The Nuclear Cluster of the Milky Way: Star Formation and Velocity Dispersion in the Central 0.5 Parsec

Context. The environment of Sagittarius A* (Sgr A*), the central black hole of the Milky Way, is the only place in the Universe where we can currently study the interaction between a nuclear star cluster and a massive black hole and infer the properties of a nuclear cluster from observations of individual stars. Aims. This work aims to explore the star formation history of the nuclear cluster and the structure of the innermost stellar cusp around Sgr A*. Methods. We combined and analysed multi epoch high quality AO observations. For the region close to Sgr A* we apply the speckle holography technique to the AO data and obtain images that are ≥50% complete down to Ks ≈ 19 within a projected radius of 5″ around Sgr A*. We used H-band images to derive extinction maps. Results. We provide Ks photometry for roughly 39 000 stars and H-band photometry for ∼11 000 stars within a field of about 40″ × 40″, centred on Sgr A*. In addition, we provide Ks photometry of ∼3000 stars in a very deep central field of 10″ × 10″, centred on Sgr A*. We find that the Ks luminosity function (KLF) is rather homogeneous within the studied field and does not show any significant changes as a function of distance from the central black hole on scales of a few 0.1 pc. By fitting theoretical luminosity functions to the KLF, we derive the star formation history of the nuclear star cluster. We find that about 80% of the original star formation took place 10 Gyr ago or longer, followed by a largely quiescent phase that lasted for more than 5 Gyr. We clearly detect the presence of intermediate-age stars of about 3 Gyr in age. This event makes up about 15% of the originally formed stellar mass of the cluster. A few percent of the stellar mass formed in the past few 100 Myr. Our results appear to be inconsistent with a quasi-continuous star formation history. The mean metallicity of the stars is consistent with being slightly super solar. The stellar density increases exponentially towards Sgr A* at all magnitudes between Ks = 15−19. We also show that the precise properties of the stellar cusp around Sgr A* are hard to determine because the star formation history suggests that the star counts can be significantly contaminated, at all magnitudes, by stars that are too young to be dynamically relaxed. We find that the probability of observing any young (non-millisecond) pulsar in a tight orbit around Sgr A* and beamed towards Earth is very low. We argue that typical globular clusters, such as they are observed in and around the Milky Way today, have probably not contributed to the nuclear cluster’s mass in any significant way. The nuclear cluster may have formed following major merger events in the early history of the Milky Way.

Download Full-text

Are ultra-diffuse galaxies Milky Way-sized?

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936821 ◽

2020 ◽

Vol 633 ◽

pp. L3 ◽

Cited By ~ 5

Author(s):

Nushkia Chamba ◽

Ignacio Trujillo ◽

Johan H. Knapen

Keyword(s):

Star Formation ◽

Milky Way ◽

Size Range ◽

Mass Density ◽

Effective Radius ◽

Density Profiles ◽

Gas Density ◽

Undesirable Consequence ◽

Definition Of ◽

Density Threshold

Now almost 70 years since its introduction, the effective or half-light radius has become a very popular choice for characterising galaxy size. However, the effective radius measures the concentration of light within galaxies and thus does not capture our intuitive definition of size which is related to the edge or boundary of objects. For this reason, we aim to demonstrate the undesirable consequence of using the effective radius to draw conclusions about the nature of faint ultra-diffuse galaxies (UDGs) when compared to dwarfs and Milky Way-like galaxies. Instead of the effective radius, we use a measure of galaxy size based on the location of the gas density threshold required for star formation. Compared to the effective radius, this physically motivated definition places the sizes much closer to the boundary of a galaxy. Therefore, considering the sizes and stellar mass density profiles of UDGs and regular dwarfs, we find that the UDGs have sizes that are within the size range of dwarfs. We also show that currently known UDGs do not have sizes comparable to Milky Way-like objects. We find that, on average, UDGs are ten times smaller in extension than Milky Way-like galaxies. These results show that the use of size estimators sensitive to the concentration of light can lead to misleading results.

