Recent Star Formation Research Articles

Revisiting the giant radio galaxy ESO 422−G028 – I. Discovery of a neutral inflow and recent star formation in a restarted giant

ABSTRACT Giant radio galaxies provide important clues into the life cycles and triggering mechanisms of radio jets. With large-scale jets spanning 1.8 Mpc, ESO 422−G028 (z = 0.038) is a giant radio galaxy that also exhibits signs of restarted jet activity in the form of pc-scale jets. We present a study of the spatially resolved stellar and gas properties of ESO 422−G028 using optical integral field spectroscopy from the Wide-Field Spectrograph (WiFeS). In addition to the majority ${\sim} 13\, \rm Gyr$ old stellar population, ESO 422−G028 exhibits a much younger (${\lesssim} 10\, \rm Myr$ old) component with an estimated mass of $10^{7.6}\, \rm M_\odot$ that is predominantly located in the north-west region of the galaxy. Unusually, the ionized gas kinematics reveal two distinct discs traced by narrow ($\sigma _{\rm H\alpha } \lt 100 \, \rm km\, s^{-1}$) and broad ($\sigma _{\rm H\alpha } \gt 150 \, \rm km\, s^{-1}$) Hα emission, respectively. Both ionized gas discs are misaligned with the axis of stellar rotation, suggesting an external origin. This is consistent with the prominent interstellar Na D absorption, which traces a $1 \!-\! 3 \, \rm M_\odot \, yr^{-1}$ inflow of neutral gas from the north. We posit that an inflow of gas – either from an accretion event or a gas-rich merger – has triggered both the starburst and the restarted jet activity, and that ESO 422−G028 is potentially on the brink of an epoch of powerful active galactic nucleus (AGN) activity.

Jet‐triggered star formation in young radio galaxies

AbstractEmission in the ultraviolet continuum is a salient signature of the hot, massive, and consequently short‐lived, stellar population that traces recent or ongoing star formation. With the aim of mapping star forming regions and morphologically separating the generic star formation from that associated with the galaxy‐scale jet activity, we obtained high‐resolution ultraviolet (UV) imaging from the Hubble Space Telescope for a sample of nine compact radio sources. Out of these, seven are known Compact Steep Spectrum (CSS) galaxies that host young, kiloparsec‐scale radio sources and hence are the best candidates for studying radio‐mode feedback on galaxy scales, while the other two form a control sample of larger sources. Extended UV emission regions are observed in six of the seven CSS sources showing close spatial alignment with the radio‐jet orientation. If other mechanisms possibly contributing to the observed UV emission are ruled out, this could be evidence in support of jet‐triggered star formation in the CSS phase of radio galaxy evolution and in turn of the “positive feedback” paradigm of host–active galactic nuclei interaction.

Early-type galaxies with recent star formation have low metallicities – evidence for star formation triggered by external processes?

ABSTRACTOn the basis of the Galaxy Evolution Explore (GALEX) ultraviolet (UV) data, many studies have demonstrated that recent star formation (RSF) is common in early-type galaxies. In particular, near-ultraviolet (NUV) light is used to investigate star formation activity in early-type galaxies, owing to its high sensitivity to the presence of young stars. This study characterized the stellar population properties of luminous early-type galaxies (Mr ≤ −20.65 mag) in the redshift range 0.005 ≤ z ≤ 0.09 on the basis of the Sloan Digital Sky Survey (SDSS) data. Initially, the most frequently used criterion of NUV $-\, r$ ≤ 5.4 was applied to identify early-type galaxies with RSF, and 19 per cent (172/913) of the sample galaxies met this criterion, in agreement with previous studies. A more robust sample of galaxies with RSF along with a stricter criterion (70 galaxies with NUV $-\, r$ ≤ 5.0) were then used for further analysis, and consequently 7.7 per cent (70/913; lower limit of the RSF fraction) of the sample galaxies were classified as early-type galaxies with RSF. These galaxies tended to exhibit higher H β absorption-line strengths and stronger emission lines than quiescent (QST, NUV $-\, r$ > 5.4) early-type galaxies. The most prominent feature of early-type galaxies with RSF identified in this study was that they were more metal-poor than QST galaxies owing to metal-poor stellar populations that were possibly formed from materials accreted from gas-rich satellites during (minor) mergers or interactions. The results strongly indicate that the observed RSF in the sample galaxies at the present epoch is mainly driven by external processes (i.e. mergers or interactions).

