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Introduction

The interstellar medium is a turbulent and compressible system (von Weizsacker 1951), in which cooling and heating determine the physical state of the gas. Interstellar cooling can be the result of line and continuum plasma emission processes, as well as of adiabatic expansion of over-pressured gas. The importance of the former depends on the amount of atoms and ions present in the flow, whereas the latter is related to thermodynamical processes. For optically thin interstellar plasmas, frequently the assumption of collisional ionization equilibrium (CIE) is used, according to which the collisional excitation of the gas is followed by photon emission, with the number of ionizations being equal to the number of (dielectronic or radiative) recombinations. However, this violates detailed balancing, since collisional excitation involves three particles, an atom or ion, an electron to collide with, and a second electron which is ejected, whereas in radiative recombination, the third particle is a photon, leaving the system.

Strictly speaking, CIE can never be maintained in an evolving plasma, although it might be a fair approximation, especially in hot (T>106 K) environments.
For lower temperatures recombinations are not synchronized with the cooling and therefore, deviations from CIE inevitably occur.

Collisional + Photo Ionization Plasma Emission Software (CPIPES)

We developed the Collisional + Photo Ionization Plasma Emission Software (CPIPES) to trace in a time-dependent fashion the thermal state of a plasma (composed by the ten most abundant elements in nature - H, He, C, N, O, Ne, Mg, Si, S, and Fe) during the MHD evolution of the intersttellar medium and determine on the spot the cooling and emission spectra. For details see Avillez (2011) and Avillez & Spitoni (2011).

The evolution of a static plasma

The evolution of the ionization structure and cooling of a static plasma (i.e., no dynamics included) is studied under three conditions:
1. Collisional ionization equilibrium (steady state)
2. Non-equilibrium ionization (time-dependent):
• isochoric
• isobaric
CIE Ionization Structure

NEI Ionization Structure

As examples of the ionization structure variation with temperature for a plasma evolving under NEI isochoric (black lines) and isobaric (red lines) conditions we show the ion fractions of Ne, Mg and Si.

Cooling Efficiency of a Static Plasma

Normalized CIE and NEI cooling efficiencies are shown in the following figure in addition to the atomic processes contributions to the cooling rates: free-free (brown), radiative (red) and dielectronic (green) recombinations, electron impact ionization including excitation auto-ionization (blue), and two-photon (orange) and line excitation excluding two-photon (magenta) emissions. The NEI cooling efficiencies shown in the right panel are due to isobaric (dashed lines) and isochoric (solid lines) conditions. Total cooling is shown by the black lines.

Metallicity Dependence

By default the E(A+M)PEC includes the ten most abundant elements in nature (H, He, C, N, O, Ne, Mg, Si, S and Fe) and the latest recommended abundances by Asplund et al. (2009).

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Tidal Disruption Events (TDES) of a solar type star by the SMBH Sgr A* (M(bh)=4x106 solar mass). When a star is gravitationally captured by the black hole it enters in a fatal orbit onto the compact object. Such orbits are characterized by the penetration parameter (b) which determines how deep the star penetrates into the tidal radius (Rt; the limit for a star to be tidally disrupted) of the SMBH and its eccentricity (ecc). Stars can approach the black hole in two different orbits: parabolic and elliptic, which can be differentiated by the eccentricity of the trajectory of the star, respectively, in a parabolic orbit the star describes a trajectory with ecc=1 and in an elliptic orbit 0&lt;ecc&lt;1. With increasing b the star attains shorter distances at the pericenter Rp, the closest distance of its orbit to the black hole, and suffers the effects of the gravitational tidal forces induced by the compact object. When the stars experiences tidal deformations it develops two tidal tails and is continuously stretched, where the envelope mass is depletted from its surface during the "pancake phase". In Figure 3, the upper panel shows a TDE of star on a parabolic orbit and in the bottom panel it shows the TDE of a star on an elliptic orbit, in both cases the star crosses Rt with b=5.

Fig. 1 - A parabolic TDE of a solar mass star by the SMBH Sgr A*, with a penetration parameter of b=5
(calculated with the Phantom SPH code by © João Rocha, 2019).

During the process of a TDE, large amounts of energy are released to the surrounding InterStellar Medium (ISM).

