Accreted stars and stellar haloes of simulated galaxies in TNG50

Abstract

We use the TNG50 cosmological hydrodynamic simulation to study the accreted stellar component and stellar haloes of isolated galaxies spanning a wide range of masses ($10^8<M_*/M_\odot<10^{11}$). We find that stars formed in the main progenitor (i.e., in-situ stars) typically dominate the inner regions as far as $\sim$10 half-light radii from the centre, implying that detecting uncontrovertible evidence for the presence of an accreted stellar halo requires probing the far outskirts of a galaxy. Stars from accreted, disrupted satellites (i.e., ex-situ stars) dominate beyond that radius (roughly $25\%$ of the virial radius, $r_{200}$), which we identify as the inner boundary of the outer stellar halo. The fraction of accreted stars decreases monotonically with decreasing galaxy mass, $M_*$, from $\sim$$20\%$ on average in $\sim$$2\times 10^{12}\, M_\odot$ haloes ($M_*\sim$$10^{11}\, M_\odot$) to $2$-$3\%$ in $\sim$$2\times 10^{10}\, M_\odot$ haloes ($M_*\sim$$10^{8}\, M_\odot$). The outer halo has a mass comparable to roughly $10\%$ of all accreted stars. Fewer than $\sim$$30\%$ of stars in the outer halo are in-situ stars, many of which originate from star-forming satellites during the late stages of disruption, especially in low-mass systems. Accreted stars are systematically more metal poor in less massive systems, which makes the outer haloes of dwarf galaxies a fertile hunting ground for extremely metal-poor stars. At given galaxy mass, the more massive stellar haloes are systematically more concentrated (smaller $R_{\rm eff}$) and have steeper density profiles (larger $n$). Our results provide a blueprint for interpreting observations of the outskirts of isolated galaxies in terms of their assembly histories.

Figures (9)

• Figure 1: Distance to the main progenitor at formation time, $r_{\rm birth}$, as function of cosmic formation time, $t_{\rm birth}$, for all star particles (black dots) bound to the example galaxy TNG50-714463 (see Fig. \ref{['fig:illustrative_example_insitu_exsitu']}). Star particles born within the main progenitor (i.e., $r_{\rm birth} < r_{\rm glx}$) are classified as in-situ; those with $r_{\rm birth} > r_{\rm glx}$ (above the threshold radius shown as a red horizontal dashed line) are defined as ex-situ, or 'accreted'. The example galaxy accreted two satellites at $t \approx 3.7$ Gyr and $t \approx 7.5$ Gyr (indicated with black arrows). Stars born in these satellites contribute $\log(M_{\mathrm{acc}}/M_{\mathrm{\odot}}) = 7.68$ to the host galaxy, for a total accreted mass fraction $f_{\rm acc}=0.053$. Stars in the outer stellar halo at $z=0$ ($r > r_{\rm glx}$, labelled 'oSH') are highlighted with empty squares, blue if they are in-situ, and red if they are accreted (see Fig. \ref{['fig:illustrative_example_insitu_exsitu']}). The gray shaded region denotes distances smaller than time-evolving force-softening length for stars and dark matter, $\epsilon_{\mathrm{DM,*}}$.
• Figure 2: Edge-on projection of a simulated dwarf galaxy (TNG50-714463, stellar and virial masses are shown in the legend, left panel), split into its in-situ (centre panel) and accreted (right panel) stellar components. Our definition of galaxy radius, $r_{\rm glx} = 0.25 ~ r_{200}= 24.25$ kpc, is indicated by the cyan circles, separating the main galaxy from its outer stellar halo. This example has an accreted mass comparable to the median value for galaxies of comparable mass ($f_{\rm acc} = 0.053$). Brighter colours indicate higher number density of star particles, while starred symbols are outer halo stars coloured according to their origin (blue for in-situ; red for accreted). Images generated with PySPHViewer BenitezLlambay2017.
• Figure 3: Accreted stellar mass fraction in 3D radial bins, $f_{\rm acc}(r)=M_{\rm acc}(r)/M_{*}(r)$, for the illustrative example galaxy TNG50-714463. The vertical green dashed line indicates the stellar half-mass radius of the galaxy. Accreted stars begin to dominate the stellar mass ($f_{\rm acc} > 0.5$) at $r \gtrsim 15.5$ kpc. This defines the 'crossing radius', $r_{\rm cross}$, of the galaxy. This radius is more than 7 times larger than the stellar half-mass radius of this galaxy, and is smaller than our adopted definition of 'galaxy radius' $r_{\rm glx}$, shown with a blue dashed line. Changes of $r_{\rm glx}$ by a factor of 2 (blue shaded regions) produce only small variations in the accreted fraction profile (grey shaded regions), with $r_{\rm cross} = 15 \pm 1$ kpc.
• Figure 4: Distance to the main progenitor at birth, $r_{\mathrm{birth}}$, vs. galactocentric distance at present time, $r$, of all star particles (excluding satellites) for 773 galaxies with virial mass $10.4 < \log(M_{200}/M_{\odot}) < 10.5$ (left) and 165 galaxies with virial mass $11.2 < \log(M_{200}/M_{\odot}) < 11.3$ (right), stacked. The dotted white diagonal line shows the 1:1 relation, i.e., stars at the same galactocentric distance as when they were born. Blue dashed vertical line indicates the median $r_{\mathrm{glx}}$, which separates the main galaxy from the outskirts (the outer stellar halo). Stars born at $r_{\rm birth} > r_{\mathrm{glx}}$ (above the red dashed horizontal line) are classified as accreted. Most in-situ stars lie close to the 1:1 line, and most accreted stars end up in the inner regions of the galaxies. Consequently, the mass of stars in the stellar halo is one order of magnitude smaller than the accreted mass fraction. The 25-75th percentile of the half-mass stellar radius of the stacks of galaxies are shown with a green interval over the 1:1 line.
• Figure 5: Distance at which accreted stars begin to dominate, $r_{\rm cross}$, for all galaxies in our sample (black dots) as a function of stellar mass (left panel) and virial mass (right panel). For comparison, we show its median (white solid line), the median stellar half-mass radius $r_{50,*}$ (green dashed line), and that of the adopted galaxy radius $r_{\rm glx}$ (black solid line). The typical galactocentric distance beyond which most stars are accreted is roughly $\sim$8 times larger than the stellar half-mass radius, and close to the galaxy radius definition, $r_{\rm glx}=0.25\, r_{200}$.