FUV Emission Research Articles

An AstroSat/UVIT study of galaxies in the cluster Abell 2199

We present the newly acquired data for an AstroSat/UVIT field centred on a face-on spiral starburst galaxy UGC 10420, located in the X-ray bright cluster Abell 2199 (z = 0.031). We have analysed the FUV BaF2 data for this field along with the archival FUV and NUV data from the GALEX mission, optical photometric data from the SDSS, spectroscopic data from the literature, and low-frequency radio data from the LoTSS survey, respectively. The stars were separated from the galaxies using the SDSS photometric pipeline classification, while the spectroscopic redshifts available for 35% of the detected UVIT sources were used to identify member galaxies of the cluster Abell 2199. We find that (a) the non-cluster galaxies are on average fainter than the cluster galaxies at fixed magnitude, (b) stars and galaxies are indistinguishable in the r vs. NUV–r plane, and (c) bright stars are ∼ 1.5 mag bluer than the galaxies in the FUV–r vs. NUV–r colour-colour plane. Besides UGC 10420 which is the only known cluster galaxy with an extended-UV disk, we identify five more galaxies with asymmetric FUV morphology and extended radio emission in this field. All the asymmetric member galaxies of Abell 2199, lie within the virial boundaries of the cluster. This observation, together with the fact that these asymmetric cluster galaxies have low-frequency radio tails or FUV emission pointing away from the cluster centre leads us to hypothesise that these galaxies are likely undergoing ram-pressure stripping (RPS) under the influence of cluster-environment related mechanisms. A comparison of optical and FUV star formation rate of UVIT detected galaxies shows enhanced star formation in half of the RPS candidates, suggesting that environment-related mechanisms may lead to a burst of star formation in RPS galaxies.Our analysis indicates the presence of at least two more groups or clusters at z ∼ 0.077 and 0.260, coincident with Abell 2199 along the line of sight of the field of view studied here.

On the origin of the anomalous gas, non-declining rotation curve, and disc asymmetries in NGC 253

ABSTRACT We present a multiwavelength (from far-ultraviolet to ${\rm H}\, \rm {\small I}$ emission) study of star formation feedback on the kinematics of the interstellar medium in the Sculptor galaxy, NGC 253. Its three well-known features (a disrupted stellar disc, a previously reported declining rotation curve, and anomalous ${\rm H}\, \rm {\small I}$ gas) are studied in a common context of disc asymmetries. About 170 h of on-source ATCA observations are collected and reduced into two versions of ${\rm H}\, \rm {\small I}$ data cubes of different angular resolution (30 arcsec/2 arcmin) and H i column density sensitivity (7.4 × 1019 cm−2/4 × 1018 cm−2). We separate the anomalous gas from the disc using a custom-made line profile fitting toolkit called FMG. Two star formation tracers (H α, FUV emission) are carefully processed and studied. We find that at R > 7.5 kpc, the star formation activity is strongly lopsided (SFRNE > SFRSW), and investigate several other properties (H α/FUV, dust temperature, stellar age, and disc stability parameters). We also find that the declining nature of the rotation curve perceived by previous studies is not intrinsic but a combined effect of kinematical asymmetries at R = 7.5–16 kpc. This is likely the consequence of star formation triggered outflow. The mass distribution and the time-scale of the anomalous gas also imply that it originates from gas outflow, which is perhaps caused by galaxy–galaxy interaction considering the crowded environment of NGC 253.

An Eclipsing Binary Comprising Two Active Red Stragglers of Identical Mass and Synchronized Rotation: A Post-mass-transfer System or Just Born That Way?

We report the discovery of 2M0056–08 as an equal-mass eclipsing binary (EB), comprising two red straggler stars (RSSs) with an orbital period of 33.9 days. Both stars have masses of ≈1.419 M ⊙, identical to within 0.2%. Both stars appear to be in the early red-giant phase of evolution; however, they are far displaced to cooler temperatures and lower luminosities compared to standard stellar models. The broadband spectral energy distribution shows NUV excess and X-ray emission, which is consistent with chromospheric and coronal emission from magnetically active stars. Indeed, the stars rotate more rapidly than typical red giants and they evince light-curve modulations due to spots. These modulations also reveal the stars to be rotating synchronously with one another. There is evidence for excess FUV emission and long-term modulations in radial velocities, although it is not clear if they are also attributable to magnetic activity or if they reveal a tertiary companion. Stellar evolution models that are modified to account for the effects of spots can reproduce the observed radii and temperatures of the RSSs. If the system possesses a white dwarf tertiary, then mass-transfer scenarios could explain the manner by which the stars came to possess such remarkably identical masses and by which they came to be synchronized. However, if the stars are presumed to have been formed as identical twins and they then managed to become tidally synchronized as they evolved toward the red-giant branch, then all of the features of the system can be explained via activity effects without requiring a complex dynamical history.

