How large are the globular cluster systems of early-type galaxies and do they scale with galaxy halo properties?

Abstract

The globular cluster systems of galaxies are well-known to extend to large galactocentric radii. Here we quantify the size of GC systems using the half number radius of 22 GC systems around early-type galaxies from the literature. We compare GC system sizes to the sizes and masses of their host galaxies. We find that GC systems typically extend to 4× that of the host galaxy size, however this factor varies with galaxy stellar mass from about 3× for M * galaxies to 5× for the most massive galaxies in the universe. The size of a GC system scales approximately linearly with the virial radius (R 200 ) and with the halo mass (M 200 ) to the 1/3 power. The GC system of the Milky Way follows the same relations as for early-type galaxies. For Ultra Diffuse Galaxies their GC system size scales with halo mass and virial radius as for more massive, larger galaxies. UDGs indicate that the linear scaling of GC system size with stellar mass for massive galaxies flattens out for low stellar mass galaxies. Our scalings are different to those reported recently by Hudson & Robison (2017).

Scaling relations for globular cluster systems in early-type galaxies

“…The dashed-dotted grey curve corresponds to the relation derived by Hudson & Robison (2018), while the dotted grey curve represents the mean ratio for both parameters from Forbes (2017). Both expressions are in agreement with our fit, considering the lack of r eff,GCS measurements for many GCSs in massive galaxies, and the dispersion of the available ones.…”

“…The change in slope for lower masses might be related with the stellar mass-size relation for galaxies, that flattens for central galaxies (e.g. Shen et al 2003; The dotted curve shows the relation between the size of galaxies and the virial radius derived by Kravtsov (2013), as represented in Forbes (2017), but scaled by the factor f eff to follow the r eff,GCS in our plot. The dashed-dotted curve corresponds to the relation fitted by Hudson & Robison (2018) between the r eff,GCS and the r 200 , applying the same scaling factor f eff .…”

“…The dotted curve shows the relation derived by Kravtsov (2013) for the galaxy size as function of the virial radius, scaled by 3.7 to consider the mean ratio be-tween galaxies and GCS sizes Forbes (2017), and by f eff . The relation is in agreement with the r eff,GCS of the GCS.…”

Early-type galaxies in low-density environments: NGC 6876 explored through its globular cluster system

“…Since we have estimations for both as well as for the GCS extension, it is of interest to see where our data falls in the proposed existing relations. Recently, Forbes (2017) suggested an approximately linear relation between GCS size and halo mass to the 1/3 power, in contraposition to the simultaneous proposal by Hudson & Robison (2018), which has a much higher slope (0.80). In the case of stellar mass, again Hudson & Robison (2018) determines a bigger slope than Forbes (2017).…”

“…Recently, Forbes (2017) suggested an approximately linear relation between GCS size and halo mass to the 1/3 power, in contraposition to the simultaneous proposal by Hudson & Robison (2018), which has a much higher slope (0.80). In the case of stellar mass, again Hudson & Robison (2018) determines a bigger slope than Forbes (2017). For the halo mass, NGC 6876 falls far from both the relation proposed by Hudson & Robison (2018) which gives an estimated result of log r GCS ∼ 1.3, and from the Forbes (2017) relation for which the size is (log r GCS ∼ 1.2).…”