Dr Jacopo Chevallard

Abstract:

We present an analysis of the optical properties of three Ultra Luminous X-ray (ULX) sources identified in NGC 925. We use Integral field unit data from the George Mitchel spectrograph in the context of the Metal-THINGS survey. The optical properties for ULX-1 and ULX-3 are presented, while the spaxel associated with ULX-2 had a low S/N, which prevented its analysis. We also report the kinematics and dimensions of the optical nebula associated with each ULX using ancillary data from the PUMA Fabry-Perot spectrograph. A BPT analysis demonstrates that most spaxels in NGC 925 are dominated by star-forming regions, including those associated with ULX-1 and ULX-3. Using the resolved gas-phase metallicities, a negative metallicity gradient is found, consistent with previous results for spiral galaxies, while the ionization parameter tends to increase radially throughout the galaxy. Interestingly, ULX-1 shows a very low gas metallicity for its galactocentric distance, identified by two independent methods, while exhibiting a typical ionization. We find that such low gas metallicity is best explained in the context of the high-mass X-ray binary population, where the low-metallicity environment favours active Roche lobe overflows that can drive much higher accretion rates. An alternative scenario invoking accretion of a low-mass galaxy is not supported by the data in this region. Finally, ULX-3 shows both a high metallicity and ionization parameter, which is consistent with the progenitor being a highly-accreting neutron star within an evolved stellar population region.Comment: Accepted by Ap

Abstract:

Among the nearest most metal-poor starburst-dwarf galaxies known, SBS 0335-052E is the most luminous in integrated nebular He II {\lambda}4686 emission. This makes it a unique target to test spectral synthesis models and spectral interpretation tools of the kind that will be used to interpret future rest-frame UV observations of primeval galaxies. Previous attempts to reproduce its He II {\lambda}4686 emission luminosity found that X-ray sources, shocks, and single Wolf-Rayet stars are not main contributors to the He II-ionizing budget; and that only metal-free single rotating stars or binary stars with a top-heavy IMF and an unphysically-low metallicity can reproduce it. We present new UV (COS) and optical (MUSE) spectra which integrate the light of four super star clusters in SBS 0335-052E. Nebular He II, [C III], C III], C IV, and O III] UV emission lines with equivalent widths between 1.7 and 5 {\AA}, and a C IV {\lambda}{\lambda}1548, 1551 P-Cygni like profile are detected. Recent extremely-metal poor shock + precursor models and binary models fail to reproduce the observed optical emission-line ratios. We use different sets of UV and optical observables to test models of constant star formation with single non-rotating stars which account for very massive stars, as blueshifted O V {\lambda}1371 absorption is present. Simultaneously fitting the fluxes of all high-ionization UV lines requires an unphysically-low metallicity. Fitting the P-Cygni like + nebular components of C IV {\lambda}{\lambda}1548, 1551 does not constrain the stellar metallicity and time since the beginning of star formation. We obtain 12+log(O/H)=7.45\pm0.04 and log(C/O)=-0.45(+0.03)(-0.04) for the galaxy. Model-testing would benefit from higher spatial resolution UV and optical spectroscopy of the galaxy.

Abstract:

The James Webb Space Telescope (JWST) will enable the detection of optical emission lines in galaxies spanning a broad range of luminosities out to redshifts z ≳ 10. Measurements of key galaxy properties, such as star formation rate and metallicity, through these observations will provide unique insight into, e.g. the role of feedback from stars and active galactic nuclei (AGNs) in regulating galaxy evolution, the co-evolution of AGNs and host galaxies, the physical origin of the ‘main sequence’ of star-forming galaxies, and the contribution by star-forming galaxies to cosmic reionization. We present an original framework to simulate and analyse observations performed with the near-infrared spectrograph (NIRSpec) on board JWST. We use the BEAGLE tool (BayEsian Analysis of GaLaxy sEds) to build a semi-empirical catalogue of galaxy spectra based on photometric spectral energy distributions of dropout galaxies in the Hubble Ultra Deep Field (HUDF). We demonstrate that the resulting catalogue of galaxy spectra satisfies different types of observational constraints on high-redshift galaxies, and use it as an input to simulate NIRSpec/prism (R ∼ 100) observations. We show that a single ‘deep’ (∼100 ks) NIRSpec/prism pointing in the HUDF will enable S/N>3 detections of multiple optical emission lines in ∼30 (∼60) galaxies at z ≳ 6 (⁠z∼4-6⁠) down to mF160W≲30 AB mag. Such observations will allow measurements of galaxy star formation rates, ionization parameters, and gas-phase metallicities within factors of 1.5, mass-to-light ratios within a factor of 2, galaxy ages within a factor of 3, and V-band attenuation optical depths with a precision of 0.3.