Molecular gas and star formation within 12 strong galactic bars observed with IRAM-30 m

Context.While some galactic bars show recent massive star formation (SF) along them, some others do not. Whether bars with low level of SF are a consequence of low star formation efficiency, low gas inflow rate, or dynamical effects remains a matter of debate.Aims.In order to study the physical conditions that enable or prevent SF, we perform a multi-wavelength analysis of 12 strongly barred galaxies with total stellar masses log10(M⋆/M⊙)∈[10.2, 11], chosen to host different degrees of SF along the bar major axis without any prior condition on gas content. We observe the CO(1–0) and CO(2–1) emission within bars with the IRAM-30 m telescope (beam sizes of 1.7–3.9 kpc and 0.9–2.0 kpc, respectively; 7–8 pointings per galaxy on average).Methods.We estimated molecular gas masses (Mmol) from the CO(1–0) and CO(2–1) emissions. SF rates (SFRs) were calculated from GALEX near-ultraviolet (UV) and WISE 12 μm images within the beam-pointings, covering the full bar extent (SFRs were also derived from far-UV and 22 μm).Results.We detect molecular gas along the bars of all probed galaxies. Molecular gas and SFR surface densities span the ranges log10(Σmol/[M⊙pc−2]) ∈ [0.4,2.4] and log10(ΣSFR/[M⊙pc−1kpc−2]]) ∈ [−3.25, −0.75], respectively. The star formation efficiency (SFE; i.e., SFR/Mmol) in bars varies between galaxies by up to an order of magnitude (SFE ∈[0.1, 1.8] Gyr−1). On average, SFEs are roughly constant along bars. SFEs are not significantly different from the mean value in spiral galaxies reported in the literature (∼0.43 Gyr−1), regardless of whether we estimateMmolfrom CO(1–0) or CO(2–1). Interestingly, the higher the total stellar mass of the host galaxy, the lower the SFE within their bars. In particular, the two galaxies in our sample with the lowest SFE and ΣSFR(NGC 4548 and NGC 5850, SFE ≲ 0.25 Gyr−1, ΣSFR ≲ 10−2.25 M⊙yr−1kpc−2,M⋆ ≳ 1010.7 M⊙) are also those hosting massive bulges and signs of past interactions with nearby companions.Conclusions.We present a statistical analysis of the SFE in bars for a sample of 12 galaxies. The SFE in strong bars is not systematically inhibited (either in the central, middle, or end parts of the bar). Both environmental and internal quenching are likely responsible for the lowest SFEs reported in this work.

The Star Formation History of a Post-starburst Galaxy Determined from Its Cluster Population

