Nearby Spirals Research Articles

Ancient stellar populations in the outskirts of nearby grand-design spirals: Investigation of their star formation histories

Context. The main sequence (MS) of star-forming galaxies (SFGs) is the tight relation between the galaxy stellar mass (M⋆) and its star formation rate (SFR) and was observed up to z ∼ 6. The MS relation can be used as a reference for understanding the differences among galaxies, which are characterised by different rates of stellar production (starbursts, SFGs, and passive galaxies), and those inside a galaxy that is made up of different components (bulge, disk, and halo). To investigate peculiar features found in our galaxies sample in more depth, we focus here on their star formation history (SFH). Aims. The SFHs are a fundamental tool for revealing the galaxy path from the earlier stages of formation to the present time. The various phases of galaxy evolution are imprinted on the source spectrum globally and locally. Thus, we are able to interpret the dynamical origin of the spirals quantitatively and distinguish between in situ or ex situ formation processes. Methods. We performed a spectral energy distribution fitting procedure that accounted for the energetic balance between UV (observed) and far-IR (optically obscured) radiation on a sample of eight nearby face-on grand-design spiral galaxies from the DustPedia sample. This approach allowed us to study the spatially resolved MS of the sample and to recover the past SFH by accounting for attenuation due to the presence of dust. By exploiting the BAGPIPES code, we constrained the SFHs for each galaxy with a delayed exponentially declining model to derive their mass-weighted age (tMW). Results. The spiral galaxies in our sample have similar radial tMW trends overall. A central old region (tMW up to ∼7 Gyr, consistent with the presence of a bulge for various systems) is followed by younger regions in which the disks are still forming stars (tMW ∼ 4 Gyr). At larger distances from the centre of the galaxies, tMW increases mildly in general. Strikingly, in two galaxies (NGC4321 and NGC5194), we found a steep increase in tMW that reached levels similar to those of the bulge. These old stellar populations in the very galaxy outskirts, which are also detectable as “quenched rings” below the spatially resolved MS, is unexpected. We discuss their potential origin by considering the different gas phases (HI and H2) of the source with the most prominent quenched ring, NGC4321, and argue for two main possibilities: (1) some environmental effect (e.g. starvation) could affect the outer edge of the galaxies or (2) the circumgalactic medium of sources outside of high-density clusters might have stopped to supply pristine gas to the galaxy (e.g. if its specific angular moment is too high for being accreted).

Enhanced Star Formation Efficiency in the Central Regions of Nearby Quasar Hosts

We combine Atacama Large Millimeter/submillimeter Array and Multi Unit Spectroscopic Explorer observations tracing the molecular gas, millimeter continuum, and ionized gas emission in six low-redshift (z ≲ 0.06) Palomar–Green (PG) quasar host galaxies to investigate their ongoing star formation at roughly kiloparsec-scale resolution. The AGN contribution to the cold dust emission and the optical emission-line flux is carefully removed to derive spatial distributions of the star formation rate (SFR), which, complemented with the molecular gas data, enables the mapping of the depletion time (t dep). We report ubiquitous star formation activity within the quasar host galaxies, with the majority of the ongoing star formation occurring in the galactic center. The rise of the SFR surface density (ΣSFR) toward the nucleus is steeper than that observed for the cold molecular gas surface density, reaching values up to ΣSFR ≈ 0.15–0.80 M ⊙ yr−1 kpc−2. The gas in the nuclear regions is converted into stars at a shortened depletion time (t dep ≈ 0.2–2.0 Gyr), suggesting that those zones can be deemed as starbursts. At large galactocentric radius, we find that the ongoing star formation takes place within spiral arms or H ii region complexes, with an efficiency comparable to that reported for nearby inactive spirals (t dep ≈ 1.8 Gyr). We find no evidence of star formation activity shutoff in the PG quasar host galaxies. On the contrary, these observations shed light on how the central environments of galaxies hosting actively accreting supermassive black holes build up stellar mass.

