Recent Burst Of Star Formation Research Articles

GA-NIFS: the interplay between merger, star formation and chemical enrichment in MACS1149-JD1 at z=9.11 with JWST/NIRSpec

Abstract We present JWST/NIRSpec integral-field spectroscopy observations of the z ∼ 9.11 lensed galaxy MACS1149-JD1, as part of the GA-NIFS programme. The data was obtained with both the G395H grating (R∼ 2700) and the prism (R∼ 100). This target shows a main elongated UV-bright clump and a secondary component detected in continuum emission at a projected distance of 2 kpc. The R2700 data trace the ionised-gas morpho-kinematics in between the two components, showing an elongated emission mainly traced by [O iii] λ5007. We spatially resolve [O ii] λλ3726,3729, [O iii] λλ4959,5007, and [O iii] λ4363, which enable us to map the electron density (ne ∼ 1.0 × 103 cm−3), temperature (Te ∼ 1.6 × 104 K), and direct-method gas-phase metallicity (-1.2 to -0.7 dex solar). A spatially resolved full-spectrum modelling of the prism indicates a north-south gas metallicity and stellar age gradient between the two components. We found 3-σ evidence of a spatially resolved anti-correlation of the gas-phase metallicity and the star formation rate density, which is likely driven by gas inflows, enhancing the star formation in JD1. We employ high-z sensitive diagnostic diagrams to rule out the presence of a strong AGN in the main component. These findings show the unambiguous presence of two distinct stellar populations, with the majority of the mass ascribed to an old star formation burst, as suggested by previous works. We disfavour the possibility of a rotating-disc nature for MACS1149-JD1; we favour a merger event that has led to a recent burst of star formation in two separate regions, as supported by high values of [O iii] λ5007/Hβ, ionised gas velocity dispersion, and gas-phase metallicity.

BST1047+1156: A (Failing) Ultradiffuse Tidal Dwarf in the Leo I Group

We use deep Hubble Space Telescope imaging to study the resolved stellar populations in BST1047+1156, a gas-rich, ultradiffuse dwarf galaxy found in the intragroup environment of the Leo I galaxy group. While our imaging reaches approximately two magnitudes below the tip of the red giant branch at the Leo I distance of 11 Mpc, we find no evidence for an old red giant sequence that would signal an extended star formation history for the object. Instead, we clearly detect the red and blue helium-burning sequences of its stellar populations, as well as the fainter blue main sequence, all indicative of a recent burst of star formation having taken place over the past 50–250 Myr. Comparing to isochrones for young metal-poor stellar populations, we infer this post-starburst population to be moderately metal-poor, with metallicity [M/H] in the range −1 to −1.5. The combination of a young, moderately metal-poor post starburst population and no old stars motivates a scenario in which BST1047 was recently formed during a weak burst of star formation in gas that was tidally stripped from the outskirts of the neighboring massive spiral M96. BST1047's extremely diffuse nature, lack of ongoing star formation, and disturbed H i morphology all argue that it is a transitory object, a “failing tidal dwarf” in the process of being disrupted by interactions within the Leo I group. Finally, in the environment surrounding BST1047, our imaging also reveals the old, metal-poor ([M/H] = − 1.3 ± 0.2) stellar halo of M96 at a projected radius of 50 kpc.

The Puzzling Properties of the MACS1149-JD1 Galaxy at z = 9.11

We analyze new JWST NIRCam and NIRSpec data on the redshift 9.11 galaxy MACS1149-JD1 (hereafter JD1). Our NIRCam imaging data reveal that JD1 comprises three spatially distinct components. Our spectroscopic data indicate that JD1 appears dust free but is already enriched, . We also find that the carbon and neon abundances in JD1 are below the solar abundance ratio. Particularly the carbon under-abundance is suggestive of recent star formation where Type II supernovae have already enriched the interstellar medium (ISM) in oxygen but intermediate mass stars have not yet enriched the ISM in carbon. A recent burst of star formation is also revealed by the star formation history derived from NIRCam photometry. Our data do not reveal the presence of a significant amount of old populations, resulting in a factor of ∼7× smaller stellar mass than previous estimates. Thus, our data support the view that JD1 is a young galaxy.

