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While the slope of the dust attenuation curve ($\delta$) is found to correlate with effective dust attenuation ($A_V$) as obtained through spectral energy distribution (SED) fitting, it remains unknown how the fitting degeneracies shape this relation. We examine the degeneracy effects by fitting SEDs of a sample of local star-forming galaxies (SFGs) selected from the Galaxy And Mass Assembly survey, in conjunction with mock galaxy SEDs of known attenuation parameters. A well-designed declining starburst star formation history is adopted to generate model SED templates with intrinsic UV slope ($\beta_0$) spanning over a reasonably wide range. The best-fitting $\beta_0$ for our sample SFGs shows a wide coverage, dramatically differing from the limited range of $\beta_0, Comment: 21 pages, 13 figures, accepted for publication in the MNRAS, Comments welcome!

From near-infrared spectroscopic measurements of the MgII emission line doublet, we estimate the black hole (BH) mass of the quasar, SMSS J215728.21-360215.1, as being (3.4 +/- 0.6) x 10^10 M_sun and refine the redshift of the quasar to be z=4.692. SMSS J2157 is the most luminous known quasar, with a 3000A luminosity of (4.7 +/- 0.5) x 10^47 erg/s and an estimated bolometric luminosity of 1.6 x 10^48 erg/s, yet its Eddington ratio is only ~0.4. Thus, the high luminosity of this quasar is a consequence of its extremely large BH -- one of the most massive BHs at z > 4., 7 pages, 3 figures. Accepted for publication in MNRAS

We examine the diagnostic power of rest-frame ultraviolet (UV) nebular emission lines, and compare them to more commonly used rest-frame optical emission lines, using the test case of a single star-forming knot of the bright lensed galaxy RCSGA 032727-132609 at redshift z~1.7. This galaxy has complete coverage of all the major rest-frame UV and optical emission lines from Magellan/MagE and Keck/NIRSPEC. Using the full suite of diagnostic lines, we infer the physical properties: nebular electron temperature (T_e), electron density (n_e), oxygen abundance (log(O/H)), ionisation parameter (log(q)) and interstellar medium (ISM) pressure (log(P/k)). We examine the effectiveness of the different UV, optical and joint UV-optical spectra in constraining the physical conditions. Using UV lines alone we can reliably estimate log(q), but the same is difficult for log(O/H). UV lines yield a higher (~1.5 dex) log(P/k) than the optical lines, as the former probes a further inner nebular region than the latter. For this comparison, we extend the existing Bayesian inference code IZI, adding to it the capability to infer ISM pressure simultaneously with metallicity and ionisation parameter. This work anticipates future rest-frame UV spectral datasets from the James Webb Space Telescope (JWST) at high redshift and from the Extremely Large Telescope (ELT) at moderate redshift., 29 pages, 13 figures, 6 tables, Accepted for publication in MNRAS

We study the relation between the metallicity of quasar broad line region (BLR) and black hole (BH) mass ($10^{7.5}M_{\odot} \sim 10^{10}M_{\odot}$) and quasar bolometric luminosity ($10^{44.6}erg/s \sim 10^{48} erg/s$) using a sample of $\sim$130,000 quasars at $2.25\leq z\leq 5.25$ from Sloan Digital Sky Survey (SDSS) Data Release 12 (DR12). We generate composite spectra by stacking individual spectra in the same BH mass (bolometric luminosity) and redshift bins and then estimate the metallicity of quasar BLR using metallicity-sensitive broad emission-line flux ratios based on the photoionization models. We find a significant correlation between quasar BLR metallicity and BH mass (bolometric luminosity) but no correlation between quasar BLR metallicity and redshift. We also compare the metallicity of quasar BLR and that of host galaxies inferred from the mass-metallicity relation of star-forming galaxy at $z\sim2.3$ and $z\sim3.5$. We find quasar BLR metallicity is 0.3 $\sim$ 1.0 dex higher than their host galaxies. This discrepancy cannot be interpreted by the uncertainty due to different metallicity diagnostic methods, mass-metallicity relation of galaxy, metallicity gradient in quasar host galaxies, BH mass estimation, the effect of different spectral energy distribution (SED) models, and a few other potential sources of uncertainties. We propose a possibility that the high metallicity in quasar BLR might be caused by metal enrichment from massive star formation in the nucleus region of quasars or even the accretion disk., 15 pages, 7 figures, Has been accepted by MNRAS Main Journal