J-PLUS: a catalogue of globular cluster candidates around the M 81/M 82/NGC 3077 triplet of galaxies

ABSTRACT Globular clusters (GCs) are proxies of the formation assemblies of their host galaxies. However, few studies exist targeting GC systems of spiral galaxies up to several effective radii. Through 12-band Javalambre Photometric Local Universe Survey (J-PLUS) imaging, we study the point sources around the M 81/M 82/NGC 3077 triplet in search of new GC candidates. We develop a tailored classification scheme to search for GC candidates based on their similarity to known GCs via a principal component analysis projection. Our method accounts for missing data and photometric errors. We report 642 new GC candidates in a region of 3.5 deg2 around the triplet, ranked according to their Gaia astrometric proper motions when available. We find tantalizing evidence for an overdensity of GC candidate sources forming a bridge connecting M 81 and M 82. Finally, the spatial distribution of the GC candidates (g − i) colours is consistent with halo/intra-cluster GCs, i.e. it gets bluer as they get further from the closest galaxy in the field. We further employ a regression-tree-based model to estimate the metallicity distribution of the GC candidates based on their J-PLUS bands. The metallicity distribution of the sample candidates is broad and displays a bump towards the metal-rich end. Our list increases the population of GC candidates around the triplet by threefold, stresses the usefulness of multiband surveys in finding these objects, and provides a testbed for further studies analysing their spatial distribution around nearby (spirals) galaxies.

A multiwavelength study of nearby star-forming spiral galaxies and the clustering of star formation in M63

ABSTRACT This multiwavelength study of the star formation and star formation history (SFH) trends in the nearby universe looks at nine nearby late-type spirals. Spectral energy distributions fitted with measurements from the far-ultraviolet (FUV) to the mid-infrared are used to estimate parameters in a double-exponential SFH. Azimuthally averaged radial trends in the SFHs are shown and discussed for each galaxy. In addition to the radial analysis, catalogues of UV-selected sources are identified for each galaxy. An analysis of the clustering of the UV sources in M63 (NGC 5055) is done using a two-point correlation function. There is evidence for hierarchical star formation and clustering out to scales of several kpc. We then discuss the level of clustering in M63’s sources with differing FUV − NUV colours, and how spiral arms may play a role in the organization of star formation even in the low surface brightness regions of a galaxy.

Star Cluster Formation and Evolution in M101: An Investigation with the Legacy Extragalactic UV Survey

We present Hubble Space Telescope WFC3/UVIS (F275W, F336W) and ACS/WFC optical (F435W, F555W, and F814W) observations of the nearby grand-design spiral galaxy M101 as part of the Legacy Extragalactic UV Survey (LEGUS). Compact sources detected in at least four bands were classified by both human experts and the convolutional neural network StarcNet. Human experts classified the 2351 brightest sources, retrieving N c = 965 star clusters. StarcNet, trained on LEGUS data not including M101, classified all 4725 sources detected in four bands, retrieving N c = 2270 star clusters. The combined catalog represents the most complete census to date of compact star clusters in M101. We find that for the 2351 sources with both a visual- and machine-learning classification StarcNet is able to reproduce the human classifications at high levels of accuracy (∼80%–90%), which is equivalent to the level of agreement between human classifiers in LEGUS. The derived cluster age distribution implies a disruption rate of over 107 < τ < 108.5 yr for cluster masses ≥103.55 M ⊙ for the central region of M101 and for cluster masses ≥103.38 M ⊙ in the northwest region of the galaxy. The trends we recover are weaker than those of other nearby spirals (e.g., M51) and starbursts, consistent with the M101 environment having a lower-density interstellar medium, and providing evidence in favor of environmentally dependent cluster disruption in the central, southeast, and northwest regions of M101.

