Edge-on Galaxies Research Articles

The Volumetric Star Formation Law in the Almost Edge-on Galaxy NGC 4302 Revealed by the Atacama Large Millimeter/submillimeter Array

We observe the almost edge-on (i ∼ 90°) galaxy NGC 4302 using the Atacama Large Millimeter/submillimeter Array (CO) and Very Large Array (H i) to measure the gas disk thickness for investigating the volumetric star formation law (SFL). The recent star formation rate (SFR) is estimated based on a linear combination of IR 24 μm and Hα emissions. The measured scale heights of CO and H i increase significantly with radius. Using the scale heights along with the vertically integrated surface densities, we derive the midplane volume densities of the gas (ρ H I, , and ρ gas = ρ H I+) and the SFR (ρ SFR) and compare the volumetric SFL (ρ SFR ∝ ρ gas n) with the vertically integrated SFL (ΣSFR ∝ Σgas N). We find tight power-law correlations between the SFR and the gas (H i, H2, and the total gas) in both volume and surface densities. The power-law indices of the total gas and H i for the volumetric SFL are noticeably smaller than the indices for the vertically integrated SFL while the H2 indices for both cases are similar to each other. In terms of the star formation efficiency (SFE), we find that the molecular and total gas SFEs are roughly constant, while the atomic SFE is clearly decreasing with radius in both cases.

The survival of stellar discs in Fornax-like environments, from TNG50 to real galaxies

ABSTRACT We study the evolution of kinematically defined stellar discs in 10 Fornax-like clusters identified in the TNG50 run from the IllustrisTNG suite of cosmological simulations. We considered disc galaxies with present-day stellar mass M⋆ ≥ 3 × 108 M⊙ and follow their evolution since first entering their host cluster. Very few stellar discs survive since falling in such dense environments, ranging from 40 per cent surviving to all being disrupted. Such survival rates are consistent with what reported earlier for the two more massive, Virgo-like clusters in TNG50. In absolute terms, however, the low number of present-day disc galaxies in Fornax-like clusters could be at odds with the presence of three edge-on disc galaxies in the central regions of the actual Fornax cluster, as delineated by the Fornax3D survey. When looking at the Fornax analogues from random directions and with the same selection function of Fornax3D, the probability of finding three edge-on disc galaxies in any one Fornax-like cluster in TNG50 is rather low, albeit not impossible. We also compared the stellar-population properties near the equatorial plane derived from integral-field spectroscopy for the three edge-ons in Fornax to similar line-of-sight integrated values for present-day disc galaxies in TNG50. For one of these, the very old and metal-rich stellar population of its disc cannot be matched by any the disc galaxies in TNG50, including objects in the field. We discuss possible interpretations of these findings, while pointing to future studies on passive cluster spirals as a way to further test state-of-the-art cosmological simulations.

Kinematics of molecular gas in star-forming galaxies with large-scale ionized outflows

ABSTRACT We investigate the kinematics of the molecular gas in a sample of seven edge-on (i > 60°) galaxies identified as hosting large-scale outflows of ionized gas, using ALMA CO(1–0) observations at ∼1 kpc resolution. We build on Hogarth et al., where we find that molecular gas is more centrally concentrated in galaxies which host winds than in control objects. We perform full three-dimensional kinematic modelling with multiple combinations of kinematic components, allowing us to infer whether these objects share any similarities in their molecular gas structure. We use modelling to pinpoint the kinematic centre of each galaxy, in order to interpret their minor- and major-axis position velocity diagrams (PVDs). From the PVDs, we find that the bulk of the molecular gas in our galaxies is dynamically cold, tracing the rotation curves predicted by our symmetric, rotation-dominated models, but with minor flux asymmetries. Most notably, we find evidence of radial gas motion in a subset of our objects, which demonstrate a characteristic ‘twisting’ in their minor-axis PVDs generally associated with gas flow along the plane of a galaxy. In our highest S/N object, we include bi-symmetric radial flow in our kinematic model, and find (via the Bayesian Information Criterion) that the presence of radial gas motion is strongly favoured. This may provide one mechanism by which molecular gas and star formation are centrally concentrated, enabling the launch of massive ionized gas winds. However, in the remainder of our sample, we do not observe evidence that gas is being driven radially, once again emphasizing the variety of physical processes that may be powering the outflows in these objects, as originally noted in H21.

