Taehyun Kim

To reveal the physical origin of the changing-look (CL) phenomenon in NGC 3786, which transitioned from type 1.8/1.9 to type 1, we present an analysis of long-term spectral monitoring in the optical and near-infrared obtained with Gemini/GMOS-N and Gemini/GNIRS, respectively. Since the onset of the CL phenomenon, NGC 3786 has remained ∼1–1.5 mag brighter in the mid-infrared than in the pre-CL stage, whereas the optical continuum has changed only moderately (∼0.2–0.3 mag). Spectroscopic analysis further reveals that while the fluxes of the broad Pa β and Pa α lines were enhanced over a 2 yr follow-up period, the flux of the broad H α line remained unchanged. We propose that observed temporal variations in the continuum and line flux ratios disfavor a tidal disruption event origin. Instead, the observations can be primarily explained by a gradual change in line-of-sight extinction driven by variations in the torus covering factor, which is determined by the Eddington ratio and the accretion mode. An additional mechanism, arising from the physical conditions within the broad-line region, may partially account for the temporal evolution of the flux ratios. Our study highlights the importance of investigating the CL phenomenon in intermediate-type active galactic nuclei associated with outbursts detected only in the mid-infrared to explore the detailed structural evolution of nuclear activity.

Dark gaps, low surface brightness regions along the bar minor axis, are expected to form as a consequence of secular evolution in barred galaxies. Although several studies have proposed links between dark gap locations and dynamical resonances, the results remain inconclusive. Using DESI Legacy Imaging Survey data, we find that approximately 61% of barred galaxies exhibit pronounced dark gaps. We compare the location of dark gaps with resonance radii derived from the Tremaine–Weinberg method applied to MaNGA data for the same galaxies. Our analysis shows that dark gaps do not preferentially form at specific resonances. Instead, their locations correlate with R ≡ R CR / R Bar : slow bars tend to show shorter dark gap radii, while fast bars show longer ones. This trend reflects a tight relation between bar length and dark gap radius. However, when barred galaxies are classified by their ring morphology, certain types exhibit dark gaps that align with specific resonances. Notably, dark gaps located between the inner and outer rings are closely associated with the corotation radius. In galaxies with two dark gaps along the bar minor axis profile, the inner dark gap typically aligns with the ultraharmonic resonance, and the outer dark gap corresponds to the corotation radius. These findings suggest that some morphological types share similar R values and exhibit dark gaps near specific resonances. Thus, dark gaps may serve as proxies for dynamical resonances only in certain systems. Our findings may help explain the discrepancies observed in earlier studies.

This paper examines the reliability of the Tremaine-Weinberg (TW) method in measuring the pattern speed of barred galaxies at high redshifts. Measuring pattern speeds at high redshift may help to shed light on the time evolution of interactions between galactic bars and dark matter halos. The TW method has been extensively employed for nearby galaxies, and its accuracy in determining bar pattern speeds has been validated through numerical simulations. For nearby galaxies, the method yields acceptable results when the inclination angle of the galaxy and the position angle of the bar fall within appropriate ranges. However, the application of the TW method to high-redshift galaxies remains unexplored in both observations and simulations. For this study we generated mock observations of barred galaxies from the TNG50 cosmological simulation. These simulated observations were tailored to mimic the integral field unit (IFU) spectroscopy data that the Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST) would capture at a redshift of ɀ ≃ 1.2. By applying the TW method to these mock observations and comparing the results with the known pattern speeds, we demonstrate that the TW method performs adequately for barred galaxies whose bars are sufficiently long to be detected by JWST at high redshifts. This work opens a new avenue for applying the TW method to investigate the properties of high-redshift barred galaxies.

