arXiv subjects

Antoine Mahoro

Publications and source records attributed to Antoine Mahoro.

10 recordsLinked to original sources

arXiv2026

A Slit Mask Integral Field Unit for the Robert Stobie Spectrograph on the Southern African Large Telescope: I. Instrument Development

Integral field spectroscopy (IFS) has been added as a new observation mode to the Robert Stobie Spectrograph (RSS), the workhorse multi-mode instrument on the Southern African Large Telescope. RSS operates as an imaging spectrograph covering 320-900 nm with a spectral resolution--slit-width product of 6600 arcsec. Using fiber optics and prismatic fold mirrors, we have been able to construct compact integral field units (IFUs) that fit within the same volume as the long-slit cassettes (134 mm x 130 mm x 8 mm). These `slit mask' IFUs (SMIs) direct the telescope beam into a 2D sky-facing fiber array routed in the focal plane dimension into an 8$\arcmin$ 1D pseudo-slit, with fiber output redirected back into the spectrograph collimator. The first completed unit, SMI-200, features 303 object fibers and 24 sky fibers, providing a spatial resolution of 0.8$\arcsec$ (200 $\upmu$m core diameter) over a field of view (FOV) of 22.5$\arcsec$ x 17.6$\arcsec$. This paper describes the specific design considerations and design and fabrication strategies to maximize performance and minimize risk during construction, given the demanding and highly constrained cassette geometry. We also detail mapping and laboratory characterization of the IFU. Laboratory measurements demonstrate a total throughput of 77\%, but an effective throughput of only 55-60\% within the RSS collimator acceptance beam of f/4.2 due to losses dominated by focal ratio degradation (FRD) induced by sharp bend radii imposed by the tight cassette volume.

astro-ph.IM↗

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

AGN contribution on the morphological parameters of their host galaxies up to intermediate redshifts of z~2

The presence of Active Galaxy Nuclei (AGN) can affect the morphological classification of galaxies. This work aims to determine how the contribution of AGN affects the most used morphological parameters down to the redshift of z~2 in COSMOS-like conditions. We use a sample of > 2000 local non-active galaxies, with a well-known visual morphological classification, and add an AGN as an unresolved component that contributes to the total galaxy flux with 5%-75%. We moved all the galaxies to lower magnitudes (higher redshifts) to map the conditions in the COSMOS field, and we measured six morphological parameters. The greatest impact on morphology occurs when considering the combined effect of magnitude, redshift and AGN, with spiral galaxies being the most affected. In general, all the concentration parameters change significantly if the AGN contribution is > 25% and the magnitude > 23. We find that the GINI coefficient is the most stable in terms of AGN and magnitude/redshift, followed by the M20, Conselice-Bershady (CCON), and finally the Abraham (CABR) concentration indexes. We find that, when using morphological parameters, the combination of CABR, CCON and asymmetry is the most effective in classifying active galaxies at high-redshift, followed by the combination of CABR and GINI.

astro-ph.GA↗

arXiv2024

Emergence hour-by-hour of $r$-process features in the kilonova AT2017gfo

The spectral features in the optical/near-infrared counterparts of neutron star mergers (kilonovae, KNe), evolve dramatically on hour timescales. To examine the spectral evolution we compile a temporal series complete at all observed epochs from 0.5 to 9.4 days of the best optical/near-infrared (NIR) spectra of the gravitational-wave detected kilonova AT2017gfo. Using our analysis of this spectral series, we show that the emergence times of spectral features place strong constraints on line identifications and ejecta properties, while their subsequent evolution probes the structure of the ejecta. We find that the most prominent spectral feature, the 1$\mathrmμ$m P Cygni line, appears suddenly, with the earliest detection at 1.17 days. We find evidence in this earliest feature for the fastest kilonova ejecta component yet discovered, at 0.40-0.45$c$; while across the observed epochs and wavelengths, the velocities of the line-forming regions span nearly an order of magnitude, down to as low as 0.04-0.07$c$. The time of emergence closely follows the predictions for Sr II, due to the rapid recombination of Sr III under local thermal equilibrium (LTE) conditions. The time of transition between the doubly and singly ionised states provides the first direct measurement of the ionisation temperature, This temperature is highly consistent, at the level of a few percent, with the temperature of the emitted blackbody radiation field. Further, we find the KN to be isotropic in temperature, i.e. the polar and equatorial ejecta differ by less than a few hundred Kelvin or within 5%, in the first few days post-merger, based on measurements of the reverberation time-delay effect. This suggests that a model with very simple assumptions, with single-temperature LTE conditions, reproduces the early kilonova properties surprisingly well.

astro-ph.HE↗

arXiv2023

The [OIII] profiles of far-infrared active and non-active optically-selected green valley galaxies

We present a study of the $\rm{[OIII]λ\,5007}$ line profile in a sub-sample of 8 active galactic nuclei (AGN) and 6 non-AGN in the optically-selected green valley at $\rm{z\,<\,0.5}$ using long-slit spectroscopic observations with the 11 m Southern African Large Telescope. Gaussian decomposition of the line profile was performed to study its different components. We observe that the AGN profile is more complex than the non-AGN one. In particular, in most AGN (5/8) we detect a blue wing of the line. We derive the FWHM velocities of the wing and systemic component, and find that AGN show higher FWHM velocity than non-AGN in their core component. We also find that the AGN show blue wings with a median velocity width of approximately 600 $\rm{km\,s^{-1}}$, and a velocity offset from the core component in the range -90 to -350 $\rm{km\,s^{-1}}$, in contrast to the non-AGN galaxies, where we do not detect blue wings in any of their $\rm{[OIII]λ\,5007}$ line profiles. Using spatial information in our spectra, we show that at least three of the outflow candidate galaxies have centrally driven gas outflows extending across the whole galaxy. Moreover, these are also the galaxies which are located on the main sequence of star formation, raising the possibility that the AGN in our sample are influencing SF of their host galaxies (such as positive feedback). This is in agreement with our previous work where we studied SF, morphology, and stellar population properties of a sample of green valley AGN and non-AGN galaxies.