Linking star formation thresholds and truncations in the thin and thick disks of the low-mass galaxy UGC 7321

Thin and thick disks are found in most spiral galaxies, yet their formation scenarios remain uncertain. Whether thick disks form through slow or fast, internal or environmental, processes is unclear. The physical origin of outer truncations in thin and thick disks, observed as a drop in optical and near-infrared (NIR) surface brightness profiles, is also a much debated topic. These truncations have been linked to star formation (SF) thresholds in Milky-Way-type galaxies, but no such connection has been made for their low-mass counterparts or in thick disks. Our photometric analysis of the edge-on galaxy UGC 7321 offers a possible breakthrough. This well-studied diffuse, isolated, bulgeless, ultra-thin galaxy is thought to be under-evolved both dynamically and in SF. It is an ideal target for disentangling internal effects in the formation of thick disks and truncations. Our axial light profiles from deep far- and near-ultraviolet (GALEX) images, tracing recent SF, and optical (DESI grz) and NIR (Spitzer 3.6 μm) images, tracing old stellar populations, enable a detailed identification of an outer truncation in all probed wavelengths in both the thin and thick disks. After deprojecting to a face-on view, a sharp truncation signature is found at a stellar density of 1.5 ± 0.5 ℳ⊙ pc−2, in agreement with theoretical expectations of gas density SF thresholds. The redder colours beyond the truncation radius are indicative of stellar migration towards the outer regions. We thus show that thick disks and truncations can form via internal mechanisms alone, given the pristine nature of UGC 7321. We report the discovery of a truncation at and above the mid-plane of a diffuse galaxy that is linked to a SF threshold; this poses a constraint on physically motivated disk size measurements among low-mass galaxies.

Testing for relics of past strong buckling events in edge-on galaxies: simulation predictions and data from S4G

ABSTRACT The short-lived buckling instability is responsible for the formation of at least some box/peanut (B/P) shaped bulges, which are observed in most massive, z = 0, barred galaxies. Nevertheless, it has also been suggested that B/P bulges form via the slow trapping of stars on to vertically extended resonant orbits. The key difference between these two scenarios is that when the bar buckles, symmetry about the mid-plane is broken for a period of time. We use a suite of simulations (with and without gas) to show that when the buckling is sufficiently strong, a residual mid-plane asymmetry persists for several Gyrs after the end of the buckling phase, and is visible in simulation images. On the other hand, images of B/P bulges formed through resonant trapping and/or weak buckling remain symmetric about the mid-plane. We develop two related diagnostics to identify and quantify mid-plane asymmetry in simulation images of galaxies that are within 3° of edge-on orientation, allowing us to test whether the presence of a B/P-shaped bulge can be explained by a past buckling event. We apply our diagnostics to two nearly edge-on galaxies with B/P bulges from the Spitzer Survey of Stellar Structure in Galaxies, finding no mid-plane asymmetry, implying these galaxies formed their bulges either by resonant trapping or by buckling more than ∼5 Gyr ago. We conclude that the formation of B/P bulges through strong buckling may be a rare event in the past ∼5 Gyr.

The rotational kinetic Sunyaev–Zeldovich contribution to the temperature asymmetry toward the M 31 halo

