arXiv subjects

L. Costantin

Publications and source records attributed to L. Costantin.

At least 37 records · Page 2Linked to original sources

arXiv2024

Spatially-resolved H$α$ and ionizing photon production efficiency in the lensed galaxy MACS1149-JD1 at a redshift of 9.11

We present MIRI/JWST medium-resolution spectroscopy (MRS) and imaging (MIRIM) of the lensed galaxy MACS1149-JD1 at a redshift of $z$=9.1092$\pm$0.0002 (Universe age about 530 Myr). We detect, for the first time, spatially resolved H$α$ emission in a galaxy at a redshift above nine. The structure of the H$α$ emitting gas consists of two clumps, S and N. The total H$α$ luminosity implies an instantaneous star-formation of 5.3$\pm$0.4 $M_{\odot}$ yr$^{-1}$ for solar metallicities. The ionizing photon production efficiency, $\log(ζ_\mathrm{ion})$, shows a spatially resolved structure with values of 25.55$\pm$0.03, 25.47$\pm$0.03, and 25.91$\pm$0.09 Hz erg$^{-1}$ for the integrated galaxy, and clumps S and N, respectively. The H$α$ rest-frame equivalent width, EW$_{0}$(H$α$), is 726$^{+660}_{-182}$ Ángstrom for the integrated galaxy, but presents extreme values of 531$^{+300}_{-96}$ Ángstrom and $\geq$1951 Ángstrom for clumps S and N, respectively. The spatially resolved ionizing photon production efficiency is within the range of values measured in galaxies at redshift above six, and well above the canonical value (25.2$\pm$0.1 Hz erg$^{-1}$). The extreme difference of EW$_{0}$(H$α$) for Clumps S and N indicates the presence of a recent (<5 Myrs) burst in clump N and a star formation over a larger period of time (e.g., $\sim$50 Myr) in clump S. Finally, clump S and N show very different H$α$ kinematics with velocity dispersions of 56$\pm$4 km s$^{-1}$ and 113$\pm$33 km s$^{-1}$, likely indicating the presence of outflows or increased turbulence in the clump N. The dynamical mass, $M_\mathrm{dyn}$= (2.4$\pm$0.5)$\times$10$^{9}$ $M_{\odot}$, is within the range measured with the spatially resolved [OIII]88$μ$m line.

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arXiv2024

Clumpy star formation and an obscured nuclear starburst in the luminous dusty z=4 galaxy GN20 seen by MIRI/JWST

Dusty star-forming galaxies emit most of their light at far-IR to mm wavelengths as their star formation is highly obscured. Far-IR and mm observations have revealed their dust, neutral and molecular gas properties. The sensitivity of JWST at rest-frame optical and near-infrared wavelengths now allows the study of the stellar and ionized gas content. We investigate the spatially resolved distribution and kinematics of the ionized gas in GN20, a dusty star forming galaxy at $z$=4.0548. We present deep MIRI/MRS integral field spectroscopy of the near-infrared rest-frame emission of GN20. We detect spatially resolved \paa, out to a radius of 6 kpc, distributed in a clumpy morphology. The star formation rate derived from \paa\ (144 $\pm$ 9 \msunperyear) is only 7.7 $\pm 0.5 $\% of the infrared star formation rate (1860 $\pm$ 90 \msunperyear). We attribute this to very high extinction (A$_V$ = 17.2 $\pm$ 0.4 mag, or A$_{V,mixed}$ = 44 $\pm$ 3 mag), especially in the nucleus of GN20, where only faint \paa\ is detected, suggesting a deeply buried starburst. We identify four, spatially unresolved, clumps in the \paa\ emission. Based on the double peaked \paa\ profile we find that each clump consist of at least two sub-clumps. We find mass upper limits consistent with them being formed in a gravitationally unstable gaseous disk. The UV bright region of GN20 does not have any detected \paa\ emission, suggesting an age of more than 10 Myrs for this region of the galaxy. From the rotation profile of \paa\ we conclude that the gas kinematics are rotationally dominated and the $v_{rot}/σ_{m} = 3.8 \pm 1.4$ is similar to low-redshift LIRGs. We speculate that the clumps seen in GN20 could contribute to building up the inner disk and bulge of GN20.

