arXiv subjects

Tom Mutabazi

Publications and source records attributed to Tom Mutabazi.

11 recordsLinked to original sources

arXiv2025

The impact of selection criteria on the properties of green valley galaxies

Context: The bi-modality in the distribution of galaxies usually obtained from colour-colour or colour-stellar mass diagrams has been studied to show the difference between the galaxies in the blue cloud and in the red sequence and to define the green valley region. As a transition region, the green valley galaxies can give clues about morphological transformation of galaxies from late- to early-types, and therefore the selection of green valley is of fundamental importance. Aims: In this work, for the first time, we evaluate the selection effects of the most used green valley selection criteria. The aim is to understand how these criteria affect the identification of green valley galaxies, their properties, and their impact on galaxy evolution studies. Methods: Using the SDSS optical and GALEX ultraviolet data at redshift z < 0.1, we selected the eight most commonly used criteria based on colours, specific star formation rate, and star formation rate vs. stellar mass. We then studied the properties of the green valley galaxies (their stellar mass, star formation rate, specific star formation rate, intrinsic brightness, morphological and pectroscopic types) for each selection criterion. Results: We found that when using different criteria, we select different types of galaxies. UV-optical colour-based criteria tend to select more massive galaxies, with lower star formation rates, with higher fractions of composite and elliptical galaxies, than when using pure optical colours. Our results also show that the colour-based criteria are the most sensitive to galaxy properties, rapidly changing the selection of green valley galaxies. Conclusions: Whenever possible, we suggest avoiding the green valley colour-based selection and using other methods or a combination of several, such as the star formation rate vs. stellar mass or specific star formation rate.

astro-ph.GA↗

arXiv2025

Large scale structure constraints and matter power spectrum in $f (Q,\mathcal{L}_{m})$ gravity

In the present work, we take into account the dynamical system analysis to investigate the matter power spectrum within the framework of the $f(Q,\mathcal{L}_{m})$ gravitational theory. After obtaining autonomous dynamical system variables for two different particular pedagogical choices of $f(Q,\mathcal{L}_{m})$ models (A and B), we derive the full system of perturbation equations using the $1+3$ covariant formalism to study the matter fluctuations. We present and solve the energy density perturbation equations to obtain the energy density contrast, which decays with redshift for both models for a particular choice of model parameters. After obtaining the numerical results of the density contrast, we computed the matter spectra for each model and conducted a comparative analysis with the $\Lambda$CDM. Furthermore, by employing the Markov Chain Monte Carlo (MCMC) analysis,the model parameters were constrained using a combination of different observational data sets to improve the robustness and accuracy of the parameter estimation. Our results indicate that only model A can be compatible with the considered observational data sets.

gr-qc↗

arXiv2023

On covariant perturbations with scalar field in modified Gauss-Bonnet gravity

We investigate cosmological perturbations of f(G) gravity in the presence of a scalar field. Using the 1 + 3 covariant formalism, we present the energy overdensity perturbation equations responsible for large scale structure formation. After applying harmonic decomposition method together with the redshift transformation technique, we obtain the fully perturbed equations in redshift space. The equations are solved to study the growth of matter overdensities contrast with redshift. For both short- and long-wavelength modes, we obtain numerical results for particular functional form f(G) models and scalar field. We find that, for this choice the energy overdensity perturbations decay with increase in redshift. However, for both short- and long- wavelength modes, the perturbations which include amplitude effects due to the f(G) models with a scalar field do differ remarkably from those in Lambda CDM. The results reduce to GR results in the limit of f(G) tends to G and in the absence of scalar field.

gr-qc↗

arXiv2023

On 1 + 3 covariant perturbations of the quasi-Newtonian space-time in modified Gauss-Bonnet gravity

The consideration of a 1 + 3 covariant approach to cold dark matter universe with no shear cosmological dust model with irrotational flows is developed in the context of f (G) gravity theory in the present study. This approach reveals the existence of integrability conditions which do not appear in non-covariant treatments. We constructed the integrability conditions in modified Gauss-Bonnet f (G) gravity basing on the constraints and propagation equations. These integrability conditions reveal the linearized silent nature of quasi-Newtonian models in f (G) gravity. Finally, the linear equations for the overdensity and velocity perturbations of the quasi-Newtonian space-time were constructed in the context of modified f (G) gravity. The application of harmonic decomposition and redshift transformation techniques to explore the behaviour of the overdensity and velocity perturbations using f (G) model were made. On the other hand we applied the quasi-static approximation to study the approximated solutions on small scales which helps to get both analytical and numerical results of the perturbation equations. The analysis of the energy overdensity and velocity perturbations for both short and long wavelength modes in a dust-Gauss-Bonnet fluids were done and we see that both energy overdensity and velocity perturbations decay with redshift for both modes. In the limits to ΛCDM , it means f (G) = G the considered f (G) model results coincide with ΛCDM .

