arXiv subjects

César Esteban

Publications and source records attributed to César Esteban.

12 recordsLinked to original sources

arXiv2026

There is no single density: star-forming regions and galaxies hold more dense ionized gas than long assumed

Ionized gas fills star-forming regions and galaxies, and nearly everything we know about its temperature, pressure, mass, and composition is inferred from its emission lines [1-3]. The electron density is needed for all of these, yet a longstanding puzzle has resisted explanation: different density-sensitive lines, applied to the same gas, return values that disagree by up to two orders of magnitude. This is usually attributed either to each line tracing a physically distinct ionization zone or to imperfect atomic data [4-7]. Here we show that the disagreement is neither a flaw in the atomic data nor an ionization-stratification effect, but something more fundamental. Each diagnostic is tuned to a particular density, and when a nebula contains gas across a wide range of densities, as real nebulae do, each line reports the part of that range it is most sensitive to. The diagnostics do not measure a representative average density; they respond to different parts of a broad density distribution. This resolves the discrepancy with a simple relation between the density each line returns and the density it is most sensitive to, a relation that holds from individual H II regions to whole galaxies, near and far, and reveals that ionized nebulae contain far more dense gas than any one diagnostic implies. A nebula has no single electron density to measure, but a broad density distribution, and the masses, pressures, abundances and energetics built on the single-density assumption must be reconsidered, from nearby star-forming regions to galaxies across cosmic time.

astro-ph.GA↗

arXiv2026

Self-Consistent Direct Method for Chemical Abundances in High-z Galaxies with JWST

The unprecedented rest-frame UV and optical coverage provided by JWST enables simultaneous constraints on the electron density (n$_{\rm e}$) and temperature (T$_{\rm e}$) of ionized gas in galaxies at z>5. We present a self-consistent direct method based on multiple OIII]1661,66) and [OIII] ($λ$4363, and $λ$5007) transitions to characterize the physical conditions of the high-ionization zone. This new approach is insensitive to a wide range of n$_{\rm e}$ due to the high critical densities of the OIII] and [OIII] transitions. Applying this technique to six galaxies at z=5-9, we find electron densities up to n$_{\rm e}$$\sim 3\times 10^{5}$ cm$^{-3}$ and temperatures of T$_{\rm e}$ $\sim 20,000$ K in systems at $z>6$. Accounting for these self-consistent densities changes the derived T$_{\rm e}$ and modifies the inferred metallicities by up to 0.29 dex relative to previous estimates. We discuss the reported N/O overabundances in the high-$z$ galaxies from our sample, which arise entirely from the high N$^{3+}$/H$^{+}$ values inferred from NIV] lines. We point out that a T$_{\rm e}$-stratification, in which the N$^{3+}$ zone has a slightly higher T$_{\rm e}$ than T$_{\rm e}$([OIII]), could substantially reduce the inferred N/O. Quantitatively, if T$_{\rm e}$(N$^{3+}$) were 10\% higher than T$_{\rm e}$([OIII]), this could induce a systematic overestimation of N$^{3+}$/O$^{2+}$ of nearly 50\%. Classical N/O diagnostics such as N$^{+}$/O$^{+}$, due to their critical densities, can significantly impact the inferred N/O abundance in the presence of high-density gas, whereas N$^{2+}$/O$^{2+}$ place these galaxies closer to $z\sim0$ systems in the N/O-O/H plane. Future JWST programs with larger and more diverse samples will be essential to test the universality and robustness of these results.

