Starburst Region Research Articles

Unraveling the Mystery of the Low CO-to-H2 Conversion Factor in Starburst Galaxies: RADEX Modeling of the Antennae

CO emission has been widely used as a tracer of molecular gas mass. However, it is a long-standing issue to accurately constrain the CO-to-H2 conversion factor (α CO) that converts CO luminosity to molecular gas mass, especially in starburst galaxies. We present the first resolved α CO modeling results with multiple Atacama Large Millimeter/submillimeter Array CO and 13CO transition observations at both giant molecular cloud (GMC) scale at 150 pc and kiloparsec scale for one of the closest starburst mergers, the Antennae. By combining our CO modeling results and measurements of 350 GHz dust continuum, we find that most GMCs in the Antennae have α CO values approximately four times smaller than the commonly adopted Milky Way value (4.3). We find that α CO at GMC scales shows a strong dependence on CO intensity, 13CO/CO ratio, and GMC velocity dispersion, which is consistent with various theoretical and simulation predictions. Specifically, we suggest that the 13CO/CO line ratio and the velocity dispersion can be used to infer α CO in starburst regions. By applying our modeled α CO in GMC analyses, we find that GMCs in the Antennae are less gravitationally bound than in normal spiral galaxies, which is more consistent with what is predicted by merger simulations. At kiloparsec scale, we find that our modeled α CO values are smaller than the modeled α CO at GMC scale by 40%, which can be due to inclusion of a diffuse gas component with lower α CO values. We find a similar correlation of α CO and CO intensity at kiloparsec scales to that at GMC scales.

MIDIS: The Relation between Strong (Hβ + [O iii]) Emission, Star Formation, and Burstiness around the Epoch of Reionization

We investigate the properties of strong (Hβ + [O iii]) emitters before and after the end of the “Epoch of Reionization” from z = 8 to z = 5.5. We make use of ultradeep JWST/NIRCam imaging in the parallel field (P2) of the MIRI Deep Imaging Survey (MIDIS) in the Hubble eXtreme Deep Field (H-XDF), in order to select prominent (Hβ + [O iii]) emitters (with rest-frame equivalent width (EW0) ≳ 100 Å) at z = 5.5–7, based on their flux density enhancement in the F356W band with respect to the spectral energy distribution continuum. We complement our selection with other (Hβ + [O iii]) emitters from the literature at similar and higher (z = 7−8) redshifts. We find (nonindependent) anticorrelations between EW0(Hβ + [O iii]) and both galaxy stellar mass and age, in agreement with previous studies, and a positive correlation with specific star formation rate (sSFR). On the SFR–M ⋆ plane, the (Hβ + [O iii]) emitters populate both the star formation main sequence and the starburst region, which become indistinguishable at low stellar masses ( log10(M⋆)<7.5 ). We find tentative evidence for a nonmonotonic relation between EW0(Hβ + [O iii]) and SFR, such that both parameters correlate with each other at SFR ≳ 1 M ⊙ yr−1, while the correlation flattens out at lower SFRs. This suggests that low metallicities producing high EW0(Hβ + [O iii]) could be important at low SFR values. Interestingly, the properties of the strong emitters and other galaxies (33% and 67% of our z = 5.5–7 sample, respectively) are similar, including, in many cases, high sSFR. Therefore, it is crucial to consider both emitters and nonemitters to obtain a complete picture of the cosmic star formation activity around the Epoch of Reionization.

The stellar fundamental metallicity relation: the correlation between stellar mass, star formation rate, and stellar metallicity