Down but Not Out: Properties of the Molecular Gas in the Stripped Virgo Cluster Early-type Galaxy NGC 4526

We present Atacama Large Millimeter/submillimeter Array data on the 3 mm continuum emission, CO isotopologues (12CO, 13CO, and C18O), and high-density molecular tracers (HCN, HCO+, HNC, HNCO, CS, CN, and CH3OH) in NGC 4526. These data enable a detailed study of the physical properties of the molecular gas in a longtime resident of the Virgo Cluster; comparisons to more commonly studied spiral galaxies offer intriguing hints into the processing of molecular gas in the cluster environment. Many molecular line ratios in NGC 4526, along with our inferred abundances and CO/H2 conversion factors, are similar to those found in nearby spirals. One striking exception is the very low observed 12CO/13CO(1−0) line ratio, 3.4 ± 0.3, which is unusually low for spirals though not for Virgo Cluster early-type galaxies. We carry out radiative transfer modeling of the CO isotopologues with some archival (2−1) data, and we use Bayesian analysis with Markov Chain Monte Carlo techniques to infer the physical properties of the CO-emitting gas. We find surprisingly low [12CO/13CO] abundance ratios of and at radii of 0.4 kpc and 1 kpc. The emission from the high-density tracers HCN, HCO+, HNC, CS, and CN is also relatively bright, and CN is unusually optically thick in the inner parts of NGC 4526. These features hint that processing in the cluster environment may have removed much of the galaxy’s relatively diffuse, optically thinner molecular gas along with its atomic gas. Angular momentum transfer to the surrounding intracluster medium may also have caused contraction of the disk, magnifying radial gradients such as we find in [13CO/C18O]. More detailed chemical evolution modeling would be interesting in order to explore whether the unusual [12CO/13CO] abundance ratio is entirely an environmental effect or whether it also reflects the relatively old stellar population in this early-type galaxy.

The observability of galaxy merger signatures in nearby gas-rich spirals

ABSTRACT Galaxy mergers are crucial to understanding galaxy evolution, therefore we must determine their observational signatures to select them from large IFU galaxy samples such as MUSE and SAMI. We employ 24 high-resolution idealized hydrodynamical galaxy merger simulations based on the ‘Feedback In Realistic Environment’ (FIRE-2) model to determine the observability of mergers to various configurations and stages using synthetic images and velocity maps. Our mergers cover a range of orbital configurations at fixed 1:2.5 stellar mass ratio for two gas rich spirals at low redshift. Morphological and kinematic asymmetries are computed for synthetic images and velocity maps spanning each interaction. We divide the interaction sequence into three: (1) the pair phase; (2) the merging phase; and (3) the post-coalescence phase. We correctly identify mergers between first pericentre passage and 500 Myr after coalescence using kinematic asymmetry with 66 per cent completeness, depending upon merger phase and the field of view of the observation. We detect fewer mergers in the pair phase (40 per cent) and many more in the merging and post-coalescence phases (97 per cent). We find that merger detectability decreases with field of view, except in retrograde mergers, where centrally concentrated asymmetric kinematic features enhances their detectability. Using a cut-off derived from a combination of photometric and kinematic asymmetry, we increase these detections to 89 per cent overall, 79 per cent in pairs, and close to 100 per cent in the merging and post-coalescent phases. By using this combined asymmetry cut-off we mitigate some of the effects caused by smaller fields of view subtended by massively multiplexed integral field spectroscopy programmes.

New calibrations for estimating the N/O ratio in H ii regions

ABSTRACT We use a sample of 536 H ii regions located in nearby spirals, with a homogeneous determination of their Te-based abundances, to obtain new empirical calibrations of the N2O2, N2S2, O3N2, and N2 strong-line indices to estimate the nitrogen-to-oxygen abundance ratio when auroral lines are not detected. All indices are strongly correlated with the Te-based log (N/O) for our H ii region sample, even more strongly than with 12 + log (O/H). N2O2 is the most strongly correlated index, and the best fit to the log (N/O)–N2O2 relation is obtained with a second-order polynomial. The derived relation has a low dispersion (rms &amp;lt; 0.09 dex), being valid in the range −1.74 &amp;lt; N2O2 &amp;lt; 0.62 (or −1.81 &amp;lt; log (N/O) &amp;lt; −0.13). We have compared our calibration with previous ones and have discussed the differences between them in terms of the nature of the objects used as calibrators.