Temperature asymmetry in the cosmic microwave background (CMB) data by the Planck satellite has been discovered and analyzed toward several nearby edge-on spiral galaxies. It provides a way to probe galactic halo rotation, and to constrain the baryon fraction in the galactic halos. The frequency independence of the observed data provides a strong indication of the Doppler shift nature of the effect, due to the galactic halo rotation. It was proposed that this effect may arise from the emission of cold gas clouds populating the galactic halos. However, in order to confirm this view, other effects that might give rise to a temperature asymmetry in the CMB data, have to be considered and studied in detail. The main aim of the present paper is to estimate the contribution in the CMB temperature asymmetry data due to the free-free emission by hot gas (particularly electrons) through the rotational kinetic Sunyaev–Zeldovich (rkSZ) effect. We concentrate, in particular, on the M 31 galactic halo and compare the estimated values of the rkSZ induced temperature asymmetry with those obtained by using the SMICA pipeline of the Planck data release, already employed to project out the SZ sources and for lensing studies. As an additional consistency check, we also verified that the hot gas diffuse emission in the X-ray band does not exceed that detected in the soft X-ray band by ROSAT observations. We note that our results clearly show that the rkSZ effect gives only a minor contribution to the observed M 31 halo temperature asymmetry by Planck data.

The distribution of dust in edge-on galaxies: I. The global structure

ABSTRACT In this first paper in a series we present a study of the global dust emission distribution in nearby edge-on spiral galaxies. Our sample consists of 16 angularly large and 13 less spatially resolved galaxies selected from the DustPedia sample. To explore the dust emission distribution, we exploit the Herschel photometry in the range 100–500 $\mu $m. We employ Sérsic and 3D disc models to fit the observed 2D profiles of the galaxies. Both approaches give similar results. Our analysis unequivocally states the case for the presence of extraplanar dust in between 6 and 10 large galaxies. The results reveal that both the disc scale length and height increase as a function of wavelength between 100 and 500 $\mu $m. The dust disc scale height positively correlates with the dust disc scale length, similar to what is observed for the stellar discs. We also find correlations between the scale lengths and scale heights in the near- and far-infrared which suggest that the stellar discs and their dust counterparts are tightly connected. Furthermore, the intrinsic flattening of the dust disc is inversely proportional to the maximum rotation velocity and the dust mass of the galaxy: more massive spiral galaxies host, on average, relatively thinner dust discs. Also, there is a tendency for the dust-to-stellar scale height ratio to decrease with the dust mass and rotation velocity. We conclude that low-mass spiral galaxies host a diffuse, puffed-up dust disc with a thickness similar to that of the stellar disc.

H i Vertical Structure of Nearby Edge-on Galaxies from CHANG-ES

We study the vertical distribution of the highly inclined galaxies from the Continuum Halos in Nearby Galaxies—an EVLA Survey (CHANG-ES). We explore the feasibility of photometrically deriving the H i disk scale heights from the moment-0 images of the relatively edge-on galaxies with inclination >80°, by quantifying the systematic broadening effects and thus deriving correction equations for direct measurements. The corrected H i disk scale heights of the relatively edge-on galaxies from the CHANG-ES sample show trends consistent with the quasi-equilibrium model of the vertical structure of gas disks. The procedure provides a convenient way to derive the scale heights and can easily be applied to statistical samples in the future.

SDSS-IV MaNGA – gas rotation velocity lags in the final sample of MaNGA galaxies

ABSTRACT We consider the largest sample of 561 edge-on galaxies observed with integral field units by the MaNGA survey and find 300 galaxies where the ionized gas shows a negative vertical gradient (lag) in its rotational speed. We introduce the stop altitude as the distance to the galactic mid-plane at which the gas rotation should stop in the linear approximation. We find correlations between the lags, stop altitude and galactic mass, stellar velocity dispersion, and overall Sersic index. We do not find any correlation of the lags or stop altitude with the star formation activity in the galaxies. We conclude that low-mass galaxies (log(M*/M⊙) < 10) with low-Sersic index and with low-stellar velocity dispersion possess a wider ‘zone of influence’ in the extragalactic gas surrounding them with respect to higher mass galaxies that have a significant spherical component. We estimated the trend of the vertical rotational gradient with radius and find it flat for most of the galaxies in our sample. A small subsample of galaxies with negative radial gradients of lag has an enhanced fraction of objects with aged low-surface brightness structures around them (e.g. faint shells), which indicates that noticeable accretion events in the past affected the extraplanar gas kinematics and might have contributed to negative radial lag gradients. We conclude that an isotropic accretion of gas from the circumgalactic medium plays a significant role in the formation of rotation velocity lags.

