Distance measurements from the internal dynamics of globular clusters: Application to the Sombrero galaxy (M104)

TL;DR

This work extends the GC velocity dispersion (GCVD) distance method to the Sombrero galaxy (M104) by obtaining high-resolution spectra for 93 GCs and one UCD, deriving velocity dispersions and, for a subset with sizes, dynamical mass-to-light ratios. Using a calibrated $M_V$–$σ$ relation and a fixed slope from MW/M31 samples, the authors fit the $V$–$σ$ plane to derive a fiducial M104 distance of $D = 9.00$ Mpc with statistical and systematic uncertainties of $0.29$ and $0.26$ Mpc, respectively. They test multiple photometric samples and conversions, find an intrinsic GCVD scatter of about 6%, and demonstrate general agreement with literature distances such as the TRGB value, while noting SUCD1 as an outlier due to its peculiar properties. The results support GCVD as a competitive, independent distance indicator for extragalactic GC systems and motivate applying the method to larger samples to refine slope universality and systematics. Overall, the GCVD distance places M104 in good accord with diverse distance indicators and underscores the value of extragalactic GC kinematics for cosmic distance measurements.

Abstract

Globular clusters (GCs) are dense star clusters found in all massive galaxies. Recent work has established that they follow a tight relation between their internal stellar velocity dispersion $σ$ and luminosity, enabling accurate distance measurements. In this work, we aim to apply this GC velocity dispersion (GCVD) distance method to measure the distance to M104 (NGC 4594, the Sombrero galaxy). We have measured internal stellar velocity dispersions for 85 globular clusters (GCs) and one ultra-compact dwarf galaxy around M104 using high-resolution multi-object integrated-light spectroscopy with FLAMES/GIRAFFE on the Very Large Telescope. The measured velocity dispersions range from $σ= 4 - 30$ km s$^{-1}$, with a mean uncertainty of $Δσ= 2.5$ km s$^{-1}$. For a subset of 77 GCs with $V$-band magnitudes and reliable velocity dispersion measurements above $σ> 4$ km s$^{-1}$, we constructed the $M_V$-$σ$ relation to measure the distance to M104, finding $D=9.00\pm0.29$ (stat.)~$\pm0.26$ (sys.) Mpc. The GCs follow the Milky Way and M31 $M_V-σ$ relation closely, with the exception of the luminous ultra-compact dwarf SUCD1, which is nearly one magnitude brighter than the mean relation. 29 GCs in the sample have sizes determined from Hubble Space Telescope imaging which allowed us to determine their masses and $V$-band dynamical mass-to-light ratios (M/L$_V$). We find a mean $<M/L_V> = 2.6 M_{\odot}/L_{\odot}$ for the luminous ($M_V < -8$ mag) M 104 GCs, which is higher than the Milky Way GCs, but is reminiscent of the brightest GCs in Centaurus A. With the exception of SUCD1, the GCs of M104 follow the GCVD relation irrespective of their mass-to-light ratio.

Figures (10)

• Figure 1: Examples of FLAMES GC spectra. Original spectrum in black, pPXF fit in red. The grey regions indicate the positions of sky lines which were masked out before performing the fits. The GC identifier, the best-fitting velocity dispersion (before aperture correction) and the signal-to-noise ratio are noted in the bottom right corner of each panel.
• Figure 2: Symmetric velocity dispersion uncertainty as a function of spectral signal-to-noise (left) and $i$-band magnitude (right).
• Figure 3: Velocity dispersions versus apparent magnitudes in different filters. The orange line shows the GCVD relation at a distance of 9.00 Mpc. SUCD1 is absent from the right panels as it is not listed in the ground-based catalogue provided in Kang2022.
• Figure 4: Relationship between apparent magnitude and velocity dispersion, colour-coded with different properties. From top left to bottom right: general distribution compared with MW, M31, and Cen A GCs (all placed at a distance of 9.0 Mpc to facilitate comparison with M104), metallicities, $B-R$ colours, line-of-sight velocities, projected distances, half-light radii, mass-to-light ratios, and relaxation times. The solid line is the GCVD relation at a distance of 9.0 Mpc. SUCD1 is marked by the diamond symbol.
• Figure 5: Distribution of GC half-light radii. Top: Milky Way GCs from the catalogue of BaumgardtHilker2018. Middle panel: M31 GCs from Barmby2007, Peacock2010, and Strader2011. Bottom: Sizes of M104 GCs from Harris2010. The full catalogue of 652 GCs is shown in purple. The pink histogram shows the subsample of 29 GCs for which we have both size and dispersion measurements. The lines in each panel show the kernel densities to better visualise the shape of the distributions.