The mm-to-cm SED of spiral galaxies

The mm-to-cm range of the Spectral Energy Distribution of spiral galaxies remains largely unexplored. Its coverage is required to disentangle the contribution of dust emission, free-free and synchrotron radiation and can provide constraints on dust models, star-formation rates and ISM properties. We present the case for a synergy between NIKA2 observations of nearby spirals and those from planned and current instrumentation at the Sardinia Radio Telescope, and report on a pilot K-band program to search for Anomalous Microwave Emission, an elusive emission component which is presumably related to dust.

Massive disc galaxies too dominated by dark matter in cosmological hydrodynamical simulations

We investigated the disc-halo connection in massive (M⋆ &gt; 5 × 1010 M⊙) disc galaxies from the cosmological hydrodynamical simulations EAGLE and IllustrisTNG, and compared this connection with that inferred from the study of H I rotation curves in nearby massive spirals from the Spitzer Photometry and Accurate Rotation Curves dataset. We find that discrepancies between the simulated and observed discs arise both on global and local scales. Globally, the simulated discs inhabit halos that are a factor ∼4 (in EAGLE) and ∼2 (in IllustrisTNG) more massive than those derived from the rotation curve analysis of the observed dataset. We also used synthetic rotation curves of the simulated discs to demonstrate that the recovery of the halo masses from rotation curves are not systematically biased. We find that the simulations predict systems dominated by dark matter with stellar-to-total enclosed mass ratios that are a factor of 1.5−2 smaller than real galaxies at all radii. This is an alternative manifestation of the ‘failed feedback problem,’ since it indicates that simulated halos hosting massive discs have been too inefficient at converting their baryons into stars, possibly owing to an overly efficient stellar and/or AGN feedback implementation.

Dynamical Properties of Molecular-forming Gas Clumps in Galaxies at the Epoch of Reionization

Abstract We study the properties of molecular-forming gas clumps (MGCs) at the epoch of reionization using cosmological zoom-in simulations. We identify MGCs in a prototypical galaxy (“Althæa”) using an H2 density-based clump finder. We compare their mass, size, velocity dispersion, gas surface density, and virial parameter ( ) to observations. In Althæa, the typical MGC mass and size are M ⊙ and pc, which are comparable to those found in nearby spirals and starburst galaxies. MGCs are highly supersonic and supported by turbulence, with rms velocity dispersions of 20–100 km s−1 and pressure of (i.e., with respect to the Milky Way), similar to those found in nearby and z ∼ 2 gas-rich starburst galaxies. In addition, we perform stability analysis to understand the origin and dynamical properties of MGCs. We find that MGCs are globally stable in the main disk of Althæa. Densest regions where star formation is expected to take place in clouds and cores on even smaller scales instead have lower and Toomre Q values. Detailed studies of the star-forming gas dynamics at the epoch of reionization thus require a spatial resolution of ≲40 pc (≃ ), which is within reach with the Atacama Large (sub-)Millimeter Array and the Next Generation Very Large Array.

A panchromatic spatially resolved analysis of nearby galaxies – I. Sub-kpc-scale main sequence in grand-design spirals

ABSTRACT We analyse the spatially resolved relation between stellar mass (M⋆) and star formation rate (SFR) in disc galaxies (i.e. the main sequence, MS). The studied sample includes eight nearby face-on grand-design spirals, e.g. the descendant of high-redshift, rotationally supported star-forming galaxies. We exploit photometric information over 23 bands, from the UV to the far-IR, from the publicly available DustPedia data base to build spatially resolved maps of stellar mass and SFRs on sub-galactic scales of 0.5–1.5 kpc, by performing a spectral energy distribution fitting procedure that accounts for both the observed and obscured star formation processes, over a wide range of internal galaxy environments (bulges, spiral arms, and outskirts). With more than 30 000 physical cells, we have derived a definition of the local spatially resolved MS per unit area for discs, log (ΣSFR) = 0.82log (Σ*) − 8.69. This is consistent with the bulk of recent results based on optical IFU, using the H α line emission as an SFR tracer. Our work extends the analysis at lower sensitivities in both M⋆ and SFR surface densities, up to a factor of ∼10. The self-consistency of the MS relation over different spatial scales, from sub-galactic to galactic, as well as with a rescaled correlation obtained for high-redshift galaxies, clearly proves its universality.