Scrutiny of a very young, metal-poor star-forming Lyα emitter at z ≈ 3.7

ABSTRACT The origin of the Lyman α (Lyα) emission in galaxies is a long-standing issue: despite several processes known to originate this line (e.g. active galactic nucleus, star formation, cold accretion, shock heating), it is difficult to discriminate among these phenomena based on observations. Recent studies have suggested that the comparison of the ultraviolet (UV) and optical properties of these sources could solve the riddle. For this reason, we investigate the rest-frame UV and optical properties of Abell 2895b, a strongly lensed Lyα emitter at redshift z ∼ 3.7. From this study, we find that our target is a compact (rn ∼ 1.2 pkpc) star-forming (star formation rate ≃11 M⊙ yr−1) galaxy having a young stellar population. Interestingly, we measure a high ratio of the Hβ and the UV continuum monochromatic luminosities (L(Hβ)/L(UV) ≃ 100). Based on tracks of theoretical stellar models (starburst99 and bpass), we can only partially explain this result by assuming a recent (≲10 Myr), bursty episode of star formation and considering models characterized by binary stars, a top-heavy initial mass function and subsolar metallicities (Z ≲ 0.01 Z⊙). These assumptions also explain the observed low (C/O) abundance of our target (≃0.23(C/O)⊙). By comparing the UV and optical data sets, we find that the Lyα and UV continuum are more extended (×2) than the Balmer lines, and that the peak of the Lyα is offset (≃0.6 pkpc). The multiwavelength results of our analysis suggest that the observed Lyα emission originates from a recent star formation burst, likely taking place in an off-centre clump.

Star-forming and gas-rich brightest cluster galaxies at z ∼ 0.4 in the Kilo-Degree Survey

Brightest cluster galaxies (BCGs) are typically massive ellipticals at the centers of clusters. They are believed to experience strong environmental processing, and their mass assembly and star formation history are still debated. We have selected three star-forming BCGs in the equatorial field of the Kilo-Degree Survey (KiDS). They are KiDS 0920 (z = 0.3216), KiDS 1220 (z = 0.3886), and KiDS 1444 (z = 0.4417). We have observed them with the IRAM 30 m telescope in the first three CO transitions. We remarkably detected all BCGs at high signal-to-noise ratio, S/N ≃ (3.8 − 10.2), for a total of seven detected lines out of eight, corresponding to a success rate of 88%. This allows us to double the number of distant BCGs with clear detections in at least two CO lines. We then combined our observations with available stellar, star formation, and dust properties of the BCGs and compared them with a sample of ∼100 distant cluster galaxies with observations in CO. Our analysis yields large molecular gas reservoirs MH2 ≃ (0.5 − 1.4)×1011 M⊙, high excitation ratios r31 = LCO(3→2)′/LCO(1→0)′ ≃ (0.1 − 0.3), long depletion times τdep ≃ (2 − 4) Gyr, and high MH2/Mdust ≃ (170 − 300) for the three targeted BCGs. The excitation ratio r31 of intermediate-z BCGs, including RX1532 and M1932 from previous studies, appears to be well correlated with the star formation rate and efficiency, which suggests that excited gas is found only in highly star-forming and cool-core BCGs. By performing color-magnitude plots and a red-sequence modeling, we find that recent bursts of star formation are needed to explain the fact that the BCGs are measurably bluer than photometrically selected cluster members. To explain the global observed phenomenology, we suggest that a substantial amount of the molecular gas has been accreted by the KiDS BCGs but still not efficiently converted into stars. KiDS 1220 also shows a double-horn emission in CO(3→2), which implies a low gas concentration. The modeling of the spectrum yields an extended molecular gas reservoir of ∼9 kpc, which is reminiscent of the mature extended-disk phase observed in some local BCGs.

