arXiv subjects

Alfredo Mejía-Narváez

Publications and source records attributed to Alfredo Mejía-Narváez.

8 recordsLinked to original sources

arXiv2026

SDSS-V Local Volume Mapper (LVM): The Integrated Light and Internal Rotation of Omega Centauri

The SDSS-V Local Volume Mapper (LVM) is a wide-field integral field spectroscopic survey of the Southern Milky Way plane, the Magellanic Clouds, and nearby Local Group galaxies. We use Early Science observations of the whole body of the nearest nuclear cluster, Omega Centauri, to extend the LVM beyond its primary interstellar-medium science case. The wide LVM field allows us to precisely map $ω$ Cen's line-of-sight rotation out to $\sim 3r_{HL}$ or $15^\prime$, reaching a maximum value of $(8.4 \pm 0.8)$ km s$^{-1}$ at $r \approx 4.7^\prime$. Within the central region, comparisons with existing VLT MUSE oMEGACat data show explicitly that the unresolved-light signal is dominated by a small number of bright stars, with an effective sample size of only $\sim$12 per resolution element. Using Gaia DR3 as an external reference, we verify that the SDSS-V's LVM reduction pipeline recovers integrated stellar fluxes to 1-4 % across six magnitudes of surface brightness. Our resulting total spectrum of $ω$ Cen is one of the highest S/N integrated spectrum for any globular or nuclear star cluster. We use it to test four widely-used SSP template libraries against resolved age-metallicity ground truth from oMEGACat. All templates recover an old, metal-poor population. But, even at S/N $\sim$1300, the inferred mean ages and mean [Fe/H] vary by $\sim$7 Gyr and $\sim$0.4 dex, respectively, across libraries and wavelength ranges, reflecting a systematic floor for integrated-light studies of old multi-population systems.

astro-ph.GA↗

arXiv2026

SDSS-V LVM: A spatially resolved study of the physical conditions and the chemical abundance discrepancy in the Lagoon Nebula (M 8)

The abundance discrepancy problem refers to the systematic differences observed between chemical abundances derived from collisionally excited lines (CELs) and recombination lines (RLs) of heavy ions. It remains a major unsolved problem in the study of ionized nebulae and is quantified by the abundance discrepancy factor (ADF). In this work, we present a deep integral field spectroscopic dataset covering the entire Lagoon Nebula (M 8), obtained by the SDSS-V Local Volume Mapper project, at a spatial resolution of 0.21 pc per spaxel. This unique dataset allows us, for the first time, to investigate spatially resolved maps of oxygen RL intensities (O II V1), together with maps of H I RLs, heavy-ion CELs, and dust attenuation across a whole H II region. We map the electron temperature using CELs and RLs of $O^{2+}$, CELs of $N^{+}$, and the electron density using CELs of $S^{+}$. We derive CEL-based ionic and elemental oxygen abundances and, for the first time, a spatially resolved map of the RL-based $O^{2+}$ abundance in an H II region. These measurements enable the construction of the first spatially resolved ADF($O^{2+}$) map of an H II region and yield a global mean ADF of ~0.47 +/- 0.02 dex. Focusing on the central region of M 8, where ionization is dominated by the O-type star Her 36, we find radial variations in the ADF ranging between ~0.35-0.50 dex. Our findings provide novel constraints on the spatial behavior and origin of the abundance discrepancy in H II regions.

astro-ph.GA↗

arXiv2025

SDSS-V LVM: Resolving Physical Conditions in the Trifid Nebula

The chemical abundance of the interstellar medium sets the initial conditions for star formation and provides a probe of chemical galaxy evolution models. However, unresolved inhomogeneities in the electron temperature can lead to a systematic underestimation of the abundances. We aim to directly test this effect. We use the SDSS-V Local Volume Mapper to spatially map the physical conditions of the Trifid Nebula (M 20), a Galactic H II region ionized by a single mid-type O star, at 0.24 pc resolution. We exploit various emission lines (e.g., Hydrogen recombination lines and collisionally excited lines, including also faint auroral lines) and compute spatially resolved maps of [O II] and [S II] electron densities; [N II], [O II], [S II], [S III] electron temperatures; and the ionic oxygen abundances. We find internal variations of electron density that result from the ionization front, along with a negative radial gradient. However, we do not find strong gradients or structures in the electron temperature and the total oxygen abundance, making the Trifid Nebula a relatively homogeneous H II region at the observed spatial scale. We compare these spatially resolved properties with equivalent integrated measurements of the Trifid Nebula and find no significant variations between integrated and spatially resolved conditions. This isolated H II region, ionized by a single O-star, represents a test case of an ideal Strömgren sphere. The physical conditions in the Trifid Nebula behave as expected, with no significant differences between integrated and resolved measurements.