The stellar velocity dispersion in nearby spirals: radial profiles and correlations

Abstract The stellar velocity dispersion, σ, is a quantity of crucial importance for spiral galaxies, where it enters fundamental dynamical processes such as gravitational instability and disc heating. Here we analyse a sample of 34 nearby spirals from the Calar Alto Legacy Integral Field Area (CALIFA) spectroscopic survey, deproject the line-of-sight σ to σR and present reliable radial profiles of σR as well as accurate measurements of ⟨σR⟩, the radial average of σR over one effective (half-light) radius. We show that there is a trend for σR to increase with decreasing R, that ⟨σR⟩ correlates with stellar mass (M⋆) and tested correlations with other galaxy properties. The most significant and strongest correlation is the one with M⋆: $\langle \sigma _{R}\rangle \propto M_{\star }^{0.5}$. This tight scaling relation is applicable to spiral galaxies of type Sa–Sd and stellar mass M⋆ ≈ 109.5–1011.5 M⊙. Simple models that relate σR to the stellar surface density and disc scale length roughly reproduce that scaling, but overestimate ⟨σR⟩ significantly.

Dynamical properties of Molecular Cloud Complexes at the Epoch of Reionization

AbstractThe Atacama Large (Sub-)millimeter Array (ALMA) has provided glimpse of the interstellar medium (ISM) properties of galaxies at the Epoch of Reionization (EoR); however, detailed understanding of their internal structure is still lacking. We present properties of molecular cloud complexes (MCCs) in a prototypical galaxy at this epoch studied in cosmological zoom-in simulations (Leung et al. 2019c). Typical MCC mass and size are comparable to nearby spirals and starburst galaxies (Mgas∼106.5Mȯ and R≃45–100 pc). MCCs are highly supersonic, with velocity dispersion of σgas≃20–100 km s−1 and pressure of P/kB ≃107.6Kcm−3, which are comparable to gas-rich starburst galaxies. In addition, we perform stability analysis to understand the origin and dynamical properties of MCCs. We find that MCCs are globally stable in the main disk of Althæa. Densest regions where star formation is expected to take place in clumps and cores on even smaller scales instead have lower virial parameter and Toomre-Q values. Detailed studies of the star-forming gas dynamics at the EoR thus require a spatial resolution of &lt; 40 pc ( ≃ 0.01″), which is within reach of ALMA, to complement studies of stellar populations at EoR using the James Webb Space Telescope (JWST).

A high-resolution, dust-selected molecular cloud catalogue of M33, the Triangulum Galaxy

We present a catalogue of Giant Molecular Clouds (GMCs) in M33, extracted from cold dust continuum emission. Our GMCs are identified by computing dendrograms. We measure the spatial distribution of these clouds, and characterise their dust properties. Combining these measured properties with CO(J=2-1) and 21cm HI data, we calculate the gas-to-dust ratio (GDR) of these clouds, and from this compute a total cloud mass. In total, we find 165 GMCs with cloud masses in the range of 10$^4$-10$^7$ M$_\odot$. We find that radially, $\log_{10}(\mathrm{GDR}) = -0.043(\pm0.038) \,\mathrm{R [kpc]} + 1.88(\pm0.15)$, a much lower GDR than found in the Milky Way, and a correspondingly higher $\alpha_{\rm CO}$ factor. The mass function of these clouds follows a slope proportional to M$^{-2.84}$, steeper than many previous studies of GMCs in local galaxies, implying that M33 is poorer at forming massive clouds than other nearby spirals. Whilst we can rule out interstellar pressure as the major contributing factor, we are unable to disentangle the relative effects of metallicity and HI velocity dispersion. We find a reasonably featureless number density profile with galactocentric radius, and weak correlations between galactocentric radius and dust temperature/mass. These clouds are reasonably consistent with Larson's scaling relationships, and many of our sources are co-spatial with earlier CO studies. Massive clouds are identified at large galactocentric radius, unlike in these earlier studies, perhaps indicating a population of CO-dark gas dominated clouds at these larger distances.

