We present deep broadband imaging and long-slit spectroscopy of three compact, low-mass starburst galaxies at redshift z ~ 0.2-0.3, also referred to as Green Peas (GP). We measure physical properties ...of the ionized gas and derive abundances for several species with high precision. We find that the three GPs display relatively low extinction, low oxygen abundances, and remarkably high nitrogen-to-oxygen ratios. We also report on the detection of clear signatures of Wolf-Rayet (W-R) stars in these galaxies. We carry out a pilot spectral synthesis study using a combination of both population and evolutionary synthesis models. Their outputs are in qualitative agreement, strongly suggesting a formation history dominated by starbursts. In agreement with the presence of W-R stars, these models show that these GPs currently undergo a major starburst producing between ~4% and ~20% of their stellar mass. However, as models imply, they are old galaxies that formed most of their stellar mass several Gyr ago. The presence of old stars has been spectroscopically verified in one of the galaxies by the detection of Mg I lambdalambda5167, 5173 absorption lines. Additionally, we perform a surface photometry study based on Hubble Space Telescope data, which indicates that the three galaxies possess an exponential low surface brightness envelope. If due to stellar emission, the latter is structurally compatible with the evolved hosts of luminous blue compact dwarf (BCD)/H II galaxies, suggesting that GPs are identifiable with major episodes in the assembly history of local BCDs. These conclusions highlight the importance of these objects as laboratories for studying galaxy evolution at late cosmic epochs.
Abstract
Using VIsible MultiObject Spectrograph (VIMOS)-integral field unit (IFU) observations, we study the interstellar medium (ISM) of two star-forming dwarf galaxies, UM 461 and Mrk 600. Our aim ...was to search for the existence of metallicity inhomogeneities that might arise from infall of nearly pristine gas feeding ongoing localized star formation. The IFU data allowed us to study the impact of external gas accretion on the chemical evolution as well as the ionized gas kinematics and morphologies of these galaxies. Both systems show signs of morphological distortions, including cometary-like morphologies. We analysed the spatial variation of 12 + log(O/H) abundances within both galaxies using the direct method (Te), the widely applied HII–CHI–mistry code, as well as by employing different standard calibrations. For UM 461, our results show that the ISM is fairly well mixed, at large scales;
however, we find an off-centre and low-metallicity region with 12 + log(O/H) < 7.6 in the SW part of the brightest H ii region, using the direct method. This result is consistent with the recent infall of a metal-poor H i cloud into the region now exhibiting the lowest metallicity, which also displays localized perturbed neutral and ionized gas kinematics. Mrk 600 in contrast, appears to be chemically homogeneous on both large and small scales. The intrinsic differences in the spatially resolved properties of the ISM in our analysed galaxies are consistent with these systems being at different evolutionary stages.
The optical morphology of galaxies holds the cumulative record of their assembly history, and techniques for its quantitative characterization offer a promising avenue toward understanding galaxy ...formation and evolution. However, the morphology of star-forming galaxies is generally dictated by the youngest stellar component, which can readily overshine faint structural/morphological features in the older underlying stellar background (e.g., relics from recent minor mergers) that could hold important insights into the galaxy build-up process. Stripping off galaxy images from the emission from stellar populations younger than an adjustable age cutoff tcut can therefore provide a valuable tool in extragalactic research. RemoveYoung (ℛ𝒴), a publicly available tool that is presented here, exploits the combined power of integral field spectroscopy (IFS) and spectral population synthesis (SPS) toward this goal. Two-dimensional (2D) post-processing of SPS models to IFS data cubes with ℛ𝒴 permits computation of the spectral energy, surface brightness, and stellar surface density distribution of stellar populations older than a user-defined tcut. This suggests a variety of applications of star-forming galaxies, such as interacting or merging galaxy pairs and lower mass starburst galaxies near and far; these include blue compact and tidal dwarf galaxies.