ABSTRACT We present observational evidence for a stellar fundamental metallicity relation (FMR), a smooth relation between stellar mass, star formation rate, and the light-weighted stellar metallicity of galaxies, analogous to the well-established gas-phase FMR. We use the non-parametric software ppxf to reconstruct simultaneously the star formation and chemical-enrichment history of a representative sample of galaxies from the local MaNGA (Mapping Nearby Galaxies at Apache Point Observatory) survey. We find that (i) the metallicity of individual galaxies increases with cosmic time and (ii) at all stellar masses, the metallicity of galaxies is progressively higher, moving from the starburst region above the main sequence (MS) towards the passive galaxies below the MS, manifesting the stellar FMR. The scatter is reduced when replacing the stellar mass $M_{*}$ with $M_{*}/R_{\rm e}$ (with $R_{\rm e}$ being the effective radius), in agreement with previous results using the velocity dispersion $\sigma _{\rm e}$, which correlates with $M_{*}/R_{\rm e}$. Our results point to starvation as the main physical process through which galaxies quench, showing that metal-poor gas accretion from the intergalactic medium/circumgalactic medium – or the lack thereof – plays an important role in galaxy evolution by simultaneously shaping both their star formation and their metallicity evolutions, while outflows play a subordinate role. This interpretation is further supported by the additional finding of a young stellar FMR, tracing only the stellar populations formed in the last 300 Myr. This suggests a tight co-evolution of the chemical composition of both the gaseous interstellar medium and the stellar populations, where the gas-phase FMR is continuously imprinted on to the stars over cosmic times.

Simulating nearby disc galaxies on the main star formation sequence

Past studies have long emphasised the key role played by galactic stellar bars in the context of disc secular evolution, via the redistribution of gas and stars, the triggering of star formation, and the formation of prominent structures such as rings and central mass concentrations. However, the exact physical processes acting on those structures, as well as the timescales associated with the building and consumption of central gas reservoirs are still not well understood. We are building a suite of hydro-dynamical RAMSES simulations of isolated, low-redshift galaxies that mimic the properties of the PHANGS sample. The initial conditions of the models reproduce the observed stellar mass, disc scale length, or gas fraction, and this paper presents a first subset of these models. Most of our simulated galaxies develop a prominent bar structure, which itself triggers central gas fuelling and the building of an over-density with a typical scale of 100−1000 pc. We confirm that if the host galaxy features an ellipsoidal component, the formation of the bar and gas fuelling are delayed. We show that most of our simulations follow a common time evolution, when accounting for mass scaling and the bar formation time. In our simulations, the stellar mass of 1010 M⊙ seems to mark a change in the phases describing the time evolution of the bar and its impact on the interstellar medium. In massive discs (M⋆ ≥ 1010 M⊙), we observe the formation of a central gas reservoir with star formation mostly occurring within a restricted starburst region, leading to a gas depletion phase. Lower-mass systems (M⋆ &lt; 1010 M⊙) do not exhibit such a depletion phase, and show a more homogeneous spread of star-forming regions along the bar structure, and do not appear to host inner bar-driven discs or rings. Our results seem to be supported by observations, and we briefly discuss how this new suite of simulations can help our understanding of the secular evolution of main sequence disc galaxies.

Spectral characterisation of the extinction properties of NGC 3603 using JWST NIRSpec.

A necessary ingredient in understanding the star formation history of a young cluster is knowledge of the extinction towards the region. This has typically been done by making use of the colour-difference method with photometry, or similar methods utilising the colour-colour diagram. These approaches rely on adopting an extinction law with a given total-to-selective extinction ratio $R(V)$, or determining a value of $R(V)$ through empirical relationships. They also rely upon accurate spectral classification, reliable stellar isochrones, and separating field stars from genuine cluster members. The colour excess $E(B-V)$ can be independently determined by studying the decrements of the recombination lines produced by the nebular gas. Having access to many recombination lines from the same spectral series removes the need of adopting an extinction curve. Rather, different extinction curves can be trialled and the most appropriate one selected based on a minimum $ procedure. Using the Micro-Shutter Assembly (MSA) on board the Near InfraRed Spectrograph (NIRSpec), multi-object spectroscopy was performed, yielding 600 nebular spectra from the Galactic massive star formation region NGC 3603. The recombination line intensity ratios were used to determine independent values of $E(B-V)$. A series of extinction curves were trialled ranging from $R(V) = 2$ to $R(V) = 8$. The appropriate value of $R(V)$ was adopted based on the minimum $ procedure. The extinction characteristics of NGC 3603 are similar to other Galactic HII regions like Orion, as well as starburst regions such 30 Doradus in the Large Magellanic Cloud, in that we find a relatively large value of $R(V) = 4.8 1.06$, larger than the Galactic average of $3.1$. We find a typical value of $E(B-V) = 0.64 0.27$, significantly lower than values determined in previous studies. We also present a stacked nebular spectrum with a typical continuum S/N = $70$. This spectrum highlights the recombination lines of the HII region, several s-process elements such as Kr $III$ and Se $IV$, and molecular $H_2$ emission lines. This high S/N spectrum can act as a helpful template for identifying nebular emission lines. Using ratios of hydrogen recombination lines, we calculated the value of $R(V)$, $E(B-V)$ and $A(V)$ for $&gt; 200$ lines of sight across NGC 3603. An extinction curve with a typical value $R(V) = 4.8 1.06$ is required to explain the colour excess observed in the nebular spectra. This corresponds to a typical $E(B-V) = 0.64 0.27$. This is significantly lower than what has been found in previous extinction studies of NGC 3603.