Smallest Scale Clumpy Star Formation in Stephan’s Quintet Revealed from UV and IR Imaging

The spatial distribution and physical sizes of star-forming clumps at the smallest scales provide valuable information on hierarchical star formation (SF). In this context, we report the sites of ongoing SF at ∼120 pc along the interacting galaxies in Stephan’s Quintet compact group using AstroSat-UVIT and JWST data. Since ultraviolet radiation is a direct tracer of recent SF, we identified star-forming clumps in this compact group from the FUV imaging which we used to guide us to detect star-forming regions on JWST IR images. The FUV imaging reveals star-forming regions within which we detect smaller clumps from the higher spatial resolution images of JWST, likely produced by PAH molecules and dust ionized by FUV emission from young massive stars. This analysis reveals the importance of FUV imaging data in identifying star-forming regions in the highest spatial resolution IR imaging available.

Understanding High-Energy (UV and X-ray) Emission from AGB Stars—Episodic Accretion in Binary Systems

X-ray surveys of UV-emitting AGB stars show that ∼40% of objects with FUV emission and GALEX FUV/NUV flux ratio Rfuv/nuv ∼> 0.2 (fuvAGB stars) have variable X-ray emission characterized by very high temperatures (Tx∼35–160 MK) and luminosities (Lx∼0.002–0.2 L⊙), indicating the presence of accretion associated with a close binary companion. However, the UV-emitting AGB star population is dominated by objects with Rfuv/nuv ≲ 0.06 (nuvAGB stars), and we do not know whether the UV emission from these is intrinsic to the AGB star or extrinsic (i.e., due to binarity). In order to help distinguish between intrinsic and extrinsic models of the puzzling high-energy emission of cool AGB stars, we report results from two studies—(i) XMM-Newton X-observations of two nuvAGB stars, and (ii) simple chromosphere modeling. In study (i), we detect the one which has the lower FUV/NUV ratio, with a total Lx = 0.00027 L⊙, and a spectrum best fitted with a dominant component at Tx∼10 MK, most likely coronal emission from a main-sequence companion. Therefore, a significant fraction of nuvAGB stars may also be binaries with active, but weak accretion. Study (ii) shows that chromospheres with temperatures of ∼10,000 K can produce Rfuv/nuv≲0.06; higher ratios require hotter gas, implying active accretion.

Assimilative Mapping of Auroral Electron Energy Flux Using SSUSI Lyman‐Birge‐Hopfield (LBH) Emissions

AbstractFar ultraviolet (FUV) imaging of the aurora from space provides great insight into the dynamic coupling of the atmosphere, ionosphere, and magnetosphere on global scales. To gain a quantitative understanding of these coupling processes, the global distribution of auroral energy flux is required, but the inversion of FUV emission to derive precipitating auroral particles' energy flux is not straightforward. Furthermore, the spatial coverage of FUV imaging from Low Earth Orbit (LEO) altitudes is often insufficient to achieve global mapping of this important parameter. This study seeks to fill these gaps left by the current geospace observing system using a combination of data assimilation and machine learning techniques. Specifically, this paper presents a new data‐driven modeling approach to create instantaneous, global assimilative mappings of auroral electron total energy flux from Lyman‐Birge‐Hopfield (LBH) emission data from the Defense Meteorological System Program (DMSP) Special Sensor Ultraviolet Spectrographic Imager (SSUSI). We take a two‐step approach; the creation of assimilative maps of LBH emission using optimal interpolation, followed by the conversion to energy flux using a neural network model trained with conjunction observations of in‐situ auroral particles and LBH emission from the DMSP Special Sensor J and SSUSI instruments. The paper demonstrates the feasibility of this approach with a model prototype built with DMSP data from 17 February 2014 to 23 February 2014. This study serves as a blueprint for a future comprehensive data‐driven model of auroral energy flux that is complementary to traditional inversion techniques to take advantage of FUV imaging from LEO platforms for global assimilative mapping of auroral energy flux.