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arXiv2023

On the nature of disks at high redshift seen by JWST/CEERS with contrastive learning and cosmological simulations

Visual inspections of the first optical rest-frame images from JWST have indicated a surprisingly high fraction of disk galaxies at high redshifts. Here, we alternatively apply self-supervised machine learning to explore the morphological diversity at $z \geq 3$. Our proposed data-driven representation scheme of galaxy morphologies, calibrated on mock images from the TNG50 simulation, is shown to be robust to noise and to correlate well with the physical properties of the simulated galaxies, including their 3D structure. We apply the method simultaneously to F200W and F356W galaxy images of a mass-complete sample ($M_*/M_\odot>10^9$) at $ 3 \leq z \leq 6$ from the first JWST/NIRCam CEERS data release. We find that the simulated and observed galaxies do not exactly populate the same manifold in the representation space from contrastive learning. We also find that half the galaxies classified as disks -- either CNN-based or visually -- populate a similar region of the representation space as TNG50 galaxies with low stellar specific angular momentum and non-oblate structure. Although our data-driven study does not allow us to firmly conclude on the true nature of these galaxies, it suggests that the disk fraction at $z \geq 3$ remains uncertain and possibly overestimated by traditional supervised classifications. Deeper imaging and spectroscopic follow-ups as well as comparisons with other simulations will help to unambiguously determine the true nature of these galaxies, and establish more robust constraints on the emergence of disks at very high redshift.

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arXiv2023

New insight on the nature of cosmic reionizers from the CEERS survey

The Epoch of Reionization (EoR) began when galaxies grew in abundance and luminosity, so their escaping Lyman continuum (LyC) radiation started ionizing the surrounding neutral intergalactic medium (IGM). Despite significant recent progress, the nature and role of cosmic reionizers are still unclear: in order to define them, it would be necessary to directly measure their LyC escape fraction ($f_{esc}$). However, this is impossible during the EoR due to the opacity of the IGM. Consequently, many efforts at low and intermediate redshift have been made to determine measurable indirect indicators in high-redshift galaxies so that their $f_{esc}$ can be predicted. This work presents the analysis of the indirect indicators of 62 spectroscopically confirmed star-forming galaxies at $6 \leq z \leq 9$ from the Cosmic Evolution Early Release Science (CEERS) survey, combined with 12 sources with public data from other JWST-ERS campaigns. From the NIRCam and NIRSpec observations, we measured their physical and spectroscopic properties. We discovered that on average $6<z<9$ star-forming galaxies are compact in the rest-frame UV ($r_e \sim $ 0.4 kpc), are blue sources (UV-$β$ slope $\sim $ -2.17), and have a predicted $f_{esc}$ of about 0.13. A comparison of our results to models and predictions as well as an estimation of the ionizing budget suggests that low-mass galaxies with UV magnitudes fainter than $M_{1500} = -18$ that we currently do not characterize with JWST observations probably played a key role in the process of reionization.

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arXiv2023

Stellar metallicity from optical and UV spectral indices: Test case for WEAVE-StePS

The upcoming generation of optical spectrographs on four meter-class telescopes, with their huge multiplexing capabilities, excellent spectral resolution, and unprecedented wavelength coverage, will provide high-quality spectra for thousands of galaxies. These data will allow us to examine of the stellar population properties at intermediate redshift, an epoch that remains unexplored by large and deep surveys. We assess our capability to retrieve the mean stellar metallicity in galaxies at different redshifts and S/N, while simultaneously exploiting the UV and optical rest-frame wavelength coverage. The work is based on a comprehensive library of spectral templates of stellar populations, covering a wide range of age and metallicity values and built assuming various SFHs. We simulated realistic observations of a large sample of galaxies carried out with WEAVE at the WHT at different redshifts and S/N values. We measured all the reliable indices on the simulated spectra and on the comparison library. We then adopted a Bayesian approach to obtain the probability distribution of stellar metallicity. The analysis of the spectral indices has shown how some mid-UV indices can provide reliable constraints on stellar metallicity, along with optical indicators. The analysis of the mock observations has shown that even at S/N=10, the metallicity can be derived within 0.3 dex, in particular, for stellar populations older than 2 Gyr. Our results are in good agreement with other theoretical and observational works in the literature and show how the UV indicators can be advantageous in constraining metallicities. This is very promising for the upcoming surveys carried out with new, highly multiplexed, large-field spectrographs, such as StePS at the WEAVE and 4MOST, which will provide spectra of thousands of galaxies covering large spectral ranges at relatively high S/N.