Omnipolar mapping and vector mapping in premature ventricular contractions ablation: a comparison with bipolar mapping

Abstract Background Omnipolar Mapping, with HD Grid high-density mapping catheter, overcomes the dependency of EGM characteristics to wavefront orientation, reconstructing the highest peak-to-peak voltage attainable within the electric field and the real wavefront direction, displayed in the form of a local activation vector [1][2]. Methods Consecutive patients who underwent radiofrequency ablation of PVCs from October 2021 to August 2023 at the Hospital Clinic of Barcelona were included in the study. EAM was performed with EnsiteX mapping system, Advisor HDgrid was used as mapping catheter. An activation map was performed if enough PVC burden. The area of interest was determined by earliest activation time combined with the morphology of unipolar electrogram during activation mapping. RF was delivered at the area of interest with target temperature of 45°C and maximum power of 50W. Both bipolar and omnipolar maps were analysed retrospectively. The reviewer was blinded to the ablation set for this analysis. A window of interest was created uniformally for all maps by defining a caliper within 15 ms before the onset of PVC. Activation vectors were displayed on the map and visually analysed. Three specific patterns were identified: Starburst, Homogeneous and Chaotic. Area of the earliest activation was measured. Earliest site of bipolar and omnipolar maps and specific location of star burst pattern were tagged. The site of the effective RF ablation was identified and the distance between points was determined. Results 18 patients were included. Overall, 11(61%) were males; age: 57,9±15,2 ys, 6(33%) patients haven't structural heart disease, 4(22%) presented PVC-induced cardiomyopathy. Mean left ventricular ejection fraction was 46±10. Mean PVC burden was 27±10%. In 9(50%) patients, PVCs were ablated from RVOT, 4(22%) in LVOT, 1(5%) in the left superior fascicle, 1(5%) in the antero-lateral papillary muscle, 2(10%) in the perihissian area and 1(5%) in the aortomitral continuity. Compared with bipolar, omnipolar mapping showed a smaller area of earliest activation(2,13±1,9mm2 vs 3,34±2,1 mm2,p&amp;lt;0,001), a shorter distance of the earliest point to starburst region(4,2±4,1mm vs 7,8±4,2mm,p&amp;lt;0,001) and shorter distance to the effective ablation site(6,1±3,9 mm vs 8,33±4,2mm, p&amp;lt;0,07). No significative difference was found between the effective RF ablation site and the starburst area compared to the distance between the effective RF ablation site and the omnipolar earliest deflection (6,11±3,9mm vs 6,5±4,3mm, p=0,6). No statistical significative difference was found between the timing of the earliest deflection in both settings (33,2±12,5 ms vs 31,5±12 ms, p=0,2) Conclusions Omnipolar mapping allows a better identification of the area of interest in PVC’s ablation compared to bipolar mapping. Vector mapping is a valuable asset to identify the earliest activation point to guide PVCs catheter ablation.Retrospective EAM analysisArea/distance measurements

Resolving the Mechanical and Radiative Feedback in J1044+0353 with Keck Cosmic Web Imager Spectral Mapping