Abstract The recent star formation histories (SFHs) of post-starburst galaxies have been determined almost exclusively from detailed modeling of their composite starlight. This has provided important but limited information on the number, strength, and duration of bursts of star formation. In this work, we present a direct and independent measure of the recent SFH of the post-starburst galaxy S12 (plate-mjd-fiber for SDSS 623-52051-207; designated EAS12 in Smercina et al.) from its star cluster population. We detect clusters from high-resolution, UBR optical images taken with the Hubble Space Telescope and compare their luminosities and colors with stellar population models to estimate the ages and masses of the clusters. No clusters younger than ∼70 Myr are found, indicating star formation shut off at this time. Clusters formed ∼120 Myr ago reach masses up to a ∼few × 107 M ⊙, several times higher than similar-age counterparts formed in actively merging galaxies like the Antennae and NGC 3256. We develop a new calibration based on known properties for eight nearby galaxies to estimate the star formation rate (SFR) of a galaxy from the mass of the most massive cluster, M max. The cluster population indicates that S12 experienced an extremely intense but short-lived burst ∼120 Myr ago, with an estimated peak of 500 − 250 + 500 M ⊙ yr − 1 and duration of 50 ± 25 Myr, one of the highest SFRs estimated for any galaxy in the modern universe. The cluster population also allows us to fill in more of the backstory of S12. Prior to the recent, intense burst, S12 was forming stars at a moderate rate of ∼3–5 M ⊙ yr−1, typical of spiral galaxies, but the system experienced an earlier burst at some point, approximately 1–3 Gyr ago. While fairly uncertain, we estimate that the SFR during this earlier burst was ∼20–30 M ⊙ yr−1, similar to the current SFR in the Antennae and NGC 3256.

How stars formed in warps settle into (and contaminate) thick discs

ABSTRACT In recent years star formation has been discovered in the Milky Way’s warp. These stars formed in the warp (warp stars) must eventually settle into the plane of the disc. We use an N-body+smooth particle hydrodynamics model of a warped galaxy to study how warp stars settle into the disc. By following warp stars in angular momentum space, we show that they first tilt to partially align with the main disc in a time-scale of ${\sim} 1\mbox{$\, {\rm Gyr}$}$. Then, once differential precession halts this process, they phase mix into an axisymmetric distribution on a time-scale of ${\sim} 6 \mbox{$\, {\rm Gyr}$}$. The warp stars end up contaminating the geometric thick disc. Because the warp in our fiducial simulation is growing, the warp stars settle to a distribution with a negative vertical age gradient as younger stars settle further from the mid-plane. While vertically extended, warp star orbits are still nearly circular and they are therefore subject to radial migration, with a net movement inwards. As a result warp stars can be found throughout the disc. The density distribution of a given population of warp stars evolves from a torus to an increasingly centrally filled-in density distribution. Therefore we argue that, in the Milky Way, warp stars should be found in the Solar Neighbourhood. Moreover, settled warp stars may constitute part of the young flaring population seen in the Milky Way’s outskirts.

Galaxy evolution on resolved scales: ageing and quenching in CALIFA

ABSTRACT This work investigates the fundamental mechanism(s) that drive galaxy evolution in the local Universe. By comparing two proxies of star formation sensitive to different time-scales, such as EW($\rm H\,\alpha$) and colours like g − r, one may distinguish between smooth secular evolution (ageing) and sudden changes (quenching) in the recent star formation history of galaxies. Building upon the results obtained from a former study based on 80 000 Sloan Digital Sky Survey (SDSS) single-fibre measurements, we now focus on spatially resolved (on kpc scales) galaxies, comparing with a sample of 637 nearby objects observed by the Calar Alto Legacy Integral-Field Area survey. In general, galaxies cannot be characterized in terms of a single ‘evolutionary stage’. Individual regions within galaxies arrange along a relatively narrow ageing sequence, with some intrinsic scatter possibly due to their different evolutionary paths. These sequences, though, differ from one galaxy to another, although they are broadly consistent with the overall distribution found for the (central) SDSS spectra. We find evidence of recent quenching episodes (relatively blue colours and strong $\rm H\,\alpha$ absorption) in a small fraction of galaxies (most notably, low-mass ellipticals), on global scales and individual regions (particularly at high metallicity). However, we argue that most of the systems, over their entire extent, are compatible with a secular inside-out scenario, where the evolutionary stage correlates with both global (mass, morphology, and environment) and local (surface brightness and metallicity) properties.