Context.I Zw 18, ever since regarded as the prototypical blue compact dwarf (BCD) galaxy, is, quite ironically, the most atypical BCD known. This is because its large low-surface brightness (LSB) ...envelope is not due to an old underlying stellar host, as invariably is the case for typical BCDs, but is entirely due to extended nebular emission. Aims. Our goal is to explore I Zw 18 and its detached C component I Zw 18 C down to an unprecedently faint surface brightness μ (mag/◻″) level in order to gain further insight into the structural properties and evolutionary history of this enigmatic galaxy pair. Methods. We present a photometric analysis of the entire set of archival HST ACS V, R and I band data for I Zw 18. Results. Radial color profiles for I Zw 18 C reveal blue and practically constant colors (0 ± 0.05) down to μ ~ 27.6, and a previously undisclosed, slightly redder (V − I ≈ 0.2), stellar population in its extreme periphery (μ ~ 29). We argue that stellar diffusion over τ ~ 108 yr and the associated stellar mass filtering effect can consistently account for the observed properties of the stellar component in the outskirts of I Zw 18 C. This process, in combination with propagating star formation with a mean velocity of ~20 km s-1 can reproduce all essential characteristics of I Zw 18 C within ~τ. An extremely faint substrate of older stars cannot be ruled out but does not need to be postulated. As for I Zw 18, we find that nebular emission (ne) extends out to ~16 stellar scale lengths, shows a nearly exponential outer profile, and provides at least one third of the total optical emission. Nebular emission dominates already at μ ~ 23.5, as evident from e.g. the uniform and extremely blue (V − I ≈ –1, R − I ≈ –1.4) colors of the LSB envelope of I Zw 18. Conclusions. The case of I Zw 18 suggests caution in studies of distant galaxies in dominant stages of their evolution, rapidly assembling their stellar mass at high specific star formation rates (SSFRs). It calls attention to the fact of ne not necessarily being cospatial with the underlying ionizing and non-ionizing stellar background. It also does not have to scale with the background surface density. The prodigious energetic output during dominant phases of galaxy evolution may result in large exponential ne envelopes, extending much beyond the still compact stellar component, just like in I Zw 18. Therefore, the morphological paradigm of I Zw 18, while probably unique in the nearby Universe, may be ubiquitous among high-SSFR galaxies at high redshift. Using I Zw 18 as reference, we show that extended ne may introduce substantial observational biases and affect several of the commonly studied fundamental galaxy relations. Among others, we show that the surface brightness profiles of distant morphological analogs to I Zw 18 may be barely distinguishable from Sérsic profiles with an exponent 2 ≲ η ≲ 5, thus mimicking the profiles of massive galaxy spheroids.
ABSTRACT
This work focuses on the gas chemical composition of CGCG007-025. This compact dwarf is undergoing a galaxy wide star-forming burst, whose spatial behaviour has been observed by VLT/MUSE. We ...present a new line measurement library to treat almost 7800 voxels. The direct method chemical analysis is limited to 484 voxels with good detection of the $\mathrm{S}\, {\small III}6312\, \mathring{\rm A}$ temperature diagnostic line. The recombination fluxes are corrected for stellar absorption via a population synthesis. Additionally, we discuss a new algorithm to fit photoionization models via neural networks. The eight ionic abundances analysed show a spatial normal distribution with a $\sigma \sim 0.1\, {\rm dex}$, where only half this value can be explained by the uncertainty in the measurements. The oxygen abundance distribution is 12 + log(O/H) = 7.88 ± 0.11. The ${\it T}_{e}\mathrm{S}\, {\small III}$ and neS ii are also normally distributed. However, in the central and brightest region, the neS ii is almost thrice the mean galaxy value. This is also reflected in the extinction measurements. The ionization parameter has a distribution of ${\rm log}(U) = -2.52^{0.17}_{0.19}$. The parameter spatial behaviour agrees with the S2+/S+ map. Finally, the discrepancies between the direct method and the photoionization model fitting are discussed. In the latter technique, we find that mixing lines with uneven uncertainty magnitudes can impact the accuracy of the results. In these fittings, we recommend overestimating the minimum flux uncertainty one order below the maximum line flux uncertainty. This provides a better match with the direct method.
