High‐Precision Stellar Radial Velocities in the Galactic Center

Abstract

We present radial velocities for 85 cool stars projected onto the central parsec of the Galaxy. The majority of these velocities have relative errors of $1 km s À1 , or a factor of $30-100 smaller than those previously obtained with proper-motion or other radial velocity measurements for a similar stellar sample. The error in a typical individual stellar velocity, including all sources of uncertainty, is 1.7 km s À1 . Two similar data sets were obtained 1 month apart, and the total error in the relative velocities is 0.80 km s À1 in the case where an object is common to both data sets. The data are used to characterize the velocity distribution of the old population in the Galactic center. We find that the stars have a Gaussian velocity distribution with a mean heliocentric velocity of À10:1 AE 11:0 km s À1 (blueshifted) and a standard deviation of 100:9 AE 7:7 km s À1 ; the mean velocity of the sample is consistent with no bulk line-of-sight motion with respect to the local standard of rest. At the 1 level, the data are consistent with a symmetric velocity distribution about any arbitrary axis in the plane of the sky. We find evidence for a flattening in the distribution of late-type stars within a radius of $0.4 pc and infer a volume density distribution of r À1/4 in this region. Finally, we establish a first epoch of radial velocity measurements that can be compared with subsequent epochs to measure small accelerations (1 km s À1 yr À1 ), corresponding to the magnitude expected over a time span of several years for stars nearest to Sgr A*.

Composition of the galactic center star cluster

“…These values give an even stronger support to the proposed hole/dip in the center: we find a power law index of −0.70 ± 0.09 for the outer region, while the inner region even shows a decline towards the center (0.17 ± 0.09). This is a very interesting result, since it shows that the previously observed flattening of the density profile of the total population (Genzel et al 1996;Figer et al 2003;Genzel et al 2003;Schödel et al 2007;Zhu et al 2008) is an even stronger feature in the late type population. We can therefore assume that the stellar population in the innermost ∼0.2 pc is indeed depleted not only of bright giants, but also of fainter giants down to our magnitude limit of 15.5 mag.…”

“…To the contrary, the late-type, old stellar population, which appears to make up the vast majority of stars in the GC cluster, shows a flat or even slightly inverted power-law in projection within about 0.2 pc from Sgr A*. This means that there is some kind of hole in the late-type population near the center, as has been pointed out by Figer et al (2003). The exact cause is still not understood, but various explanations have been suggested.…”

“…evidence for a flattened distribution of late type stars within 0.4 pc (∼10 ). As Figer et al (2003) and Zhu et al (2008) point out, the flat projected surface density profile of the late-type stars implies in fact a hole/dip in the 3-dimensional distribution of the late-type stars. Schödel et al (2007) described the total population of the central parsec with a broken power law (break radius R break = 6 .0 ± 1 .0, β = −0.19 ± 0.05 within R break and β = −0.75 ± 0.10 outside of the break radius) for a magnitude limit of 17.75 and using completeness corrected data.…”

The nuclear star cluster of the Milky Way: proper motions and mass

“…(2) contains only limited information about the distribution of the mass. In addition, as emphasized by Genzel et al (2000), Figer et al (2003) and Zhu et al (2008), moment estimators must be applied cautiously in cases where data do not extend over the entire system, or where the observed sample is dominated by stars that are intrinsically far from (but projected near to) the center. Figure 11 shows the result of computing (2) as a function of the outer radius of the sample.…”

“…This leads inevitably to an under-estimation of the 3D distances of the tracer stars from the BH because the number density behaves rather like n(r) ∝ r −1.2 in the central parsec (Schödel et al 2007). The issue becomes even more important when the analysis is limited to the late-type stars near Sgr A*, whose density may be even decreasing toward the black hole (see, e.g., Figer et al 2003;Genzel et al 2003;Buchholz et al 2009). In our analysis we have assumed n(r) ∝ r 0.5 for the late-type stars near Sgr A*.…”