Distance and extinction to the Milky Way spiral arms along the Galactic centre line of sight

Context. The position of the Sun inside the disc of the Milky Way significantly hampers the study of the spiral arm structure given the high amount of dust and gas along the line of sight, and the overall structure of this disc has therefore not yet been fully characterised. Aims. We aim to analyse the spiral arms in the line of sight towards the Galactic centre (GC) in order to determine their distance, extinction, and stellar population. Methods. We use the GALACTICNUCLEUS survey, a JHKs high-angular-resolution photometric catalogue (0.2″) for the innermost regions of the Galaxy. We fitted simple synthetic colour-magnitude models to our data via χ2 minimisation. We computed the distance and extinction to the detected spiral arms. We also analysed the extinction curve and the relative extinction between the detected features. Finally, we studied extinction-corrected Ks luminosity functions (KLFs) to study the stellar populations present in the second and third spiral arm features. Results. We determined the mean distances to the spiral arms: d1 = 1.6 ± 0.2 kpc, d2 = 2.6 ± 0.2 kpc, d3 = 3.9 ± 0.3 kpc, and d4 = 4.5 ± 0.2 kpc, and the mean extinctions: AH1 = 0.35 ± 0.08 mag, AH2 = 0.77 ± 0.08 mag, AH3 = 1.68 ± 0.08 mag, and AH4 = 2.30 ± 0.08 mag. We analysed the extinction curve in the near-infrared for the stars in the spiral arms and find mean values of AJ/AH = 1.89 ± 0.11 and AH/AKs = 1.86 ± 0.11, in agreement with the results obtained for the GC. This implies that the shape of the extinction curve does not depend on distance or absolute extinction. We also built extinction maps for each spiral arm and find them to be homogeneous and that they might correspond to independent extinction layers. Finally, analysing the KLFs from the second and third spiral arms, we find that they have similar stellar populations. We obtain two main episodes of star formation: > 6 Gyr (∼60 − 70% of the stellar mass), and 1.5 − 4 Gyr (∼20 − 30% of the stellar mass), compatible with previous work. We also detect recent star formation at a lower level (∼10%) for the third spiral arm.

Star formation history for the starburst dwarf galaxy in the Local Group, IC 10

AbstractIC 10 as a starburst dwarf galaxy in the Local Group (LG) has a large population of newly formed stars that are massive and intrinsically very bright in comparison with other LG galaxies. Using the Isaac Newton Telescope (INT) with the Wide Field Camera (WFC) in the i-band and V-band, we performed an optical monitoring survey to identify the most evolved asymptotic giant branch stars (AGBs) and red supergiant stars (RSGs) in this star-forming galaxy, which can be used to determine the star formation history (SFH). The E(B - V) as an effective factor for obtaining the precise magnitude of stars is measured for each star using a 2D dust map (SFD98) to obtain a total extinction for each star in both the i-band and V-band. We obtained the photometric catalog for 53579 stars within the area of 0.07 deg2 (13.5 kpc2), of which 762 stars are classified as variable candidates after removing the foreground stars and saturated ones from our catalog. To reconstruct the SFH for IC 10, we first identified 424 long-period variable (LPV) candidates within the area of two half-light radii (2rh) from the center of the galaxy. We estimated the recent star formation rate (SFR) at ∼ 0.32 yr-1 for a constant metallicity Z = 0.0008, showing the galaxy is currently undergoing high levels of star formation. Also, a total stellar mass of is obtained within 2rh for that metallicity.

