arXiv subjects

Xiaojie Liao

Publications and source records attributed to Xiaojie Liao.

4 recordsLinked to original sources

arXiv2026

GASTRO library II: Exploring Chemical Bimodalities in Disk Galaxies with GSE-like Mergers and Massive Star-forming Clumps

We use several smoothed particle hydrodynamics+N-body models as part of the GASTRO library to study the role of high-density star-forming clumpy regions and a single merger on the formation of the $α$-rich and $α$-poor populations in the disk galaxies. These experiments are tailored to mimic what is expected to be the Gaia-Sausage/Enceladus (GSE) accretion event, which occurred circa 10 Gyr ago in the Milky Way (MW). We find that either an early clumpy phase or a retrograde merger significantly reduces the star formation rate (SFR) of the disk, giving rise to a chemical bimodality qualitatively similar to the MW's. The decrease of the SFR as the cause of the chemical bimodality is consistent with previous idealized and cosmological simulations. On the other hand, a prograde radial merger does not significantly modify the SFR of the disk, resulting in no clear chemical bimodality. We further show that stars originating from the inner regions ($R_{form}<4$ kpc) do not create the disk's chemical bimodality, although they can enhance it. Finally, only the models with an early clumpy phase can produce a significant fraction of old, age $>11$ Gyr, $α-$poor stars with disk-like orbits, similar to what has been recently observed in the MW. Our results strengthen the case of clumpy disky galaxies observed at redshift $z\approx 1-2$ as likely progenitors of our Galaxy.

astro-ph.GA↗

arXiv2025

Influence of Barlens on the Bulge Parameters in the 2D Image Decomposition

Recent observations and simulations have shown that a buckled bar in the face-on view can be considered as a combination of a long flat bar and a short round barlens (corresponding to the boxy/peanut bulge in the edge-on view). However, the barlens component can be misidentified as the bulge, potentially leading to inaccurate bulge parameter measurements in two-dimensional (2D) image decomposition. Our goal is to explore the optimal method for modeling the barlens component and to understand its impact on bulge parameter measurements in 2D image decomposition. We first analyze mock images from two different simulations (with/without bulge) to verify our decomposition method. We then apply the method to two nearby barred galaxies, NGC 1533 and NGC 7329, from the Carnegie-Irvine Galaxy Survey (CGS). Using GALFIT, we conduct 2D image decomposition by gradually increasing the complexity of model configurations. We also explore the effects of inclination by projecting the simulated galaxy to various viewing angles and analyzing the variations in bulge and barlens parameters. From the mock images, we find that the bulge-to-total ratio (B/T) could be overestimated by 50$\%$ without considering the barlens component; the Sérsic index and effective radius of the bulge are also affected to varying degrees. The decomposition results of the two CGS galaxies are consistent with our mock image tests. Uncertainties of the structural parameters of the bulge and barlens are larger at higher inclination angels due to the strong projection effect in the central region. Our findings underscore the necessity of accurately modeling the barlens, revealing that its inclusion in 2D image decomposition can lead to a decrease in B/T by $\sim$30-50$\%$, with other bulge parameters, such as the Sérsic index and effective radius, also affected.

astro-ph.GA↗

arXiv2025

Unveiling a young thick disk in the Milky Way

The thickness of a galaxy's disk provides a valuable probe of its formation and evolution history. Observations of the Milky Way and local galaxies have revealed an ubiquitous disk structure with two distinctive components: an old thick disk and a relatively young thin disk. The formation of this dual-disk structure and the mechanisms that develop the thickness of the disk are still unclear. Whether the disk thickness inherit from the birth environment or is established through secular dynamical heating after formation is under debate. In this work we identify a relatively young ($\sim$6.6 billion years old) geometric thick disk in the Milky Way, with a scale height of $0.64$ kpc at the Solar Circle. This young thick component exhibits comparable thickness and flaring strength to the canonical old thick disk but is more radially extended and systematically younger. We also identify thin disk components that formed before and after this young thick disk. Detailed analysis of the solar vicinity structure suggests that the young thick disk marks the onset of a new phase of upside-down disk formation. These findings strongly discount the role of secular dynamical heating and support a turbulent, bursty birth environment as the primary mechanism behind thick disk formation. The existence of two thick disk components suggests that the Milky Way has undergone at least two episodes of turbulent and bursty star formation, likely triggered by galaxy mergers.

astro-ph.GA↗

arXiv2024

Insight into the Galactic Bulge Chemodynamical Properties from Gaia DR3

We explore the chemodynamical properties of the Galaxy in the azimuthal velocity $V_ϕ$ and metallicity [Fe/H] space using red giant stars from Gaia Data Release 3. The row-normalized $V_ϕ$-[Fe/H] maps form a coherent sequence from the bulge to the outer disk, clearly revealing the thin/thick disk and the Splash. The metal-rich stars display bar-like kinematics while the metal-poor stars show dispersion-dominated kinematics. The intermediate-metallicity population ($-1<$[Fe/H]$<-0.4$) can be separated into two populations, one that is bar-like, i.e. dynamically cold ($σ_{V_R}\sim80$ $\rm km\ s^{-1}$) and fast rotating ($V_ϕ\gtrsim100$ $\rm km\ s^{-1}$), and the Splash, which is dynamically hot ($σ_{V_R}\sim110$ $\rm km\ s^{-1}$) and slow rotating ($V_ϕ\lesssim100$ $\rm km\ s^{-1}$). We compare the observations in the bulge region with an Auriga simulation where the last major merger event occurred $\sim10$ Gyr ago: only stars born around the time of the merger reveal a Splash-like feature in the $V_ϕ$-[Fe/H] space, suggesting that the Splash is likely merger-induced, predominantly made-up of heated disk stars and the starburst associated with the last major merger. Since the Splash formed from the proto-disk, its lower metallicity limit coincides with that of the thick disk. The bar formed later from the dynamically hot disk with [Fe/H] $>-1$ dex, with the Splash not participating in the bar formation and growth. Moreover, with a set of isolated evolving $N$-body disk simulations, we confirm that a non-rotating classical bulge can be spun up by the bar and develop cylindrical rotation, consistent with the observation for the metal-poor stars.

astro-ph.GA↗