We present integral field spectroscopy toward and around J1044+0353, a rapidly growing, low-metallicity galaxy that produces extreme [O iii] line emission. A new map of the O32 flux ratio reveals a density-bounded ionization cone emerging from the starburst. The interaction of the hydrogen-ionizing radiation, produced by the very young starburst, with a cavity previously carved out by a galactic outflow, whose apex lies well outside the starburst region, determines the pathway for global Lyman continuum (LyC) escape. In the region within a few hundred parsecs of the young starburst, we demonstrate that superbubble breakthrough and blowout contribute distinct components to the [O iii] line profile: broad and very broad emission line wings, respectively. We draw attention to the large [O iii] luminosity of the broad component and argue that this emission comes from photoionized, superbubble shells rather than a galactic wind as is often assumed. The spatially resolved He ii λ4686 nebula appears to be photoionized by young star clusters. Stellar wind emission from these stars is likely the source of line wings detected on the He ii line profile. This broader He ii component indicates slow stellar winds, consistent with an increase in stellar rotation (and a decrease in effective escape speed) at the metallicity of J1044+0353. At least in J1044+0353, the recent star formation history plays a critical role in generating a global pathway for LyC escape, and the anisotropic escape would likely be missed by direct observations of the LyC.

GOALS-JWST: Mid-infrared Molecular Gas Excitation Probes the Local Conditions of Nuclear Star Clusters and the Active Galactic Nucleus in the LIRG VV 114

The enormous increase in mid-IR sensitivity and spatial and spectral resolution provided by the JWST spectrographs enables, for the first time, detailed extragalactic studies of molecular vibrational bands. This opens an entirely new window for the study of the molecular interstellar medium in luminous infrared galaxies (LIRGs). We present a detailed analysis of rovibrational bands of gas-phase CO, H2O, C2H2, and HCN toward the heavily obscured eastern nucleus of the LIRG VV 114, as observed by NIRSpec and the medium resolution spectrograph on the Mid-InfraRed Instrument (MIRI MRS). Spectra extracted from apertures of 130 pc in radius show a clear dichotomy between the obscured active galactic nucleus (AGN) and two intense starburst regions. We detect the 2.3 μm CO bandheads, characteristic of cool stellar atmospheres, in the star-forming regions, but not toward the AGN. Surprisingly, at 4.7 μm, we find highly excited CO (T ex ≈ 700–800 K out to at least rotational level J = 27) toward the star-forming regions, but only cooler gas (T ex ≈ 200 K) toward the AGN. We conclude that only mid-infrared pumping through the rovibrational lines can account for the equilibrium conditions found for CO and H2O in the deeply embedded starbursts. Here, the CO bands probe regions with an intense local radiation field inside dusty young massive star clusters or near the most massive young stars. The lack of high-excitation molecular gas toward the AGN is attributed to geometric dilution of the intense radiation from the bright point source. An overview of the relevant excitation and radiative transfer physics is provided in an appendix.

JWST Observations of Starbursts: Polycyclic Aromatic Hydrocarbon Emission at the Base of the M82 Galactic Wind

We present new observations of the central 1 kpc of the M82 starburst obtained with the James Webb Space Telescope near-infrared camera instrument at a resolution θ ∼ 0.″05–0.″1 (∼1–2 pc). The data comprises images in three mostly continuum filters (F140M, F250M, and F360M), and filters that contain [Fe ii] (F164N), H2 v = 1 → 0 (F212N), and the 3.3 μm polycyclic aromatic hydrocarbon (PAH) feature (F335M). We find prominent plumes of PAH emission extending outward from the central starburst region, together with a network of complex filamentary substructures and edge-brightened bubble-like features. The structure of the PAH emission closely resembles that of the ionized gas, as revealed in Paschen α and free–free radio emission. We discuss the origin of the structure, and suggest the PAHs are embedded in a combination of neutral, molecular, and photoionized gas.

