Far-infrared metallicity diagnostics: application to local ultraluminous infrared galaxies

The abundance of metals in galaxies is a key parameter that permits to distinguish between different galaxy formation and evolution models. Most of the metallicity determinations are based on optical line ratios. However, the optical spectral range is subject to dust extinction and, for high-z objects (z > 3), some of the lines used in optical metallicity diagnostics are shifted to wavelengths not accessible to ground-based observatories. For this reason, we explore metallicity diagnostics using far-infrared (far-IR) line ratios which can provide a suitable alternative in such situations. To investigate these far-IR line ratios, we modelled the emission of a starburst with the photoionization code cloudy. The most sensitive far-IR ratios to measure metallicities are the [O iii]52 μm and 88 μm to [N iii]57 μm ratios. We show that this ratio produces robust metallicities in the presence of an active galactic nucleus and is insensitive to changes in the age of the ionizing stellar. Another metallicity-sensitive ratio is the [O iii]88 μm/[N ii]122 μm ratio, although it depends on the ionization parameter. We propose various mid- and far-IR line ratios to break this dependence. Finally, we apply these far-IR diagnostics to a sample of 19 local ultraluminous IR galaxies (ULIRGs) observed with Herschel and Spitzer. We find that the gas-phase metallicity in these local ULIRGs is in the range $0.7<Z_{\rm gas}/$ Z⊙ < 1.5, which corresponds to $8.5 <12 + \log ({\rm O / H}) < 8.9$. The inferred metallicities agree well with previous estimates for local ULIRGs and this confirms that they lie below the local mass–metallicity relation.

Emission-Line Diagnostics for Galaxies

I will give an overview of the emission-line diagnostics used for star-forming galaxies. I will review the UV, optical, and IR diagnostics that can be used to yield information about the ISM conditions, star-formation properties, and power sources in GRB hosts, both globally, and with spatially-resolved data. I will discuss wide integral field spectroscopy and AO-led integral field spectroscopy on current and future telescopes, focusing on the insights to be gained on the properties of GRB sites.

Theory of G2 cloud multi-wavelength emission

An object called G2 was recently discovered moving towards the supermassive black hole in the Galactic center. G2 emits infrared (IR) lines and continuum, which allows constraining its properties. The question is still unresolved whether G2 has a central windy star or it is a coreless cloud. Assuming the object is a cloud originating near the apocenter I perform line/continuum IR diagnostics, revisit estimates of non-thermal emission from pericenter passage, and speculate about future observational prospects. This work is partially reported in Shcherbakov (2013) and partially consists of new ideas discussed at the conference.