Comparison of star formation histories of AGN and non-AGN galaxies

We used active galactic nuclei (AGNs) with X-ray luminosities, LX, 2 − 10 keV ∼ 1042.5 − 44 erg s−1, from the COSMOS-Legacy survey that lie within the UltraVISTA region and cross-matched them with the LEGA-C catalogue. The latter provides measurements of the calcium break, Dn4000, and Hδ Balmer line that allow us to study the stellar populations of AGNs and compare them with a galaxy reference catalogue. Our samples consist of 69 AGNs and 2176 non-AGN systems, within 0.6 < z < 1.3, that satisfy the same photometric selection criteria. We constructed the spectral energy distributions (SEDs) of both population and used the CIGALE code to investigate the effect of the two indices in the SED fitting process. Our analysis shows that the inclusion of Dn4000 and Hδ allows CIGALE to better constrain the ages of the stellar populations. Furthermore, we find an increase of the estimated stellar masses by ∼0.2 dex, in particular for systems with young stars (Dn4000 < 1.5), when the two indices are included in the SED fitting. We then compare the Dn4000 and Hδ of AGNs with sources in the reference catalogue, accounting for the different stellar mass of the two populations. Our analysis reveals that low-to-moderate LX AGNs tend to reside in galaxies with older stellar populations and are less likely to have experienced a recent star formation burst compared to galaxies in the control sample. Finally, we compared the two populations as a function of their morphology (bulge-dominated, BD, versus non-BD) and compactness (mass-to-size ratio). A similar fraction of AGN and non-AGN systems are classified as non-BD (∼70%). Our analysis shows that BD AGN tend to have younger stellar populations compared to BD non-AGN systems. On the other hand, non-BD AGNs have, on average, older stellar populations and are less likely to have experienced a burst compared to non-BD sources in the reference sample. Furthermore, AGNs tend to prefer more compact systems compared to non-AGNs.

Dust depletion of metals from local to distant galaxies

Large fractions of metals are missing from the observable gas-phase in the interstellar medium (ISM) because they are incorporated into dust grains. This phenomenon is called dust depletion. It is important to study the depletion of metals into dust grains in the ISM to investigate the origin and evolution of metals and cosmic dust. We characterize the dust depletion of several metals from the Milky Way to distant galaxies. We collected measurements of ISM metal column densities from absorption-line spectroscopy in the literature, and in addition, we determined Ti and Ni column densities from a sample of 70 damped Lyman-α absorbers (DLAs) toward quasars that were observed at high spectral resolution with the Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). We used relative ISM abundances to estimate the dust depletion of 18 metals (C, P, O, Cl, Kr, S, Ge, Mg, Si, Cu, Co, Mn, Cr, Ni, Al, Ti, Zn, and Fe) for different environments (the Milky Way, the Magellanic Clouds, and DLAs toward quasars and towards gamma-ray bursts). We observed overall linear relations between the depletion of each metal and the overall strength of the dust depletion, which we traced with the observed [Zn/Fe]. The slope of these dust depletion sequences correlates with the condensation temperature of the various elements, that is, the more refractory elements show steeper depletion sequences. In the neutral ISM of the Magellanic Clouds, small deviations from linearity are observed as an overabundance of the α-elements Ti, Mg, S, and an underabundance of Mn, including for metal-rich systems. The Ti, Mg, and Mn deviations completely disappear when we assume that all systems in our sample of OB stars observed toward the Magellanic Clouds have an α-element enhancement and Mn underabundance, regardless of their metallicity. This may imply that the Magellanic Clouds have recently been enriched in α-elements, potentially through recent bursts of star formation. We also observe an S overabundance in all local galaxies, which is an effect of ionization due to the contribution of their H ii regions to the measured S ii column densities. The observed strong correlations of the depletion sequences of the metals all the way from low-metallicity quasi-stellar object DLAs to the Milky Way suggest that cosmic dust has a common origin, regardless of the star formation history, which, in contrast, varies significantly between these different galaxies. This supports the importance of grain growth in the ISM as a significant process of dust production.

