150 kpc) the density is 2–3 times larger close to the plane of the host galaxy disk versus above or below the disk, so satellites that orbit at low galactocentric latitudes may be more efficiently quenched.
Export citation and abstract BibTeX RIS
Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
1. Introduction
Satellite galaxies are gravitationally bound to the host galaxy, and orbit within the host galaxy's virial radius. Their infall into the host environment causes them to hydrodynamically interact with the host's circumgalactic medium (CGM), a reservoir of gas extending throughout the halo, through processes like ram pressure stripping, which removes gas from the satellite. A consequence of this hydrodynamic interaction is quenching—cessation of star formation—in these satellite galaxies once the gas that fuels star formation has been removed.
Together, the Milky Way (MW), Andromeda (M31), and their satellite galaxies make up the Local Group (LG). LG satellite dwarf galaxies (M* ≈ 105−9 M⊙) are mostly quenched, except for the most massive satellites (M* ≈ 108−9 M⊙) like the LMC and SMC (Wetzel et al. 2015). In contrast, isolated dwarf galaxies with masses as low as M* ≈ 107 M⊙ are star-forming (Geha et al. 2012). The MW/M31 host environment clearly plays a prominent role in regulating galaxy formation on dwarf scales.
2. Methods
To understand the relationship between the host halo and satellite quenching, we must build a quantitative understanding of the host environment and its effects on satellites. Here we characterize the gaseous halo of a simulated MW-like galaxy in order to inform future modeling of the role of the host halo in quenching satellite galaxies. We analyze a MW-mass galaxy from the FIRE simulations, m12i, which has a total mass of 1.2 × 1012 M⊙ and a stellar mass of 5.5 × 1010 M⊙ (Wetzel et al. 2016; Samuel et al. 2020).
We use the gas particle data from the simulation at six snapshots from z = 0–1.76 separated by roughly 2 Gyr. The ram-pressure experienced by an infalling satellite is given by the equation P ∝ ρ v2. We analyze the number density as a function of distance from the host and galactocentric latitude over the last 10 Gyr in order to model the role of the host halo gas in quenching satellites in future work. At each snapshot we select gas particles within 30–300 kpc of the host to capture the majority of the CGM while excluding regions too near the disk.
3. Results
3.1. Radial Profile at z = 0
The left panel of Figure 1 depicts the median number density in 10 kpc bins versus distance from the host. We find that the number density decreases with increasing distance. The distinct slopes in the inner and outer halo show that the density changes drastically near ≈140 kpc. The non-uniformity of the upper scatter shows that the density in each bin is highly variable, with clumps of gas scattering to densities 1–2 orders of magnitude higher than the median. We fit the median density with Astropy's BrokenPowerLaw1D given in Equation (1). The fitting parameters obtained for the best-fit line are A = 2.75 × 10−5, xbreak = 139.2 kpc, α1 = 1.64, α2 = 2.56.
Figure 1. Left: Radial Profile of m12i with respect to distance from the host at z = 0. The solid line is the median, the inner scatter is 68th percentile and the outer scatter is 95th percentile. The dashed line is the best-fit (Equation (1)). Middle: Evolution of the radial profile of m12i with respect to distance from the host over time from z = 0–1.76. Approaching z = 0, the density in the outer halo decreases monotonically, thus, ram pressure stripping in this halo has decreased over time. Right: Gas density of m12i with respect to latitude at z = 0 at various distances. In the inner halo (<150 kpc), the median density is about 2–3 times higher at low latitudes (−25° to 25°) compared to higher latitudes.
Download figure:
Standard image High-resolution image
3.2. Time Evolution of Radial Profile
In the middle panel of Figure 1, we show the median density profile of m12i with respect to distance from the host over time from z = 0–1.76, i.e., present day to 10 Gyr ago, to study the evolution of a MW-like halo. We observe that the density of the host halo generally decreases over time i.e., the halo gas is denser at z = 1.76 than z = 1.04 and so on. We conclude that the ram pressure stripping experienced by a satellite galaxy will vary depending on when it falls into the host halo environment, with an earlier infalling satellite experiencing more ram pressure.
3.3. Density with Respect to Galactocentric Latitude
Density also varies throughout the halo as a function of galactocentric latitude (β) at a given radius. Equation (2) demonstrates how to measure β using a Cartesian coordinate system centered on the host, with the z-axis aligned with the minor axis of the host disk. In the right panel of Figure 1, we use four distance bins to study how latitude differs at different distances from the host. In all distance bins, the number density is higher at lower latitudes (−25° to 25°) and decreases as latitude increases on both ends, i.e., approaches −90° and 90°. Therefore, satellites that experience close passages or pericenters at low latitudes will be more effectively stripped of their gas versus satellites that have pericenters at high latitudes.
4. Conclusions
We examined the halo gas density of a simulated MW-like galaxy (m12i) and analyzed the number density as a function of distance from the host, time and latitude. We measured higher densities in the inner halo at z = 0. We find that the halo density of m12i has generally decreased over time and is higher at low latitudes at z = 0. Thus, a satellite experiencing close pericenters, approaching at lower latitudes or having fallen in earlier likely experienced more ram pressure stripping and quenched more efficiently.