Prof. Dimitra Rigopoulou

Abstract:

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 [OIII]52 μm and 88 μm to [NIII]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 [OIII]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 < Zgas/Z⊙ < 1.5, which corresponds to 8.5 < 12 + log(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.

Abstract:

We present the first detection of the ortho-H2O 4₂₃ - 3₃₀ transition at 448 GHz in space. We observed this transition in the local (z = 0.010) luminous infrared (IR) galaxy ESO 320-G030 (IRAS F11506-3851) using the Atacama Large Millimeter/submillimeter Array (ALMA). The water 4₂₃ - 3₃₀ emission, which originates in the highly obscured nucleus of this galaxy, is spatially resolved over a region of ~65 pc in diameter and shows a regular rotation pattern compatible with the global molecular and ionized gas kinematics. The line profile is symmetric and well fitted by a Gaussian with an integrated flux of 37.0 ± 0.7 Jy km s^-1 . Models predict this water transition as a potential collisionally excited maser transition. On the contrary, in this galaxy, we find that the 4₂₃ - 3₃₀ emission is primarily excited by the intense far-IR radiation field present in its nucleus. According to our modeling, this transition is a probe of deeply buried galaxy nuclei thanks to the high dust optical depths (τ 100μm > 1, NH > 10^24 cm^-2 ) required to efficiently excite it.