gr-qc↗

arXiv2022

Perturbations in the interacting vacuum

In this study, we present the evolution of cosmological perturbations in a universe consisting of standard matter and interacting vacuum. We use the $1 + 3$ covariant formalism in perturbation framework and consider two different models for the interacting vacuum namely a linear interacting model and interaction with creation pressure model. For both models, we derive the evolution equations governing the growth of linear perturbations for both radiation and dust dominated Universe. We find numerical solutions in appropriate limits namely long and short wavelengths. For both models, the perturbations grow with time (decay with redshift), showing that structure formation is possible in an accelerated cosmic background. The perturbation amplitudes and their relative scalings with those of standard model of cosmology depend on the values of the interaction parameters considered, and in a way that can be used to constrain the models using existing and future large scale structure data. In the vanishing limits of the coupling parameters of the interaction, we show that standard cosmology, both background and perturbed, is recovered.

gr-qc↗

arXiv2021

Cosmological perturbations in f(G) gravity

We explore cosmological perturbations in a modified Gauss-Bonnet f(G) gravity, using a 1+3 covariant formalism. In such a formalism, we define gradient variables to get perturbed linear evolution equations. We transform these linear evolution equations into ordinary differential equations using a spherical harmonic decomposition method. The obtained ordinary differential equations are time-dependent and then transformed into redshift dependent. After these transformations, we analyze energy-density perturbations for two-fluid systems, namely for a Gauss-Bonnet field-dust system and for a Gaus-Bonnet field-radiation system for three different pedagogical f(G) models: trigonometric, exponential, and logarithmic. For the Gauss-Bonnet field-dust system, energy-density perturbations decay with an increase in redshift for all three models. For the Gauss-Bonnet field-radiation system, the energy-density perturbations decay with an increase in redshift for all of the three f(G) models for long-wavelength modes whereas for short-wavelength modes, the energy-density perturbations decay with increasing redshift for the logarithmic and exponential f(G) models and oscillate with decreasing amplitude for the trigonometric f(G) model.

gr-qc↗

arXiv2021

The Distance and Peculiar Velocity of the Norma cluster (ACO 3627) using the Near-Infrared $J$ and $K_s$-band Fundamental Plane Relations

We report distance measurements for the Norma cluster based on the near-infrared $J$- and $K_s$-band Fundamental Plane (FP) relations. Our simultaneous $J$ and $K_s$-band photometry analyses were performed using 31 early-type galaxies in the nearby Norma cluster obtained using the 1.4 m InfraRed Survey Facility (IRSF) at the South African Astronomical Observatory. Our final $K_s$-band FP sample consists of 41 early-type galaxies from the Norma cluster observed using the IRSF and the New Technology Telescope (NTT) at the European Southern Observatory. This is the largest cluster sample used for peculiar velocity studies in the Great Attractor region to date. From the $K_s$-band FP, we find a distance to the Norma cluster of $4915 \pm 121$ km s$^{-1}$. The implied peculiar velocity for Norma is $44 \pm 151$ km s$^{-1}$ which further supports a small peculiar velocity for the Norma cluster.