We present a comparative study of the relation between the aperture-based gas-phase metallicity and three structural parameters of star-forming galaxies: mass ($\mathrm{M \equiv M_*}$), average potential ($\Phi \equiv \mathrm{M_*/R_e}$) and average surface mass density ($\Sigma \equiv \mathrm{M_*/R_e^2}$; where $\mathrm{R_e}$ is the effective radius). We use a volume-limited sample drawn from the publicly available SDSS DR7, and base our analysis on aperture-matched sampling by selecting sets of galaxies where the SDSS fibre probes a fixed fraction of $\mathrm{R_e}$. We find that between 0.5 and 1.5 $\mathrm{R_e}$, the gas-phase metallicity correlates more tightly with $\Phi$ than with either $\mathrm{M}$ or $\Sigma$, in that for all aperture-matched samples, the potential-metallicity relation has (i) less scatter, (ii) higher Spearman rank correlation coefficient and (iii) less residual trend with $\mathrm{R_e}$ than either the mass-metallicity relation and the average surface density-metallicity relation. Our result is broadly consistent with the current models of gas enrichment and metal loss. However, a more natural explanation for our findings is a local relation between the gas-phase metallicity and escape velocity., Comment: Accepted by MNRAS; 17 pages, 11 figures, 1 table

We investigate the evolution of the ionisation parameter of star-forming galaxies using a high-redshift ($z\sim 1.5$) sample from the FMOS-COSMOS survey and matched low-redshift samples from the Sloan Digital Sky Survey. By constructing samples of low-redshift galaxies for which the stellar mass ($\mathrm{M}_{*}$), star formation rate (SFR) and specific star formation rate (sSFR) are matched to the high-redshift sample we remove the effects of an evolution in these properties. We also account for the effect of metallicity by jointly constraining the metallicity and ionisation parameter of each sample. We find an evolution in the ionisation parameter for main-sequence, star-forming galaxies and show that this evolution is driven by the evolution of sSFR. By analysing the matched samples as well as a larger sample of $z

We investigate the energy sources of random turbulent motions of ionised gas from H$\alpha$ emission in eight local star-forming galaxies from the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey. These galaxies satisfy strict pure star-forming selection criteria to avoid contamination from active galactic nuclei (AGN) or strong shocks/outflows. Using the relatively high spatial and spectral resolution of SAMI, we find that -- on sub-kpc scales our galaxies display a flat distribution of ionised gas velocity dispersion as a function of star formation rate (SFR) surface density. A major fraction of our SAMI galaxies shows higher velocity dispersion than predictions by feedback-driven models, especially at the low SFR surface density end. Our results suggest that additional sources beyond star formation feedback contribute to driving random motions of the interstellar medium (ISM) in star-forming galaxies. We speculate that gravity, galactic shear, and/or magnetorotational instability (MRI) may be additional driving sources of turbulence in these galaxies., Comment: 11 pages, 5 figures, 3 tables. Accepted by MNRAS

Star-forming galaxies at $z > 1$ exhibit significantly different properties to local galaxies of equivalent stellar mass. Not only are high-redshift star-forming galaxies characterized by higher star formation rates and gas fractions than their local counterparts, they also appear to host star-forming regions with significantly different physical conditions, including greater electron densities. To understand what physical mechanisms are responsible for the observed evolution of star-forming conditions we have assembled the largest sample of star-forming galaxies at $z\sim 1.5$ with emission-line measurements of the $\mathrm{[OII]} \lambda \lambda 3726,3729$ doublet. By comparing our $z\sim 1.5$ sample to local galaxy samples with equivalent distributions of stellar mass, star formation rate and specific star formation rate we investigate the proposed evolution in electron density and its dependence on global properties. We measure an average electron density of $114_{-27}^{+28} \, \mathrm {cm}^{-3} $ for our $z\sim 1.5$ sample, a factor of five greater than the typical electron density of local star-forming galaxies. However, we find no offset between the typical electron densities of local and high-redshift galaxies with equivalent star-formation rates. Our work indicates that the average electron density of a sample is highly sensitive to the star formation rates, implying that the previously observed evolution is mainly the result of selection effects., Comment: 16 pages, 6 Figures