The merger fraction of post-starburst galaxies in UNIONS

ABSTRACT Post-starburst galaxies (PSBs) are defined as having experienced a recent burst of star formation, followed by a prompt truncation in further activity. Identifying the mechanism(s) causing a galaxy to experience a post-starburst phase therefore provides integral insight into the causes of rapid quenching. Galaxy mergers have long been proposed as a possible post-starburst trigger. Effectively testing this hypothesis requires a large spectroscopic galaxy survey to identify the rare PSBs as well as high-quality imaging and robust morphology metrics to identify mergers. We bring together these critical elements by selecting PSBs from the overlap of the Sloan Digital Sky Survey and the Canada–France Imaging Survey and applying a suite of classification methods: non-parametric morphology metrics such as asymmetry and Gini-M20, a convolutional neural network trained to identify post-merger galaxies, and visual classification. This work is therefore the largest and most comprehensive assessment of the merger fraction of PSBs to date. We find that the merger fraction of PSBs ranges from 19 per cent to 42 per cent depending on the merger identification method and details of the PSB sample selection. These merger fractions represent an excess of 3–46× relative to non-PSB control samples. Our results demonstrate that mergers play a significant role in generating PSBs, but that other mechanisms are also required. However, applying our merger identification metrics to known post-mergers in the IllustrisTNG simulation shows that 70 per cent of recent post-mergers (≲200 Myr) would not be detected. Thus, we cannot exclude the possibility that nearly all PSBs have undergone a merger in their recent past.

Evidence of deep mixing in IRS 7, a cool massive supergiant member of the Galactic nuclear star cluster

ABSTRACT The centre of the Milky Way contains stellar populations spanning a range in age and metallicity, with a recent star formation burst producing young and massive stars. Chemical abundances in the most luminous stellar member of the nuclear star cluster (NSC), IRS 7, are presented for 19F, 12C, 13C, 14N, 16O, 17O, and Fe from a local thermodynamic equilibrium analysis based on spherical modelling and radiative transfer with a 25-M⊙ model atmosphere, whose chemistry was tailored to the derived photospheric abundances. We find IRS 7 to be depleted heavily in both 12C (∼–0.8 dex) and 16O (∼–0.4 dex), while exhibiting an extremely enhanced 14N abundance (∼+1.1 dex), which are isotopic signatures of the deep mixing of CNO-cycled material to the stellar surface. The 19F abundance is also heavily depleted by ∼1 dex relative to the baseline fluorine of the NSC, providing evidence that fluorine along with carbon constrain the nature of the deep mixing in this very luminous supergiant. The abundances of the minor isotopes 13C and 17O are also derived, with ratios of 12C/13C ∼ 5.3 and 16O/17O ∼ 525. The derived abundances for IRS 7, in conjunction with previous abundance results for massive stars in the NSC, are compared with rotating and non-rotating models of massive stars and it is found that the IRS 7 abundances overall follow the behaviour predicted by stellar models. The depleted fluorine abundance in IRS 7 illustrates, for the first time, the potential of using the 19F abundance as a mixing probe in luminous red giants.

Characterizing epochs of star formation across the Milky Way disc using age–metallicity distributions of GALAH stars

ABSTRACT We provide a detailed map of the ages and metallicities of turn-off stars in the Milky Way disc based on data from GALAH DR3 and Gaia EDR3. From this map, we identify previously undetected features in the age–metallicity distribution of disc stars and interpret these results as indicating a three-phase formation history of the Milky Way. In the first phase, inner disc stars form along a single age–metallicity sequence and are today kinematically hot. The end of this phase is marked by a local minimum in the inner disc age distribution 10 Gyr ago. At this time, we find the stellar populations to transition from high to low alpha-element abundances and from high to low vertical velocity dispersion. In the second phase, stars form across the disc with outwardly decreasing metallicity. In this phase, inner disc stars form at supersolar metallicities in a continuation of the early age–metallicity relation, while outer disc stars begin forming at metallicities at least 0.5 dex lower. Finally, the third phase is associated with a recent burst of star formation across the local disc marked by a local minimum in the age–metallicity distribution 4–6 Gyr ago. Future quantitative comparisons between the observed age–metallicity distribution and those of simulated galaxies could help constrain the processes driving each of the star formation phases.

