arXiv subjects

Zhen Wan

Publications and source records attributed to Zhen Wan.

59 records · Page 4Linked to original sources

arXiv2019

The Great Escape: Discovery of a nearby 1700 km/s star ejected from the Milky Way by Sgr A*

We present the serendipitous discovery of the fastest Main Sequence hyper-velocity star (HVS) by the Southern Stellar Stream Spectroscopic Survey (S5). The star S5-HVS1 is a $\sim 2.35$ M$_\odot$ A-type star located at a distance of $\sim 9$ kpc from the Sun and has a heliocentric radial velocity of $1017\pm 2.7$ km/s without any signature of velocity variability. The current 3-D velocity of the star in the Galactic frame is $1755\pm50$ km/s. When integrated backwards in time, the orbit of the star points unambiguously to the Galactic Centre, implying that S5-HVS1 was kicked away from Sgr A* with a velocity of $\sim 1800$ km/s and travelled for $4.8$ Myr to its current location. This is so far the only HVS confidently associated with the Galactic Centre. S5-HVS1 is also the first hyper-velocity star to provide constraints on the geometry and kinematics of the Galaxy, such as the Solar motion $V_{y,\odot}= 246.1\pm 5.3$ km/s or position $R_0=8.12\pm 0.23$ kpc. The ejection trajectory and transit time of S5-HVS1 coincide with the orbital plane and age of the annular disk of young stars at the Galactic centre, and thus may be linked to its formation. With the S5-HVS1 ejection velocity being almost twice the velocity of other hyper-velocity stars previously associated with the Galactic Centre, we question whether they have been generated by the same mechanism or whether the ejection velocity distribution has been constant over time.

astro-ph.GA↗

arXiv2019

The Pristine Inner Galaxy Survey (PIGS) I: Tracing the kinematics of metal-poor stars in the Galactic bulge

Our Galaxy is known to contain a central boxy/peanut-shaped bulge, yet the importance of a classical, pressure-supported component within the central part of the Milky Way is still being debated. It should be most visible at low metallicity, a regime that has not yet been studied in detail. Using metallicity-sensitive narrow-band photometry, the Pristine Inner Galaxy Survey (PIGS) has collected a large sample of metal-poor ([Fe/H] < -1.0) stars in the inner Galaxy to address this open question. We use PIGS to trace the metal-poor inner Galaxy kinematics as function of metallicity for the first time. We find that the rotational signal decreases with decreasing [Fe/H], until it becomes negligible for the most metal-poor stars. Additionally, the velocity dispersion increases with decreasing metallicity for -3.0 < [Fe/H] < -0.5, with a gradient of -44 $\pm$ 4 km$\,$s$^{-1}\,$dex$^{-1}$. These observations may signal a transition between Galactic components of different metallicities and kinematics, a different mapping onto the boxy/peanut-shaped bulge for former disk stars of different metallicities and/or the secular dynamical and gravitational influence of the bar on the pressure-supported component. Our results provide strong constraints on models that attempt to explain the properties of the inner Galaxy.

astro-ph.GA↗

arXiv2019

The Globular Cluster Population of NGC 1052-DF2: Evidence for Rotation

Based upon the kinematics of ten globular clusters, it has recently been claimed that the ultra-diffuse galaxy, NCD 1052-DF2, lacks a significant quantity of dark matter. Dynamical analyses have generally assumed that this galaxy is pressure supported, with the relatively small velocity dispersion of the globular cluster population indicating the deficit of dark matter. However, the presence of a significant rotation of the globular cluster population could substantially modify this conclusion. Here we present the discovery of such a signature of rotation in the kinematics of NGC 1052-DF2's globular clusters, with a velocity amplitude of $\sim12.44^{+4.40}_{-5.16}$ km/s, which, through Bayesian model comparison, represents a marginally better fit to the available kinematic data; note that this rotation is distinct from, and approximately perpendicular to, the recently identified rotation of the stellar component of NGC 1052-DF2. Assuming this truly represents an underlying rotation, it is shown that the determined mass depends upon the inclination of the rotational component and, with a moderate inclination, the resultant mass to light ratio can exceed $M/L\sim10$.

astro-ph.GA↗

arXiv2019

Two major accretion epochs in M31 from two distinct populations of globular clusters

Large galaxies grow through the accumulation of dwarf galaxies. In principle it is possible to trace this growth history using the properties of a galaxy's stellar halo. Previous investigations of the galaxy M31 (Andromeda) have shown that outside a radius of 25 kpc the population of halo globular clusters is rotating in alignment with the stellar disk, as are more centrally located clusters. The M31 halo also contains coherent stellar substructures, along with a smoothly distributed stellar component. Many of the globular clusters outside 25 kpc are associated with the most prominent substructures, while others are part of the smooth halo. Here we report a new analysis of the kinematics of these globular clusters. We find that the two distinct populations are rotating with perpendicular orientations. The rotation axis for the population associated with the smooth halo is aligned with the rotation axis for the plane of dwarf galaxies that encircles M31. We interpret these separate cluster populations as arising from two major accretion epochs, likely separated by billions of years. Stellar substructures from the first epoch are gone, but those from the more recent second epoch still remain.

astro-ph.GA↗

arXiv2018

Galactic cartography with SkyMapper: I. Population sub-structure and the stellar number density of the inner halo

The stars within our Galactic halo presents a snapshot of its ongoing growth and evolution, probing galaxy formation directly. Here, we present our first analysis of the stellar halo from detailed maps of Blue Horizontal Branch (BHB) stars drawn from the SkyMapper Southern Sky Survey. To isolate candidate BHB stars from the overall population, we develop a machine-learning approach through the application of an Artificial Neural Network (ANN), resulting in a relatively pure sample of target stars. From this, we derive the absolute $u$ magnitude for the BHB sample to be $\sim2\ mag$, varying slightly with $(v-g)_0$ and $(u-v)_0$ colours. We examine the BHB number density distribution from 5272 candidate stars, deriving a double power-law with a break radius of $r_s = 11.8\pm0.3\ kpc$, and inner and outer slopes of $α_{in} = -2.5\pm0.1$ and $α_{out} = -4.5\pm0.3$ respectively. Through isochrone fitting of simulated BHB stars, we find a colour-age/metallicity correlation, with older/more metal-poor stars being bluer, and establish a parameter to indicate this age (or metallicity) variation. Using this, we construct the three-dimensional population distribution of BHB stars in the halo and identify significant substructure. Finally, in agreement with previous studies, we also identify a systemic age/metallicity shift spanning $\sim3\ kpc$ to $ \sim20\ kpc$ in galactocentric distance.

astro-ph.GA↗