We use deep integral field spectroscopy data from the CALIFA survey to study the warm interstellar medium (wim) over the entire extent and optical spectral range of 32 nearby early-type galaxies ...(ETGs). We find that faint nebular emission is extended in all cases, and its surface brightness decreases roughly as ∝ r−α. The large standard deviation in the derived α (1.09 ± 0.67) argues against a universal power-law index for the radial drop-off of nebular emission in ETGs. Judging from the properties of their extranuclear component, our sample ETGs span a broad, continuous sequence with respect to their α, Hα equivalent width (EW) and Lyman continuum (Lyc) photon leakage fraction (plf). We propose a tentative subdivision into two groups: Type i ETGs are characterized by rather steep Hα profiles (α ≃ 1.4), comparatively large (≳1 Å), nearly radially constant EWs, and plf ≃ 0. Photoionization by post-AGB stars appears to be the main driver of extended nebular emission in these systems, with nonthermal sources being potentially important only in their nuclei. Typical properties of type ii ETGs are shallower Hα profiles (α ≃ 0.8), very low (≲0.5 Å) EWs with positive radial gradients, and a mean plf ≳ 0.7, rising to ≳0.9 in their centers. Such properties point to a low, and inwardly decreasing wim density and/or volume filling factor. We argue that, because of extensive Lyc photon leakage, emission-line luminosities and EWs are reduced in type ii ETG nuclei by at least one order of magnitude. Consequently, the line weakness of these ETGs is by itself no compelling evidence for their containing merely “weak”(sub-Eddington accreting) active galactic nuclei (AGN). In fact, Lyc photon escape, which has heretofore not been considered, may constitute a key element in understanding why many ETGs with prominent signatures of AGN activity in radio continuum and/or X-ray wavelengths show only faint emission lines and weak signatures of AGN activity in their optical spectra. The Lyc photon escape, in conjunction with dilution of nuclear EWs by line-of-sight integration through a triaxial stellar host, can systematically impede detection of AGN in gas-poor galaxy spheroids through optical emission-line spectroscopy, thereby leading to an observational bias. We further find that type i&ii ETGs differ little (≲0.4 dex) in their mean BPT line ratios, which in both cases are characteristic of LINERs and are, within their uncertainties, almost radius-independent. This potentially hints at a degeneracy of the projected, luminosity-weighted BPT ratios in the LINER regime, for the specific 3D properties of the wim and the ionizing photon field in ETGs.
The goal of population spectral synthesis (pss; also referred to as inverse, semi-empirical evolutionary- or fossil record approach) is to decipher from the spectrum of a galaxy the mass, age and ...metallicity of its constituent stellar populations. This technique, which is the reverse of but complementary to evolutionary synthesis, has been established as fundamental tool in extragalactic research. It has been extensively applied to large spectroscopic data sets, notably the SDSS, leading to important insights into the galaxy assembly history. However, despite significant improvements over the past decade, all current pss codes suffer from two major deficiencies that inhibit us from gaining sharp insights into the star-formation history (SFH) of galaxies and potentially introduce substantial biases in studies of their physical properties (e.g., stellar mass, mass-weighted stellar age and specific star formation rate). These are i) the neglect of nebular emission in spectral fits, consequently; ii) the lack of a mechanism that ensures consistency between the best-fitting SFH and the observed nebular emission characteristics of a star-forming (SF) galaxy (e.g., hydrogen Balmer-line luminosities and equivalent widths-EWs, shape of the continuum in the region around the Balmer and Paschen jump). In this article, we present fado (Fitting Analysis using Differential evolution Optimization) – a conceptually novel, publicly available pss tool with the distinctive capability of permitting identification of the SFH that reproduces the observed nebular characteristics of a SF galaxy. This so-far unique self-consistency concept allows us to significantly alleviate degeneracies in current spectral synthesis, thereby opening a new avenue to the exploration of the assembly history of galaxies. The innovative character of fado is further augmented by its mathematical foundation: fado is the first pss code employing genetic differential evolution optimization. This, in conjunction with various other currently unique elements in its mathematical concept and numerical realization (e.g., mid-analysis optimization of the spectral library using artificial intelligence, test for convergence through a procedure inspired by Markov chain Monte Carlo techniques, quasi-parallelization embedded within a modular architecture) results in key improvements with respect to computational efficiency and uniqueness of the best-fitting SFHs. Furthermore, fado incorporates within a single code the entire chain of pre-processing, modeling, post-processing, storage and graphical representation of the relevant output from pss, including emission-line measurements and estimates of uncertainties for all primary and secondary products from spectral synthesis (e.g., mass contributions of individual stellar populations, mass- and luminosity-weighted stellar ages and metallicities). This integrated concept greatly simplifies and accelerates a lengthy sequence of individual time-consuming steps that are generally involved in pss modeling, further enhancing the overall efficiency of the code and inviting to its automated application to large spectroscopic data sets.
