Browsing by Author "Gerhard, Ortwin"

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    Bar-driven Gas Dynamics of M31
    (2024) Feng, Zi-Xuan; Li, Zhi; Shen, Juntai; Gerhard, Ortwin; Saglia, R. P.; Blana, Matias; Li, Hui; Jing, Yingjie
    The large-scale gaseous shocks in the bulge of M31 can be naturally explained by a rotating stellar bar. We use gas dynamical models to provide an independent measurement of the bar pattern speed in M31. The gravitational potentials of our simulations are from a set of made-to-measure models constrained by stellar photometry and kinematics. If the inclination of the gas disk is fixed at i = 77 degrees, we find that a low pattern speed of 16-20 km s-1 kpc-1 is needed to match the observed position and amplitude of the shock features, as shock positions are too close to the bar major axis in high omega b models. The pattern speed can increase to 20-30 km s-1 kpc-1 if the inner gas disk has a slightly smaller inclination angle compared with the outer one. Including subgrid physics such as star formation and stellar feedback has minor effects on the shock amplitude, and does not change the shock position significantly. If the inner gas disk is allowed to follow a varying inclination similar to the H i and ionized gas observations, the gas models with a pattern speed of 38 km s-1 kpc-1, which is consistent with stellar-dynamical models, can match both the shock features and the central gas features.
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    Large-scale Hydrodynamical Shocks as the Smoking-gun Evidence for a Bar in M31
    (2022) Feng, Zi-Xuan; Li, Zhi; Shen, Juntai; Gerhard, Ortwin; Saglia, R. P.; Blana, Matias
    The formation and evolutionary history of M31 are closely related to its dynamical structures, which remain unclear due to its high inclination. Gas kinematics could provide crucial evidence for the existence of a rotating bar in M31. Using the position-velocity diagram of [O III] and H I, we are able to identify clear sharp velocity jump (shock) features with a typical amplitude over 100 km s(-1) in the central region of M31 (4.6 kpc x 2.3 kpc, or 20' x 10'). We also simulate gas morphology and kinematics in barred M31 potentials and find that the bar-induced shocks can produce velocity jumps similar to those in [O III]. The identified shock features in both [O III] and H I are broadly consistent, and they are found mainly on the leading sides of the bar/bulge, following a hallmark pattern expected from the bar-driven gas inflow. Shock features on the far side of the disk are clearer than those on the near side, possibly due to limited data coverage on the near side, as well as to obscuration by the warped gas and dust layers. Further hydrodynamical simulations with more sophisticated physics are desired to fully understand the observed gas features and to better constrain the parameters of the bar in M31.
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    The Hercules stream as seen by APOGEE-2 South
    (2018) Hunt, Jason A. S.; Bovy, Jo; Perez-Villegas, Angeles; Holtzman, Jon A.; Sobeck, Jennifer; Chojnowski, Drew; Santana, Felipe A.; Palicio, Pedro A.; Wegg, Christopher; Gerhard, Ortwin; Almeida, Andres; Bizyaev, Dmitry; Fernandez-Trincado, Jose G.; Lane, Richard R.; Pelope Longa-Pena, Pen; Majewski, Steven R.; Pan, Kaike; Roman-Lopes, Alexandre
    The Hercules stream is a group of comoving stars in the solar neighbourhood, which can potentially be explained as a signature of either the outer Lindblad resonance (OLR) of a fast Galactic bar or the corotation resonance (CR) of a slower bar. In either case, the feature should be present over a large area of the disc. With the recent commissioning of the APOGEE-2 Southern spectrograph we can search for the Hercules stream at (l, b)=(270 degrees, 0), a direction in which the Hercules stream, if caused by the bar's OLR, would be strong enough to be detected using only the line-of-sight velocities. We clearly detect a narrow, Hercules-like feature in the data that can be traced from the solar neighbourhood to a distance of about 4 kpc. The detected feature matches well the line-of-sight velocity distribution from the fast-bar (OLR) model. Confronting the data with a model where the Hercules stream is caused by the CR of a slower bar leads to a poorer match, as the corotation model does not predict clearly separated modes, possibly because the slow-bar model is too hot.