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arXiv2023

MIDIS: Strong (Hb + [OIII]) and Ha emitters at redshift $z \simeq 7-8$ unveiled with JWST/NIRCam and MIRI imaging in the Hubble eXtreme Deep Field (XDF)

We make use of \textit{JWST} medium and broad-band NIRCam imaging, along with ultra-deep MIRI $5.6 \rm μm$ imaging, in the Hubble eXtreme Deep Field (XDF) to identify prominent line emitters at $z\simeq 7-8$. Out of a total of 58 galaxies at $z\simeq 7-8$, we find 18 robust candidates ($\simeq$31\%) for (H$β$ + [OIII]) emitters, based on their enhanced fluxes in the F430M and F444W filters, with EW$_{0}$(H$β$ +[OIII]) $\simeq 87 - 2100$ Å. Among these emitters, 16 lie in the MIRI coverage area and 12 exhibit a clear flux excess at $5.6 \, \rm μm$, indicating the simultaneous presence of a prominent H$α$ emission line with EW$_{0}$(H$α$) $\simeq 200-3000$ Å. This is the first time that H$α$ emission can be detected in individual galaxies at $z>7$. The H$α$ line, when present, allows us to separate the contributions of H$β$ and [OIII] to the (H$β$ +[OIII]) complex, and derive H$α$-based star formation rates (SFRs). We find that in most cases [OIII]/H$β> 1$. Instead, two galaxies have [OIII]/H$β< 1$, indicating that the NIRCam flux excess is mainly driven by H$β$. This could potentially imply extremely low metallicities. Most prominent line emitters are very young starbursts or galaxies on their way to/from the starburst cloud. They make for a cosmic SFR density $\rm log_{10}(ρ_{SFR_{Hα}}) \simeq -2.35$, which is about a quarter of the total value ($\rm log_{10}(ρ_{SFR_{tot}}) \simeq -1.76$) at $z\simeq 7-8$. Therefore, the strong H$α$ emitters likely had a significant role in reionization.

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arXiv2023

Delving deep: a population of extremely dusty dwarfs observed by JWST

We take advantage of the NIRCam photometric observations available as part of the Cosmic Evolution Early Release Science survey (CEERS) to identify and analyse very red sources in an effort to discover very dusty star forming galaxies. We select red galaxies as objects with a S/N>3 at 4.4 $μ$m and a S/N<2 in all JWST and HST filters at $λ\leq2μ$m, which corresponds to [F200W]-[F444W]>1.2 considering CEERS depths. This selection is ideal to identify very dusty (Av>1 mag) galaxies with stellar masses between $10^6$ to $10^{10}\, \rm M_{\odot}$ at z<5, more massive dusty galaxies at z=5-18 and galaxies at z>18 due to the Lyman absorption, independently of their dust extinction. Our sample of F200W-dropouts contains no strong candidates at z>6.5, instead it consists almost completely (~81%) of z<2 low-mass galaxies, with a median stellar mass of $10^{7.3} \rm M_{\odot}$. These galaxies show an exceptional dust extinction with median value of Av=4.9 mag, completely unexpected given their low stellar mass. The remaining galaxies, which are at z<6.5, show similar large dust extinction (Av>1), but they are generally more massive $>10^{7.5}\rm M_{\odot}$.

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arXiv2023

Galaxy Morphology from $z\sim6$ through the eyes of JWST

We analyze the Near Infrared ($\sim0.8-1μ$m) rest-frame morphologies of galaxies with $\log M_*/M_\odot>9$ in the redshift range $0 10.5$) at $z\sim5$, and bulge-dominated galaxies also exist at these early epochs, confirming a rich and evolved morphological diversity of galaxies $\sim1$ Gyr after the Big Bang. Finally, we find that the morphology-quenching relation is already in place for massive galaxies at $z>3$, with massive quiescent galaxies ($\log M_*/M_\odot>10.5$) being predominantly bulge-dominated.

