Astronomical instrumentation

Abstract:

We explore a sample of 1492 galaxies with measurements of the mean stellar population properties and the spin parameter proxy, λRe⁠, drawn from the SAMI Galaxy Survey. We fit a global [α/Fe]–σ relation, finding that [α/Fe]=(0.395±0.010)log10(σ)−(0.627±0.002)⁠. We observe an anti-correlation between the residuals Δ[α/Fe] and the inclination-corrected λeoRe⁠, which can be expressed as Δ[α/Fe]=(−0.057±0.008)λeoRe+(0.020±0.003)⁠. The anti-correlation appears to be driven by star-forming galaxies, with a gradient of Δ[α/Fe]∼(−0.121±0.015)λeoRe⁠, although a weak relationship persists for the subsample of galaxies for which star formation has been quenched. We take this to be confirmation that disc-dominated galaxies have an extended duration of star formation. At a reference velocity dispersion of 200 km s−1, we estimate an increase in half-mass formation time from ∼0.5 Gyr to ∼1.2 Gyr from low- to high-λeoRe galaxies. Slow rotators do not appear to fit these trends. Their residual α-enhancement is indistinguishable from other galaxies with λeoRe⪅0.4⁠, despite being both larger and more massive. This result shows that galaxies with λeoRe⪅0.4 experience a similar range of star formation histories, despite their different physical structure and angular momentum.

Abstract:

We explore how observations relate to the physical properties of the emitting galaxies by post-processing a pair of merging z ∼ 2 galaxies from the cosmological, hydrodynamical simulation NEWHORIZON, using LCARS (Light from Cloudy Added to RAMSES) to encode the physical properties of the simulated galaxy into H α emission line. By carrying out mock observations and analysis on these data cubes, we ascertain which physical properties of the galaxy will be recoverable with the HARMONI spectrograph on the European Extremely Large Telescope (ELT). We are able to estimate the galaxy’s star formation rate and dynamical mass to a reasonable degree of accuracy, with values within a factor of 1.81 and 1.38 of the true value. The kinematic structure of the galaxy is also recovered in mock observations. Furthermore, we are able to recover radial profiles of the velocity dispersion and are therefore able to calculate how the dynamical ratio varies as a function of distance from the galaxy centre. Finally, we show that when calculated on galaxy scales the dynamical ratio does not always provide a reliable measure of a galaxy’s stability against gravity or act as an indicator of a minor merger.

Abstract:

Compact obscured nuclei (CONs) are mainly found in local luminous and ultraluminous infrared galaxies (U/LIRGs). In the local Universe, these sources are generally selected through the detection of the HCN–vib (3-2) emission line at submillimetre wavelengths. In this work, we present a diagnostic method to select deeply buried nuclei based on mid-infrared (mid-IR) polycyclic aromatic hydrocarbons (PAHs) and mid-IR continuum ratios. Using Spitzer InfraRed Spectrograph (IRS) spectra of a representative sample of local ULIRGs (z <  0.27), we examine their PAH and underlying continuum emission ratios. For deeply embedded sources, we find that the 9.7 μm silicate absorption band has a particularly pronounced effect on the 11.3 μm PAH feature. The low flux level in the nuclear silicate absorption band enhances the 11.3 μm PAH feature contrast (high PAH equivalent width) compared to that of the other PAH features. The technique has been extended to include the use of the underlying 11.3/12.7 and 11.3/6.2 μm continuum ratios. However, the latter are affected by the extinction coming from both the host galaxy and the nuclear region, whereas the foreground (host-galaxy) extinction is cancelled out when using the PAH equivalent width ratios. We apply our method to local U/LIRGs from the HERUS and GOALS samples and classify 14 ULIRGs and 10 LIRGs as CON candidates, which corresponds to 30% of the ULIRGs and 7% of the LIRGs from these samples. We find that the observed continuum ratios of CON-dominated sources can be explained by assuming torus models with a tapered disc geometry and a smooth dust distribution. This suggests that the nuclear dusty structure of deeply obscured galaxy nuclei has an extremely high dust coverage. Finally, we demonstrate that the use of mid-IR colour–colour diagrams is an effective way to select CON-dominated sources at different redshifts. In particular, the combination of filters of the James Webb Space Telescope/Mid-Infrared Instrument will enable the selection of CONs out to z ∼ 1.5. This will allow the selection of CONs to be extended to high redshifts where U/LIRGs are more numerous.