A study of outer disc stellar populations of face-on star-forming galaxies in SDSS-IV MaNGA: causes of H α deficiency

ABSTRACT Integral field unit spectra of face-on star-forming galaxies from the MaNGA survey are stacked in radial bins so as to reach an S/N high enough to measure emission lines and Lick indices out to 2.5–3Re. Two-thirds of galaxies have stellar populations in the outer discs that are older, more metal poor, and less dusty than in the inner discs. Recent bursts of star formation have occurred more frequently in the outer disc, but extinction-corrected H α equivalent widths are significantly lower at fixed Dn(4000) in these regions. I examine the properties of a subset of galaxies with the most H α-deficient outer discs. These regions contain young stellar populations that must have formed within the last 0.5 Gyr, but have extinction-corrected H α values well below the values predicted for a standard Kroupa initial mass function. The H α-deficient galaxies have flat Dn(4000) and H δA profiles with little radial fluctuation, indicating that star formation has occurred extremely uniformly across the entire disc. The H α line profiles indicate that the ionized gas kinematics is also very regular across the disc. The main clue to the origin of the H α deficiency is that it sets in at the same radius where the dust extinction abruptly decreases, suggesting a mode of star formation deficient in massive stars in quiescent, H i-dominated gas. Finally, I have carried out a search for galaxies with signatures of unusual H α kinematics and find that 15 per cent of the sample exhibits evidence for significant ionized gas that is displaced from the systemic velocity of the disc.

The interacting nature of dwarf galaxies hosting superluminous supernovae

Context. Type I superluminous supernovae (SLSNe I) are rare, powerful explosions whose mechanism and progenitors remain elusive. Several studies have shown a preference for SLSNe I to occur in low-metallicity, actively star-forming dwarf galaxies. Aims. We investigate whether the host galaxies of SLSNe I show increased evidence for interaction. Galaxy interaction can trigger star formation and provide favourable conditions for these exceptional explosions to take place. Methods. Based on SLSN host galaxy images obtained with the Hubble Space Telescope (HST), we narrowed down a sample of 42 images obtained in the rest-frame ultraviolet over the redshift range between 0 < z < 2. The number of host galaxy companions was measured by counting the number of objects detected within a given projected radius from the host. As a comparison, we used two different Monte Carlo-based methods to estimate the expected average number of companion objects in the same HST images, as well as a sample of 32 dwarf galaxies that have hosted long gamma-ray bursts (GRBs). Results. About 50% of SLSN I host galaxies have at least one major companion (within a flux ratio of 1:4) within 5 kpc. The average number of major companions per SLSN I host galaxy is 0.70−0.14+0.19. Our two Monte Carlo comparison methods yield a lower number of companions for random objects of similar brightness in the same image or for the SLSN host after randomly redistributing the sources in the same image. The Anderson-Darling test shows that this difference is statistically significant (p-value < 10−3) independent of the redshift range. The same is true for the projected distance distribution of the companions. The SLSN I hosts are, thus, found in areas of their images, where the object number density is greater than average. The SLSN I hosts have more companions than GRB hosts (0.44−0.13+0.25 companions per host distributed over 25% of the hosts) but the difference is not statistically significant. The difference between their separations is, however, marginally significant with SLSN companions being closer, on average, than those of GRBs. Conclusions. The dwarf galaxies hosting SLSNe I are often part of interacting systems. This suggests that SLSNe I progenitors are formed after a recent burst of star formation. Low metallicity alone cannot explain this tendency.

