Galaxy shape
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Understanding Galaxy Shapes: Insights from Recent Research
Triaxial Ellipsoids in Galaxy Shapes
Spiral and Elliptical Galaxies in the Sloan Digital Sky Survey
The shapes of galaxies, particularly spiral and elliptical types, have been extensively studied using data from the Sloan Digital Sky Survey (SDSS). Research indicates that elliptical galaxies are generally oblate spheroids, with a correlation between their luminosity and ellipticity. Specifically, the minor to middle axis ratios range from 0.41 for less luminous ellipticals to 0.76 for more luminous ones. This suggests that more luminous elliptical galaxies tend to be rounder1. In contrast, spiral galaxies exhibit significant variations in shape due to the presence of dust, with their intrinsic shapes being consistent with flat discs. The thickness to diameter ratio for these spirals is approximately 21%, and their face-on ellipticity is notably affected by dust extinction1.
Extremely Metal-Poor Galaxies
Extremely metal-poor galaxies (XMPs) provide another perspective on galaxy shapes. These galaxies, which are thought to be in the process of accreting gas from the cosmic web, tend to be triaxial. Smaller XMPs are relatively thicker compared to larger ones, with the shortest axis being about 0.4 times the longest axis for small galaxies and 0.15 for large ones. This triaxiality may be influenced by factors such as gas accretion and star formation processes2.
Brightest Cluster Galaxies and Their Shapes
Brightest cluster galaxies (BCGs) have been found to be strongly triaxial, with triaxiality parameters ranging from 0.39 to 0.72. These galaxies are more spherical in their central regions but become flatter at larger radii. This shape distribution aligns well with simulations of massive galaxies and their dark matter halos, suggesting a potential link to the nature of dark matter3.
Evolution of Galaxy Shapes in CANDELS
The evolution of galaxy shapes has been modeled using data from the CANDELS survey. Findings indicate that galaxies tend to evolve from prolate (elongated) shapes at low stellar masses and high redshifts to discy (oblate) shapes at higher stellar masses and lower redshifts. This transition is thought to be driven by a compaction event that makes the galaxy center baryon-dominated4.
Intrinsic Shapes from Kinematic Data
Using kinematic data from the SAMI Galaxy Survey, researchers have inferred the intrinsic shapes of galaxies. Most galaxies are found to be oblate axisymmetric, with their shapes varying based on their rotational support. High-spin galaxies are more flattened and axisymmetric, while low-spin galaxies exhibit higher triaxiality. These intrinsic shapes are linked to the galaxies' formation and merger histories6.
Environmental Effects on Galaxy Shapes
The environment also plays a significant role in shaping galaxies. Spiral galaxies in denser environments or closer to the center of their groups tend to have more circular discs compared to those in less dense environments. Central spiral galaxies in groups are generally thinner. For elliptical galaxies, those in groups are more spherical than their counterparts in the field, although their shape does not significantly depend on their position within the group or the local density10.
Conclusion
The study of galaxy shapes reveals a complex interplay between intrinsic properties, environmental factors, and evolutionary processes. From the triaxial nature of elliptical and XMP galaxies to the environmental influences on spiral galaxies, these findings enhance our understanding of galaxy formation and evolution. Future research, particularly with advanced simulations and deeper surveys, will continue to shed light on these fascinating cosmic structures.
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Most relevant research papers on this topic
The shapes of galaxies in the Sloan Digital Sky Survey
The intrinsic shapes of spiral galaxies in the SDSS DR6 are consistent with flat discs, with more luminous galaxies having thicker and rounder discs, and both elliptical and spiral galaxies tending to be rounder for larger galaxies.
Highly CitedInferring the 3D Shapes of Extremely Metal-poor Galaxies from Sets of Projected Shapes
Extremely metal-poor galaxies have triaxial stellar components with an intermediate axis 0.7 times the longest axis, and smaller galaxies are relatively thicker, reflecting slow rotation, stellar feedback, or recent gas accretion.
Intrinsic Shapes of Brightest Cluster Galaxies
Brightest cluster galaxies are strongly triaxial, with more spherical shapes in the central regions but flattening out at large radii, potentially probing the nature of dark matter.
The evolution of galaxy shapes in CANDELS: from prolate to discy
Galaxies tend to evolve from prolate to discy, with prolate galaxies at low M-* and high redshifts and discy at high M-* and low redshifts, potentially due to a compaction event.
Galaxy shape measurement with convolutional neural networks
CNNs accurately predict galaxy shapes from wide-field survey images, improving speed and precision by more than 4 orders of magnitudes compared to traditional methods.
The SAMI Galaxy Survey: the intrinsic shape of kinematically selected galaxies
Most galaxies are oblate axisymmetric, with high-spin systems being more intrinsically flattened and axisymmetric, and their intrinsic shape is linked to their formation and merger histories.
Galaxy shapes and intrinsic alignments in the MassiveBlack-II simulation
The shapes of stellar and dark matter in galaxy subhalos are more round for less massive subhalos and at lower redshifts, with larger misalignment between the two components in central subhalos.
Highly CitedProspects for recovering galaxy intrinsic shapes from projected quantities
Galaxy intrinsic shape is correlated with specific angular momentum, but the simple relationship between $Psi_rm int$ and intrinsic shape is not valid for Illustris galaxies.
The Three-Dimensional Shapes of Galaxy Clusters
Galaxy clusters are better approximated as triaxial objects, allowing for lower concentration parameter inference compared to spherical analysis, but simulations still rely on spherical analysis of triaxial halos.
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