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arXiv2023

Uncovering the stellar structure of the dusty star-forming galaxy GN20 at z=4.055 with MIRI/JWST

Luminous infrared galaxies at high redshifts ($z$>4) include extreme starbursts that build their stellar mass over short periods of time (>100 Myr). These galaxies are considered to be the progenitors of massive quiescent galaxies at intermediate redshifts ($z\sim$2) but their stellar structure and buildup is unknown. Here, we present the first spatially resolved near-infrared imaging of GN20, one of the most luminous dusty star-forming galaxies known to date, observed at an epoch when the Universe was only 1.5 Gyr old. The 5.6$μ$m image taken with the JWST Mid-Infrared Instrument (MIRI/JWST) shows that GN20 is a very luminous galaxy (M$_\mathrm{1.1μm,AB}$=$-$25.01), with a stellar structure composed of a conspicuous central source and an extended envelope. The central source is an unresolved nucleus that carries 9% of the total flux. The nucleus is co-aligned with the peak of the cold dust emission, and offset by 3.9 kpc from the ultraviolet stellar emission. The diffuse stellar envelope is similar in size to the clumpy CO molecular gas distribution. The centroid of the stellar envelope is offset by 1 kpc from the unresolved nucleus, suggesting GN20 is involved in an interaction or merger event supported by its location as the brightest galaxy in a proto-cluster. The stellar size of GN20 is larger by a factor of about 3-5 than known spheroids, disks, and irregulars at $z\sim$4, while its size and low Sérsic index are similar to those measured in dusty, infrared luminous galaxies at $z\sim$2 of the same mass. GN20 has all the ingredients necessary for evolving into a massive spheroidal quiescent galaxy at intermediate $z$: it is a large, luminous galaxy at $z$=4.05 involved in a short and massive starburst centred in the stellar nucleus and extended over the entire galaxy, out to radii of 4 kpc, and likely induced by the interaction or merger with a member of the proto-cluster.

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arXiv2023

The bar rotation rate as a diagnostic of dark matter content in the centre of disc galaxies

We investigate the link between the bar rotation rate and dark matter content in barred galaxies by concentrating on the cases of the lenticular galaxies NGC4264 and NGC4277. These two gas-poor galaxies have similar morphologies, sizes, and luminosities. But, NGC4264 hosts a fast bar, which extends to nearly the corotation, while the bar embedded in NGC4277 is slow and falls short of corotation. We derive the fraction of dark matter $f_{\rm DM, bar}$ within the bar region from Jeans axisymmetric dynamical models by matching the stellar kinematics obtained with the MUSE integral-field spectrograph and using SDSS images to recover the stellar mass distribution. We build mass-follows-light models as well as mass models with a spherical halo of dark matter, which is not tied to the stars. We find that the inner regions of NGC4277 host a larger fraction of dark matter ($f_{\rm DM, bar} = 0.53 \pm 0.02$) with respect to NGC4264 ($f_{\rm DM, bar} = 0.33 \pm 0.04$) in agreement with the predictions of theoretical works and the findings of numerical simulations, which have found that fast bars live in baryon-dominated discs, whereas slow bars experienced a strong drag from the dynamical friction due to a dense DM halo. This is the first time that the bar rotation rate is coupled to $f_{\rm DM, bar}$ derived from dynamical modelling.

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arXiv2023

The Fornax3D project: Environmental effects on the assembly of dynamically cold disks in Fornax cluster galaxies