MIDIS: JWST NIRCam and MIRI Unveil the Stellar Population Properties of Lyα Emitters and Lyman-break Galaxies at z ≃ 3–7

We study the stellar population properties of 182 spectroscopically confirmed (MUSE/VLT) Lyα emitters (LAEs) and 450 photometrically selected Lyman-break galaxies (LBGs) at z = 2.8–6.7 in the Hubble Extreme Deep Field. Leveraging the combined power of Hubble Space Telescope and JWST NIRCam and MIRI observations, we analyze their rest-frame UV-through-near-IR spectral energy distributions, with MIRI playing a crucial role in robustly assessing the LAEs’ stellar masses and ages. Our LAEs are low-mass objects (log10(M⋆/M⊙)≃7.5) with little or no dust extinction (E(B − V) ≃ 0.1) and a blue UV continuum slope (β ≃ −2.2). While 75% of our LAEs are young (<100 Myr), the remaining 25% have significantly older stellar populations (≥100 Myr). These old LAEs are statistically more massive, less extinct, and have lower specific star formation rate than young LAEs. Besides, they populate the plane of M ⋆ versus star formation rate along the main sequence of star-forming galaxies, while young LAEs populate the starburst region. The comparison between the LAEs’ properties and those of a stellar-mass-matched sample of LBGs shows no statistical difference between these objects, except for the LBGs’ redder UV continuum slope and marginally larger E(B − V) values. Interestingly, 48% of the LBGs have ages <10 Myr and are classified as starbursts, but lack detectable Lyα emission. This is likely due to H i resonant scattering and/or dust-selective extinction. Overall, we find that JWST observations are crucial in determining the properties of LAEs and shedding light on their comparison with LBGs.

Absorption of Millimeter-band CO and CN in the Early Universe: Molecular Clouds in the Radio Galaxy B2 0902+34 at Redshift 3.4

Using the Karl G. Jansky Very Large Array, we have detected absorption lines due to carbon monoxide, CO(J = 0 → 1), and the cyano radical, CN(N = 0 → 1), associated with radio galaxy B2 0902+34 at redshift z = 3.4. The detection of millimeter-band absorption observed 1.5 Gyr after the Big Bang facilitates studying molecular clouds down to gas masses inaccessible to emission-line observations. The CO absorption in B2 0902+34 has a peak optical depth of τ ≥ 8.6% and consists of two components, one of which has the same redshift as previously detected 21 cm absorption of neutral hydrogen (H i) gas. Each CO component traces an integrated H2 column density of NH2 ≳ 3 × 1020 cm−2. CN absorption is detected for both CO components, as well as for a blueshifted component not detected in CO, with CO/CN line ratios ranging from ≲0.4 to 2.4. We discuss the scenario that the absorption components originate from collections of small and dense molecular clouds that are embedded in a region with more diffuse gas and high turbulence, possibly within the influence of the central active galactic nucleus or a starburst region. The degree of reddening in B2 0902+34, with rest-frame color B − K ∼ 4.2, is lower than the very red colors (B − K > 6) found among other known redshifted CO absorption systems at z < 1. Nevertheless, when including the many nondetections from the literature, a potential correlation between the absorption-line strength and B − K color is evident, giving weight to the argument that the red colors of CO absorbers are due to a high dust content.

Modeling the spectral energy distribution of starburst galaxies

Context. Analyzing multiwavelength observations of galaxies from the far-ultraviolet to the millimeter domains provides a wealth of information on the physical properties of galaxies and their evolution across cosmic time. Existing or upcoming ground-based or space-borne facilities with enhanced sensitivities and resolutions open an unprecedented window on the galaxy evolution in the early Universe. However, the derivation of galaxy properties from nebular emission lines is not trivial because the interstellar medium in a galaxy may be patchy, and emission might originate both from starburst emission regions and from partially covered photon-dominated regions. Aims. We model both the nebular continuum emission and the line emission of the spectral energy distribution for galaxies exhibiting both a HII region-like emission and emission like that from a photon-dominated regions to account for the partial shielding of the starburst emission region by dense clouds. Methods. Nebular galactic emission was modeled from far-ultraviolet to millimeter ranges in a two-sector model with an HII region and a photon-dominated region. The partial overlap of the HII region by the photon-dominated region was accounted for by a covering factor. We generated grids of emission spectra using the Cloudy photoionization code for our two-sector model. Results. We compared our models with spectral lines from different samples of galaxies for which we mixed characteristic emission from starburst regions and denser regions. We show that the infrared line ratios can constrain the density, metallicity, photoionization parameter, and the covering factor. We also built infrared diagnostic diagrams based on different infrared line ratios in which the galaxy location contains information about its physical conditions. Conclusions. The two-sector model that couples starburst emission regions and photon-dominated regions can span the existing observations. We implement the resulting emission line libraries in the CIGALE galaxy spectral energy distribution code to help interpret spectrophotometric observations.

