Prof. Dimitra Rigopoulou

Abstract:

Merging luminous galaxies are ideal laboratories for studying some of the most extreme astrophysical phenomena. The local (z=0.1232) obscured quasar J1356+1026 has two nuclei, North and South (J1356N and J1356S), but despite numerous efforts, J1356S had not yet been confirmed as an AGN . Based on the superb sensitivity and spatial resolution of the MIRI/MRS instrument on board the JWST, we present new evidence suggesting that J1356S may indeed host an AGN with ̊m log L_ ̊m bol =43.4±^ 0.6 _ 0.5 erg s^ -1 . This is supported by the detection of strong coronal line emission at this location and by a spectral shape that differs from that of J1356N and those of the narrow-line region. Aided by the spatially resolved information of MIRI/MRS and VLT/SINFONI, we also found that the high-ionization gas, traced by the coronal lines Ne v 14.3 μm and Si vi 1.963 μm, extends from sim13-15.5 kpc. This is likely a lower limit of the true extension, as suggested by the comparison with optical imaging from HST. The extended Ne v emission can be accounted for by photoionization from the quasar in J1356N in a relatively low-density environment, ranging from ̊m n_ełeq 2000-3800 cm^ -3 in J1356N and ̊m n_ełeq 600-1200 cm^ -3 in J1356S and the narrow-line region, as measured from the Ne v 14.3 μm and 24.3 μm lines.

Abstract:

With the rise of integral field spectroscopy (IFS), we are currently dealing with large amounts of spatially resolved data, whose analysis has become challenging, especially when observing complex objects such as nearby galaxies. We aim to develop a method of automatically separating regions with different physical properties (ionisation, kinematics, etc.) within the central parts (1" ∼ 160,pc, on average) of galaxies. This could allow us to better understand the systems and provide an initial characterisation of the main ionisation sources affecting its evolution. We developed an unsupervised hierarchical clustering algorithm to analyse data cubes based on spectral similarity. It clusters spaxels together with similar spectra, which is useful to disentangle regions affected by different processes, such as ionisation sources. We applied this method to a sample of 15 nearby (distances $<100,Mpc) galaxies: 7 from the Galaxy Activity, Torus, and Outflow Survey (GATOS) and 8 archival sources, all observed with the medium-resolution spectrometer (MRS) of the Mid-Infrared Instrument (MIRI) on board the James Webb Space Telescope (JWST). The sample spans sources of various morphologies, active galactic nucleus (AGN) types, and/or starbursts. From the clusters, we computed their median spectrum and measured the line and continuum properties. We used these measurements to train random forest models and create several empirical mid-IR diagnostic diagrams for the MRS channel 3 wavelength range, ranging from 11.5 to 18μ$m, which includes among others the bright Ne,II , Ne,III , and Ne,V lines, several H_2 transitions, and PAH features. The clustering technique allows one to differentiate emission coming from an AGN, a nuclear starburst, the disc and star-forming (SF) regions in the galaxies, and other composite regions, potentially ionised by several sources simultaneously. This is supported by the results from the empirical diagnostic diagrams, which are indeed able to separate physically distinct regions. This innovative method serves as a tool to identify regions of interest in any data cube prior to an in-depth analysis of the sources. In a future work, we shall explore other wavelength ranges and a larger sample that would help us to obtain statistically significant conclusions.

Abstract:

Luminous infrared (IR) galaxies are key sites of obscured stellar mass assembly at z > 0.5. Their star formation rates (SFRs) are often estimated using the luminosities of the 6.2 and 11.2 μm polycyclic aromatic hydrocarbon (PAH) features or those of the [Ne ii] and [Ne iii] fine-structure lines, as they are minimally affected by obscuration. It is uncertain whether the calibration of these features as SFR tracers depends on the starburst bolometric luminosity or the level of active galactic nucleus (AGN) activity. We here investigate the relationship between the luminosities of PAH and neon lines with SFR for highly luminous objects using radiative transfer modeling and archival observations of 42 local ultraluminous (≥1012L⊙) IR galaxies (ULIRGs). We find that PAH and [Ne ii] features arise mainly in star-forming regions, with small contributions from the AGN or host, but that the [Ne iii] line has a mixed contribution from both star formation and AGN activity. We present relations between LPAH and L[Ne II], and both starburst luminosity and SFR. We find relations for lower-luminosity (LIR ≃ 1010–1012L⊙) systems underestimate the SFRs in local ULIRGs by up to ∼1 dex. The 6.2 and 11.2 μm PAH features, and the [Ne ii] line, are thus good tracers of SFR in ULIRGs. We do not find that a more luminous AGN affects the relationship between SFR and PAH or neon luminosity but that it can make PAH emission harder to discern. Our results and derived relations are relevant to studies of star-forming and composite galaxies at z < 3 with the James Webb Space Telescope.