Angular momentum and local gravitational instability in galaxy discs: does Q correlate with j or M ?

ABSTRACT We introduce a new diagnostic for exploring the link between angular momentum and local gravitational instability in galaxy discs. Our diagnostic incorporates the latest developments in disc instability research, is fully consistent with approximations that are widely used for measuring the stellar specific angular momentum, j⋆ = J⋆/M⋆, and is also very simple. We show that such a disc instability diagnostic hardly correlates with j⋆ or M⋆, and is remarkably constant across spiral galaxies of any given type (Sa–Sd), stellar mass ($M_{\star }=10^{9.5}\mbox{--}10^{11.5}\, \mbox{M}_{\odot }$), and velocity dispersion anisotropy (σ$z$⋆/σR⋆ = 0–1). The fact that M⋆ is tightly correlated with star formation rate, molecular gas mass (Mmol), metallicity (12 + log O/H), and other fundamental galaxy properties thus implies that nearby star-forming spirals self-regulate to a quasi-universal disc stability level. This not only proves the existence of the self-regulation process postulated by several star formation models, but also raises important caveats.

No evidence for small disk-like bulges in a sample of late-type spirals

About 20% of low-redshift galaxies are late-type spirals with a small or no bulge component. Although they are the simplest disk galaxies in terms of structure and dynamics, the role of the different physical processes driving their formation and evolution is not yet fully understood. We investigated whether small bulges of late-type spirals follow the same scaling relations traced by ellipticals and large bulges and if they are disk-like or classical bulges. We derived the photometric and kinematic properties of 9 nearby late-type spirals. To this aim, we analyzed the surface brightness distribution from the i-band images of the Sloan Digital Sky Survey and obtained the structural parameters of the galaxies from a two-dimensional photometric decomposition. We measured the line-of-sight stellar velocity distribution within the bulge effective radius from the long-slit spectra taken with high spectral resolution at the Telescopio Nazionale Galileo. We used the photometric and kinematic properties of the sample bulges to study their location in the Fundamental Plane, Kormendy, and Faber-Jackson relations defined for ellipticals and large bulges. We found that our sample bulges satisfy some of the photometric and kinematic prescriptions for being considered disk-like bulges such as small sizes and masses with nearly exponential light profiles, small bulge-to-total luminosity ratios, low stellar velocity dispersions, and ongoing star formation. However, each of them follows the same scaling relations of ellipticals, massive bulges, and compact early-type galaxies so they cannot be classified as disk-like systems. We find a single population of galaxy spheroids that follow the same scaling relations, where the mass seems to lead to a smooth transition in the photometric and kinematic properties from less massive bulges to more massive bulges and ellipticals.

What drives gravitational instability in nearby star-forming spirals? The impact of CO and H i velocity dispersions

The velocity dispersion of cold interstellar gas, sigma, is one of the quantities that most radically affect the onset of gravitational instabilities in galaxy discs, and the quantity that is most drastically approximated in stability analyses. Here we analyse the stability of a large sample of nearby star-forming spirals treating molecular gas, atomic gas and stars as three distinct components, and using radial profiles of sigma_CO and sigma_HI derived from HERACLES and THINGS observations. We show that the radial variations of sigma_CO and sigma_HI have a weak effect on the local stability level of galaxy discs, which remains remarkably flat and well above unity, but is low enough to ensure (marginal) instability against non-axisymmetric perturbations and gas dissipation. More importantly, the radial variation of sigma_CO has a strong impact on the size of the regions over which gravitational instabilities develop, and results in a characteristic instability scale that is one order of magnitude larger than the Toomre length of molecular gas. Disc instabilities are driven, in fact, by the self-gravity of stars at kpc scales. This is true across the entire optical disc of every galaxy in the sample, with few exceptions. In the linear phase of the disc instability process, stars and molecular gas are strongly coupled, and it is such a coupling that ultimately triggers local gravitational collapse/fragmentation in the molecular gas.