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arXiv2026

The DESIRED temperature-metallicity relations in star-forming regions: probing the Galactic radial and azimuthal metallicity distributions

We analyse a sample of 225 star-forming regions from the DESIRED-E project, each with simultaneous determinations of the electron temperature from ionized nitrogen and oxygen, $T_{\rm e}$([NII]) and $T_{\rm e}$([OIII]), respectively. We derive new empirical relations connecting the gas-phase metallicity to the global electron temperature, $T_{\rm e}$(H$^+$), as determined via radio observations. We establish two calibrations: one assuming a homogeneous temperature distribution ($t^2 = 0$, the ``direct method''), and another accounting for internal temperature fluctuations ($t^2 > 0$). Applying these calibrations to 460 radio observations of Galactic HII~regions spanning Galactocentric distances from $\sim0.1$ to 16 kpc, we determine the radial O/H gradient in the Milky Way under both assumptions. We further compare these nebular gradients to independent metallicity estimates from young O- and B-type stars and Cepheid variables. We find that the $t^2 > 0$ calibration yields a gradient in excellent agreement with stellar-based determinations, whereas the $t^2 = 0$ method underestimates metallicities by up to $\sim$0.3 dex. This discrepancy cannot be reconciled by invoking oxygen depletion onto dust grains or nucleosynthetic processing via the CNO cycle in massive stars. We also find that one widely used relation in the literature, assuming $t^2 = 0$, produces an excessively steep gradient -- likely due to the use of outdated atomic data and pre-CCD observations. Finally, we explore potential azimuthal variations in the Galactic metallicity distribution driven by the presence of the spiral arms, finding no evidence for variations larger than $\sim$0.1 dex with respect to the general radial gradient.

astro-ph.GA↗

arXiv2024

On the chlorine abundance in H II regions

Chlorine (Cl) is a chemical element of the group of the halogens and is between the 17th and the 20th most abundant elements in the Solar System. It is thought to be produced from the capture of a proton or neutron by specific alpha-element isotopes during both hydrostatic and explosive oxygen burning, though some contribution may come from Type Ia supernovae. Cl lines are quite rare in stellar spectra, so most of the information available about its abundance comes from analyzing the emission lines of ionized nebulae, especially the collisionally excited lines of Cl2+ ([Cl III] λλ5518,5538). Our goal is to accurately determine the Cl abundance in H II regions, and gather more information about its nucleosynthetic origin. For this work we used a sample of observations that encompasses the deepest spectra of H II regions available in the literature, from both the Milky Way and other galaxies in the local Universe, covering a range of oxygen (O) abundances, 12+log(O/H), from 7.18 to 8.70. As a first step, we determine the most representative electron temperature of the zone of the nebulae where the Cl2+ ion lies. To this aim we used a grid of photoionization models and diagnostics valid for other ions, as that parameter cannot be determined directly through [Cl III] lines. We then computed the total Cl abundance using different sets of ionization correction factors to account for the contribution from unseen ionization stages.

astro-ph.GA↗

arXiv2023

Temperature inhomogeneities in Mrk71 can not be discarded

In a very recent work, [1] claim that the scenario of temperature inhomogeneities proposed by [2] ($t2$ > 0) is not able to explain the O$^{2+}$/H$^{+}$ abundance discrepancy observed between the calculations based on the optical [OIII] collisional excited lines (CELs) and the OII recombination lines (RLs) in the star forming galaxy Mrk71. In this work, we show that conclusions of [1] depend on several assumptions on the absolute flux calibration, reddening correction and the adopted electron density. In fact, using the data of [1] in a different way and even considering their 1σ uncertainties, it is possible to reach the opposite conclusion, consistent with $t2$ = $0.097 ^{+0.008}_{-0.009}$. Therefore, the existence of temperature inhomogeneities causing the O$^{2+}$/H$^{+}$ abundance discrepancy in Mrk71 can not be ruled out.