Context. We present deep spectroscopy of a large sample of low-metallicity emission-line galaxies. Aims. The main goal of this study is to derive element abundances in these low-metallicity galaxies. ...Methods. We analyze 121 VLT spectra of H ii regions in 46 low-metallicity emission-line galaxies. Of these spectra 83 are archival VLT/FORS1+UVES spectra of H ii regions in 31 low-metallicity emission-line galaxies that are studied for the first time with standard direct methods to determine the electron temperatures, the electron number densities, and the chemical abundances. Results. The oxygen abundance of the sample lies in the range 12 + log O/H = 7.2−8.4. We confirm previous findings that Ne/O increases with increasing oxygen abundance, likely because of a higher depletion of oxygen in higher-metallicity galaxies. The Fe/O ratio decreases from roughly solar at the lowest metallicities to about one tenth of solar, indicating that the degree of depletion of iron into dust grains depends on metallicity. The N/O ratio in extremely low-metallicity galaxies with 12 + log O/H < 7.5 shows a slight increase with decreasing oxygen abundance, which could be the signature of enhanced production of primary nitrogen by rapidly rotating stars at low metallicity. We present the first empirical relation between the electron temperature derived from S iiiλ6312/λ9069 or N iiλ5755/λ6583 and the one derived from O iiiλ4363/λ(4959+5007) in low-metallicity galaxies. We also present an empirical relation between te derived from O iiλ3727/(λ7320 + λ7330) or S iiλ4068/(λ6717 + λ6730) and O iiiλ4363/λ(4959+5007). The electron number densities Ne(Cl iii) and Ne(Ar iv) were derived in a number of objects and are found to be higher than Ne(O ii) and Ne(S ii). This has potential implications for the derivation of the pregalactic helium abundance. In a number of objects, the abundances of C++ and O++ could be derived from recombination lines. Our study confirms the discrepancy between abundances found from recombination lines (RLs) and collisionally excited lines (CELs) and that C/O increases with O/H.
We reveal the importance of ongoing in situ star formation in the brightest cluster galaxy (BCG) in the massive cool-core CLASH cluster MACS 1931.8-2635 at a redshift of
z
= 0.35 by analysing ...archival VLT-MUSE optical integral field spectroscopy. Using a multi-wavelength approach, we assessed the stellar and warm ionised medium components, which were spatially resolved by the VLT-MUSE spectroscopy, and linked them to the molecular gas by incorporating sub-mm ALMA observations. We measured the fluxes of strong emission lines such as: O
II
λ
3727, H
β
, O
III
λ
5007, H
α
, N
II
λ
6584, and S
II
λ
6718, 6732, which allowed us to determine the physical conditions of the warm ionised gas, such as electron temperature, electron density, extinction, ionisation parameter, (O/H) gas metallicities, star formation rates, and gas kinematics, as well as the star formation history of the system. Our analysis reveals the ionising sources in different regions of the galaxy. The ionised gas flux brightness peak corresponds to the location of the supermassive black hole in the BCG and the system shows a diffuse warm ionised gas tail extending 30 kpc in the north-east direction. The ionised and molecular gas are co-spatial and co-moving, with the gaseous component in the tail likely falling inward, providing fuel for star formation and accretion-powered nuclear activity. The gas is ionised by a mix of star formation and other energetic processes which give rise to LINER-like emission, with active galactic nuclei emission dominant only in the BCG core. We measured a star formation rate of ∼97
M
⊙
yr
−1
, with its peak at the BCG core. However, star formation accounts for only 50–60% of the energetics needed to ionise the warm gas. The stellar mass growth of the BCG at
z
< 0.5 is dominated either by in situ star formation generated by thermally unstable intracluster medium cooling or by dry mergers, with these mechanisms accounting for the build-up of 20% of the stellar mass of the system. Our measurements reveal that the most central regions of the BCG contain the lowest gas-phase oxygen abundance, whereas the H
α
arm exhibits slightly more elevated values, suggesting the transport of gas out to large distances from the centre as a result of active galactic nuclei outbursts. The galaxy is a dispersion-dominated system that is typical for massive, elliptical galaxies. The gas and stellar kinematics are decoupled, with the gaseous velocity fields being more closely related to the bulk motions of the intracluster medium.
Context. The morphological, spectroscopic, and kinematical properties of the warm interstellar medium (wim) in early-type galaxies (ETGs) hold key observational constraints to nuclear activity and ...the buildup history of these massive, quiescent systems. This article centers on a 2D investigation of the wim component in 32 nearby (?150 Mpc) ETGs from CALIFA, complementing a previous 1D analysis of the same sample. The analysis presented here includes Ha intensity and equivalent width (EW) maps and radial profiles, diagnostic emission-line ratios, and ionized-gas and stellar kinematics. It is supplemented by t-ratio maps, which are a more efficient means to quantify the role of photoionization by the post-AGB stellar component than alternative mechanisms (e.g., AGN, low-level star formation). This study adds further observational evidence for a considerable heterogeneity among ETGs with regard to the physical properties and 2D kinematics of their extended wim component, and it clearly shows that a comprehensive understanding of these systems requires IFS studies over their entire optical extent.
PDF