We analyze bars formed in N-body simulations to investigate two key aspects of stellar kinematic structure of barred galaxies: the angular distributions of the radial and azimuthal components of stellar velocities, and the impact of bars on rotation curves. We find that stars on bar-supporting x ₁-like orbits exhibit characteristic sawtooth-like radial velocity patterns and archlike tangential velocity patterns as a function of azimuth. In contrast, stars on box and disk orbits show little azimuthal variation, effectively smoothing the overall velocity distribution. When averaged over all orbital families, the resulting kinematics are broadly consistent with the bisymmetric model of Sellwood and Spekkens, with the amplitudes of bar-induced velocity perturbations increasing with bar strength. In addition, bars amplify the radial pressure gradient associated with enhanced random stellar motions, leading to a noticeable reduction in the mean rotational velocity. This effect becomes more pronounced with increasing bar strength, resulting in a shallower rotation curve within the bar region. We discuss our results in the context of the kinematic properties of observed barred galaxies.

We investigate two barred galaxies with nuclear structures, NGC 6951 and NGC 7716, to examine whether they host slow bars. Using Gemini/GMOS long-slit spectroscopy, we calculate the bar pattern speed with the Tremaine-Weinberg method and detect kinematically decoupled nuclear disks in both galaxies. We also measure the bar length and strength using Pan-STARRs images and identify a nuclear ring in NGC 6951 and a nuclear bar in NGC 7716 from HST/PC images. Our results indicate that NGC 6951 hosts a slow, long, and strong bar, which likely evolved through interactions with the dark matter halo and contributed to the formation of both the nuclear disk and ring. We also find hints of a rapidly rotating oval structure within the primary bar, although it is not clearly seen in the imaging data. In contrast, the primary bar in NGC 7716 is too weak to be classified as a barred galaxy, while its nuclear disk and nuclear bar are unusually large, possibly due to tidal interactions or the weakness of the primary bar. These findings suggest that slow bars may be more observed in galaxies with nuclear structures and highlight the often underappreciated role of galaxy interactions in bar evolution.

We investigate two barred galaxies with nuclear structures, NGC 6951 and NGC 7716, to examine whether they host slow bars. Using Gemini/GMOS long-slit spectroscopy, we calculate the bar pattern speed with the Tremaine–Weinberg method and detect kinematically decoupled nuclear disks in both galaxies. We also measure the bar length and strength using Pan-STARRs images and identify a nuclear ring in NGC 6951 and a nuclear bar in NGC 7716 from Hubble Space Telescope/Planetary Camera images. Our results indicate that NGC 6951 hosts a slow, long, and strong bar, which likely evolved through interactions with the dark matter halo and contributed to the formation of both the nuclear disk and ring. We also find hints of a rapidly rotating oval structure within the primary bar, although it is not clearly seen in the imaging data. In contrast, the primary bar in NGC 7716 is too weak to be classified as a barred galaxy, while its nuclear disk and nuclear bar are unusually large, possibly due to tidal interactions or the weakness of the primary bar. These findings suggest that slow bars may be more observed in galaxies with nuclear structures and highlight the often underappreciated role of galaxy interactions in bar evolution.

We analyze bars formed in $N$-body simulations to investigate two key aspects of stellar kinematic structure of barred galaxies: the angular distributions of the radial and azimuthal components of stellar velocities, and the impact of bars on rotation curves. We find that stars on bar-supporting $x_1$-like orbits exhibit characteristic sawtooth-like radial velocity patterns and arch-like tangential velocity patterns as a function of azimuth. In contrast, stars on box and disk orbits show little azimuthal variation, effectively smoothing the overall velocity distribution. When averaged over all orbital families, the resulting kinematics are broadly consistent with the bisymmetric model of Spekkens & Sellwood, with the amplitudes of bar-induced velocity perturbations increasing with bar strength. In addition, bars amplify the radial pressure gradient associated with enhanced random stellar motions, leading to a noticeable reduction in the mean rotational velocity. This effect becomes more pronounced with increasing bar strength, resulting in a shallower rotation curve within the bar region. We discuss our results in the context of the kinematic properties of observed barred galaxies.