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The SPH implementation of the galaxy merger simulation requires gravity for the interaction of multiple particles (e.g., stars, gas, dark matter) where the particles interact hydrodynamically between them and the gas interacts with itself. The evolution of the galaxy merger simulation in Phantom SPH requires gravity for the interaction of multiple particles (e.g., stars, gas, dark matter) where the particles interact hydrodynamically between them and the gas interacts with itself. In order to obtain a Milky Way-type galaxy with a stellar disc, a stellar bulge and a dark matter halo this simulatin was implemented with GalacTics, where the stellar disc was duplicated in the $x=y$ plane so this way there are no coincidences with star particles and the gas disc can be created taking 10$\%$ of the total stellar mass. The gas halo was encapsulated within the dark matter halo with a temperature profile and a $\beta$-profile\footnote{Observed surface brightness that constrains the gas density distribution}, where the mass from the hot gas halo was taken from the dark matter particles to keep a total halo mass. The merger of the galaxies was set with a distance of 70 kpc between them and approaching each other on a parabolic trajectory.

Fig. 2 - The Galaxy merger of two Milky Way type galaxies that evolve through 1.5 Gyrs (calculated with the Phantom SPH code by © João Rocha, 2019).

The component breakdown for each galaxy is presented in the Table 1, below:

Tab. 1 - The Galaxy merger breakdown components of two Milky Way type galaxies (© João Rocha, 2019).

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Based on a mixture of theoretical and empirical ingredients, spectral synthesis is a powerful tool in Extragalactic Astronomy which aims to analyse the spectral building blocks of a galaxy and to probe their underlying physical phenomena. In fact, spectral synthesis has proved over the decades to be essential in the study of the physical conditions and mechanisms responsible for galaxy formation and evolution (e.g. Heavens, Jimenez & Lahav 2000; Kauffmann et al. 2003c; Heavens et al. 2004; Cid Fernandes et al. 2005; Tojeiro et al. 2007). Codes like FADO (Gomes & Papaderos 2017) estimate the main physical properties of galaxies, for instance, stellar (e.g. total mass, mean age, mean metallicity) and nebular properties (e.g. mean metallicity, extinction). This contributes to the ever-expanding understanding of galaxy evolution by reconstructing, for instance, the star-formation histories (SFHs) of galaxies.

Using a large sample of mock galaxy spectra, Cardoso, Gomes & Papaderos (2016, 2017) showed that the introduction of a non-thermal continuum (e.g. AGN) parameterised by a simple power-lar on otherwise purely-stellar galaxy spectra (Fig. 1), computed with the evolutionary synthesis code REBETIKO (Papaderos & Gomes, in prep.), can lead to severe biases on the estimated stellar properties (e.g. up to ~2 dex in mass, ~4 dex in mean age and ~0.6 dex in mean metallicity) when using a purely-stellar population synthesis approach (e.g. Cid Fernandes et al. 2005).

Fig. 3 - Synthetic spectra for a continuous SFH, solar metallicity and ages between 1 Myr and 15 Gyr (© Leandro Cardoso, 2019).

Moreover, Cardoso, Gomes & Papaderos (2019) also showed that nebular emission rising from HII regions in mock star-forming galaxies alone can also lead to similarly significant biases. These results are rendered particularly relevant after realising that, until very recently, all spectroscopic spectral synthesis codes (e.g. Cid Fernandes et al. 2005; Ocvirck et al. 2006ab; Koleva et al. 2007; Tojeiro et al. 2009; Leja et al. 2017), which are commonly cited in the literature and were thoroughly applied to large-scale local universe survey (e.g. SDSS, MaNGA), only assume stellar population models as spectral building blocks. However, Cardoso, Gomes & Papaderos (2019) also showed that FADO can reliably recover the main stellar properties (Fig. 2), due to its ability to self-consistently fit both the stellar and nebular continua. This improvement will lead to a major shift in the interpretation of how star-forming galaxies were formed and evolved along cosmic time.

Fig. 4 - Mean stellar age (left-hand side) and metallicity (right-hand side) weighted by light (light-shaded lines) and mass (dark-shaded lines) from STARLIGHT (red lines) and FADO (blue lines) has a function of the age of models with continuous SFH (© Leandro Cardoso, 2019).