Prof. Dimitra Rigopoulou

Abstract:

The [N ii] 122 and 205 μm transitions are powerful tracers of the ionized gas in the interstellar medium. By combining data from 21 galaxies selected from the Herschel KINGFISH and Beyond the Peak surveys, we have compiled 141 spatially resolved regions with a typical size of ∼1 kpc, with observations of both [N ii] far-infrared lines. We measure [N ii] 122/205 line ratios in the ∼0.6-6 range, which corresponds to electron gas densities of n e ∼ 1-300 cm-3, with a median value of n e = 30 cm-3. Variations in the electron density within individual galaxies can be as high as a factor of ∼50, frequently with strong radial gradients. We find that n e increases as a function of infrared color, dust-weighted mean starlight intensity, and star-formation rate (SFR) surface density (ΣSFR). As the intensity of the [N ii] transitions is related to the ionizing photon flux, we investigate their reliability as tracers of the SFR. We derive relations between the [N ii] emission and SFR in the low-density limit and in the case of a log-normal distribution of densities. The scatter in the correlation between [N ii] surface brightness and ΣSFR can be understood as a property of the n e distribution. For regions with n e close to or higher than the [N ii] line critical densities, the low-density limit [N ii]-based SFR calibration systematically underestimates the SFR because the [N ii] emission is collisionally quenched. Finally, we investigate the relation between [N ii] emission, SFR, and n e by comparing our observations to predictions from the MAPPINGS-III code.

Abstract:

The Far InfraRed Spectroscopic EXplorer (FIRSPEX) is a novel concept for an astronomy satellite mission that will revolutionise our understanding of the properties of the Interstellar Medium (ISM) and star formation through velocity resolved spectroscopic observations at multi-terahertz frequencies. FIRSPEX comprises a fully cryogenic (~4K) heterodyne payload and a ~1.2 m primary antenna to scan the sky in a number of discreet spectroscopic channels delivering 3- dimensional spectral information. The spectral range selected contains important molecular, atomic and ionic species; the majority of which cannot be observed from the ground. FIRSPEX is UK led with additional contributions from partners throughout Europe. FIRSPEX opens up a relatively unexplored parameter space that will produce an enormously significant scientific legacy by focusing on the properties of the multi-phase ISM, the assembly of molecular clouds in our Galaxy and the onset of star formation topics which are fundamental to our understanding of galaxy evolution.

Abstract:

We use 3035 Herschel-SPIRE 500$\mu$m sources from 20.3 sq deg of sky in the HerMES Lockman, ES1 and XMM-LSS areas to estimate the star-formation rate density at z = 1-6. 500 mu sources are associated first with 350 and 250 mu sources, and then with Spitzer 24 mu sources from the SWIRE photometric redshift catalogue. The infrared and submillimetre data are fitted with a set of radiative-transfer templates corresponding to cirrus (quiescent) and starburst galaxies. Lensing candidates are removed via a set of colour-colour and colour-redshift constraints. Star-formation rates are found to extend from < 1 to 20,000 Mo/yr. Such high values were also seen in the all-sky IRAS Faint Source Survey. Star-formation rate functions are derived in a series of redshift bins from 0-6, combined with earlier far-infrared estimates, where available, and fitted with a Saunders et al (1990) functional form. The star-formation-rate density as a function of redshift is derived and compared with other estimates. There is reasonable agreement with both infrared and ultraviolet estimates for z < 3, but we find higher star-formation-rate densities than ultraviolet estimates at z = 3-6. Given the considerable uncertainties in the submillimetre estimates, we can not rule out the possibility that the ultraviolet estimates are correct. But the possibility that the ultraviolet estimates have seriously underestimated the contribution of dust-shrouded star-formation can also not be excluded.