PHL 6625: A Minor Merger-Associated QSO Behind NGC 247

PHL 6625 is a luminous quasi-stellar object (QSO) at z = 0.3954 located behind the nearby galaxy NGC 247 (z = 0.0005). Hubble Space Telescope (HST) observations revealed an arc structure associated with it. We report on spectroscopic observations with the Very Large Telescope (VLT) and multiwavelength observations from the radio to the X-ray band for the system, suggesting that PHL 6625 and the arc are a close pair of merging galaxies, instead of a strong gravitational lens system. The QSO host galaxy is estimated to be (4-28) x 10^10 M_sun, and the mass of the companion galaxy of is estimated to be M_* = (6.8 +/- 2.4) x 10^9 M_sun, suggesting that this is a minor merger system. The QSO displays typical broad emission lines, from which a black hole mass of about (2-5) x 10^8 M_sun and an Eddington ratio of about 0.01-0.05 can be inferred. The system represents an interesting and rare case where a QSO is associated with an ongoing minor merger, analogous to Arp 142.Comment: ApJ to appea

On Variation of Light Curves and Broad Emission Lines for Periodic QSOs from co-rotating Supermassive binary black holes in elliptical orbits

Context. Periodic QSOs are considered as candidates of supermassive binary black hole (BBH) systems in galactic centers. Further confirmation of these candidates may require different lines of observational evidences. Aims. Assuming the Doopler boosting scenario, in this paper we investigate the (coherent) variations of both broad emission lines (BELs) and continuum light curves for active BBH systems surrounding by a circumbinary broad line region (cBLR) and focus on their dependence on the eccentric orbital configuration. Methods. We calculate the variation of continuum light and the Doppler enhanced/weakened photoionization of each BLR cloud according to the motion of BBHs in elliptical orbits, and finally obtain the coherent variation of the continuum and BELs. Results. We find that both the amplitude and variation pattern of the continuum light curves and the evolution of the BEL profiles sensitively depend on the eccentric orbital configuration of BBH systems. If only the secondary BH is active, both the variation amplitudes of continuum light curves and BELs increase with increasing BBH inclination angles and orbital eccentricities, but decrease with increasing BBH mass ratio. If both BHs are active, the asymmetry in the ionization of BLR clouds at different areas caused by the Doppler boosting effect of the secondary BH is weakened due to that of the primary BH at the opposite direction, which leads to systematically smaller variation amplitudes of both continuum light curves and BELs than the cases with only secondary BH activated. Conclusions. The coherent variations of the BEL profiles with the continuum light for those periodic QSOs provide an important way to confirm the existence of BBHs in their center. Future joint analysis of the light curves and multi-epoch observed BEL profiles for periodic QSOs may lead to the identification of a number of BBH systems.Comment: 13 pages, 9 figures, accepted for publication in A&

Broad-line region configuration of the supermassive binary black hole candidate PG1302-102 in the relativistic Doppler boosting scenario

PG1302-102 is thought to be a supermassive binary black hole (BBH) system according to the periodical variations of its optical and UV photometry, which may be interpreted as being due to the relativistic Doppler boosting of the emission mainly from the disk around the secondary black hole (BH) modulated by its orbital motion. In this paper, we investigate several broad emission lines of PG1302-102 using archived UV spectra obtained by IUE, GALEX, and Hubble, to reveal the broad-line region (BLR) emission properties of this BBH system under the Doppler boosting scenario. We find that the broad lines Ly$\alpha$, NV, CIV, and CIII] all show Gaussian profiles, and none of these lines exhibits obvious periodical variation. Adopting a simple model for the BLR, we perform Markov chain Monte Carlo fittings to these broad lines, and find that the BLR must be viewed at an orientation angle of $\sim33^{\circ}$, close to face-on. If the Doppler boosting interpretation is correct, then the BLR is misaligned with the BBH orbital plane by an angle of $\sim51^\circ$, which suggests that the Doppler boosted continuum variation has little effect on the broad-line emission and thus does not lead to periodical line variation. We further discuss the possible implications for such a BLR configuration with respect to the BBH orbital plane.Comment: 9 pages, 6 figures, matches A&A version (only minor changes

Recovering Stellar Population Parameters via Two Full-Spectrum Fitting Algorithms in the Absence of Model Uncertainties