A physical picture for the acoustic resonant drag instability

ABSTRACT Mixtures of gas and dust are pervasive in the Universe, from active galactic nuclei (AGNs) and molecular clouds to protoplanetary discs. When the two species drift relative to each other, a large class of instabilities can arise, called ‘resonant drag instabilities’ (RDIs). The most famous RDI is the streaming instability, which plays an important role in planet formation. On the other hand, acoustic RDIs, the simplest kind, feature in the winds of cool stars, AGNs, or starburst regions. Unfortunately, owing to the complicated dynamics of two coupled fluids (gas and dust), the underlying physics of most RDIs is mysterious. In this paper, we develop a clear physical picture of how the acoustic RDI arises and support this explanation with transparent mathematics. We find that the acoustic RDI is built on two coupled mechanisms. In the first, the converging flows of a sound wave concentrate dust. In the second, a drifting dust clump excites sound waves. These processes feed into each other at resonance, thereby closing an unstable feedback loop. This physical picture helps decide where and when RDIs are most likely to happen, and what can suppress them. Additionally, we find that the acoustic RDI remains strong far from resonance. This second result suggests that one can simulate RDIs without having to fine-tune the dimensions of the numerical domain.

SDSS-IV MaNGA: how do star formation histories affect gas-phase abundances?

ABSTRACT Gas-phase abundances in galaxies are the products of those galaxies’ evolutionary histories. The star formation history (SFH) of a region might therefore be expected to influence that region’s present day gaseous abundances. Here, we employ data from the MaNGA survey to explore how local gas metallicities relate to SFHs of galaxy regions. We combine MaNGA emission line measurements with SFH classifications from absorption line spectra to compare gas-phase abundances in star-forming regions with those in regions classified as starburst, post-starburst, and green valley. We find that starburst regions contain gas that is more pristine than in normal star-forming regions, in terms of O/H and N/O; we further find that post-starburst regions (which have experienced stochastic SFHs) behave very similarly to ordinary star-forming regions (which have experienced far smoother SFHs) in O/H–N/O space. We argue from this that gas is diluted significantly by pristine infall but is then re-enriched rapidly after a starburst event, making gas-phase abundances insensitive to the precise form of the SFH at late times. We also find that green valley regions possess slightly elevated N/O abundances at a given O/H; this is potentially due to a reduced star formation efficiency in such regions, but it could also point to late-time rejuvenation of green valley regions in our sample.

HST FUV Spectroscopy of Super Star Cluster A in the Green Pea Analog Mrk 71: Revealing the Presence of Very Massive Stars

Mrk 71 is a low-metallicity (Z = 0.16 Z ☉) starburst region in the local dwarf galaxy NGC 2366, hosting two super star clusters (SSCs A and B), and it is recognized as a Green Pea (GP) analog with SSC A responsible for the GP properties. We present STIS and FOS far-ultraviolet (FUV) spectra of the embedded SSC Mrk 71-A obtained with the Hubble Space Telescope. The STIS FUV spectrum shows the characteristic features of very massive stars (VMS; masses >100 M ⊙) and we derive an age of 1 ± 1 Myr by comparison with the Charlot & Bruzual suite of spectral population synthesis models with upper mass limits of 300 and 600 M ⊙. We compare the STIS spectrum with all known SSC spectra exhibiting VMS signatures: NGC 5253-5, R136a, NGC 3125-A1, and the z = 2.37 Sunburst cluster. We find that the cluster mass-loss rates and wind velocities, as characterized by the C iv P Cygni profiles and the He ii emission line strengths, are very similar over Z = 0.16–0.4 Z☉. This agrees with predictions that the optically thick winds of VMS will be enhanced near the Eddington limit and show little metallicity dependence. We find very strong damped Lyα absorption with N(H i) =1022.2 cm−2 associated with Mrk 71-A. We discuss the natal environment of this young SSC in terms of radiatively driven winds, catastrophic cooling, and recent models where the cluster is surrounded by highly pressurized clouds with large neutral columns.