Probing the structure of a massive filament: ArTéMiS 350 and 450 μm mapping of the integral-shaped filament in Orion A

Context. The Orion molecular cloud is the closest region of high-mass star formation. It is an ideal target for investigating the detailed structure of massive star-forming filaments at high resolution and the relevance of the filament paradigm for the earliest stages of intermediate- to high-mass star formation. Aims. Within the Orion A molecular cloud, the integral-shaped filament (ISF) is a prominent, degree-long structure of dense gas and dust with clear signs of recent and ongoing high-mass star formation. Our aim is to characterise the structure of this massive filament at moderately high angular resolution (8′′ or ~0.016 pc) in order to measure the intrinsic width of the main filament, down to scales well below 0.1 pc, which has been identified as the characteristic width of filaments. Methods. We used the ArTéMiS bolometer camera at APEX to map a ~0.6 × 0.2 deg2 region covering OMC-1, OMC-2, and OMC-3 at 350 and 450 μm. We combined these data with Herschel-SPIRE maps to recover extended emission. The combined Herschel-ArTéMiS maps provide details on the distribution of dense cold material, with a high spatial dynamic range, from our 8′′ resolution up to the transverse angular size of the map, ~10–15′. By combining Herschel and ArTéMiS data at 160, 250, 350, and 450 μm, we constructed high-resolution temperature and H2 column density maps. We extracted radial intensity profiles from the column density map in several representative portions of the ISF, which we fitted with Gaussian and Plummer models to derive their intrinsic widths. We also compared the distribution of material traced by ArTéMiS with that seen in the higher-density tracer N2H+(1–0) that was recently observed with the ALMA interferometer. Results. All the radial profiles that we extracted show a clear deviation from a Gaussian, with evidence for an inner plateau that had not previously been seen clearly using Herschel-only data. We measure intrinsic half-power widths in the range 0.06–0.11 pc. This is significantly larger than the Gaussian widths measured for fibres seen in N2H+, which probably only traces the dense innermost regions of the large-scale filament. These half-power widths are within a factor of two of the value of ~0.1 pc found for a large sample of nearby filaments in various low-mass star-forming regions, which tends to indicate that the physical conditions governing the fragmentation of pre-stellar cores within transcritical or supercritical filaments are the same over a large range of masses per unit length.

Dense molecular gas properties on 100 pc scales across the disc of NGC 3627

ABSTRACT It is still poorly constrained how the densest phase of the interstellar medium varies across galactic environment. A large observing time is required to recover significant emission from dense molecular gas at high spatial resolution, and to cover a large dynamic range of extragalactic disc environments. We present new NOrthern Extended Millimeter Array (NOEMA) observations of a range of high critical density molecular tracers (HCN, HNC, HCO+) and CO isotopologues (13CO, C18O) towards the nearby (11.3 Mpc) strongly barred galaxy NGC 3627. These observations represent the current highest angular resolution (1.85 arcsec; 100 pc) map of dense gas tracers across a disc of a nearby spiral galaxy, which we use here to assess the properties of the dense molecular gas, and their variation as a function of galactocentric radius, molecular gas, and star formation. We find that the HCN(1–0)/CO(2–1) integrated intensity ratio does not correlate with the amount of recent star formation. Instead, the HCN(1–0)/CO(2–1) ratio depends on the galactic environment, with differences between the galaxy centre, bar, and bar-end regions. The dense gas in the central 600 pc appears to produce stars less efficiently despite containing a higher fraction of dense molecular gas than the bar ends where the star formation is enhanced. In assessing the dynamics of the dense gas, we find the HCN(1–0) and HCO+(1–0) emission lines showing multiple components towards regions in the bar ends that correspond to previously identified features in CO emission. These features are cospatial with peaks of Hα emission, which highlights that the complex dynamics of this bar-end region could be linked to local enhancements in the star formation.

The central region of the enigmatic Malin 1

Malin 1, being a class of giant low surface galaxies, continues to surprise us even today. The HST/F814W observation has shown that the central region of Malin 1 is more like a normal SB0/a galaxy, while the rest of the disk has the characteristic of a low surface brightness system. The AstroSat/UVIT observations suggest scattered recent star formation activity all over the disk, especially along the spiral arms. The central \(9''\) (\(\sim 14\,\hbox {kpc}\)) region, similar to the size of the Milky Way’s stellar disk, has a number of far-UV clumps—indicating recent star-formation activity. The high resolution UVIT/F154W image reveals far-UV emission within the bar region (\(\sim 4\,\hbox {kpc}\))—suggesting the presence of hot, young stars in the bar. These young stars from the bar region are perhaps responsible for producing the strong emission lines such as H\(\alpha \), [Oii] seen in the SDSS spectra. Malin 1B, a dwarf early-type galaxy, is interacting with the central region and probably responsible for inducing the recent star-formation activity in this galaxy.

