arXiv subjects

Qikang Feng

Publications and source records attributed to Qikang Feng.

10 recordsLinked to original sources

arXiv2026

The Epoch of the GSE Merger: Insights from the Splash and Thick Disk Age Distributions

The epoch of the Gaia-Sausage-Enceladus (GSE) merger, the last major merger experienced by the Milky Way, is crucial for reconstructing the Galaxy's evolutionary history. This event dynamically heated the disk, scattering some stars into the halo and producing the so-called Splash. Yet the observed age distribution of the Splash is systematically older than that of the thick disk from which it is thought to originate, posing a puzzling inconsistency. In this work, we show that this apparent discrepancy can be naturally explained once stellar age uncertainties are taken into account. The GSE truncated the intrinsic age distribution of the Splash at the merger epoch, while measurement errors introduce an Eddington-like bias that systematically shifts the truncated distribution toward older ages. Importantly, the magnitude of this shift depends on both the thick-disk star formation history and the merger epoch, thereby offering a new way to constrain the timing of the merger. By combining the observed Splash age distribution with the peak of thick-disk star formation, we infer a GSE merger epoch of $10.1^{+0.2}_{-0.2}$ Gyr ago, providing one of the tightest constraints to date. This result offers new insight into the Milky Way's accretion history and opens a path toward more robust reconstructions of its early evolution.

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arXiv2026

RV and TTV Measurements of Two Transiting Long-Period Giants around TOI-4600

TOI-4600b and c, originally identified by the Transiting Exoplanet Survey Satellite (TESS) and reported by I. Mireles et al. (2023), are a rare pair of transiting long-period giant planets ($\rm P_b=82.7$ days, $\rm P_c=482.8$ days) orbiting an early K dwarf. In this work, we refine the orbital parameters of the TOI-4600 system by combining new TESS photometry, ground-based transit follow-up, and radial velocity (RV) observations from MAROON-X. We obtain improved constraints on planetary masses and eccentricities, and update other parameters, such as the stellar age. For TOI-4600b, we measure a mass of $M_p = 74.7^{+4.7}_{-4.4}\,M_{\oplus}$ and an eccentricity of $e=0.153^{+0.020}_{-0.018}$, and $M_p = 212.53^{+13.26}_{-13.03}\,M_{\oplus}$ and $e=0.219^{+0.015}_{-0.018}$ for TOI-4600c. We find significant transit timing variations (TTV) in both planets, with semi-amplitudes of approximately $1$\,hr. We derive Transit Spectroscopy Metric values of 16.87 for TOI-4600b and 10.09 for TOI-4600c, indicating that both planets are promising JWST targets for studying the atmospheres of temperate and cold Jupiters, a relatively poorly characterized sample thus far. These updated parameters and TTV ephemerides are important for planning and interpreting future photometric, spectroscopic, and dynamical studies of the TOI-4600 system.

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arXiv2026

An Old, Low-mass, Metal-poor Hypervelocity Star Candidate Consistent with a Galactic Center Origin

We report the discovery of DESI-HVS1, a hypervelocity star (HVS) candidate identified from DESI DR1 spectroscopy and Gaia DR3 astrometry. DESI-HVS1 is an old, low-mass, metal-poor F-type star with a mass of $0.8\,M_\odot$, an age of $\sim14.1$~Gyr, and $\mathrm{[Fe/H]}=-1.6$. It is located at a heliocentric distance of $3.77^{+0.39}_{-0.36}$~kpc and has a Galactocentric total velocity of $523^{+46}_{-47}\,\mathrm{km\,s^{-1}}$, marginally exceeding the local escape speed, corresponding to an unbound probability of $P_{\rm ub} \sim 50\%$. Backward orbit integrations show that DESI-HVS1 had a closest approach to the Galactic center (GC) of $0.40^{+0.23}_{-0.11}\,\mathrm{kpc}$, with a velocity of $682^{+22}_{-35}\,\mathrm{km\,s^{-1}}$ and a flight time of $12.89^{+0.92}_{-0.74}\,\mathrm{Myr}$. The reconstructed orbit exhibits a clear perigalactic turning point and only a single crossing of the Galactic midplane ($P_{\rm cross} > 0.95$). These properties suggest that DESI-HVS1 is most naturally explained by the Hills mechanism, although alternative scenarios cannot be entirely ruled out. Its discovery provides the first strong evidence for an old, low-mass HVS candidate consistent with a GC origin, indicating that the apparent dominance of young, massive GC-origin HVSs is likely a consequence of observational selection effects.