Mapping the core of the Tarantula Nebula with VLT-MUSE – III. A template for metal-poor starburst regions in the visual and far-ultraviolet

ABSTRACT We present the integrated VLT-MUSE spectrum of the central 2 × 2 arcmin2 (30 × 30 pc2) of NGC 2070, the dominant giant H ii region of the Tarantula Nebula in the Large Magellanic Cloud, together with an empirical far-ultraviolet spectrum constructed via LMC template stars from the ULLYSES survey and Hubble Tarantula Treasury Project UV photometry. NGC 2070 provides a unique opportunity to compare results from individual stellar populations (e.g. VLT FLAMES Tarantula Survey) in a metal-poor starburst region to the integrated results from the population synthesis tools Starburst99, Charlot &amp; Bruzual, and BPASS. The metallicity of NGC 2070 inferred from standard nebular strong line calibrations is ∼0.4 ± 0.1 dex lower than obtained from direct methods. The Hα inferred age of 4.2 Myr from Starburst99 is close to the median age of OB stars within the region, although individual stars span a broad range of 1–7 Myr. The inferred stellar mass is close to that obtained for the rich star cluster R136 within NGC 2070, although this contributes only 21 per cent to the integrated far-UV continuum. He ii λ1640 emission is dominated by classical WR stars and main sequence WNh + Of/WN stars. Around 18 per cent of the NGC 2070 far UV continuum flux arises from very massive stars with ≥100 M⊙, including several very luminous Of supergiants. None of the predicted population synthesis models at low metallicities are able to successfully reproduce the far-UV spectrum of NGC 2070. We attribute issues to the treatment of mass-loss in very massive stars, the lack of contemporary empirical metal-poor templates, plus WR stars produced via binary evolution.

Spatially resolved Kennicutt–Schmidt relation at z ≈ 7 and its connection with the interstellar medium properties

ABSTRACT We exploit moderately resolved [O $\scriptstyle \rm III$], [C $\scriptstyle \rm II$] and dust continuum ALMA observations to derive the gas density (n), the gas-phase metallicity (Z), and the deviation from the Kennicutt–Schmidt (KS) relation (κs) on $\approx \, \rm sub-kpc$ scales in the interstellar medium (ISM) of five bright Lyman Break Galaxies at the Epoch of Reionization (z ≈ 7). To do so, we use GLAM, a state-of-art, physically motivated Bayesian model that links the [C $\scriptstyle \rm II$]and [O $\scriptstyle \rm III$]surface brightness (Σ[CII], Σ[OIII]) and the SFR surface density (ΣSFR) to n, κs, and Z. All five sources are characterized by a central starbursting region, where the Σgas versus ΣSFR align ≈10 × above the KS relation (κs ≈ 10). This translates into gas depletion times in the range tdep ≈ 80 − 250 Myr. The inner starbursting centres are characterized by higher gas density (log (n/cm−3) ≈ 2.5–3.0) and higher metallicity (log (Z/Z⊙) ≈ −0.5) than the galaxy outskirts. We derive marginally negative radial metallicity gradients (∇log Z ≈ −0.03 ± 0.07 dex/kpc), and a dust temperature (Td ≈ 32 − 38 K) that anticorrelates with the gas depletion time.

A resolved study of the inner regions of nearby galaxies with an excess of young massive stars: missing link in the AGN–starburst connection?

