Shenli Tang

We present a spectroscopic follow-up program targeting closely-separated dual quasar candidates selected from imaging of SDSS quasars with the Subaru Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP). Using two-dimensional image decomposition, our selection identifies PSF-like companions within 0″.6–4″separation (≲ 30 kpc) around the SDSS quasar. We newly confirm six broad-line dual quasars and eleven offset quasars (quasar-galaxy pairs), spanning 1.5 < z < 3.3 for the duals and predominantly z < 0.6 for the offset systems. No obvious lensed quasars were discovered from this program. We obtained 99 spectra of these candidates from NTT/EFOSC2, Gemini/GMOS-N, Keck/NIRES, and Subaru/FOCAS. From the spectra, we measure the emission-line properties of these dual black holes (BH). At z > 1.5, the confirmed duals exhibit high black hole mass (M_BH =10^8.5–10¹⁰M_⊙) with high bolometric luminosities (L_bol =10^45.5–10^47.5 erg s⁻¹), yet accrete at moderate Eddington ratios (λ_Edd =0.01–0.4). From the spectroscopically-confirmed samples, we estimate the dual fraction of SDSS quasars with separations of 0.6″–4″ to be 0.2 %-1.2 % at z < 0.8, 0.08 %-0.24 % at 0.8 < z < 1.5, and 0.06 % at 1.5 < z < 3.3. These values are broadly consistent with other recent optical studies, but lower than theoretical expectations of a rising dual fraction at cosmic noon. However, we note that these fractions, especially at high z, still need a more accurate assessment of selection and observation effects.

During cosmic noon ( z ∼ 1–3), when both star formation and black hole growth peaked, galaxy mergers are predicted to trigger dual active galactic nuclei that eventually coalesce as supermassive black hole (SMBH) binaries. However, observations of dual quasars with sub-5 kpc separations—the critical phase preceding final coalescence—have remained challenging due to angular resolution limitations. We present the discovery and confirmation of two subarcsecond dual quasars at z > 1, selected from 59,025 Sloan Digital Sky Survey (SDSS) quasars, which fall within the footprint of the Hyper Suprime-Cam Survey. Using high-resolution Hubble Space Telescope imaging and slitless spectroscopy, we confirmed SDSS J1625+4309 ( z = 1.647, separation 0 . ″ 55/4.7 kpc) and SDSS J0229−0514 ( z = 3.174, separation 0 . ″ 42/3.2 kpc), probing the sub-5 kpc separation regime. Through the novel combination of WFC3/IR direct imaging (F140W) and grism spectroscopy (G141), we resolve both components morphologically and spectroscopically confirm their dual nature via detection of H β +[O iii ] and Mg ii emission lines in each nucleus. Two-dimensional image decomposition reveals distinct host galaxy morphologies: J1625+4309 shows an extended, disturbed structure ( R _e = 4.7 kpc) indicative of an ongoing major merger, while J0229−0514 exhibits a compact host ( R _e = 1.4 kpc) suggesting an advanced coalescence stage. Black hole mass estimates based on virial relations yield M BH ∼ 1 0 8.1 – 1 0 8.7 M ⊙ with line-of-sight velocity offsets of (0.7 ± 0.1) × 10 ³ km s ⁻¹ and (1.0 ± 0.2) × 10 ³ km s ⁻¹ , respectively. These confirmations directly constrain the frequency and properties of close dual quasars, opening new avenues for studying SMBH mergers at cosmic noon.

We present a rest-ultraviolet to infrared spectral energy distribution (SED) analysis of 63 heavily reddened quasars (HRQs) at redshifts z=0.7-2.7 and with dust extinctions E(B-V)=0.4-1.8. Our analysis demonstrates that SEDs with red optical and blue UV continua are very common in HRQs, with more than 82 per cent of the sample showing a UV-excess relative to the reddened quasar continuum. We model the SEDs by combining a reddened quasar and an unobscured scattered light component, though contributions from a star-forming host galaxy cannot be ruled out. The average scattering fraction is small (0.3 per cent). Higher scattering fractions are ruled out by the (i-K)=2.5 colour-cut used to select HRQs which pre-dates the discovery of the JWST "Little Red Dot" (LRD) population. Hence, LRDs generally have bluer UV continua. Nevertheless, four HRQs satisfy the LRD UV/optical continuum slope selections and are therefore massive, cosmic noon analogues of LRDs. Analysis of the near-infrared SEDs of HRQs reveals a deficit of hot dust relative to blue quasars, similar to what is observed in LRDs. This suggests HRQs trace a phase where strong AGN feedback processes eject dust from the inner torus. The UV scattering fraction of HRQs is weakly correlated with the amount of hot dust emission and anti-correlated with the line-of-sight extinction, E(B-V). This is consistent with the hot dust acting as the scattering medium, and the line-of-sight extinction being dominated by dust on interstellar medium scales in the host galaxy.