Clustered star formation toward Berkeley 87/ON2

Context. Disentangling line-of-sight alignments of young stellar populations is crucial for observational studies of star-forming complexes. This task is particularly problematic in a Cygnus-X subregion where several components, located at different distances, overlap: the Berkeley 87 young massive cluster, the poorly known [DB2001] Cl05 embedded cluster, and the ON2 star-forming complex, which in turn is composed of several H II regions. Aims. We provide a methodology for building an exhaustive census of young objects that can consistently treat large differences in extinction and distance. Methods. OMEGA2000 near-infrared observations of the Berkeley 87/ON2 field were merged with archival data from Gaia, Chandra, Spitzer, and Herschel and with cross-identifications from the literature. To address the incompleteness effects and selection biases that arise from the line-of-sight overlap, we adapted existing methods for extinction estimation and young object classification. We also defined the intrinsic reddening index, Rint, a new tool for separating intrinsically red sources from those whose infrared color excess is caused by extinction. Finally, we introduce a new method for finding young stellar objects based on Rint. Results. We find 571 objects whose classification is related to recent or ongoing star formation. Together with other point sources with individual estimates of distance or extinction, we compile a catalog of 3005 objects to be used for further membership work. A new distance for Berkeley 87, (1673 ± 17) pc, is estimated as a median of 13 spectroscopic members with accurate Gaia EDR3 parallaxes. Conclusions. The flexibility of our approach, especially regarding the Rint definition, allows overcoming photometric biases caused by large variations in extinction and distance in order to obtain homogeneous catalogs of young sources. The multiwavelength census that results from applying our methods to the Berkeley 87/ON2 field will serve as a basis for disentangling the overlapped populations.

The Fundamental Plane in the LEGA-C Survey: Unraveling the M/L Ratio Variations of Massive Star-forming and Quiescent Galaxies at z ∼ 0.8

Abstract We explore the connection between the kinematics, structures and stellar populations of massive galaxies at 0.6 < z < 1.0 using the fundamental plane (FP). Combining stellar kinematic data from the Large Early Galaxy Astrophysics Census (LEGA-C) survey with structural parameters measured from deep Hubble Space Telescope imaging, we obtain a sample of 1419 massive ( log ( M * / M ⊙ ) > 10.5 ) galaxies that span a wide range in morphology, star formation activity, and environment, and therefore is representative of the massive galaxy population at z ∼ 0.8. We find that quiescent and star-forming galaxies occupy the parameter space of the g-band FP differently and thus have different distributions in the dynamical mass-to-light ratio (M dyn/L g ), largely owing to differences in the stellar age and recent star formation history, and to a lesser extent, the effects of dust attenuation. In contrast, we show that both star-forming and quiescent galaxies lie on the same mass FP at z ∼ 0.8, with a comparable level of intrinsic scatter about the plane. We examine the variation in M dyn/M * through the thickness of the mass FP, finding no significant residual correlations with stellar population properties, Sérsic index, or galaxy overdensity. Our results suggest that, at fixed size and velocity dispersion, the variations in M dyn/L g of massive galaxies reflect an approximately equal contribution of variations in M */L g , and variations in the dark matter fraction or initial mass function.

Star cluster survival in dark matter haloes: an old cluster in Eridanus II?