Download Full-text

Probing non-spherical dark halos in the Galactic dwarf satellites

Proceedings of the International Astronomical Union ◽

10.1017/s1743921313006789 ◽

2013 ◽

Vol 9 (S298) ◽

pp. 411-411

Author(s):

Kohei Hayashi ◽

Masashi Chiba

Keyword(s):

Dark Matter ◽

Cold Dark Matter ◽

Milky Way ◽

Velocity Dispersion ◽

Line Of Sight ◽

Spherical Structure ◽

Spherical Models ◽

Galactic Satellites ◽

Best Fitting

AbstractWe construct axisymmetric mass models for dwarf spheroidal (dSph) galaxies in the Milky Way to obtain realistic limits on the non-spherical structure of their dark halos. This is motivated by the fact that the observed luminous parts of the dSphs are actually non-spherical and cold dark matter models predict non-spherical virialized dark halos on sub-galactic scales. Applying these models to line-of-sight velocity dispersion profiles along three position angles in six Galactic satellites, we find that the best fitting cases for most of the dSphs yield not spherical but oblate and flattened dark halos. We also find that the mass of the dSphs enclosed within inner 300 pc varies depending on their total luminosities, contrary to the conclusion of previous spherical models. This suggests the importance of considering non-spherical shapes of dark halos in dSph mass models.

Download Full-text

Resolved Gas Kinematics in a Sample of Low-Redshift High Star-Formation Rate Galaxies

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2016.3 ◽

2016 ◽

Vol 33 ◽

Cited By ~ 10

Author(s):

Mathew Varidel ◽

Michael Pracy ◽

Scott Croom ◽

Matt S. Owers ◽

Elaine Sadler

Keyword(s):

Star Formation ◽

Velocity Gradient ◽

Velocity Dispersion ◽

Star Formation Rate ◽

Formation Rate ◽

Local Effect ◽

Line Of Sight ◽

Mean Velocity ◽

Gas Kinematics ◽

Integral Field Spectroscopy

AbstractWe have used integral field spectroscopy of a sample of six nearby (z ~ 0.01–0.04) high star-formation rate ($\text{SFR} \sim 10\hbox{--}40$$\text{M}_\odot \text{ yr$^{-1}$}$) galaxies to investigate the relationship between local velocity dispersion and star-formation rate on sub-galactic scales. The low-redshift mitigates, to some extent, the effect of beam smearing which artificially inflates the measured dispersion as it combines regions with different line-of-sight velocities into a single spatial pixel. We compare the parametric maps of the velocity dispersion with the Hα flux (a proxy for local star-formation rate), and the velocity gradient (a proxy for the local effect of beam smearing). We find, even for these very nearby galaxies, the Hα velocity dispersion correlates more strongly with velocity gradient than with Hα flux—implying that beam smearing is still having a significant effect on the velocity dispersion measurements. We obtain a first-order non parametric correction for the unweighted and flux weighted mean velocity dispersion by fitting a 2D linear regression model to the spaxel-by-spaxel data where the velocity gradient and the Hα flux are the independent variables and the velocity dispersion is the dependent variable; and then extrapolating to zero velocity gradient. The corrected velocity dispersions are a factor of ~ 1.3–4.5 and ~ 1.3–2.7 lower than the uncorrected flux-weighted and unweighted mean line-of-sight velocity dispersion values, respectively. These corrections are larger than has been previously cited using disc models of the velocity and velocity dispersion field to correct for beam smearing. The corrected flux-weighted velocity dispersion values are σm ~ 20–50 km s−1.

Download Full-text

STELLAR VELOCITY DISPERSION AND ANISOTROPY OF THE MILKY WAY INNER HALO

The Astrophysical Journal ◽

10.1088/0004-637x/813/2/89 ◽

2015 ◽

Vol 813 (2) ◽

pp. 89 ◽

Cited By ~ 16

Author(s):

Charles King III ◽

Warren R. Brown ◽

Margaret J. Geller ◽

Scott J. Kenyon

Keyword(s):

Milky Way ◽

Velocity Dispersion ◽

Stellar Velocity

Download Full-text

Star Formation in the Milky Way: The Infrared View

Cosmic Rays in Star-Forming Environments - Astrophysics and Space Science Proceedings ◽

10.1007/978-3-642-35410-6_4 ◽

2013 ◽

pp. 29-40 ◽

Cited By ~ 1

Author(s):

Alberto Noriega-Crespo

Keyword(s):

Star Formation ◽

Milky Way

Download Full-text

On the Origin of a Rotating Metal-poor Stellar Population in the Milky Way Nuclear Cluster

The Astrophysical Journal ◽

10.3847/2041-8213/abb245 ◽

2020 ◽

Vol 901 (2) ◽

pp. L29 ◽

Cited By ~ 1

Author(s):

Manuel Arca Sedda ◽

Alessia Gualandris ◽

Tuan Do ◽

Anja Feldmeier-Krause ◽

Nadine Neumayer ◽

...

