Progress has been made in the structural study of the Andromeda Galaxy (M31).

Recently, the Shanghai Astronomical Observatory of the Chinese Academy of Sciences and Shanghai Jiao Tong University have made important progress in the study of the structure of the Andromeda Galaxy (M31). The study proposes a new way of searching for shock waves in interstellar gas observation data, thus giving independent evidence that M31 galaxy is a barred spiral galaxy rather than a normal spiral galaxy, and using fluid dynamics simulation to reproduce the main observational characteristics of shock waves in gas. The results were published in The Astrophysical Journal. The results were selected as a research highlight by the American Astronomical Society’s (AAS) Nova website on July 25, with the headline “A Bar in the Andromeda Galaxy” as the title of “A Bar in the Andromeda Galaxy”.

Spiral galaxies are divided into normal spiral galaxies and barred spiral galaxies. The bar at the center of a barred galaxy is a long strip of stars that is an important internal factor driving the long-term slow-moving evolution of spiral galaxies. M31 is the closest spiral galaxy to us. For a long time, astronomers have tried to determine the galaxy morphology of M31 to give its position in the famous Hubble Galaxy classification map. M31, on the other hand, is almost a lateral galaxy relative to our line of sight, making it difficult to determine the structural features of its interior directly from the image of the galaxy. Previously, astronomers proposed that M31 might contain a galaxy bar based on the distortion of the star’s isotropic lines, but this phenomenon may not only be produced by the rod, but also by an ellipsoidal nuclear ball that does not rotate. Gas observation data also suggest that M31 may have rods, such as significant non-circular motion of gases, distorted zero velocity lines, etc. However, other mechanisms such as the process of merging with another galaxy can also lead to similar characteristics. As a galaxy with significant classical nuclear sphere composition, there is controversy over whether M31 is a barred spiral galaxy, and determining the internal structure of M31 will help astronomers better explain the structural evolution of our nearby galaxies.

To solve this problem, the research team proposed a new idea of searching for shock waves in interstellar gas observation data, thus giving independent evidence that the M31 galaxy is a barred spiral galaxy rather than an ordinary spiral galaxy, and using fluid dynamics simulations to reproduce the main observational characteristics of shock waves in gas.

When the rods of spiral galaxies drive interstellar media inflows, they trigger shock waves. Such shock waves produce one of the most notable features of barred spiral galaxies — the formation of a pair of dust bands on the leading side of the rod that is about the same scale as the bar. Shen Juntai, a professor at Shanghai Jiao Tong University, said that the shock wave will show a sharp speed jump on the position-velocity diagram. This violent velocity jump feature can be captured and resolved by an integral field unit (IFU). If these shock characteristics conform to the laws of barred spiral galactic shocks, then it can be clearly proved that there are rods in the M31 galaxy.

Based on this idea, the research team used the latest integral field of view spectrometer VIRUS-W to emit ionized oxygen gas emission lines in M31[OIII]The observational data, combined with the data of neutral hydrogen atom gas (HI), extracted the position-gaze velocity map perpendicular to different slices in the direction of the main axis of the galaxy disk, and finally identified the M31 galaxy by edge detection algorithm[OIII]and shock signatures in HI data (Figure 1). The study found that these shock features are more regularly distributed on scales of the order of thousandsecec (kpc) (Figure 2). Currently, the latest stellar dynamics models suggest that the rod spindle angle of the M31 galaxy differs from the disk spindle by about 17 degrees. If this hypothesis holds, then the shock wave features are indeed mainly distributed on the leading side of the bar, which is consistent with theoretical expectations of bar-spinning galaxies. “We found that these shock features are mainly distributed in the nuclear sphere region of M31, with the strongest speed jump of more than 170 km/s and a velocity gradient of 1.2 km/sec.” Feng Zixuan, a doctoral student at the Shanghai Astronomical Observatory, said, “The question we face is whether numerical simulations of fluids based on the rod potential field of rotating galaxies can reproduce such sharp shock characteristics. “The study combined with the latest stellar dynamics models to simulate different rod speeds, effective sound velocities of gases, and gas motion from an observational perspective, and finally obtained a model that is basically consistent with the observations (Figure 3). The shock position and velocity jump characteristics in the model are basically the same as those in the observations, and the model rod has a rotational speed of 20 km/sec/parseec and an effective sound velocity of 30 km/sec for gases. The azimuth and gas disc inclination of the rod spindle are 54.7 degrees and 77 degrees, respectively. The study also tested the analogy of an ellipsoidal nuclear ball structure with a non-rotating rod and found that a non-rotating rod could not produce shock waves and did not have obvious speed jump characteristics. These findings further suggest that M31 has a rotating centerbar rather than a static ellipsoidal nuclear ball.

Figure 1. The M31 galaxy is on our side[OIII]Shock characteristics. Secondary ionized oxygen with a wavelength of about 500 nanometers[OIII]The dual emission line is the forbidden line in the visible spectrum, which can only occur in a very low-density cosmic environment, and is one of the most important emission lines in the wavelength range of the VIRUS-W spectrometer. Data points represent[OIII]of observations, the color represents the flow density. Each subgraph corresponds to a slice perpendicular to the disk spindle. The X represents the position of the slice on the disk spindle. The black curve represents the result of the data points being smoothed. The thick red, thin, and dashed lines represent the strongest, stronger, and weaker shock characteristics, respectively. Most shock features are distributed at the distal end of the galaxy (below the disk spindle).

Figure 2.[OIII]and hi shock characteristics in the spatial distribution of the M31 galaxy. Background optical images are derived from the Hubble Space Telescope, subaru telescope, and Mayall telescope. The red circle and the blue triangle represent each other[OIII]and the location of the shock wave in HI. The signs represented by solid, hollow, and imaginary boxes represent the strongest, stronger, and weaker shock characteristics, respectively. The dashed line represents the bar spindle orientation in the latest dynamic model.

Figure 3. Gas dynamics model of the M31 galaxy. The left shows the gas ish density distribution after the M31 model is projected to the sky plane (upper left corner) and before projection (lower left corner). The pink circle and purple triangle are represented separately[OIII]and the location of shock waves in HI data. The right side shows the location-looking velocity plot for the different slices, with the black data points representing the velocity distribution of the gases in the model, and the red and blue data points representing each other[OIII]and OBSERVATIONal data of HI. The lower right corner is an enlarged version of the image at the shock in the upper right corner, and the dotted line represents the rod model without rotation.

This study provides clear observational evidence of the rod structure of the M31 galaxy, which helps to reveal the structure formation and dynamic evolution history of M31, and the determination of the rod structure of M31 will help astronomers better explain the structural evolution of our nearby galaxies. The researchers will compare the observed gas characteristics with more gas dynamics simulations in follow-up studies in order to better elucidate the gas characteristics of the M31 center and clarify the main parameters of the M31 rod.

The research work has been supported by the National Natural Science Foundation of China, the Ministry of Science and Technology, and Shanghai Jiao Tong University. The numerical simulations for this work use the Cluster Cluster of the Shanghai Astronomical Observatory and the Gravity Cluster of the Department of Astronomy of Shanghai Jiao Tong University. The Ortwin Gerhard group at the Institute of Extraterrestrial Physics of the Max Planck Association for the Advancement of Science in Germany participated in the research. (Source: Shanghai Astronomical Observatory, Chinese Academy of Sciences)

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