Are superthin galaxies low-surface-brightness galaxies seen edge-on? The star formation probe

ABSTRACT Superthin galaxies (STs) are edge-on disc galaxies with strikingly high planar-to-vertical axes ratios of ∼10–20 with no bulge component, and central surface brightness in the B band > 23 mag arcsec−2 comparable to low-surface-brightness galaxies (LSBs). Although STs and LSBs have similar dynamical, stellar, and atomic hydrogen (H i) masses on an average, it is tricky to conclude if they constitute the same galaxy population, given the edge-on and face-on orientations of the STs and the LSBs, respectively. We systematically study star formation rate (SFR) in a sample of STs and LSBs using SED fitting of photometric data in 10 bands including GALEX: FUV, NUV, SDSS: u,g,r,i,z and 2MASS: J, H, Ks using stellar population synthesis models employing the publicly available software magphys (Multi-Wavelength Analysis of Galaxy Physical Properties). The estimated median SFRs for LSBs and STs are $0.4^{+2.2}_{-0.3}$$ and $0.2^{+0.9}_{-0.2}\, {\rm M}_{\odot }{\rm yr}^{-1}$, respectively. Our calculations indicate that this deficit in the SFR of an ST can be attributed to inclination and opacity effects. Therefore, we conclude that STs and LSBs have equal intrinsic SFR over and above other physical properties, which possibly implies that STs are just LSBs seen in edge-on.

SDSS IV MaNGA: characteristics of edge-on galaxies with a counter-rotating gaseous disc

ABSTRACT Counter-rotating components in galaxies are one of the most direct forms of evidence for past gas accretion or merging. We discovered 10 edge-on disc gaseous counter-rotators in a sample of 523 edge-on galaxies identified in the final MaNGA (Mapping Nearby Galaxies at APO) IFU sample. The counter-rotators tend to located in small groups. The gaseous counter-rotators have intermediate stellar masses and and located in the green valley and red sequence of the colour–magnitude diagram. The average vertical extents of the stellar and ionized gas discs are the same as for the rest of the sample while their radial gas and stellar distributions are more centrally concentrated. This may point at angular momentum loss during the formation process of the counter-rotating discs. The counter-rotators have low gas and dust content, weak emission-line strengths, and low star formation rates. This suggests that the formation of counter-rotators may be an efficient way to quench galaxies. One counter-rotator, SDSS J080016.09+292817.1 (Galaxy F), has a post-starburst region and a possible AGN at the centre. Another counter-rotator, SDSS J131234.03+482159.8 (Galaxy H), is identified as a potential ongoing galaxy interaction with its companion satellite galaxy, a gas-rich spiral galaxy. This may be representative case of a gaseous counter-rotator forming through a merger origin. However, tidal distortions expected in mergers are only found in a few of the galaxies and we cannot rule out direct gas accretion as another formation mechanism.

The Fornax3D project: intrinsic correlations between orbital properties and the stellar initial mass function

ABSTRACT Variations of the stellar initial mass function (IMF) in external galaxies have been inferred from a variety of independent probes. Yet the physical conditions causing these variations remain largely unknown. In this work, we explore new spatially resolved measurements of the IMF for three edge-on lenticular galaxies in the Fornax cluster. We utilize existing orbit-based dynamical models in order to fit the new IMF maps within an orbital framework. We find that, within each galaxy, the high-angular momentum disc-like stars exhibit an IMF which is rich in dwarf stars. The centrally concentrated pressure-supported orbits exhibit similarly dwarf-rich IMF. Conversely, orbits at large radius which have intermediate angular momentum exhibit IMF which are markedly less dwarf-rich relative to the other regions of the same galaxy. Assuming that the stars which reside, in the present-day, on dynamically hot orbits at large radii are dominated by accreted populations, we interpret these findings as a correlation between the dwarf-richness of a population of stars, and the mass of the host in which it formed. Specifically, deeper gravitational potentials would produce more dwarf-rich populations, resulting in the relative deficiency of dwarf stars which originated in the lower mass accreted satellites. The central and high-angular momentum populations are likely dominated by in situ stars, which were formed in the more massive host itself. There are also global differences between the three galaxies studied here, of up to ∼0.3 dex in the IMF parameter ξ. We find no local dynamical or chemical property which alone can fully account for the IMF variations.