Using mock spectra based on Vazdekis/MILES library fitted within the wavelength region 3600–7350 Å, we analyse the bias and scatter on the resulting physical parameters induced by the choice of fitting algorithms and observational uncertainties, but avoid effects of those model uncertainties. We consider two full-spectrum fitting codes: PPXF and STARLIGHT, in fitting for stellar population age, metallicity, mass-to-light ratio, and dust extinction. With PPXF, we find that both the bias μ in the population parameters and the scatter σ in the recovered logarithmic values follows the expected trend μ ∝ σ ∝ 1/(S/N)⁠. The bias increases for younger ages and systematically makes recovered ages older, M*/Lr larger and metallicities lower than the true values. For reference, at S/N = 30, and for the worst case (t = 108 yr), the bias is 0.06 dex in M*/Lr, 0.03 dex in both age and [M/H]. There is no significant dependence on either E(B − V) or the shape of the error spectrum. Moreover, the results are consistent for both our 1-SSP (simple stellar population) and 2-SSP tests. With the STARLIGHT algorithm, we find trends similar to PPXF, when the input E(B − V) \u3c 0.2 mag. However, with larger input E(B − V), the biases of the output parameter do not converge to zero even at the highest S/N and are strongly affected by the shape of the error spectra. This effect is particularly dramatic for youngest age (t = 108 yr), for which all population parameters can be strongly different from the input values, with significantly underestimated dust extinction and [M/H], and larger ages and M*/Lr. Results degrade when moving from our 1-SSP to the 2-SSP tests. The STARLIGHT convergence to the true values can be improved by increasing Markov Chains and annealing loops to the ‘slow mode’. For the same input spectrum, PPXF is about two order of magnitudes faster than STARLIGHT’s ‘default mode’ and about three order of magnitude faster than STARLIGHT’s ‘slow mode’

SDSS-IV MaNGA: Stellar M/L gradients and the M/L-colour relation in galaxies

The stellar mass-to-light ratio gradient in SDSS r-band ∇(M*/Lr) of a galaxy depends on its mass assembly history, which is imprinted in its morphology and gradients of age, metallicity, and stellar initial mass function (IMF). Taking a MaNGA sample of 2051 galaxies with stellar masses ranging from 109 to 1012M⊙ released in SDSS DR15, we focus on face-on galaxies, without merger and bar signatures, and investigate the dependence of the 2D ∇(M*/Lr) on other galaxy properties, including M*/Lr-colour relationships by assuming a fixed Salpeter IMF as the mass normalization reference. The median gradient is ∇M*/Lr ∼ −0.1 (i.e. the M*/Lr is larger at the centre) for massive galaxies, becomes flat around M* ∼ 1010M⊙ and change sign to ∇M*/Lr ∼ 0.1 at the lowest masses. The M*/Lr inside a half-light radius increases with increasing galaxy stellar mass; in each mass bin, early-type galaxies have the highest value, while pure-disc late-type galaxies have the smallest. Correlation analyses suggest that the mass-weighted stellar age is the dominant parameter influencing the M*/Lr profile, since a luminosity-weighted age is easily affected by star formation when the specific star formation rate (sSFR) inside the half-light radius is higher than 10−3 Gyr−1. With increased sSFR gradient, one can obtain a steeper negative ∇(M*/Lr). The scatter in the slopes of M*/L-colour relations increases with increasing sSFR, for example, the slope for post-starburst galaxies can be flattened to 0.45 from the global value 0.87 in the M*/L versus g − r diagram. Hence converting galaxy colours to M*/L should be done carefully, especially for those galaxies with young luminosity-weighted stellar ages, which can have quite different star formation histories

SDSS-IV MaNGA:environmental dependence of stellar age and metallicity gradients in nearby galaxies

We present a study on the stellar age and metallicity distributions for 1105 galaxies using the STARLIGHT software on MaNGA integral field spectra. We derive age and metallicity gradients by fitting straight lines to the radial profiles, and explore their correlations with total stellar mass M*, NUV-r colour and environments, as identified by both the large scale structure (LSS) type and the local density. We find that the mean age and metallicity gradients are close to zero but slightly negative, which is consistent with the inside-out formation scenario. Within our sample, we find that both the age and metallicity gradients show weak or no correlation with either the LSS type or local density environment. In addition, we also study the environmental dependence of age and metallicity values at the effective radii. The age and metallicity values are highly correlated with M* and NUV-r and are also dependent on LSS type as well as local density. Low-mass galaxies tend to be younger and have lower metallicity in low-density environments while high-mass galaxies are less affected by environment.Comment: 18 pages, 24 figures, accepted for publication in MNRA