astro-ph.GA↗

arXiv2023

Temperature inhomogeneities cause the abundance discrepancy in H II regions

HII regions, ionized nebulae where massive star formation has taken place, exhibit a wealth of emission lines that are the fundamental basis for estimating the chemical composition of the Universe. For more than 80 years, a discrepancy of at least a factor of two between heavy-element abundances derived with collisional excited lines (CELs) and the weaker recombination lines (RLs) has thrown our absolute abundance determinations into doubt. Heavy elements regulate the cooling of the interstellar gas, being essential to the understanding of several phenomena such as nucleosynthesis, star formation and chemical evolution. In this work, we use the best available deep optical spectra of ionized nebulae to analyze the cause of this abundance discrepancy problem. We find for the first time general observational evidence in favor of the temperature inhomogeneities within the gas, quantified by t2. The temperature inhomogeneities inside H II regions are affecting only the gas of high ionization degree and producing the abundance discrepancy problem. This work implies that the metallicity determinations based on CELs must be revised, as they can be severely underestimated, especially in the regions of lower metallicity, such as the JWST high-z galaxies. We present methods to estimate these corrections, which will be critical for robust interpretations of the chemical composition of the Universe over cosmic time.

astro-ph.GA↗

arXiv2020

On the radial abundance gradients of nitrogen and oxygen in the inner Galactic disc

We present optical spectra of nine Galactic H II regions observed with the 10.4 m Gran Telescopio Canarias telescope and located at Galactocentric distances (RG) from 4 to 8 kpc. The distances of the objects have been revised using Gaia DR2 parallaxes. We determine the electron temperature for all the nebulae, which allows a precise computation of their ionic abundances. We have included published data of an additional sample of Galactic H II regions, providing a final data set of 42 objects. The shape of the radial gradients of O/H and N/H is linear and constant, discarding any substantial change of the slope, at least for RG between 4 and 17 kpc. The small dispersion of the O/H and N/H values with respect to the computed gradients implies the absence of significant azimuthal variations of the chemical abundances, at least in the quadrant covered by our observations. We find an almost flat N/O versus O/H diagram relation. This result is not observed in other nearby spiral galaxy except M31. Finally, we compare our computed gradients with those obtained using far infrared (FIR) spectra. We confirm the significant offset in the N/O distribution between the optical and FIR observations. Possible explanations involve ionization correction factors and the strong dependence on density of the abundance determinations based on FIR lines.

astro-ph.GA↗

arXiv2020

The Galactic radial abundance gradients of C, N, O, Ne, S, Cl and Ar from deep spectra of H II regions

We present a reassessment of the radial abundance gradients of C, N, O, Ne, S, Cl and Ar in the Milky Way using deep spectra of 33 H II regions gathered from the literature, covering Galactocentric distances from 6 to 17 kpc. The distances of the objects have been revised using Gaia parallaxes. We recalculate the physical conditions and ionic abundances in an homogeneous way using updated atomic data. All the objects have direct determination of the electron temperature, permitting to derive their precise ionic abundances. We analyze and compare different ICF schemes for each element in order to obtain the most confident total abundances. Due to the revised distances, our results do not support previous claims about a possible flattening of the O/H gradient in the inner Galactic disk. We find that the Galactic N/O gradient is rather flat, in contrast to what has been found in other spiral galaxies. The slope of the gradients of some elements is sensitive to the ICF scheme used, especially in the case of Ne. The dispersion around the fit for the gradients of C, N, O, S, Cl and Ar is of the order of the typical uncertainties in the determination of the abundances, implying the absence of significant inhomogeneities in the chemical composition of the ionized gas phase of the ISM. We find flat gradients of log(S/O) and log(Cl/O) and very shallow or flat ones for log(Ne/O) and log(Ar/O), consistent with a lockstep evolution of Ne, S, Cl and Ar with respect to O.

astro-ph.GA↗

arXiv2015

The Fe/Ni ratio in ionized nebulae: clues on dust depletion patterns

We perform a homogeneous analysis of the Fe/Ni abundance ratio in eight Galactic planetary nebulae (PNe) and three Galactic H II regions that include the Orion nebula, where we study four nebular zones and one shocked region. We use [Fe ii], [Fe iii], and [Ni iii] lines, and ionization correction factors (ICFs) that account for the unobserved ions. We derive an ICF for nickel from an extensive grid of photoionization models. We compare our results with those derived by other authors for 16 neutral clouds in the solar neighbourhood with available Fe/Ni ratios in the literature. We find an excellent agreement between the ionized nebulae and the diffuse clouds, with both types of regions showing a clear correlation between the Fe/Ni ratios and the iron and nickel depletion factors. The trend shows that the objects with a relatively low depletion have near solar Fe/Ni ratios whereas at higher depletions the Fe/Ni ratio increases with the depletion. Our results confirm that, compared to iron atoms, nickel ones are more efficiently stuck to the dust grains in ambients where dust formation or growth have been more efficient.