ABSTRACT We have selected galaxies with very high levels of H α emission (EQW(H α) &amp;gt;700 Å.) in their central regions from the final data release of the MaNGA survey. Our study focuses on 14 very well-resolved nearby galaxies with stellar masses in the range 9.5 &amp;lt; log M*/(M⊙) &amp;lt; 11.5. We investigate a variety of procedures for selecting galaxy regions that are likely to harbour excess populations of young massive stars, finding that selection in the 2D space of extinction-corrected H α EQW and [S iii]/[S ii] line ratio produces the best results. By comparing stacked spectra covering these regions with stacked spectra covering normal starburst regions with 100 Å&amp;lt;EQW(H α) &amp;lt;200 Å, we obtain the following main results: (1) Clear signatures of excess Wolf–Rayet stars are found in half of the H α excess regions, (2) galaxy regions containing excess Wolf–Rayet stars are more often associated with the presence of high-ionization emission lines characteristic of accreting black holes. Excess [Ne iii] is detected in 4 out of 8 of the WR regions and there are tentative [Fe x] detections in 2 galaxies. (3) Regions of the galaxy with excess Wolf–Rayet stars are located where the interstellar medium has larger ionized gas turbulent velocities and higher neutral gas overdensities. We make a first attempt to constrain changes in the high-mass end of the stellar initial mass function (IMF) using the HR-pyPopStar evolutionary population synthesis models that include high-wavelength-resolution theoretical atmosphere libraries for Wolf–Rayet stars.

Molecular and ionized gas in tidal dwarf galaxies: the spatially resolved star formation relation

ABSTRACT Tidal dwarf galaxies (TDGs) are low-mass objects that form within tidal and/or collisional debris ejected from more massive interacting galaxies. We use CO(1–0) observations from Atacama Large Millimeter/submillimeter Array and integral-field spectroscopy from Multi-Unit Spectroscopic Explorer to study molecular and ionized gas in three TDGs: two around the collisional galaxy NGC 5291 and one in the late-stage merger NGC 7252. The CO and H α emission is more compact than the H i emission and displaced from the H i dynamical centre, so these gas phases cannot be used to study the internal dynamics of TDGs. We use CO, H i, and H α data to measure the surface densities of molecular gas (Σmol), atomic gas (Σatom), and star formation rate (ΣSFR), respectively. We confirm that TDGs follow the same spatially integrated ΣSFR–Σgas relation of regular galaxies, where Σgas = Σmol + Σatom, even though they are H i dominated. We find a more complex behaviour in terms of the spatially resolved ΣSFR–Σmol relation on subkpc scales. The majority ($\sim 60~{{\ \rm per\ cent}}$) of star-forming regions in TDGs lie on the same ΣSFR–Σmol relation of normal spiral galaxies but show a higher dispersion around the mean. The remaining fraction of star-forming regions ($\sim 40~{{\ \rm per\ cent}}$) lie in the starburst region and are associated with the formation of massive super star clusters, as shown by Hubble Space Telescope images. We conclude that the local star formation activity in TDGs proceeds in a hybrid fashion, with some regions comparable to normal spiral galaxies and others to extreme starbursts.

The cluster initial mass function of the M82 disc super star clusters

ABSTRACT The presence of a population of a large number (∼400) of almost coeval (100–300 Myr) super star clusters (SSCs) in the disc of M82 offers an opportunity to construct the Cluster Initial Mass Function (CIMF) from the observed present-day Cluster Mass Function (CMF). We carry out the dynamical and photometric evolution of the CMF assuming that the clusters move in circular orbits under the gravitational potential of the host galaxy using the semi-analytical simulation code Evolve Me a Cluster of StarS. We explore power-law and lognormal functions for the CIMFs, and populate the clusters in the disc assuming uniform, power-law, and exponential radial distribution functions. We find that the observed CMF is best produced by a CIMF that is power law in form with an index of 1.8, for a power-law radial distribution function. More importantly, we establish that the observed turn-over in the present-day CMF is the result of observational incompleteness rather than due to dynamically induced effects, or an intrinsically lognormal CIMF, as was proposed for the fossil starburst region B of this galaxy. Our simulations naturally reproduce the mass–radius relation observed for a sub-sample of M82 SSCs.