ABSTRACT The star formation history and the internal dynamics of Milky Way satellite galaxies are often complicated. In the last years, a substantial fraction of the known faint dwarf satellites have been studied. Some of them show embedded stellar substructures, such as star clusters and even globular star clusters. In this work, we study Eridanus II, a dwarf spheroidal satellite that hosts a star cluster, using published and archival data from the Hubble Space Telescope Advanced Camera for Surveys. We employ a Bayesian hierarchical method to infer the star formation history of Eridanus II. We find that the bulk of the stars in Eridanus II are very old ($13.5_{-1}^{+0.5}$ Gyr) and quite metal-poor (Z = 0.000 01). We do not find any evidence of the presence of an intermediate age or young population in Eri II. We cannot date the embedded star cluster as a separate entity, but we find it likely that the cluster has a similar age and metallicity as the bulk of the stars in Eri II. The existence of an old star cluster in a dark matter dominated old metal-poor dwarf galaxy is of major importance to cast light on the dark matter distribution within dwarf galaxies. The existence of intermediate age stars is required by the recent detection of carbon stars in Eri II. Since no recent star formation is detected, blue-straggler fusions of lower mass stars are the most likely origin of the carbon star progenitors.

The nuclear environment of NGC 2442: a Compton-thick low-luminosity AGN

ABSTRACT The detailed study of nuclear regions of galaxies is important because it can help understanding the active galactic nucleus (AGN) feedback mechanisms, the connections between the nuclei and their host galaxies, and ultimately the galaxy formation processes. We present the analysis of an optical data cube of the central region of the galaxy NGC 2442, obtained with the integral field unit (IFU) of the Gemini Multi-Object Spectrograph (GMOS). We also performed a multiwavelength analysis, with Chandra data, XMM–Newton and NuSTAR spectra, and Hubble Space Telescope (HST) images. The analysis revealed that the nuclear emission is consistent with a Low Ionization Nuclear Emission-line Region (LINER) associated with a highly obscured compact hard X-ray source, indicating a Compton-thick AGN. The HST image in the F658N filter (H α) reveals an arched structure corresponding to the walls of the ionization cone of the AGN. The gas kinematic pattern and the high gas velocity dispersion values in the same region of the ionization cone suggest an outflow emission. The stellar archaeology results indicate the presence of only old stellar populations (∼ 10 Gyr), with high metallicity (z = 0.02 and 0.05), and the absence of recent star formation in the central region of NGC 2442, which is possibly a consequence of the AGN feedback, associated with the detected outflow, shutting off star formation. NGC 2442 is a late-type galaxy similar to the Milky Way, and comparisons show that the main difference between them is the presence of a low-luminosity AGN.

Brought to Light. II. Revealing the Origins of Cloaked Spiral Features in Cluster Passive Dwarf Galaxies

Abstract In our companion paper (Brought to Light I: Michea et al.), we reveal spectacular spiral-galaxy-like features in deep optical imaging of nine Virgo early-type dwarf galaxies, hidden beneath a dominating smooth stellar disk. Using a new combination of approaches, we find that bar- and spiral-like features contribute 2.2%–6.4% of the total flux within 2 R eff. In this study, we conduct high-resolution simulations of cluster harassment of passive dwarf galaxies. Following close pericenter passages of the cluster core, tidal triggering generates features in our model disks that bear a striking resemblance to the observed features. However, we find the disks must be highly rotationally supported (V peak/σ 0 ∼ 3), much higher than typically observed. We propose that some early-type dwarfs may contain a few percent of their mass in a cold, thin disk that is buried in the light of a hot, diffuse disk and only revealed when they undergo tidal triggering. The red optical colors of our sample do not indicate any recent significant star formation, and our simulations show that very plunging pericenter passages (r peri < 0.25r vir) are required for tidal triggering. Thus, many cluster early-type dwarfs with less-plunging orbits may host a yet-undetected cold stellar disk component. We discuss possible origin scenarios and consider why similar-mass star-forming galaxies in the field are significantly more thin-disk dominated than in our cluster sample.

