An Extremely Top-Heavy IMF in the Galactic Center Stellar Disks (0908.2177v2)

Abstract: We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the spectroscopic detection of early and late type stars to m_K >= 16, more than 2 magnitudes deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O- and B- stars reside in two strongly warped disks between 0.8" and 12" from SgrA*, as well as a central compact concentration (the S-star cluster) centered on SgrA*. The later type B stars (m_K>15) in the radial interval between 0.8" and 12" seem to be in a more isotropic distribution outside the disks. The observed dearth of late type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best fit power law of dN/dm ~ m^-0.45+/-0.3. Since at least the WR/O-stars were formed in situ in a single star formation event ~6 Myrs ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 0.8" and of the early-type stars at distances beyond 12" are compatible with a standard Salpeter/Kroupa IMF (best fit power law of dN/dm ~ m^-2.15+/-0.3).

• The paper demonstrates that the stellar mass function in the Galactic Center disks follows a power-law index of -0.45, indicating a striking top-heavy distribution.
• It employs adaptive optics-assisted spectroscopy to expand the early-type star sample to 177, enabling detailed insights into stellar dynamics near Sgr A*.
• The findings imply a massive, short-lived starburst about 6 million years ago, questioning standard star formation models under extreme tidal forces.

Analysis of an Extremely Top-Heavy Initial Mass Function in the Galactic Center Stellar Disks

The paper by Bartko et al. explores the stellar population in the central parsec of the Galaxy, particularly focusing on the initial mass function (IMF) in the stellar disks near the supermassive black hole, Sgr A*. Utilizing high-resolution spectroscopic data from the ESO/VLT SINFONI instrument, the authors have significantly expanded the sample of identified early-type stars, reaching unprecedented depths in their observations.

The central premise of the paper lies in the characterization of the initial mass function of stars in the Galactic Center (GC). The authors find that the stellar mass function for the early-type stars in the two warped disks between 0.8" and 12" from Sgr A* is extremely top-heavy. The best fit follows a power law of $\mathrm{d}N/\mathrm{d}m \propto m^{-0.45\pm0.3}$, distinctly deviating from the standard Salpeter/Kroupa IMF often observed in other regions of the Galaxy.

Key Findings

1. Spectroscopic Observations and Sample Size: The paper leverages adaptive optics-assisted observations to detect early and late-type stars with a significant increase in the detection limit compared to previous data sets. This has resulted in a robust sample size of 177 bona fide early-type stars, facilitating a detailed analysis.
2. Stellar Distribution and Dynamics: The majority of the detected Wolf-Rayet (WR), O-, and B-type stars are distributed within two highly warped disks and a central compact S-star cluster near Sgr A*. The B-type stars in the more extended radial regions (0.8" to 12") display an isotropic distribution, contrasting with the structured nature of the disk stars.
3. Absence of Late-Type Stars: The observed paucity of late-type stars near the central black hole (even accounting for stellar collisions) presents a puzzling dynamic. This absence contributes to the lack of a classical stellar cusp in the GC, challenging existing models of stellar dynamics around supermassive black holes.
4. Top-Heavy Initial Mass Function: The mass function of disk stars indicates a top-heavy IMF, suggesting that the star formation in this region favored the formation of massive stars. The observed IMF of the disk stars is indicative of a powerful, short-lived starburst that occurred roughly 6 million years ago.
5. Contrast with External Regions: Beyond 12", the IMF of field stars aligns with a typical Salpeter/Kroupa power-law slope $\mathrm{d}N/\mathrm{d}m \propto m^{-2.15\pm0.3}$, supporting the existence of significant differences in the star formation processes within and beyond the disks.

Implications and Future Directions

The revelation of a top-heavy IMF in the nuclear stellar disks implies significant constraints on the star formation histories and conditions in the GC. If star formation is indeed occurring so close to the supermassive black hole, it challenges the conventional understanding of star formation under extreme tidal forces. The mass segregation and dynamical processing unique to the GC environment can potentially alter the typical star formation pathways, resulting in an environment fostering more massive stars.

Future research avenues could focus on characterizing the mechanisms that lead to such a top-heavy IMF and dissecting the gas dynamics preceding star formation in these environments. Additionally, understanding the absence of late-type stars and its correlation to dynamical heating or previous disruptive events remains a critical mystery to unravel. The paper opens pathways to explore multifaceted questions about star formation in the most extreme environments in the universe, influencing both theoretical models of star clusters around supermassive black holes and practical approaches in extragalactic observations.