Stellar Halos from the The Dragonfly Edge-on Galaxies Survey

We present the primary results from the Dragonfly Edge-on Galaxies Survey, an exploration of the stellar halos of twelve nearby (d < 25 Mpc) edge-on disk galaxies with the Dragonfly Telephoto Array. The edge-on orientation of these galaxies allows their stellar halos to be explored with minimal obscuration by or confusion with the much brighter disk light. Galaxies in the sample span a range of stellar masses from 109.68 to 1010.88 M ⊙. We confirm that the wide range of stellar halo mass fractions previously seen for Milky Way–mass galaxies is also found among less massive spiral galaxies. The scatter in stellar halo mass fraction is large, but we do find a significant positive correlation between stellar halo mass fraction and total stellar mass when the former is measured beyond five half-mass radii. Reasonably good agreement is found with predictions from cosmological hydrodynamical simulations, although observed stellar halo fractions appear to be somewhat lower than expected from these simulations.

A Bayesian Population Model for the Observed Dust Attenuation in Galaxies

Dust plays a pivotal role in determining the observed spectral energy distribution (SED) of galaxies. Yet our understanding of dust attenuation is limited and our observations suffer from the dust–metallicity–age degeneracy in SED fitting (single galaxies), large individual variances (ensemble measurements), and the difficulty in properly dealing with uncertainties (statistical considerations). In this study, we create a population Bayesian model to rigorously account for correlated variables and non-Gaussian error distributions and demonstrate the improvement over a simple Bayesian model. We employ a flexible 5D linear interpolation model for the parameters that control dust attenuation curves as a function of stellar mass, star formation rate (SFR), metallicity, redshift, and inclination. Our setup allows us to determine the complex relationships between dust attenuation and these galaxy properties simultaneously. Using Prospector fits of nearly 30,000 3D–Hubble Space Telescope galaxies, we find that the attenuation slope (n) flattens with increasing optical depth (τ), though less so than in previous studies. τ increases strongly with SFR, though when , τ remains roughly constant over a wide range of stellar masses. Edge-on galaxies tend to have larger τ than face-on galaxies, but only for , reflecting the lack of triaxiality for low-mass galaxies. Redshift evolution of dust attenuation is strongest for low-mass, low-SFR galaxies, with higher optical depths but flatter curves at high redshift. Finally, n has a complex relationship with stellar mass, highlighting the intricacies of the star–dust geometry. We have publicly released software for users to access our population model.

CHANG-ES – XXVII. A radio/X-ray catalogue of compact sources in and around edge-on galaxies

ABSTRACT We present catalogues of discrete, compact radio sources in and around the discs of 35 edge-on galaxies in the Continuum Halos in Nearby Galaxies – an EVLA Survey. The sources were extracted using the pybdsf program at both 1.6 GHz (L band) and 6.0 GHz (C band) from matching resolution (≈3 arcsec) data. We also present catalogues of X-ray sources from Chandra data sets for 27 of the galaxies. The sources at the two radio frequency bands were positionally cross-correlated with each other, and the result cross-correlated with the X-ray sources. All catalogues are included for download with this paper. We detect a total of 2507 sources at L band and 1413 sources at C band. 75 sources have been successfully cross-correlated in both radio bands plus X-ray. Three new nuclear sources are candidates for low luminosity active galactic nuclei in NGC 3877, NGC 4192, and NGC 5792; the one in NGC 3877 also appears to be variable. We also find new nuclear sources in two companion galaxies: NGC 4435 (companion to NGC 4438) and NGC 4298 (companion to NGC 4302). We have also discovered what appears to be a foreground double star; each star has X-ray emission and there is radio emission at both L band and C band in between them. This could be a colliding wind binary system. Suggestions for follow-up studies are offered.

