We show that in order to minimize the uncertainties in the N and O abundances of low-mass, low-metallicity (O/ H 1/5 solar) emission-line galaxies, it is necessary to employ separate parameterizations for inferring T e (N + ) and T e (O + ) from T e (O +2 ). In addition, we show that for the above systems, the ionization correction factor (ICF ) for obtaining N/O from N + /O + , where the latter is derived from optical emission-line flux ratios, is ICF h i ¼ 1:08 AE 0:09. These findings are based on state-of-the-art single-star H ii region simulations, employing our own modeled stellar spectra as input. Our models offer the advantage of having matching stellar and nebular abundances. In addition, they have O/ H as low as 1/50 solar (lower than any past work), as well as log ( N/O) and log (C/O) fixed at characteristic values of À1.46 and À0.7, respectively. The above results were used to rederive N and O abundances for a sample of 68 systems with 12 þ log (O/H) 8:1, whose dereddened emission-line strengths were collected from the literature. The analysis of the log ( N/O) versus 12 þ log (O/ H ) diagram of the above systems shows that (1) the largest group of objects forms the well-known N/O plateau with a value for the mean (and its statistical error) of À1:43 þ0:0084 À0:0085 , (2) the objects are distributed within a range in log ( N/O) of À1.54 to À1.27 in Gaussian fashion around the mean with a standard deviation of ¼ þ0:071 À0:084 , and (3) a 2 analysis suggests that only a small amount of the observed scatter in log ( N /O) is intrinsic.

Chemical evolution models and Monte Carlo simulation techniques have been combined for the first time to study the distribution of blue compact galaxies on the N/O plateau. Each simulation comprises 70 individual chemical evolution models. For each model, input parameters relating to a galaxy's star formation history (bursting or continuous star formation, star formation efficiency), galaxy age, and outflow rate are chosen randomly from ranges predetermined to be relevant. Predicted abundance ratios from each simulation are collectively overplotted onto the data to test its viability. We present our results both with and without observational scatter applied to the model points. Our study shows that most trial combinations of input parameters, including a simulation comprising only simple models with instantaneous recycling, are successful in reproducing the observed morphology of the N/O plateau once observational scatter is added. Therefore, simulations that include delay of nitrogen injection are no longer favored over those that propose that most nitrogen is produced by massive stars, if only the plateau morphology is used as the principal constraint. The one scenario that clearly cannot explain plateau morphology is one in which galaxy ages are allowed to range below 250 Myr. We conclude that the present data for the N/O plateau are insufficient by themselves for identifying the portion of the stellar mass spectrum most responsible for cosmic nitrogen production.

We present results from a half-day observation by XMM-Newton of the nucleus of the nearby Seyfert 2 galaxy NGC 6300. The X-ray spectrum of the nucleus consists of a heavily absorbed hard component dominating the 3-10 keV band and a soft component seen in the 0.2-2 keV band. In the hard band, the spectrum is well fitted by a power-law model with photon index of 1:83 AE 0:08 attenuated by a Compton-thin absorber (N H ' 2:2 ; 10 23 cm À2 ). A narrow iron line is detected at 6:43 þ0:01 À0:02 keV with an equivalent width of $150 eV; the line velocity width is marginally resolved to be $ 60 eV. The soft emission can be modeled as a power law and may be emission scattered by surrounding plasma. Rapid and high-amplitude variability is observed in the hard X-ray band, whereas both the iron line and the soft emission show no significant variability. It is suggested that the nucleus has experienced an overall long-term trend of decreasing hard X-ray intensity on a timescale of years. We discuss the origins of the spectral components.