The Environments of CO Cores and Star Formation in the Dwarf Irregular Galaxy WLM

Abstract The low metallicities of dwarf irregular galaxies (dIrr) greatly influence the formation and structure of molecular clouds. These clouds, which consist primarily of H2, are typically traced by CO, but low-metallicity galaxies are found to have little CO despite ongoing star formation. In order to probe the conditions necessary for CO core formation in dwarf galaxies, we have used the catalog of Rubio et al. for CO cores in WLM, a Local Group dwarf with an oxygen abundance that is 13% of solar. Here we aim to characterize the galactic environments in which these 57 CO cores formed. We grouped the cores together based on proximity to each other and strong FUV emission, examining properties of the star-forming region enveloping the cores and the surrounding environment where the cores formed. We find that high H i surface density does not necessarily correspond to higher total CO mass, but regions with higher CO mass have higher H i surface densities. We also find the cores in star-forming regions spanning a wide range of ages show no correlation between age and CO core mass, suggesting that the small size of the cores is not due to fragmentation of the clouds with age. The presence of CO cores in a variety of different local environments, along with the similar properties between star-forming regions with and without CO cores, leads us to conclude that there are no obvious environmental characteristics that drive the formation of these CO cores.

A UVIT Look at Star Formation in Nearby Interacting/Merging Galaxies

AbstractThe interactions and mergers of gas rich galaxies are known to produce star formation which often leads to nuclear activity as well. The star formation is ideally mapped using FUV and NUV emission, since UV traces star formation for longer timescales compared to Hα emission. It is also emitted over a broader range of stellar masses in galaxies. In this study we present FUV and NUV observations of merging and interacting galaxies in our nearby universe conducted using the UVIT. We present the example of a merging system MRK212 that has dual AGN and the triple AGN system NGC7733-7734. The UV emission is associated with the tidal arms, individual nuclei, resonance rings, nuclear spirals as well as AGN/stellar feedback. We also find that radio emission is often closely associated with the UV emission, arising from both star formation as well as AGN activity, and perhaps kpc-scale AGN feedback. We find that a comparison of optical IFU imaging with FUV in NGC7733-7734 reveals unique properties associated with the interaction including the third AGN buried in a tidal arm.

Dorado and its member galaxies II: A UVIT picture of the NGC 1533 substructure

Dorado is a nearby (17.69 Mpc) strongly evolving galaxy group in the Southern Hemisphere. We are investigating the star formation in this group. This paper provides a FUV imaging of NGC 1533, IC 2038 and IC 2039, which form a substructure, south west of the Dorado group barycentre. FUV CaF2-1 UVIT-Astrosat images enrich our knowledge of the system provided by GALEX. In conjunction with deep optical wide-field, narrow-band H\(\alpha \) and 21-cm radio images we search for signatures of the interaction mechanisms looking in the FUV morphologies and derive the star formation rate. The shape of the FUV luminosity profile suggests the presence of a disk in all three galaxies. FUV emission is detected out to the optical size for IC 2038, and in compact structures corresponding to H\(\alpha \) and H ii bright features in NGC 1533. A faint FUV emission, without an optical counterpart, reminiscent of the H i structure that surrounds the outskirts of NGC 1533 and extends up to IC 2038/2039, is revealed above the local background noise.

Discovery of an Extremely Short Duration Flare from Proxima Centauri Using Millimeter through Far-ultraviolet Observations

Abstract We present the discovery of an extreme flaring event from Proxima Cen by the Australian Square Kilometre Array Pathfinder (ASKAP), Atacama Large Millimeter/submillimeter Array (ALMA), Hubble Space Telescope (HST), Transiting Exoplanet Survey Satellite (TESS), and the du Pont Telescope that occurred on 2019 May 1. In the millimeter and FUV, this flare is the brightest ever detected, brightening by a factor of >1000 and >14,000 as seen by ALMA and HST, respectively. The millimeter and FUV continuum emission trace each other closely during the flare, suggesting that millimeter emission could serve as a proxy for FUV emission from stellar flares and become a powerful new tool to constrain the high-energy radiation environment of exoplanets. Surprisingly, optical emission associated with the event peaks at a much lower level with a time delay. The initial burst has an extremely short duration, lasting for <10 s. Taken together with the growing sample of millimeter M dwarf flares, this event suggests that millimeter emission is actually common during stellar flares and often originates from short burst-like events.