Luminous and Variable Stars in M31 and M33. IV. Luminous Blue Variables, Candidate LBVs, B[e] Supergiants, and the Warm Hypergiants: How to Tell Them Apart*

Abstract In this series of papers we have presented the results of a spectroscopic survey of luminous stars in the nearby spirals M31 and M33. Here, we present spectroscopy of 132 additional stars. Most have emission-line spectra, including luminous blue variables (LBVs) and candidate LBVs, Fe ii emission line stars, the B[e] supergiants, and the warm hypergiants. Many of these objects are spectroscopically similar and are often confused with each other. We examine their similarities and differences and propose the following guidelines that can be used to help distinguish these stars in future work. (1) The B[e] supergiants have emission lines of [O i] and [Fe ii] in their spectra. Most of the spectroscopically confirmed sgB[e] stars also have warm circumstellar dust in their spectral energy distributions (SEDs). (2) Confirmed LBVs do not have the [O i] emission lines in their spectra. Some LBVs have [Fe ii] emission lines, but not all. Their SEDs show free–free emission in the near-infrared but no evidence for warm dust. Their most important and defining characteristic is the S Dor-type variability. (3) The warm hypergiants spectroscopically resemble the LBVs in their dense wind state and the B[e] supergiants. However, they are very dusty. Some have [Fe ii] and [O i] emission in their spectra like the sgB[e] stars, but are distinguished by their A- and F-type absorption-line spectra. In contrast, the B[e] supergiant spectra have strong continua and few if any apparent absorption lines. Candidate LBVs should share the spectral characteristics of the confirmed LBVs with low outflow velocities and the lack of warm circumstellar dust.

DYNAMO-HST survey: clumps in nearby massive turbulent discs and the effects of clump clustering on kiloparsec scale measurements of clumps

We present $\sim$100 pc resolution Hubble Space Telescope H$\alpha$ images of 10 galaxies from the DYnamics of Newly-Assembled Massive Objects (DYNAMO) survey of low-$z$ turbulent disk galaxies, and use these to undertake the first detailed systematic study of the effects of resolution and clump clustering on observations of clumps in turbulent disks. In the DYNAMO-{\em HST} sample we measure clump diameters spanning the range $d_{clump} \sim 100-800$~pc, and individual clump star formation rates as high as $\sim5$~M$_{\odot}$~yr$^{-1}$. DYNAMO clumps have very high SFR surface densities, $\Sigma_{SFR}\sim 15$~M$_{\odot}$~yr$^{-1}$~kpc$^{-2}$, $\sim100\times$ higher than in H{\sc ii} regions of nearby spirals. Indeed, SFR surface density provides a simple dividing line between massive star forming clumps and local star forming regions, where massive star forming clumps have $\Sigma_{SFR}> 0.5$~M$_{\odot}$~yr$^{-1}$~kpc$^{-2}$. When degraded to match the observations of galaxies in $z\sim 1-3$ surveys, DYNAMO galaxies are similar in morphology and measured clump properties to clumpy galaxies observed in the high-$z$ Universe. Emission peaks in the simulated high-redshift maps typically correspond to multiple clumps in full resolution images. This clustering of clumps systematically increases the apparent size and SFR of clumps in 1~kpc resolution maps, and decreases the measured SFR surface density of clumps by as much as a factor of 20$\times$. From these results we can infer that clump clustering is likely to strongly effect the measured properties of clumps in high-$z$ galaxies, which commonly have kiloparsec scale resolution.