astro-ph.GA↗

arXiv2022

Stellar populations of a sample of far-IR AGN and non-AGN green valley galaxies

We present a study on the stellar populations and stellar ages of sub-sample of far-infrared AGN and non-AGN green valley analysed in Mahoro et al. (2017,2019) at 0.6 < z < 1.0 using the data from the COSMOS field. We used long-slit spectroscopy and derived stellar populations and stellar ages using the stellar population synthesis code "STARLIGHT" and analysed the available Lick/IDS indices, such as Dn4000 and $\rm{Hδ_{A}}$. We find that both FIR AGN and non-AGN green valley galaxies are dominated by intermediate stellar populations 67 % and 53 %, respectively. The median stellar ages for AGN and non-AGN are log t = 8.5 [yr] and log t = 8.4 [yr], respectively. We found that majority of our sources (62 % of AGN and 66 % non-AGN) could have experienced bursts and continuous star formation. In addition, most of our FIR AGN (38 %) compared to FIR non-AGN (27 %) might have experienced a burst of SF more than 0.1 Gyr ago. We also found that our FIR AGN and non-AGN green valley galaxies have similar quenching time-scales of ~ 70 Myr. Therefore, the results obtained here are in line with our previous results where we do not find that our sample of FIR AGN in the green valley shows signs of negative AGN feedback, as has been suggested previously in optical studies.

astro-ph.GA↗

arXiv2020

Properties of green valley galaxies in relation to their selection criteria

The distribution of galaxies has been studied to show the difference between the blue cloud and red sequence and to define the green valley region. However, there are still many open questions regarding the importance of the green valley for understanding the morphological transformation and evolution of galaxies, how galaxies change from late-type to early-type and the role of AGN in galaxy formation and evolution scenario. The work focused on studying in more details the properties of green valley galaxies by testing the six most used selection criteria, differences between them, and how they may affect the main results and conclusions. The main findings are that, by selecting the green valley galaxies using different criteria, we are selecting different types of galaxies in terms of their stellar masses, sSFR, SFR, spectroscopic classification and morphological properties, where the difference was more significant for colour criteria than for sSFR and SFR vs. stellar-mass criteria.

astro-ph.GA↗

arXiv2020

Properties of X-ray detected far-IR AGN in the green valley

In this study, we analysed active galactic nuclei in the "green valley" by comparing active and non-active galaxies using data from the COSMOS field. We found that most of our X-ray detected active galactic nuclei with far-infrared emission have star formation rates higher than the ones of normal galaxies of the same stellar mass range.

astro-ph.GA↗

arXiv2019

Star formation in far-IR AGN and non-AGN galaxies in the green valley. II. Morphological analysis

This paper studies morphological properties of 103 green valley FIR active and 2609 non-active galaxies presented in Mahoro et al. 2017. The photometric data from the COSMOS survey were used, and the morphological parameters, such as Abraham and Conselice-Bershady concentration indices, Gini, M20 moment of light, and asymmetry, were analysed taking into account public catalogues. Furthermore, a visual classification of galaxies was performed. We found that the fraction of peculiar galaxies with clear signs of interactions and mergers is significantly higher in AGN (38%) than non-AGN (19%) green valley galaxies, while non-AGN galaxies from our sample are predominantly spirals (46%). We found that the largest fraction of our green valley galaxies is located on the main-sequence (MS) of star formation, independently on morphology, which is in contrast with most of previous studies carried out in optical. We also found that FIR AGN green valley galaxies have significantly higher star formation rates in all analysed morphological types. Therefore, our results suggest that interactions and mergers obtained in the high fraction of FIR AGN contribute significantly to high star formation rates observed in the selected sample, but are not the only mechanism responsible for enhancing star formation, and others such as AGN positive feedback could contribute as well. In future we will study in more details the possibility of AGN positive feedback through the spectroscopic analysis of public and our SALT data.

astro-ph.GA↗

arXiv2017

Star formation of far-IR AGN and non-AGN galaxies in the green valley: possible implication of AGN positive feedback

In this paper we present the star formation properties of $\rm{I_{subaru}\leq}$ 23 X-ray detected AGN and non-AGN galaxies in the green valley with far-IR (FIR) emission, using data from the COSMOS field. We measured star formation rates (SFR) using FIR Herschel/PACS data and we observed the location of AGN and non-AGN galaxies on the main-sequence of star formation. We went a step further in analysing the importance of AGN in quenching star formation in the green valley, the region proposed to be the transitional phase in galaxy evolution where galaxies are moving from later- to earlier-types. We found that most of our green valley X-ray detected AGN with far-IR emission have SFRs higher than the ones of inactive galaxies at fixed stellar mass ranges, the result that is different when considering optical data. These FIR AGN have still very active star formation, being located either on or above the main-sequence of star formation (in total 82\% of our sample). Therefore, they do not show signs of star formation quenching, but rather its enhancement. Our results may suggest that for X-ray detected AGN with FIR emission if there is an influence of AGN feedback on the star formation in the green valley the scenario of AGN positive feedback seem to take place, rather than the negative one.

astro-ph.GA↗