astro-ph.SR↗

arXiv2007

Small scale behavior of the physical conditions and the abundance discrepancy in the Orion nebula

We present results of long-slit spectroscopy in several positions of the Orion nebula. Our goal is to study the spatial distribution of a large number of nebular quantities, including line fluxes, physical conditions and ionic abundances at a spatial resolution of about 1". We find that protoplanetary disks (proplyds) show prominent spikes of T([N II]) probably produced by collisional deexcitation due to the high electron densities found in these objects. Herbig-Haro objects show also relatively high T([N II]) but probably produced by local heating due to shocks. We also find that the spatial distribution of pure recombination O II and [O III] lines is fairly similar, in contrast to that observed in planetary nebulae. The abundance discrepancy factor (ADF) of O^{++} remains rather constant along the slit positions, except in some particular small areas of the nebula where this quantity reaches somewhat higher values, in particular at the location of the most conspicuous Herbig-Haro objects: HH 202, HH 203, and HH 204. There is also an apparent slight increase of the ADF in the inner 40" around theta^1 Ori C. We find a negative radial gradient of T([O III]) and T([N II]) in the nebula based on the projected distance from theta^1 Ori C. We explore the behavior of the ADF of O^{++} with respect to other nebular quantities, finding that it seems to increase very slightly with the electron temperature. Finally, we estimate the value of the mean-square electron temperature fluctuation, the so-called t^2 parameter. Our results indicate that the hypothetical thermal inhomogeneities --if they exist-- should be smaller than our spatial resolution element.

astro-ph↗

arXiv2007

Interactions and star formation activity in Wolf-Rayet galaxies

We present the main results of the PhD Thesis carried out by López-Sánchez (2006), in which a detailed morphological, photometrical and spectroscopical analysis of a sample of 20 Wolf-Rayet (WR) galaxies was realized. The main aims are the study of the star formation and O and WR stellar populations in these galaxies and the role that interactions between low surface companion objects have in the triggering of the bursts. We analyze the morphology, stellar populations, physical conditions, chemical abundances and kinematics of the ionized gas, as well as the star-formation activity of each system.

astro-ph↗

arXiv2004

Massive star formation and tidal structures in HCG 31

We present new broad-band optical and near-infrared CCD imaging together with deep optical intermediate-resolution spectroscopy of the Hickson Compact Group 31. We analyze the morphology and colors of the stellar populations of the galaxies, as well as the kinematics, physical conditions and chemical composition of the ionized gas in order to get a more complete view on the origin and evolution of the system. We estimate the ages of the most recent star formation bursts of the system, finding an excellent consistency among the values obtained with different indicators and starburst models. We find that member F hosts the youngest starburst of the group, showing a substantial population of Wolf-Rayet stars. The chemical abundances are fairly similar in all the members of the group despite their very different absolute magnitudes. We argue that the use of traditional metallicity-luminosity relations based on the absolute $B$-magnitude is not appropriate for dwarf starburst galaxies, because their luminosity is dominated by the transient contribution of the starburst to the blue luminosity. We think that members E and F of the group are candidate tidal dwarf galaxies because of their high metallicity, their kinematics, and the absence of underlying old stellar populations. Finally, we propose that HCG~31 is suffering several almost simultaneous interaction processes. The most relevant of these processes are: (a) the merging of members A and C, that would have produced two optical tidal tails; and (b) a fly-by encounter between G and the A+C complex, that would have produced an \ion{H}{1} tidal tail from the stripping of the external gas of A+C, from which members F and E have originated.

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