astro-ph.GA↗

arXiv2025

SDSS-V LVM: Collisionless Shocks in the Supernova Remnant RCW86

The supernova remnant (SNR) RCW86 is among the few SNRs with Balmer-emission lines containing broad and narrow spectral components that trace fast, non-radiative shocks in partially-ionized gas.\ These are invaluable laboratories for collisionless shock physics, especially for poorly-understood phenomena like electron-ion equilibration, and shock precursors. Here we present the first $\sim$0.3 pc spatial scale integral field unit (IFU) observations of the southwestern RCW86 shock, obtained as part of the Sloan Digital Sky Survey-V Local Volume Mapper (SDSS-V LVM). The forward shock, clearly visible as thin filaments in narrowband images, have broad H$α$ components, indicating shock velocities varying from 500--900 km/s in the south to 1000--1500 km/s in the north. The varying velocity widths and broad-to-narrow intensity ratios show that electrons and ions have lower equilibration ($T_e/T_p \rightarrow 0.1$) in faster ($>$800 km/s) shocks, in line with previous studies. The broad components are generally redshifted from the narrow components by $\lesssim$100 km/s, likely due to shock-obliquity or non-Maxwellian post-shock distributions. We observe high extinction-corrected Balmer-decrements of 3--5 in the narrow components, indicating that conversion of Ly$β$ photons to H$α$ is more efficient than Ly$γ$ to H$β$. Broad HeII$λ$4686 was marginally ($\gtrsim$2$σ$) detected in the southern shock, meaning the shock is impacting gas with high ($>$30--100\%) neutral fraction. We also find the first evidence of an intermediate H$α$ component in RCW86, with $Δ$V(FWHM) = 193--207 km/s, likely due to a neutral precursor. We also briefly discuss the southwestern radiative shock, and lay out the exciting future of studying astrophysical shocks with LVM.

astro-ph.HE↗

arXiv2022

CoSHA: Code for Stellar properties Heuristic Assignment -- for the MaStar stellar library

We introduce \cosha{}: a Code for Stellar properties Heuristic Assignment. In order to estimate the stellar properties, \cosha{} implements a Gradient Tree Boosting algorithm to label each star across the parameter space ($T_\mathrm{eff}$, $\log{g}$, $[\mathrm{Fe}/\mathrm{H}]$, and $[α/\mathrm{Fe}]$). We use \cosha{} to estimate these stellar atmospheric parameters of $22\,$k unique stars in the MaNGA Stellar Library (MaStar). To quantify the reliability of our approach, we run both internal tests using the Göttingen Stellar Library (GSL, a theoretical library) and the first data release of MaStar, and external tests by comparing the resulting distributions in the parameter space with the APOGEE estimates of the same properties. In summary, our parameter estimates span in the ranges: $T_\mathrm{eff}=[2900,12000]\,$K, $\log{g}=[-0.5,5.6]$, $[\mathrm{Fe}/\mathrm{H}]=[-3.74,0.81]$, $[α/\mathrm{Fe}]=[-0.22,1.17]$. {We report internal (external) uncertainties of the properties of $σ_{T_\mathrm{eff}}\sim43\,(240)\,$K, $σ_{\log{g}}\sim0.2\,(0.4)$, $σ_{[\mathrm{Fe}/\mathrm{H}]}\sim0.16\,(0.24)$, $σ_{[α/\mathrm{Fe}]}\sim0.09\,(0.08)$.} These uncertainties are comparable to those of other methods with similar objectives. Despite the fact that \cosha{} is not aware of the spatial distribution of these physical properties in the Milky Way, we are able to recover the main trends known in the literature. The catalog of physical properties for MaStar can be accessed in \url{http://ifs.astroscu.unam.mx/MaStar}.