We present a study of the molecular gas in five closely-separated (R⊥ < 20 kpc) dual quasars (Lbol ≳ 1044 erg s−1) at redshifts 0.4 < z < 0.8 with the Atacama Large Millimeter/submillimeter Array. The dual quasar phase represents a distinctive stage during the interaction between two galaxies for investigating quasar fueling and feedback effects on the gas reservoir. The dual quasars were selected from the Sloan Digital Sky Survey and Subaru/Hyper Suprime-Cam Subaru Strategic Program, with confirmatory spectroscopic validation. Based on the detection of the CO J=2–1 emission line with Band 4, we derive key properties including CO luminosities, line widths, and molecular gas masses for these systems. Eight quasars, of the ten in pairs, have line detections exceeding 5σ that result in molecular gas masses (Mmolgas) between 109.7 − 10.7 M⊙. The molecular gas-to-stellar mass ratios (μmolgas) of these dual quasars are between 18-50% with the exception of 301%, which are similar to the single quasars in mergers and inactive star-forming galaxies. The results indicate no clear evidence of molecular gas depletion attributed to dual quasar activities on global scale. However, intriguing gas features in certain systems appear to show possible signatures of AGN feedback effect.

We present deep Subaru/FOCAS spectra for two extreme emission line galaxies (EELGs) at z ∼ 1 with strong [O iii] λ5007 emission lines, exhibiting equivalent widths (EWs) of 290 5 − 578 + 946 Å and 200 0 − 159 + 188 Å, comparable to those of EELGs at high redshifts that are now routinely identified with JWST spectroscopy. Adding a similarly large [O iii] EW ( 250 8 − 689 + 1487 Å) EELG found at z ∼ 2 in the JWST CEERS survey to our sample, we explore the physical origins of the large [O iii] EWs of these three galaxies with the Subaru spectra and various public data including JWST/NIRSpec, NIRCam, and MIRI data. While there are no clear signatures of active galactic nuclei (AGNs) identified by the optical line diagnostics, we find that two out of two galaxies covered by the MIRI data show strong near-infrared excess in the spectral energy distributions (SEDs), indicating obscured AGNs. Because none of the three galaxies show clear broad Hβ lines, the upper limits on the flux ratios of broad Hβ to [O iii] lines are small, ≲0.15 that are comparable with Seyfert 1.8–2.0 galaxies. We conduct Cloudy modeling with the stellar and AGN incident spectra, allowing a wide range of parameters, including metallicities and ionization parameters. We find that the large [O iii] EWs are not self-consistently reproduced by the spectra of stars or unobscured AGNs, but obscured AGNs that efficiently produce O⁺⁺ ionizing photons with weak nuclear and stellar continua that are consistent with the SED shapes.

We present a deep X-Shooter rest-frame UV to optical spectral analysis of the heavily reddened quasar, ULASJ2315+043 at z=2.566, known to reside in a major-merger host galaxy. The rest-frame optical is best-fit by a dust-reddened quasar E(B-V)_QSO = 1.55 with black-hole mass log10(Hbeta, MBH [M_sol]) = 10.26 +\- 0.05, bolometric luminosity L_Bol = 10^48.16 erg s^-1 and Eddington-scaled accretion rate log10(λ_Edd) = -0.19. We find remarkable similarities between ULASJ2315+043 and the high-redshift Little Red Dots (LRDs). The rest-frame UV cannot be explained by a dusty quasar component alone and requires an additional blue component consistent with either a star-forming host galaxy or scattered AGN light. We detect broad high-ionisation emission lines in the rest-UV, supporting the scattered light interpretation for the UV excess. The scattering fraction represents just 0.05% of the total luminosity of ULASJ2315+043. Analysis of the mid infra-red SED suggests an absence of hot dust on torus-scales similar to what is observed for LRDs. The obscuring medium is therefore likely on galaxy scales. We detect narrow, blueshifted associated absorption line systems in CIV, NV, SiIV and SiIII. There is evidence for significant high-velocity (>1000 km s^-1) outflows in both the broad and narrow line regions as traced by CIV and [OIII] emission. The kinetic power of the [OIII] wind is e_ion = 10^44.61 erg s^-1 ~ 0.001 L_Bol. ULASJ2315+043 is likely in an important transition phase where star formation, black-hole accretion and multi-phase gas flows are simultaneously occurring.