Detection of young (≤20 Myr) stellar populations in apparently quenched low-mass galaxies using red spectral line indices

ABSTRACT We report on the detection of a small contribution (around and below 1 per cent in mass) from young stellar components with ages ≤20 Myr in low-mass galaxies purposely selected from the MaNGA survey to be already-quenched systems. Among the sample of 28 galaxies, 8 of them show signatures of having suffered a very recent burst of star formation. The detection has been done through the analysis of line-strength indices in the red spectral range [5700,8800] Å. The increasing contribution of red supergiants to this red regime is responsible for a deviation of the index measurements with respect to their position within the model grids in the standard spectral range [3600,5700] Å. We demonstrate that a combination of red indices, as well as a qualitative assessment of the mean luminosity-weighted underlying stellar population, is required in order to distinguish between a true superyoung population and other possible causes of this deviation, such as abundance ratio variations. Our result implies that many presumably quenched low-mass galaxies actually contain gas that is triggering some level of star formation. They have, therefore, either accreted external gas, internally recycled enough gas from stellar evolution to trigger new star formation, or they kept a gas reservoir after the harassment or stripping process that quenched them in the first place. Internal processes are favoured since we find no particular trends between our non-quenched galaxies and their environment, although more work is needed to fully discard an external influence.

VIS3COS

We present spectroscopic observations of 466 galaxies in and around a superstructure at z ∼ 0.84 targeted by the VIMOS Spectroscopic Survey of a Supercluster in the COSMOS field (VIS3COS). We use [OII]λ3727, Hδ, and Dn4000 to trace recent, medium-, and long-term star formation histories and investigate the effect of stellar mass and local environment on them. By studying trends in individual and composite galaxy spectra, we find that stellar mass and environment play a role in the observed galactic properties. Galaxies with low stellar mass (10 < log10(M⋆/M⊙) < 10.5) in the field show the strongest Hδ absorption. Similarly, the massive population (log10(M⋆/M⊙) > 11) shows an increase in Hδ absorption strengths in intermediate-density environments (e.g. filaments). Galaxies with intermediate stellar mass (10.5 < log10(M⋆/M⊙) < 11) have similar Hδ absorption profiles in all environments, but show an indication of enhanced [OII] emission in intermediate-density environments. This indicates that field galaxies with low stellar mass and filament galaxies with high stellar mass are more likely to have experienced a recent burst of star formation, while galaxies of the intermediate stellar-mass show an increase of star formation at filament-like densities. We also find that the median [OII] equivalent width (|EW[OII]|) decreases from 27 ± 2 Å to 2.0+0.5−0.4 Å and Dn4000 increases from 1.09 ± 0.01 to 1.56 ± 0.03 with increasing stellar mass (from ∼109.25 to ∼1011.35 M⊙). For the dependence on the environment, we find that at fixed stellar mass, |EW[OII]| is tentatively lower in environments with higher density. We find for Dn4000 that the increase with stellar mass is sharper in denser environments, which indicates that these environments may accelerate galaxy evolution. Moreover, we find higher Dn4000 values in denser environments at fixed stellar mass, suggesting that galaxies are on average older and/or more metal rich in these dense environments. This set of tracers depicts a scenario where the most massive galaxies have, on average, the lowest specific star formation rates and the oldest stellar populations (age ≳ 1 Gyr, showing a mass-downsizing effect). We also hypothesize that the observed increase in star formation (higher EW[OII]|, higher specific star formation rate) at intermediate densities may lead to quenching because we find that the quenched fraction increases sharply from the filament to cluster-like regions at similar stellar masses.

The Star Formation History in the Solar Neighborhood as Told by Massive White Dwarfs

Abstract White dwarfs (WDs) are the remnants of low- and intermediate-mass stars. Because of electron degeneracy, their evolution is just a simple gravothermal process of cooling. Recently, thanks to Gaia data, it has been possible to construct the luminosity function of massive ( ) WDs in the solar neighborhood ( pc). Because the lifetime of their progenitors is very short, the birth times of both parents and daughters are very close and facilitate the reconstruction of an (effective) star formation rate. This rate started growing from zero during the early Galaxy and reached a maximum 6–7 Gyr ago. It declined and ∼5 Gyr ago started to climb once more, reaching a maximum 2–3 Gyr ago; it has decreased since then. There are some traces of a recent star formation burst, but the method used here is not appropriate for recently born WDs.

