Distance measurements from the internal dynamics of globular clusters: Application to the Sombrero galaxy (M 104)

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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>σ</mml:mi> </mml:math> 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 M 104 (NGC 4594, the Sombrero galaxy). We have measured internal stellar velocity dispersions for 85 globular clusters (GCs) and one ultra-compact dwarf galaxy around M 104 using high-resolution multi-object integrated-light spectroscopy with FLAMES/GIRAFFE on the Very Large Telescope. The measured velocity dispersions range from <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>σ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>4</mml:mn> <mml:mo>−</mml:mo> <mml:mn>30</mml:mn> </mml:mrow> </mml:math> km s, with a mean uncertainty of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>Δ</mml:mi> <mml:mi>σ</mml:mi> <mml:mo>=</mml:mo> <mml:mn>2.5</mml:mn> </mml:mrow> </mml:math> km s. For a subset of 77 GCs with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>V</mml:mi> </mml:math> -band magnitudes and reliable velocity dispersion measurements above <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>σ</mml:mi> <mml:mo>&gt;</mml:mo> <mml:mn>4</mml:mn> </mml:mrow> </mml:math> km s, we constructed the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>V</mml:mi> </mml:msub> </mml:math> - <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>σ</mml:mi> </mml:math> relation to measure the distance to M 104, finding <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>D</mml:mi> <mml:mo>=</mml:mo> <mml:mn>9.00</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.29</mml:mn> </mml:mrow> </mml:math> (stat.) <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>±</mml:mo> <mml:mn>0.26</mml:mn> </mml:mrow> </mml:math> (sys.) Mpc. The GCs follow the Milky Way and M 31 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>V</mml:mi> </mml:msub> <mml:mo>−</mml:mo> <mml:mi>σ</mml:mi> </mml:mrow> </mml:math> 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mi>V</mml:mi> </mml:math> -band dynamical mass-to-light ratios (M/L). We find a mean <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mo>&lt;</mml:mo> <mml:mi>M</mml:mi> <mml:mi>/</mml:mi> <mml:msub> <mml:mi>L</mml:mi> <mml:mi>V</mml:mi> </mml:msub> <mml:mo>&gt;</mml:mo> <mml:mo>=</mml:mo> <mml:mn>2.6</mml:mn> <mml:mspace width="0.167em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mi>/</mml:mi> <mml:msub> <mml:mi>L</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> </mml:mrow> </mml:math> for the luminous ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>V</mml:mi> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mo>−</mml:mo> <mml:mn>8</mml:mn> </mml:mrow> </mml:math> 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 M 104 follow the GCVD relation irrespective of their mass-to-light ratio.