Gas inflows in the polar ring of NGC 4111: the birth of an AGN

ABSTRACT We have used Hubble Space Telescope (HST) images, SAURON Integral Field Spectroscopy (IFS), and adaptative optics assisted Gemini NIFS near-infrared K-band IFS to map the stellar and gas distribution, excitation and kinematics of the inner few kpc of the nearby edge-on S0 galaxy NGC 4111. The HST images map its ≈450 pc diameter dusty polar ring, with an estimated gas mass ≥107 M⊙. The NIFS data cube maps the inner 110 pc radius at ≈7 pc spatial resolution, revealing a ≈220 pc diameter polar ring in hot (2267 ± 166 K) molecular H2 1–0 S(1) gas embedded in the polar ring. The stellar velocity field shows disc-dominated kinematics along the galaxy plane both in the SAURON large scale and in the NIFS nuclear-scale data. The large-scale [O iii] λ5007 Å velocity field shows a superposition of two disc kinematics: one similar to that of the stars and another along the polar ring, showing non-circular motions that seem to connect with the velocity field of the nuclear H2 ring, whose kinematics indicate accelerated inflow to the nucleus. The estimated mass inflow rate is enough not only to feed an active galactic nucleus (AGN) but also to trigger circumnuclear star formation in the near future. We propose a scenario in which gas from the polar ring, which probably originated from the capture of a dwarf galaxy, is moving inwards and triggering an AGN, as supported by the local X-ray emission, which seems to be the source of the H2 1–0 S(1) excitation. The fact that we see neither near-UV nor Br γ emission suggests that the nascent AGN is still deeply buried under the optically thick dust of the polar ring.

B/PS bulges in DESI Legacy edge-on galaxies – I. Sample building

ABSTRACTWe present the biggest up-to-date sample of edge-on galaxies with boxy/peanut-shaped (B/PS) bulges and X-structures. The sample was prepared using images from the Dark Energy Spectroscopic Instrument (DESI) Legacy catalogue and contains about 2000 galaxies. To find suitable candidates in catalogue, we made the assumption that the residues (original images minus model) of galaxies with B/PS bulges should exhibit a characteristic X-shape. Galaxies with such features were selected by eye and then used as input data for a neural network training, which was applied to a bigger sample of edge-on galaxies. Using the available data and the photometric models from the literature, we investigated the observational and statistical properties of the sample created. Comparing the B/D ratios for galaxies with and without B/PS bulges, we found that the B/D ratio for galaxies from our sample is statistically higher, with typical values in the range ≈0.2–0.5 depending on the decomposition procedure. We studied how the opening angles φ of the X-structure and the length of its rays are distributed in the formed sample and found them to be consistent with previous measurements and predictions from N-body models, e.g. $\varphi \gtrsim 25{^\circ }$, but measured here for a much larger number of galaxies. We found a sharp increase in the B/PS bulge fraction for stellar masses log M⋆ ≳ 10.4, but for edge-on galaxies, which complements the results of previous works. The sample can be used in future work to test various bar models and their relationship with B/PS bulges, as well as to study their stability and evolution.

Hα emission from gaseous structures above galactic discs

Abstract Extraplanar structures of ionized gas are detected in numerous edge-on galaxies. Whether HII regions or supernovae (SNe) responsible for its ionization is still debated. Here, we consider the H α {\rm{H}}\alpha emissivity from shells of superbubbles driven by SNe explosions in a stellar cluster located either in the midplane or above the midplane of the disc. We found that these shells form filamentary structures emitting brightly in H α {\rm{H}}\alpha line at height of ∼ 0.6 \sim 0.6 – 1.6 kpc 1.6\hspace{0.33em}{\rm{kpc}} above the disc depending on cluster location and gas metallicity. The H α {\rm{H}}\alpha surface brightness ranges within ∼ 1 0 − 17 \sim 1{0}^{-17} – 1 0 − 16 erg s − 1 cm − 2 arcsec − 2 1{0}^{-16}\hspace{0.33em}{\rm{erg}}\hspace{0.33em}{{\rm{s}}}^{-1}\hspace{0.33em}{{\rm{cm}}}^{-2}\hspace{0.33em}{{\rm{arcsec}}}^{-2} are close to the values observed in edge-on galaxies. We conclude that the H α {\rm{H}}\alpha emission of extraplanar structures can be explained by recombination in collisionally ionized gas of unstable shells driven by multiple SNe.

