Kavli Affiliate: Yingjie Peng
| First 5 Authors: Bitao Wang, Yingjie Peng, Michele Cappellari, Hua Gao, Houjun Mo
| Summary:
It is not straightforward to physically interpret the apparent morphology of
galaxies. Recent observations by James Webb Space Telescope (JWST) revealed a
dominant galaxy population at high redshifts ($z>2$) that were visually
classified as discs for their flattened shapes and/or exponential light
profiles. The extensively accepted interpretation is that they are dynamically
cold discs supported by bulk rotation. However, it is long known that flattened
shapes and exponential profiles are not exclusive for rotating disc structure.
To break degeneracy and assess the rotational support of typical high-$z$
galaxies in the JWST samples, those with active star formation and stellar
masses $mathrm{lg}(mathcal{M}_{star}/mathcal{M}_{odot})sim9$, we study
the kinematics of their equal-mass counterparts at $z=0$. While these local
star-forming low-mass galaxies are photometrically similar to real dynamically
cold discs, they are not supported by ordered rotation but primarily by random
motion, and their flattened shapes result largely from tangential orbital
anisotropy. Given the empirical and theoretical evidence that young galaxies
are dynamically hotter at higher redshifts, our results suggest that the
high-$z$ JWST galaxies may not be cold discs but are dynamically warm/hot
galaxies with flattened shapes driven by anisotropy. While both having low
rotational support, local low-mass galaxies possess oblate shapes, contrasting
the prolate shapes (i.e. cigar-like) of low-mass systems at high redshifts.
Such shape transition (prolate$Rightarrow$oblate) indicates an associated
change in orbital anisotropy (radial$Rightarrow$tangential), with roots likely
in the assembly of their host dark matter halos.
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