At fixed galaxy stellar mass, there is a clear observational connection between structural asymmetry and offset from the star forming main sequence, ΔSFMS. Herein, we use the TNG50 simulation to investigate the relative roles of major mergers (stellar mass ratios μ ≥ 0.25), minor (0.1 ≤ μ < 0.25), and mini mergers (0.01 ≤ μ < 0.1) in driving this connection amongst star forming galaxies (SFGs). We use dust radiative transfer post-processing with SKIRT to make a large, public collection of synthetic Hyper Suprime-Cam Subaru Strategic Program (HSC-SSP) images of simulated TNG galaxies over 0.1 ≤ z ≤ 0.7 with log (M_⋆/M_⊙) ≥ 9 (∼750k images). Using their instantaneous SFRs, known merger histories/forecasts, and HSC-SSP asymmetries, we show (1) that TNG50 SFGs qualitatively reproduce the observed trend between ΔSFMS and asymmetry and (2) a strikingly similar trend emerges between ΔSFMS and the time-to-coalescence for mini mergers. Controlling for redshift, stellar mass, environment, and gas fraction, we show that individual mini merger events yield small enhancements in SFRs and asymmetries that are sustained on long timescales (at least ∼3 Gyr after coalescence, on average) – in contrast to major/minor merger remnants which peak at much greater amplitudes but are consistent with controls only ∼1 Gyr after coalescence. Integrating the boosts in SFRs and asymmetries driven by μ ≥ 0.01 mergers since z = 0.7 in TNG50 SFGs, we show that mini mergers are responsible for (i) 55 per cent of all merger-driven star formation and (ii) 70 per cent of merger-driven asymmetric structure. Due to their relative frequency and prolonged boost timescales, mini mergers dominate over their minor and major counterparts in driving star formation and asymmetry in SFGs.

How does the host galaxy morphology influence a central quasar or vice versa? We address this question by measuring the asymmetries of 2424 SDSS quasar hosts at 0.2 < z < 0.8 using broad-band (grizy) images from the Hyper Suprime-Cam Subaru Strategic Program. Control galaxies (without quasars) are selected by matching the redshifts and stellar masses of the quasar hosts. A two-step pipeline is run to decompose the PSF and Sérsic components, and then measure asymmetry indices (A_CAS, A_outer, and A_shape) of each quasar host and control galaxy. We find a mild correlation between host asymmetry and AGN bolometric luminosity (L_bol) for the full sample (spearman correlation of 0.37) while a stronger trend is evident at the highest luminosities (L_bol > 45). This then manifests itself into quasar hosts being more asymmetric, on average, when they harbor a more massive and highly accreting black hole. The merger fraction also positively correlates with L_bol and reaches up to 35% for the most luminous. Compared to control galaxies, quasar hosts are marginally more asymmetric (excess of 0.017 in median at 9.4σ level) and the merger fractions are similar (⁠|$\sim 16.5~{{\%}}$|⁠). We quantify the dependence of asymmetry on optical band which demonstrates that mergers are more likely to be identified with the bluer bands and the correlation between L_bol and asymmetry is also stronger in such bands. We stress that the band dependence, indicative of a changing stellar population, is an important factor in considering the influence of mergers on AGN activity.

We report on a spectroscopic program to search for dual quasars using Subaru Hyper Suprime-Cam (HSC) images of SDSS quasars, which represent an important stage during galaxy mergers. Using Subaru/FOCAS and Gemini-N/GMOS, we identify three new physically associated quasar pairs having projected separations less than 20 kpc, out of 26 observed candidates. These include the discovery of the highest-redshift (z = 3.1) quasar pair with a separation M _BH ∼ 10⁸⁻⁹ M _⊙), bolometric luminosities (log L _bol ∼ 44.5–47.5 erg s^–1) and Eddington ratios (0.01–0.3) similar to single SDSS quasars. We measure the stellar mass of their host galaxies based on 2D image decomposition of the five-band (grizy) optical emission and assess the mass relation between supermassive black holes (SMBHs) and their hosts. Dual SMBHs appear to have elevated masses relative to their host galaxies. Thus, mergers may not necessarily align such systems onto the local mass relation, as suggested by the Horizon-AGN simulation. This study suggests that dual luminous quasars are triggered by mergers prior to the final coalescence of the two SMBHs, resulting in early mass growth of the black holes relative to their host galaxies.