The multifarious ionization sources and disturbed kinematics of extraplanar gas in five low-mass galaxies

Aims.We investigate the origin of the extraplanar diffuse ionized gas (eDIG) and its predominant ionization mechanisms in five nearby (17–46 Mpc) low-mass (109–1010M⊙) edge-on disk galaxies: ESO 157-49, ESO 469-15, ESO 544-27, IC 217, and IC 1553.Methods.We acquired Multi Unit Spectroscopic Explorer (MUSE) integral field spectroscopy and deep narrowband Hαimaging of our sample galaxies. To investigate the connection between in-plane star formation and eDIG, we measure the star formation rates (SFRs) and perform a photometric analysis of our narrowband Hαimaging. Using our MUSE data, we investigate the origin of eDIG via kinematics, specifically the rotation velocity lags. We also construct standard diagnostic diagrams and emission-line maps (EW(Hα), [N II]/Hα, [S II]//Hα, [O III]/Hβ) and search for regions consistent with ionization by hot low-mass evolved stars (HOLMES) and shocks.Results.We measure eDIG scale heights ofhzeDIG = 0.59−1.39 kpc and find a positive correlation between them and specific SFRs. In all galaxies, we also find a strong correlation between extraplanar and midplane radial Hαprofiles. These correlations along with diagnostic diagrams suggest that OB stars are the primary driver of eDIG ionization. However, we find regions consistent with mixed OB–HOLMES and OB–shock ionization in all galaxies and conclude that both HOLMES and shocks may locally contribute to the ionization of eDIG to a significant degree. From Hαkinematics, we find rotation velocity lags above the midplane with values between 10 and 27 km s−1kpc−1. While we do find hints of an accretion origin for the ionized gas in ESO 157–49, IC 217, and IC 1553, overall the ionized gas kinematics of our galaxies do not match a steady galaxy model or any simplistic model of accretion or internal origin for the gas.Conclusions.Despite our galaxies’ similar structures and masses, our results support a surprisingly composite image of ionization mechanisms and a multifarious origin for the eDIG. Given this diversity, a complete understanding of eDIG will require larger samples and composite models that take many different ionization and formation mechanisms into account.

A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE)

Using the VESTIGE survey, a deep narrow-band Hαimaging survey of the Virgo cluster carried out at the CFHT with MegaCam, we discovered a long and diffuse tail of ionised gas in the edge-on late-type galaxy NGC 4330. This peculiar feature indicates an ongoing ram pressure stripping event able to remove the gas in the outer region of the disc. Tuned hydrodynamic simulations suggest that the ram pressure stripping event is occurring almost face-on, making NGC 4330 the ideal candidate for studying the effects of the perturbation in the direction perpendicular to the disc plane. We present here two new independent sets of Fabry-Perot observations (R≃ 10 000) with the purpose of understanding the effects of the ram pressure stripping process on the kinematics of the ionised gas. Despite their limited sensitivity to the diffuse gas emission, the data allowed us to measure the velocity and the velocity dispersion fields over the galaxy disc and in several features at the edges or outside the stellar disc formed after the ram pressure stripping event. We constructed the position-velocity diagrams and the rotation curves of the galaxy using three different techniques. The data show, consistent with the hydrodynamic simulations, that the galaxy has an inner solid-body rotation up to ∼2.4 kpc, with non-circular streaming motions outside this radius and in the several external features formed during the interaction of the galaxy with the surrounding intracluster medium. The data also indicate a decrease in the rotational velocity of the gas with increasing distance from the galaxy disc along the tails, suggesting a gradual but not linear loss of angular momentum in the stripped gas. Consistent with a ram pressure stripping scenario, thei-band image shows a boxy shape at the south-west edge of the disc, where the stellar orbits might have been perturbed by the modification of the gravitational potential well of the galaxy due to the displacement of the gas in thezdirection.