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arXiv2025

The rotation curve of the Milky Way measured by Classical Cepheids from Gaia DR3

We determine the rotation curve (RC) of the Milky Way in the range 6 < R < 18 kpc using a sample of 903 carefully selected classical Cepheids with precise proper motions and high-quality radial velocities from \emph{Gaia} DR3. Their distances can be accurately measured from the well-known Period-Wesenheit relations. The RC is computed from the three-dimensional velocity components of these Cepheids. Generally, the RC shows a slight decline with distance from the Galactic center. On top of this general trend, the newly constructed RC shows a dip around R ~ 10-11 kpc, followed by a bump around R ~ 13-14 kpc. This feature has also been reported in other RC measurements, mostly in RCs traced by young tracers like Cepheids. To better constrain the Milky Way mass, an averaged RC is then constructed by combining measurements from this work and previous efforts. Due to the ambiguous nature of the dip-and-bump feature, this averaged RC is constructed only within the radial range where the RC appears to be less influenced by this feature. By using this averaged RC, we determine the circular velocity at solar position and also build a parameterized mass model of our Milky Way. The result for the circular velocity at the solar position is $V_c(R_0) = 236.8 \pm 0.8\ \mathrm{km\,s^{-1}}$, which is in good agreement with previous measurements. The local dark matter density and the enclosed dark matter halo mass within 18 kpc are estimated from the averaged RC under different baryonic models, yielding a series of consistent results: a local density of $0.33-0.40\ \mathrm{GeV\,cm^{-3}}$ and an enclosed mass of $1.19-1.45 \times 10^{11}\ M_\odot$.

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arXiv2025

A 3D Chemodynamical Census of Inner-Galaxy Metal-poor Giants to [Fe/H]~-3.5

The earliest assembly of the Milky Way remains poorly understood, yet the spatial, chemical, and kinematic properties of its most metal-poor stars provide a unique fossil record of its proto-Galaxy phase. Understanding how this ancient component formed is essential for linking near-field Galactic archaeology to high-redshift galaxy evolution. We construct the currently largest 3D map of inner-Galaxy metal-poor giants by combining several narrow/medium-band photometric surveys, reaching metallicities down to $\mathrm{[Fe/H]}\sim-3.5$. Our final sample contains 5,095,676 giants, including 1,717,610 stars with $\mathrm{[Fe/H]}<-1$. Across $-4\le \mathrm{[Fe/H]}<-1$, the density distribution reveals a centrally concentrated, flattened spheroidal component extending to $r_{\rm gc}\sim15$ kpc, together with a prominent overdensity near $X\sim-5$ kpc that is dominated by metal-poor stars on disklike orbits, with a kinematically hot background also present. The selection-function-corrected metallicity distribution function shows a distinct, very metal-poor component around $\mathrm{[Fe/H]}\sim-2.7$ that becomes most prominent at 1$<r_{\rm gc}<$3 kpc. Stars with $-3.5\lesssim\mathrm{[Fe/H]}\lesssim-1.4$ exhibit weak net rotation and low rotational support within $r_{\rm gc}<15$ kpc. Finally, we briefly note that the centrally enhanced very metal-poor component could be qualitatively consistent with one or more early dissipative build-up episodes (e.g., high-$z$ compaction/"blue-nugget" phases) as one possible interpretation.

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arXiv2025

The Milky Way is a less massive galaxy--new estimates of the Milky Way's local and global stellar masses

Stellar mass is the most fundamental property of a galaxy. While it has been robustly measured for millions of external galaxies, it remains poorly constrained for the Milky Way because of the strong selection effect from our inside perspective. In this work, we reconstruct the intrinsic vertical mass density profile in the solar neighborhood and the radial mass density profile across the entire Galaxy using data from the Gaia and APOGEE surveys, following careful correction for the selection function. The local density profile exhibits strong north-south asymmetry in the geometric thick disk regime and increases steeply toward the disk mid-plane, favoring an exponential model over the sech$^2$ model. Integrating the local vertical density profile yields a surface stellar mass density of 31.563$\pm$2.813(syst.)$\pm$0.024(stoch.)~${\rm M_{\odot}pc^{-2}}$, of which 25.074 and 6.489~${\rm M_{\odot}pc^{-2}}$ correspond to living stars and stellar remnants, respectively. The radial surface mass density profile {of the Milky Way} shares the same flat inner component as observed in the brightness profile. With this mass density profile {and local mass density from Gaia}, we derive a new estimate of the total stellar mass of the Milky Way of 2.607$\pm$0.353(syst.)$\pm$0.085(stoch.)${\rm \times10^{10}M_{\odot}}$, a factor of two lower than the previous results. This discrepancy arises primarily from the inner disk profile, which was previously unavailable and extrapolated from the outer disk profile. The lower stellar mass estimate of the Milky Way significantly reduces its rarity in terms of supermassive black hole mass among external galaxies and implies a larger dark matter-to-baryon mass ratio in the inner Galaxy.