We apply a population-orbit superposition method to 16 galaxies in the Fornax cluster observed with MUSE/VLT in the context of the Fornax3D project. By fitting the luminosity distribution, stellar kinematics, and age and metallicity maps simultaneously, we obtained the internal stellar orbit distribution, as well as the age and metallicity distribution of stars on different orbits for each galaxy. Based on the model, we decompose each galaxy into a dynamically cold disk (orbital circularity $λ_z\ge0.8$) and a dynamically hot non-disk component (orbital circularity $λ_z<0.8$), and obtain the surface-brightness, age, and metallicity radial profiles of each component. The galaxy infall time into the cluster is strongly correlated with galaxy cold-disk age with older cold disks in ancient infallers. We quantify the infall time $t_{\rm infall}$ of each galaxy with its cold-disk age using a correlation calibrated with TNG50 cosmological simulations. For galaxies in the Fornax cluster, we found that the luminosity fraction of cold disk in galaxies with $t_{\rm infall}>8$ Gyr are a factor of $\sim 4$ lower than in more recent infallers while controlling for total stellar mass. Nine of the 16 galaxies have spatially extended cold disks, and most of them show positive or zero age gradients; stars in the inner disk are $\sim 2-5$ Gyr younger than that in the outer disk, in contrast to the expectation of inside-out growth. Our results indicate that the assembly of cold disks in galaxies is strongly affected by their infall into clusters, by either removal of gas in outer regions or even tidally stripping or heating part of the pre-existing disks. Star formation in outer disks can stop quickly after the galaxy falls into the cluster, while star formation in the inner disks can last for a few Gyrs more, building the positive age gradient measured in cold disks.

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arXiv2023

WEAVE-StePS. A stellar population survey using WEAVE at WHT

The upcoming new generation of optical spectrographs on four-meter-class telescopes will provide valuable opportunities for forthcoming galaxy surveys through their huge multiplexing capabilities, excellent spectral resolution, and unprecedented wavelength coverage. WEAVE is a new wide-field spectroscopic facility mounted on the 4.2 m William Herschel Telescope in La Palma. WEAVE-StePS is one of the five extragalactic surveys that will use WEAVE during its first five years of operations. It will observe galaxies using WEAVE MOS (~950 fibres across a field of view of ~3 deg2 on the sky) in low-resolution mode (R~5000, spanning the wavelength range 3660-9590 AA). WEAVE-StePS will obtain high-quality spectra (S/N ~ 10 per AA at R~5000) for a magnitude-limited (I_AB = 20.5) sample of ~25,000 galaxies, the majority selected at z>=0.3. The survey goal is to provide precise spectral measurements in the crucial interval that bridges the gap between LEGA-C and SDSS data. The wide area coverage of ~25 deg2 will enable us to observe galaxies in a variety of environments. The ancillary data available in each observed field (including X-ray coverage, multi-narrow-band photometry and spectroscopic redshift information) will provide an environmental characterisation for each observed galaxy. This paper presents the science case of WEAVE-StePS, the fields to be observed, the parent catalogues used to define the target sample, and the observing strategy chosen after a forecast of the expected performance of the instrument for our typical targets. WEAVE-StePS will go back further in cosmic time than SDSS, extending its reach to encompass more than ~6 Gyr, nearly half of the age of the Universe. The spectral and redshift range covered by WEAVE-StePS will open a new observational window by continuously tracing the evolutionary path of galaxies in the largely unexplored intermediate-redshift range.

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arXiv2022

A slow bar in the lenticular barred galaxy NGC 4277

Aims: We characterised the properties of the bar hosted in lenticular galaxy NGC 4277, which is located behind the Virgo cluster. Methods: We measured the bar length and strength from the surface photometry obtained from the broad-band imaging of the Sloan Digital Sky Survey and we derived the bar pattern speed from the stellar kinematics obtained from the integral-field spectroscopy performed with the Multi Unit Spectroscopic Explorer at the Very Large Telescope. We also estimated the co-rotation radius from the circular velocity, which we constrained by correcting the stellar streaming motions for asymmetric drift, and we finally derived the bar rotation rate. Results: We found that NGC 4277 hosts a short ($R_{bar}=3.2^{+0.9}_{-0.6}$ kpc), weak ($S_{bar}=0.21 \pm 0.02$), and slow ($R=1.8^{+0.5}_ {-0.3}$) bar and its pattern speed ($Ω_{bar}=24.7\pm3.4$ km s$^{-1}$ kpc$^{-1}$) is amongst the best-constrained ones ever obtained with the Tremaine-Weinberg (TW) method with relative statistical errors of $\sim0.2$. Conclusions: NGC 4277 is the first clear-cut case of a galaxy hosting a slow stellar bar ($R>1.4$ at more than a 1$σ$ confidence level) measured with the model-independent TW method. A possible interaction with the neighbour galaxy NGC 4273 could have triggered the formation of such a slow bar and/or the bar could be slowed down due to the dynamical friction with a significant amount of dark matter within the bar region.