Keyword(s):

Stellar Population ◽

Milky Way ◽

Nuclear Cluster

Download Full-text

Autonomous Gaussian decomposition of the Galactic Ring Survey

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038479 ◽

2020 ◽

Vol 640 ◽

pp. A72

Author(s):

M. Riener ◽

J. Kainulainen ◽

J. D. Henshaw ◽

H. Beuther

Keyword(s):

Milky Way ◽

Velocity Dispersion ◽

Galactic Plane ◽

Gas Emission ◽

Spiral Arms ◽

Precise Knowledge ◽

Significant Difference ◽

The Impact ◽

Co Emission ◽

Spiral Arm

Knowledge about the distribution of CO emission in the Milky Way is essential to understanding the impact of the Galactic environment on the formation and evolution of structures in the interstellar medium. However, our current insight as to the fraction of CO in the spiral arm and interarm regions is still limited by large uncertainties in assumed rotation curve models or distance determination techniques. In this work we use the Bayesian approach from Reid et al. (2016, ApJ, 823, 77; 2019, ApJ, 885, 131), which is based on our most precise knowledge at present about the structure and kinematics of the Milky Way, to obtain the current best assessment of the Galactic distribution of 13CO from the Galactic Ring Survey. We performed two different distance estimates that either included (Run A) or excluded (Run B) a model for Galactic features, such as spiral arms or spurs. We also included a prior for the solution of the kinematic distance ambiguity that was determined from a compilation of literature distances and an assumed size-linewidth relationship. Even though the two distance runs show strong differences due to the prior for Galactic features for Run A and larger uncertainties due to kinematic distances in Run B, the majority of their distance results are consistent with each other within the uncertainties. We find that the fraction of 13CO emission associated with spiral arm features ranges from 76 to 84% between the two distance runs. The vertical distribution of the gas is concentrated around the Galactic midplane, showing full-width at half-maximum values of ~75 pc. We do not find any significant difference between gas emission properties associated with spiral arm and interarm features. In particular, the distribution of velocity dispersion values of gas emission in spurs and spiral arms is very similar. We detect a trend of higher velocity dispersion values with increasing heliocentric distance, which we, however, attribute to beam averaging effects caused by differences in spatial resolution. We argue that the true distribution of the gas emission is likely more similar to a combination of the two distance results discussed, and we highlight the importance of using complementary distance estimations to safeguard against the pitfalls of any single approach. We conclude that the methodology presented in this work is a promising way to determine distances to gas emission features in Galactic plane surveys.

Download Full-text

Central Molecular Zone of the Milky Way: Star Formation in an extreme Environment

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316007444 ◽

2015 ◽

Vol 11 (S315) ◽

pp. 163-166

Author(s):

Jens Kauffmann

Keyword(s):

Star Formation ◽

Milky Way ◽

Spatial Scales ◽

Extreme Environment ◽

Average Density ◽

High Mass ◽

Density Structure ◽

Star Formation Activity ◽

Order Of Magnitude ◽

Sgr A

AbstractThe Central Molecular Zone (CMZ; inner ~100 pc) hosts some of the most dense and massive molecular clouds of the Milky Way. These clouds might serve as local templates for dense clouds seen in nearby starburst galaxies or in the early universe. The clouds have a striking feature: they form stars at a very slow pace, considering their mass and high average density. Here we use interferometer data from ALMA and the SMA to show that this slow star formation is a consequence of the cloud density structure: CMZ clouds have a very flat density structure. They might, for example, exceed the average density of the Orion A molecular cloud by an order of magnitude on spatial scales ~5 pc, but CMZ “cores” of ~0.1 pc radius have masses and densities lower than what is found in the Orion KL region. This absence of highest–density gas probably explains the suppression of star formation. The clouds are relatively turbulent, and ALMA observations of H2CO and SiO indicate that the turbulence is induced by high–velocity shocks. We speculate that these shocks might prevent the formation of high–mass cores. It has been argued that the state of CMZ clouds depends on their position along the orbit around Sgr A*. Our incomplete data indicate no evolution in the density structure, and only a modest evolution in star formation activity per unit mass.

Download Full-text