Star-forming galaxies at low-redshift in the SHARDS survey

Context. The physical processes driving the evolution of star formation (SF) in galaxies over cosmic time still present many open questions. Recent galaxy surveys allow now to study these processes in great detail at intermediate redshift (0 ≤ z ≤ 0.5). Aims. We build a complete sample of star-forming galaxies and analyze their properties, reaching systems with low stellar masses and low star formation rates (SFRs) at intermediate-to-low redshift. Methods. We use data from the SHARDS multiband survey in the GOODS-North field. Its depth (up to magnitude ⟨m3σ⟩~ 26.5) and its spectro-photometric resolution (R ~ 50) provides us with an ideal dataset to search for emission line galaxies (ELGs). We develop a new algorithm to identify low-redshift (z < 0.36) ELGs by detecting the [OIII]5007 and Hα emission lines simultaneously. We fit the spectral energy distribution (SED) of the selected sample, using a model with two single stellar populations. Results. We find 160 star-forming galaxies for which we derive equivalent widths (EWs) and absolute fluxes of both emission lines. We detect EWs as low as 12 Å, with median values for the sample of ~35 Å in [OIII]5007 and ~56 Å in Hα, respectively. Results from the SED fitting show a young stellar population with low median metallicity (36% of the solar value) and extinction (AV ~ 0.37), with median galaxy stellar mass ~108.5 M⊙. Gas-phase metallicities measured from available spectra are also low. ELGs in our sample present bluer colours in the UVJ plane than the median colour-selected star-forming galaxy in SHARDS. We suggest a new V-J colour criterion to separate ELGs from non-ELGs in blue galaxy samples. In addition, several galaxies present high densities of O-type stars, possibly producing galactic superwinds, which makes them interesting targets for follow-up spectroscopy. Conclusions. We have demonstrated the efficiency of SHARDS in detecting low-mass ELGs (~2 magnitudes deeper than previous spectroscopic surveys in the same field). The selected sample accounts for 20% of the global galaxy population at this redshift and luminosity, and is characterized by young SF bursts with sub-solar metallicities and low extinction. However, robust fits to the full SEDs can only be obtained including an old stellar population, suggesting the young component is built up by a recent burst of SF in an otherwise old galaxy.

Investigation of dust attenuation and star formation activity in galaxies hosting GRBs

Context. The gamma-ray bursts hosts (GRBHs) are excellent targets to study the extinction properties of dust and its effects on the global emission of distant galaxies. The dust extinction curve is measured along the GRB afterglow line of sight and the analysis of the spectral energy distribution (SED) of the host galaxy gives access to the global dust attenuation of the stellar light. Aims. In this pilot study we gather information on dust extinction in GRBHs to compare the properties of the extinction curve to those of the dust obscuration affecting the total stellar light of the host galaxy. Assuming the extinction curve to be representative of the dust properties, we aim to investigate which dust-stars geometries and local dust distribution in the inter stellar medium (ISM) can reproduce the observed attenuation curve. Methods. We selected a sample of 30 GRBs for which the extinction curve along the GRB afterglow line-of-sight (l.o.s.) is measured in the rest-frame ultraviolet (UV) up to optical and we analysed the properties of the extinction curve as a function of the host galaxy properties. From these 30 GRBs, we selected seven GRBHs with a good rest-frame UV to near-infrared (NIR) spectral coverage for the host. The attenuation curve was derived by fitting the SEDs of the GRBH sample with the CIGALE SED fitting code. Different star formation histories (SFH) were studied to recover the star formation rates (SFR) derived using Hα luminosities. Implications for the dust-stars geometries in the ISM are inferred by a comparison with radiative transfer simulations. Results. The most extinguished GRBs are preferentially found in the more massive hosts and the UV bump is preferentially found in the most extinguished GRB l.o.s. Five out of seven hosts are best fitted with a recent burst of star formation, leading to lower stellar mass estimates than previously found. The average attenuation in the host galaxies is about 70% of the amount of extinction along the GRB l.o.s. We find a great variety in the derived attenuation curves of GRBHs, the UV slope can be similar, flatter or even steeper than the extinction curve slope. Half of the attenuation curves are consistent with the Calzetti attenuation law and there is evidence of a UV bump in only one GRBH. We find that the flatter (steeper) attenuation curves are found in galaxies with the highest (lowest) SFR and stellar masses. The comparison of our results with radiative transfer simulations leads to a uniform distribution of dust and stars in a very clumpy ISM for half the GRBHs and various dust-stars geometries for the second half of the sample.