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arXiv2022

From naked spheroids to disky galaxies: how do massive disk galaxies shape their morphology?

We investigate the assembly history of massive disk galaxies and describe how they shape their morphology through cosmic time. Using SHARDS and HST data, we modeled the surface brightness distribution of 91 massive galaxies at redshift $0.14<z\leq 1$ in the wavelength range $0.5-1.6$ $μ$m, deriving the uncontaminated spectral energy distributions of their bulges and disks separately. This spectrophotometric decomposition allows us to compare the stellar populations properties of each component in individual galaxies. We find that the majority of massive galaxies ($\sim85\%$) builds inside-out, growing their extended stellar disk around the central spheroid. Some bulges and disks could start forming at similar epochs, but these bulges grow more rapidly than their disks, assembling $80\%$ of their mass in $\sim0.7$ Gyr and $\sim3.5$ Gyr, respectively. Moreover, we infer that both older bulges and older disks are more massive and compact than younger stellar structures. In particular, we find that bulges display a bimodal distribution of mass-weighted ages, i.e., they form in two waves. In contrast, our analysis of the disk components indicates that they form at $z\sim1$ for both first and second-wave bulges. This translates to first-wave bulges taking longer in acquiring a stellar disk ($5.2$ Gyr) compared to second-wave less-compact spheroids ($0.7$ Gyr). We do not find distinct properties (e.g., mass, star formation timescale, and mass surface density) for the disks in both types of galaxies. We conclude that the bulge mass and compactness mainly regulate the timing of the stellar disk growth, driving the morphological evolution of massive disk galaxies.

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arXiv2022

The PUMA project. III. Incidence and properties of ionised gas disks in ULIRGs, associated velocity dispersion and its dependence on starburstiness

A classical scenario suggests that ULIRGs transform colliding spiral galaxies into a spheroid dominated early-type galaxy. Recent high-resolution simulations have instead shown that, under some circumstances, rotation disks can be preserved during the merging process or rapidly regrown after coalescence. Our goal is to analyze in detail the ionised gas kinematics in a sample of ULIRGs to infer the incidence of gas rotational dynamics in late-stage interacting galaxies and merger remnants. We analysed MUSE data of a sample of 20 nearby (z<0.165) ULIRGs, as part of the "Physics of ULIRGs with MUSE and ALMA" (PUMA) project. We found that 27% individual nuclei are associated with kpc-scale disk-like gas motions. The rest of the sample displays a plethora of gas kinematics, dominated by winds and merger-induced flows, which make the detection of rotation signatures difficult. On the other hand, the incidence of stellar disk-like motions is ~2 times larger than gaseous disks, as the former are probably less affected by winds and streams. The eight galaxies with a gaseous disk present relatively high intrinsic gas velocity dispersion (sigma = 30-85 km/s), rotationally-supported motions (with gas rotation velocity over velocity dispersion vrot/sigma > 1-8), and dynamical masses in the range (2-7)x1e10 Msun. By combining our results with those of local and high-z disk galaxies from the literature, we found a significant correlation between sigma and the offset from the main sequence (MS), after correcting for their evolutionary trends. Our results confirm the presence of kpc-scale rotating disks in interacting galaxies and merger remnants, with an incidence going from 27% (gas) to ~50% (stars). The ULIRGs gas velocity dispersion is up to a factor of ~4 higher than in local normal MS galaxies, similar to high-z starbursts as presented in the literature.

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arXiv2021

Differential attenuation in star-forming galaxies at 0.3 $\lesssim$ $z$ $\lesssim$ 1.5 in the SHARDS/CANDELS field