astro-ph.GA↗

arXiv2020

Properties of flares and CMEs on EV Lac: Possible erupting filament

Flares and CMEs are very powerful events in which energetic radiation and particles are ejected within a short time. These events thus can strongly affect planets that orbit these stars. This is particularly relevant for planets of M-stars, because these stars stay active for a long time during their evolution and yet potentially habitable planets orbit at short distance. Unfortunately, not much is known about the relation between flares and CMEs in M-stars as only very few CMEs have so far been observed in M-stars. In order to learn more about flares and CMEs on M-stars we monitored the active M-star EV Lac spectroscopically at high resolution. We find 27 flares with energies between 1.6E31 and 1.4E32 erg in Halpha during 127 hours of spectroscopic monitoring and 49 flares with energies between 6.3E31 and 1.1E33 erg during the 457 hours of TESS observation. Statistical analysis shows that the ratio of the continuum flux in the TESS-band to the energy emitted in Halpha is 10.408 +/- 0.026. Analysis of the spectra shows an increase in the flux of the He II 4686 Å line during the impulsive phase of some flares. In three large flares, we detect a continuum source with a temperature between 6 900 and 23 000 K. In none of the flares we find a clear CME event indicating that these must be very rare in active M-stars. However, in one relatively weak event, we found an asymmetry in the Balmer lines of ~220 km/s which we interpret as a signature of an erupting filament.

astro-ph.SR↗

arXiv2020

A search for new $γ$-ray blazars from infrared selected candidates

We present a systematic study of gamma-ray blazar candidates based on a sample of 40 objects taken from the WIBR catalogue. By using a likelihood analysis, 26 of the 40 sources showed significant gamma-ray signatures $\geq3σ$. Using high-energy test statistics (TS) maps, we confirm 8 sources, which are completely new, and show another 15 promising $γ$-ray candidates. The results from this analysis show that a multi-frequency approach can help to improve the current description of the gamma-ray sky.

astro-ph.HE↗

arXiv2020

Determination of K4000 of potential blazar candidates among EGRET unidentified gamma-ray sources

Blazars are radio-loud Active Galactic Nuclei (AGN) with relativistic jets oriented towards the observer's line-of-sight. Based on their optical spectra, blazars may be classified as flat-spectrum radio quasars (FSRQs) or BL Lacs. FSRQs are more luminous blazars with both narrow and broad emission and absorption lines, while BL Lacs are less luminous and featureless. Recent studies show that blazars dominate (93%) the already-identified EGRET sources (142), suggesting that among the unidentified sources (129) there could still be faint blazars. Due to the presence of a strong non-thermal component inside their jets, blazars are found to display a weaker depression at 4000 Å (K4000<0.4). In this study, we aimed at determining the K4000 break for a selected sample among the potential blazar candidates from unidentified EGRET sources to confirm their blazar nature. We used two blazar candidates, 3EGJ1800-0146 and 3EGJ1709-0817 associated with radio counterparts, J1802-0207 and J1713-0817, respectively. Their optical counterparts were obtained through spectroscopic observations using Robert Stobie spectrograph (RSS) at the Southern African Large Telescope (SALT) in South Africa. The observed Ca II H & K lines depression at 4000 Å in spectra of these sources show a shallow depression, K4000=0.35+-0.02 and 0.24+-0.01, respectively, suggesting that these sources are blazar candidates. Moreover, the redshifts z=0.165 and 0.26 measured in their spectra confirm the extragalactic nature of these sources. Keywords: line: identification, radiation mechanisms: non-thermal, techniques: spectroscopic, galaxies: active, galaxies: jets, galaxies: BL Lacertae objects

astro-ph.HE↗

arXiv2020

High-resolution spectroscopy of flares and CMES on AD Leo*

Flares and coronal mass ejections (CMEs) are important for the evolution of the atmospheres of planets and their potential habitability, particularly for planets orbiting M stars at a distance < 0.4 AU. Detections of CMEs on these stars have been sparse, and previous studies have therefore modelled their occurrence frequency by scaling up solar relations. However, because the topology and strength of the magnetic fields on M stars is different from that of the Sun, it is not obvious that this approach works well. We used a large number of high-resolution spectra to study flares, CMEs, and their dynamics of the active M dwarf star AD Leo. The results can then be used as reference for other M dwarfs. We obtained more than 2000 high-resolution spectra (R~35000) of the highly active M dwarf AD Leo, which is viewed nearly pole on. Using these data, we studied the behaviour of the spectral lines Halpha , Hbeta , and He I 5876 in detail and investigated asymmetric features that might be Doppler signatures of CMEs. We detected numerous flares. The largest flare emitted 8.32 x 10 31 erg in Hbeta and 2.12 x 10 32 erg in Halpha . Although the spectral lines in this and other events showed a significant blue asymmetry, the velocities associated with it are far below the escape velocity. Although AD Leo shows a high level of flare activity, the number of CMEs is relatively low. It is thus not appropriate to use the same flare-to-CME relation for M dwarfs as for the Sun.

astro-ph.SR↗