astro-ph.SR↗

arXiv2022

Spectral Evidence of Solar Neighborhood Analogs in CALIFA Galaxies

We introduce a novel non-parametric method to find solar neighborhood analogs (SNAs) in extragalactic IFS surveys. The main ansatz is that the physical properties of the solar neighborhood (SN) should be encoded in its optical stellar spectrum. We assume that our best estimate of such spectrum is the one extracted from the analysis performed by the Code for Stellar properties Heuristic Assignment (\textsc{CoSHA}) from the MaStar stellar library. It follows that finding SNAs in other galaxies consist in matching, in a $χ^2$ sense, the SN reference spectrum across the optical extent of the observed galaxies. We apply this procedure to a selection of CALIFA galaxies, by requiring a close to face-on projection, relative isolation and non-AGN. We explore how the local and global properties of the SNAs (stellar age, metallicity, dust extinction, mass-to-light ratio, stellar surface mass and star-formation densities and galactocentric distance) and their corresponding host galaxies (morphological type, total stellar mass, star-formation rate, effective radius) compare with those of the SN and the Milky Way (MW). We find that SNAs are located preferentially in S(B)a~--~S(B)c galaxies, in a ring-like structure, which radii seems to scale with the galaxy size. Despite the known sources of systematics and errors, {most} properties present a considerable agreement with the literature on the SN. We conclude that the solar neighborhood is relatively common in our sample of SNAs. Our results warrants a systematic exploration of correlations among the physical properties of the SNAs and their host galaxies. We reckon that our method should inform current models of the galactic habitable zone in our MW and other galaxies.

astro-ph.GA↗

arXiv2022

Chemical evolution history of MaNGA galaxies

We show the results of a study using the spectral synthesis technique study for the full MaNGA sample showing their Chemical Enrichment History (ChEH) as well as the evolution of the stellar mass-metallicity relation (MZR) over cosmic time. We find that the more massive galaxies became enriched first and the lower mass galaxies did so later, producing a change in the MZR which becomes shallower in time. Separating the sample into morphology and star-forming status bins some particularly interesting results appear: The mass dependency of the MZR becomes less relevant for later morphological types, to the extent that it inverts for Sd/Irr galaxies, suggesting that morphology is at least as important a factor as mass in chemical evolution. The MZR for the full sample shows a flattening at the high-mass end and another at the low-mass range, but the former only appears for retired galaxies while the latter only appears for star-forming galaxies. We also find that the average metallicity gradient is currently negative for all mass bins but for low mass galaxies it was inverted at some point in the past, before which all galaxies had a positive gradient. We also compare how diverse the ChEHs are in the different bins considered as well as what primarily drives the diversity: How much galaxies become enriched or how quickly they do so.

astro-ph.GA↗

arXiv2015

On the recovery of galaxy properties from SED fitting solutions

We explore the ability of four different inverse population synthesis codes to recover the physical properties of galaxies from their spectra by SED fitting. Three codes, DynBaS, TGASPEX, and GASPEX, have been implemented by the authors and are described in detail in the paper. STARLIGHT, the fourth code, is publicly available. DynBaS selects dynamically a different spectral basis to expand the spectrum of each target galaxy; TGASPEX uses an unconstrained age basis, whereas GASPEX and STARLIGHT use for all fits a fixed spectral basis selected a priori by the code developers. Variable and unconstrained basis reflect the peculiarities of the fitted spectrum and allow for simple and robust solutions to the problem of extracting galaxy parameters from spectral fits. We assemble a Synthetic Spectral Atlas of Galaxies (SSAG), comprising 100,000 galaxy spectra corresponding to an equal number of star formation histories based on the recipe of Chen et al. (2012). We select a subset of 120 galaxies from SSAG with a colour distribution similar to that of local galaxies in the seventh data release (DR7) of the Sloan Digital Sky Survey (SDSS) and produce 30 random noise realisations for each of these spectra. For each spectrum we recover the mass, mean age, metallicity, internal dust extinction, and velocity dispersion characterizing the dominant stellar population in the problem galaxy. All methods produce almost perfect fits to the target spectrum, but the recovered physical parameters can differ significantly. Our tests provide a quantitative measure of the accuracy and precision with which these parameters are recovered by each method. From a statistical point of view all methods yield similar precisions, whereas DynBaS produces solutions with minimal systematic biases in the distributions of residuals for all of these parameters.

astro-ph.GA↗