We use a sample of 706 galaxies, selected as [OII]$λ$3727 ([OII]) emitters in the Survey for High-$z$ Absorption Red and Dead Sources (SHARDS) on the CANDELS/GOODS-N field, to study the differential attenuation of the nebular emission with respect to the stellar continuum. The sample includes only galaxies with a counterpart in the infrared and $\mathrm{log}_{10}(M_{*}/\mathrm{M}_{\odot})$ $>$ 9, over the redshift interval 0.3 $\lesssim$ $z$ $\lesssim$ 1.5. Our methodology consists in the comparison of the star formation rates inferred from [OII] and H$α$ emission lines with a robust quantification of the total star-forming activity (${SFR}_{\mathrm{TOT}}$) that is independently estimated based on both infrared and ultraviolet (UV) luminosities. We obtain $f$$=$$E(B-V)_{\mathrm{stellar}}$/$E(B-V)_{\mathrm{nebular}}$ $=$ 0.69$^{0.71}_{0.69}$ and 0.55$^{0.56}_{0.53}$ for [OII] and H$α$, respectively. Our resulting $f$-factors display a significant positive correlation with the UV attenuation and shallower or not-significant trends with the stellar mass, the $SFR_{\mathrm{TOT}}$, the distance to the main sequence, and the redshift. Finally, our results favour an average nebular attenuation curve similar in shape to the typical dust curve of local starbursts.

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arXiv2021

Near-infrared spectroscopic indices for unresolved stellar populations. II. Index measurements

We measured the equivalent width of a large set of near-infrared (NIR, 0.8--2.4$ \ μ$m) line-strength indices in the XShooter medium-resolution spectra of the central regions of 14 galaxies. We found that two aluminum indices Al at 1.31 $μ$m and Al1 at 1.67 $μ$m and the two CO indices CO1 at 1.56 $μ$m and CO4 at 1.64 $μ$m are tightly correlated with the velocity dispersion. Moreover, the NIR Al and CO1 indices show strong correlations with the optical Mg2 and Mgb indices, which are usually adopted as $α$/Fe-enhancement diagnostics. The molecular FeH1 index at 1.58 $μ$m tightly correlates with the optical and [MgFe]' indices, which are used as total metallicity diagnostics. The NIR Pa$β$ index at 1.28 $μ$m has a behaviour similar to the optical H$β$ index, which is a diagnostic of mean age. We defined two new composite indices, and [AlFeH], as possible candidates to be used as NIR diagnostics of total metallicity and $α$/Fe enhancement. The NIR index has a strong correlation with the optical Mg2 and Mgb indices, while the [AlFeH] index is tightly correlated with the optical and [MgFe]' indices. The distribution of the data points in the NIR Pa$β$- and Pa$β$-[AlFeH] diagrams mimic that in the optical [MgFe]'-H$β$ and the Mgb- diagrams, which are widely used to constraint the properties of the unresolved stellar populations. We concluded that some NIR line-strength indices could be useful in studying stellar populations as well as in fine-tuning stellar population models.

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arXiv2021

A duality in the origin of bulges and spheroidal galaxies

Studying the resolved stellar populations of the different structural components which build massive galaxies directly unveils their assembly history. We aim at characterizing the stellar population properties of a representative sample of bulges and pure spheroids in massive galaxies ($M_{\star}>10^{10}$ M$_{\odot}$) in the GOODS-N field. We take advantage of the spectral and spatial information provided by SHARDS and HST data to perform the multi-image spectro-photometrical decoupling of the galaxy light. We derive the spectral energy distribution separately for bulges and disks in the redshift range $0.14<z\leq1$ with spectral resolution $R\sim50$. Analyzing these SEDs, we find evidences of a bimodal distribution of bulge formation redshifts. We find that 33% of them present old mass-weighted ages, implying a median formation redshift $z_{\rm{form}}={6.2}_{-1.7}^{+1.5}$. They are relics of the early Universe embedded in disk galaxies. A second wave, dominant in number, accounts for bulges formed at median redshift $z_{\rm{form}}={1.3}_{-0.6}^{+0.6}$. The oldest (1$^{\rm{st}}$-wave) bulges are more compact than the youngest. Virtually all pure spheroids (i.e., those without any disk) are coetaneous with the 2$^{\rm{nd}}$-wave bulges, presenting a median redshift of formation $z_{\rm{form}}={1.1}_{-0.3}^{+0.3}$. The two waves of bulge formation are not only distinguishable in terms of stellar ages, but also in star formation mode. All 1$^{\rm st}$-wave bulges formed fast at $z\sim6$, with typical timescales around 200 Myr. A significant fraction of the 2$^{\rm{nd}}$-wave bulges assembled more slowly, with star formation timescales as long as 1 Gyr. The results of this work suggest that the centers of massive disk-like galaxies actually harbor the oldest spheroids formed in the Universe.

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