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arXiv2025

Revisiting the Proper Motions of M31 and M33 Using Massive Supergiant Stars with Gaia DR3

The proper motions (PMs) of M31 and M33 are key to understanding the Local Group's dynamical evolution. However, measurement discrepancies between Gaia blue and red samples, regarding whether the transverse velocity is remarkable, introduce significant ambiguity. In this work, we remeasure the systemic PMs of M31 and M33 using massive supergiant stars from Gaia Data Release 3. Clean disk tracers are selected via color-color diagrams, with foreground contaminants removed through kinematic and astrometric cuts. We identify the discrepancy in M31's blue and red samples as arising from systematic differences between Gaia's 5-parameter (5p) and 6-parameter (6p) astrometric solutions. The 6p solution, applied to sources lacking accurate color information, relies on a pseudo-color approximation, leading to lower precision and larger uncertainties. Two key limitations of the 6p solution are: 1) degraded astrometric accuracy for very red sources (GBP - GRP > 2.6); 2) significant PM zero-point offsets. In our sample, red sources are dominated by the 6p solution, while blue sources include a substantial fraction of 5p sources; this mismatch drives the observed discrepancy. By excluding extreme red sources and calibrating PM zero-points separately for 5p and 6p sources using background quasars, we reduce the discrepancy, bringing blue and red measurements into agreement within 1 sigma. We ultimately report the most robust Gaia-based PMs using high-quality 5p sources. For M31, we obtain ({\mu}_{\alpha}*, {\mu}_{\delta})_M31 = (45.9 +/- 8.1, -20.5 +/- 6.6) {\mu}as/yr, consistent with, but more precise than, the HST result. For M33, we find ({\mu}_{\alpha}*, {\mu}_{\delta})_M33 = (45.3 +/- 9.7, 26.3 +/- 7.3) {\mu}as/yr, agreeing with VLBA measurement within 1.5 sigma. These results support a first infall scenario for M33.

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arXiv2024

Reply to Comment on "A slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp"

In this reply, we present a comprehensive analysis addressing the concerns raised by Dehnen et al. (2024) regarding our recent measurement of the disk warp precession using the `motion-picture' method (Huang et al. 2024). We carefully examine the impact of ignoring the twist of the disk warp and the so-called $R$-$\tau$ correlation on the estimation of the precession rate. The results indicate that the effect is minor and does not exceed the systematic and statistical uncertainties. Using N-body+SPH simulation data, we confirm that the `motion-picture' technique is effective in measuring retrograde precession of disk warp in stellar populations younger than 170 Myr, similar to classical Cepheids. Therefore, the overall conclusions of Huang et al. (2024) remain robust.

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arXiv2024

A slightly oblate dark matter halo revealed by a retrograde precessing Galactic disk warp

The shape of the dark matter (DM) halo is key to understanding the hierarchical formation of the Galaxy. Despite extensive efforts in recent decades, however, its shape remains a matter of debate, with suggestions ranging from strongly oblate to prolate. Here, we present a new constraint on its present shape by directly measuring the evolution of the Galactic disk warp with time, as traced by accurate distance estimates and precise age determinations for about 2,600 classical Cepheids. We show that the Galactic warp is mildly precessing in a retrograde direction at a rate of $\omega = -2.1 \pm 0.5 ({\rm statistical}) \pm 0.6 ({\rm systematic})$ km s$^{-1}$ kpc$^{-1}$ for the outer disk over the Galactocentric radius [$7.5, 25$] kpc, decreasing with radius. This constrains the shape of the DM halo to be slightly oblate with a flattening (minor axis to major axis ratio) in the range $0.84 \le q_{\Phi} \le 0.96$. Given the young nature of the disk warp traced by Cepheids (less than 200 Myr), our approach directly measures the shape of the present-day DM halo. This measurement, combined with other measurements from older tracers, could provide vital constraints on the evolution of the DM halo and the assembly history of the Galaxy.

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arXiv2024

The Kinematic and Chemical Properties of the Close-in Planet Host Star 8 UMi

A recent study by Hon et al. reported that a close-in planet around the red clump star, 8 UMi, should have been engulfed during the expansion phase of its parent star's evolution. They explained the survival of this exoplanet through a binary-merger channel for 8 UMi. The key to testing this formation scenario is to derive the true age of this star: is it an old "imposter" resulting from a binary merger, or a genuinely young red clump giant? To accomplish this, we derive kinematic and chemical properties for 8 UMi using astrometric data from {\it Gaia} DR3 and the element-abundance pattern measured from a high-resolution ($R \sim 75,000$) spectrum taken by SOPHIE. Our analysis shows that 8 UMi is a normal thin-disk star with orbital rotation speed of $\it{V}_\mathrmϕ=\mathrm{244.96 km s^{-1}}$, and possesses a Solar metallicity ([Fe/H] $= -0.05 \pm 0.07$) and $α$-element abundance ratio ([$α$/Fe] $= +0.01 \pm 0.03$). By adopting well-established relationships between age and space velocities/elemental abundances, we estimate a kinematic age of $3.50^{+3.00}_{-2.00}$ Gyr, and a chemical age of $3.25^{+2.50}_{-1.50}$ Gyr from [C/N] and $3.47 \pm 1.96$ Gyr from [Y/Mg] for 8 UMi, respectively. These estimates are consistent with the isochrone-fitting age ($1.90^{+1.15}_{-0.30}$ Gyr) of 8 UMi, but are all much younger than the timescale required in a binary-merger scenario. This result challenges the binary-merger model; the existence of such a closely orbiting exoplanet around a giant star remains a mystery yet to be resolved.

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