The Stellar Population in the Galactic Center

The Closest Look at Young S-Stars near the Black Hole

Within a distance of 0.04 pc from the the super massive black hole (SMBH) in the center of our Galaxy exists a group of young stars. Given how inhospitable the region is for star formation, their presence is more puzzling the younger we estimate their ages (Eisenhauer et al. 2005, Martins et al. 2008). It is highly unlikely that the young S-stars formed at their present location, since the SMBH’s tidal forces are too strong to allow star formation at these distances. With all the observational constraints and theoretical complexities the question of the origin and distribution of young stars in the GC has become one of the most remarkable issues in this field.

Combined spectra (2004-2016) of eight young S-stars (K<15.5 mag) in K-band

Combined spectra (2004-2016) of eight young S-stars (K <15.5 mag) in H-band. The most prominent absorption features are He I(1.7002μm) and Brackett lines (Br10-Br15).


By co-adding up to 105 hr of spectra we have obtained high signal-to-noise H- and K-band spectra of eight brightest stars in the field orbiting the SMBH. The deep H- and K-band spectra of S2 (K-band S/N: 480 and H-bandS/N: 280), S4, and also combined spectrum of other fainter S-stars, disclose a clear Stark broadening in the Brackett lines that implies high surface gravity of S-stars. This finding is established by our detailed stellar atmospheric+evolutionary model analysis, which employs line profiles of the complete Brackett series(excellent indicators of gravity)in the H- and K-bands.

HR diagram of young bright S-stars in theTeff−log(L) plane. The black crosses mark the fitted stellar parameters for the S-stars on the HR diagram in theTeff−log(L) plane. The stellar models for the Milky way (Brott et al. 2011) and solar metallicity rotating Geneva isochrones (Ekstrom et al. 2012) of 3-40 Myr areshown in respectively solid blue and dashed orange lines.

We derive an age of 6.6(+4.7- 3.4) Myr for S2. With higher uncertainties, we estimate the age range of the other studied S-stars to be less than 15 Myr. The relatively low ages for these S-stars favor a scenario in which the stars formed in a local disk rather than a field binary-disruption scenario that occurred over a longer period of time. Although the proposed scenarios so far show that a disk origin for S-stars is possible, it is unclear whether the necessary conditions predicted by different scenarios are fulfilled in the GC.

Spectroscopic Detection of a Cusp of Late-Type Stars around the Central Black Hole in the Milky Way


Theoretical stellar dynamics predicts the formation of a dense stellar cusp of old stars within a dynamically relaxed cluster around a massive black hole. Such a cusp has so far escaped unambiguous observational confirmation specially within the central 0.5 pc (Bartko et al. 2010). Due to the high extinction and extreme stellar crowding, Particularly the increasing density of young stars toward the center, it is observationally challenging to confirm whether or not a stellar cusp exists.

By co-adding spectroscopic observations taken over a decade, we identify new late-type stars within the central 0.2 pc of the Galaxy. The unique advantage of our spectroscopic study is that through an individual age estimation, we can select a stellar population that is old enough to have undergone dynamical relaxation.

Example combined spectra of giant stars within the central 0.2 pc. By combining multi-epoch AO-assisted spectroscopic observations we study stars in the vicinity of SgrA* down to 17.5 mag. We detect the first five warm giants of G2-G8III type in the region. High S/N spectra enable us to assign temperatures from the CO bandheads of these stars.



Surface density profiles of late-type stars (mK<17) out to distances of 10 arcsec (0.4 pc).We find a cusp structure in the surface number density of the old giants population within 0.02-0.4 pc described by a single power law with an exponent Γ= 0.34 ± 0.04 (blue dashed line). For the central 1 arcsec(red star), we use our star count with100% photometric and spectral classification completeness. For larger radii(2–10 arcsec)around SgrA*we use the data from Bartko et al. 2010­correctedfor the small fraction of young stars. The gray points illustrate distribution of the brightand faint giants from Gallego-Cano et al.(2018).

The updated star count, based on individual spectral classification, is used to reconstruct the surface density profile of giant stars. Our study, for the first time, finds a cusp in the surface number density of the spectroscopically identified old (>3 Gyr) giants population (mK<17) within 0.02-0.4 pc described by a single power law with an exponent Γ= 0.34 ± 0.04.


At larger distances (r > 1"), we find another population of stars: Even more massive, even younger O- or WR-type stars. A large fraction of these stars move coherently, the most prominent feature being a warped disk formed by the clock-wise moving stars (Paumard et al. 2006, Bartko et al. 2009). Except for eight stars, we cannot detect the weak accelerations anymore at these radii. With 2D positions, proper motion and radial velocity we can measures only five dynamic quantities, where six would fully determine the orbit. However, taking an ensemble of stars, we can show in a statistical sense that a large fraction of them moves in a disk. The following plot is the same projection as for the S-stars, and shows the probability distribution of the angular momentum vector as constrained by the dynamic data of the young, massive stars between 1" < r < 3.5" and with mK < 14.5. One can see the (significant) overdensity defining the clockwise stellar disk.

Probability density distribution for the orientations of the angular momentum vectors of the young, massive stars in the Galactic Center with mK < 14.5 and 1" < r < 4". There is a preferred direction, defining the clockwise disk of stars.

Further, we have shown that the IMF of these massive young stars is very top-heavy (Bartko et al. 2010)

Luminosity and mass function of the young, massive stars in the Galactic Center. The stars in the disk regime (blue) have a very top-heavy mass function, opposite to the S-stars (red) and the stars at larger radii (black).

The stars in the clockwise disk are even younger than the S-stars. Yet, their formation history appears to be solved: They have formed roughly 6 million years ago from a massive (around 100000 solar masses) molecular cloud falling into the Galactic Center. Simulations show that the gas will circularize, and form stars in a very unusual mode. The simulations show that such event results in the observed disk structure, the top-heavy mass function and the observed density profile.

Simulation of the infall of a massive molecular cloud into the Galactic Center region (Bonnell & Rice 2009).


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