BST1047+1156: An Extremely Diffuse and Gas-rich Object in the Leo I Group

Abstract We report the detection of diffuse starlight in an extragalactic H i cloud in the nearby Leo I galaxy group. We detect the source, BST1047+1156, in both broadband optical and the Galaxy Evolution Explorer (GALEX) ultraviolet (UV) light. Spanning ∼2 kpc in radius, it has a peak surface brightness of μ B = 28.8 mag arcsec−2, making it the lowest surface brightness object ever detected via integrated light. Although the object is extremely gas rich, with a gas fraction of f g = 0.99, its peak H i column density is well below levels where star formation is typically observed in galaxies. Nonetheless, BST1047+1156 shows evidence for young stellar populations: along with the detected UV emission, the object is extremely blue, with B − V = 0.14 ± 0.09. The object has two tidal tails and is found embedded within diffuse gas connecting the spiral galaxy M96 to the group’s extended H i Leo Ring. The nature of BST1047+1156 is unclear. It could be a disrupting tidal dwarf, recently spawned from star formation triggered in the Leo I group’s tidal debris. Alternatively, the object may have been a pre-existing galaxy—the most extreme example of a gas-rich field low surface brightness galaxy known to date—which had a recent burst of star formation triggered by encounters in the group environment.

J0811+4730: the most metal-poor star-forming dwarf galaxy known

We report the discovery of the most metal-poor dwarf star-forming galaxy (SFG) known to date, J0811+4730. This galaxy, at a redshift z=0.04444, has a Sloan Digital Sky Survey (SDSS) g-band absolute magnitude M_g = -15.41 mag. It was selected by inspecting the spectroscopic data base in the Data Release 13 (DR13) of the SDSS. LBT/MODS spectroscopic observations reveal its oxygen abundance to be 12 + log O/H = 6.98 +/- 0.02, the lowest ever observed for a SFG. J0811+4730 strongly deviates from the main-sequence defined by SFGs in the emission-line diagnostic diagrams and the metallicity - luminosity diagram. These differences are caused mainly by the extremely low oxygen abundance in J0811$+$4730, which is ~10 times lower than that in main-sequence SFGs with similar luminosities. By fitting the spectral energy distributions of the SDSS and LBT spectra, we derive a stellar mass of M* = 10^6.24 - 10^6.29 Msun (statistical uncertainties only), and we find that a considerable fraction of the galaxy stellar mass was formed during the most recent burst of star formation.

Unusual void galaxy DDO 68: implications of theHST-resolved photometry

DDO68 (UGC5340) is an unusual dwarf galaxy with extremely low gas metallicity (12+log(O/H) = 7.14) residing in the nearby Lynx-Cancer void. Despite its apparent isolation, it shows both optical and HI morphological evidence for strong tidal disturbance. Here, we study the resolved stellar populations of DDO68 using deep images from the HST archive. We determined a distance of 12.75+-0.41 Mpc using the tip of the red giant branch (TRGB). The star formation history reconstruction reveals that about 60 per cent of stars formed during the initial period of star formation, about 12-14 Gyr ago. During the next 10 Gyr DDO68 was in the quenched state, with only slight traces of star formation. The onset of the most recent burst of star formation occurred about 300 Myr ago. We find that young populations with ages of several million to a few hundred million years are widely spread across various parts of DDO68, indicating an intense star formation episode with a high mean rate of 0.15 Msun/yr. A major fraction of the visible stars in the whole system (~80 per cent) have low metallicities: Z = Zsun/50 - Zsun/20. The properties of the northern periphery of DDO68 can be explained by an ongoing burst of star formation induced by the minor merger of a small, gas-rich, extremely metal-poor galaxy with a more typical dwarf galaxy. The current TRGB-based distance of DDO68 implies a total negative peculiar velocity of ~500 km/s.