Millisecond pulsars modify the radio-star-formation-rate correlation in quiescent galaxies

The observed correlation between the far-infrared and radio luminosities of galaxies illustrates the close connection between star formation and cosmic-ray production. Intriguingly, recent gamma-ray observations indicate that recycled/millisecond pulsars (MSPs), which do not trace recent star formation, may also efficiently accelerate cosmic-ray electrons. We study the contribution of MSPs to the galactic non-thermal radio emission, finding that they can dominate the emission from massive quiescent galaxies. This model can explain recent LOFAR observations that found a peculiar radio excess in galaxies with high stellar masses and low star-formation rates. We show that MSP-based models provide a significantly improved fit to LOFAR data. We discuss the implications for the radio-FIR correlation, the observation of radio excesses in nearby galaxies, and local electron and positron observations.

Recovering the origins of the lenticular galaxy NGC 3115 using multiband imaging

ABSTRACT A detailed study of the morphology of lenticular galaxies is an important way to understand how this type of galaxy is formed and evolves over time. Decomposing a galaxy into its components (disc, bulge, bar, ...) allows recovering the colour gradients present in each system, its star formation history, and its assembly history. We use galfitm to perform a multiwavelength structural decomposition of the closest lenticular galaxy, NGC 3115, resulting in the description of its stellar light into several main components: a bulge, a thin disc, a thick disc, and also evidence of a bar. We report the finding of central bluer stellar populations in the bulge, as compared to the colour of the galaxy outskirts, indicating either the presence of an active galactic nucleus (AGN) and/or recent star formation activity. From the spectral energy distribution results, we show that the galaxy has a low luminosity AGN component, but even excluding the effect of the nuclear activity, the bulge is still bluer than the outer-regions of the galaxy, revealing a recent episode of star formation. Based on all of the derived properties, we propose a scenario for the formation of NGC 3115 consisting of an initial gas-rich merger, followed by accretions and feedback that quench the galaxy, until a recent encounter with the companion KK084 that reignited the star formation in the bulge, provoked a core displacement in NGC 3115 and generated spiral-like features. This result is consistent with the two-phase formation scenario, proposed in previous studies of this galaxy.

Star formation quenching stages of active and non-active galaxies

The mechanisms that bring galaxies to strongly reduce their star formation activity (star-formation quenching) remain poorly understood. To better study galaxy evolution, we propose a classification based on maps of ionised hydrogen distribution traced by the kiloparsec-resolved, equivalent width of Hα maps, and the nuclear activity of the galaxies using information from the Baldwin-Philips-Terlevich diagnostic diagrams. Using these tools, we group a sample of 238 galaxies from the Calar Alto Legacy Integral Field spectroscopy Area survey in six quenching stages (QSs): (i) objects dominated by recent star formation; (ii) systems that present a quiescent-nuclear-ring structure in their centre; (iii) galaxies that are centrally quiescent; (iv) galaxies with no clear pattern in their ionisation gas distribution (mixed); (v) systems that posses only a few star-forming regions (nearly retired), or (vi) galaxies that are completely quiescent (fully retired). Regarding their nuclear activity, we further divide the galaxies into two groups: active systems that host a weak or strong active galactic nucleus (AGN) at their centre, and non-active objects. Galaxies grouped into quenching-stage classes occupy specific locations on the star-formation-rate versus stellar mass diagram. The ‘blue cloud’ is populated by the star-forming and the quiescent-nuclear-ring galaxies, the ‘green valley’ is populated by centrally quiescent and mixed systems, and the ‘red sequence’ by the nearly- and fully retired objects. Generally, galaxies that host a weak or strong AGN show properties comparable to those of their non-active counterparts at the same QSs, except for the AGN-hosting star-forming systems. The degree of star-formation quenching increases along the present emission-line pattern sequence from star-forming to fully retired. The proposed emission-line classes reinforce the ‘inside-out’ quenching scenario, which foresees that the suppression of star formation begins from the central regions of the galaxies.