Accretion-powered Outflows in AGB Stars

AbstractOne of the big challenges for 21st century stellar astrophysics is the impact of binary interactions on stellar evolution. Such interactions are believed to play a key role in the death throes of 1-8 M⊙ stars, as they evolve from the AGB stars into Planetary Nebulae. X-ray surveys of UV-emitting AGB stars show that ∼40% of objects with FUV emission and GALEX FUV/NUV flux ratios ≳0.2 have variable X-ray emission characterized by very high temperatures (Tx∼35-160 MK) and luminosities (Lx∼0.002-0.2L⊙). We hypothesize that such AGB stars have accretion and (accretion-powered) outflows associated with a close binary companion. UV spectroscopy with HST/STIS of our brightest object (Y Gem) shows the presence of infalling and outflowing gas, providing direct kinematic confirmation of this hypothesis. However, the UV-emitting AGB star population is dominated by objects with little or no FUV emission, and we do not know whether the UV emission from these is intrinsic to the AGB star or extrinsic (i.e., due to binarity). Here we present the first results from a large grid of simple chromospheric models to help discriminate between the intrinsic and extrinsic mechanisms of UV emission for AGB stars.

A Catalog of Holes and Shells in the Interstellar Medium of the LITTLE THINGS Dwarf Galaxies

Abstract We present a catalog of holes and shells in the neutral atomic hydrogen (H i) of 41 gas-rich dwarf galaxies in LITTLE THINGS (Local Irregulars That Trace Luminosity Extremes, The H i Nearby Galaxy Survey). We analyzed their properties as part of an investigation into the relation between star formation and structures and kinematics in the H i of small galaxies. We confirmed 306 holes between 38 pc (our resolution limit) and 2.3 kpc, with expansion velocities up to 30 km s−1. The global star formation rates (SFRs) measured by Hα and far-UV (FUV) emission are consistent with those estimated from the energy required to create the cataloged holes in our sample. Although we found no obvious correlation between global SFRs and the H i surface and volume porosities of our sample, two of the four galaxies with the lowest porosity and the two galaxies with the highest porosity have no recent star formation as measured by Hα and FUV emission.

A catalogue of 108 extended planetary nebulae observed by GALEX

We present the ultraviolet (UV) imaging observation of planetary nebulae (PNe) using archival data of Galaxy Evolution Explorer (GALEX). We found 358 PNe detected by GALEX in near-UV (NUV). We have compiled a catalogue of 108 extended PNe with sizes greater than 8 00 and provided the angular diameters for all the 108 extended PNe in NUV and 28 in FUV from the GALEX images considering 3{\sigma} surface brightness level above the background. Of the 108 PNe, 74 are elliptical, 24 are circular and 10 are bipolar in NUV with most being larger in the UV than in the radio, H{\alpha} or optical. We derived luminosities for 33 PNe in FUV (L FUV ) and 89 PNe in NUV (L NUV ) and found that most of the sources are very bright in UV. The FUV emission of the GALEX band includes contribution from prominent emission lines N IV] (1487 {\AA}), CIV (1550 {\AA}), and O III] (1661 {\AA}) whereas the NUV emission includes C III] (1907 {\AA}) and C II (2325 {\AA}) for PNe of all excitation classes. The other emission lines seen in low excitation PNe are O IV] (1403 {\AA}) and N III(1892 {\AA}) in FUV, and O II (2470 {\AA}) and Mg II (2830 {\AA}) in NUV. Similarly the emission lines O V (1371 {\AA}) and He II (1666 {\AA}) strongly contribute in FUV for high and medium excitation PNe but not for low excitation PNe. A mixture of other emission lines seen in all excitation PNe. We have also provided images of 34 PNe in NUV and 9 PNe in FUV.

Far-ultraviolet Activity Levels of F, G, K, and M Dwarf Exoplanet Host Stars

We present a survey of far-ultraviolet (FUV; 1150 - 1450 Ang) emission line spectra from 71 planet-hosting and 33 non-planet-hosting F, G, K, and M dwarfs with the goals of characterizing their range of FUV activity levels, calibrating the FUV activity level to the 90 - 360 Ang extreme-ultraviolet (EUV) stellar flux, and investigating the potential for FUV emission lines to probe star-planet interactions (SPIs). We build this emission line sample from a combination of new and archival observations with the Hubble Space Telescope-COS and -STIS instruments, targeting the chromospheric and transition region emission lines of Si III, N V, C II, and Si IV. We find that the exoplanet host stars, on average, display factors of 5 - 10 lower UV activity levels compared with the non-planet hosting sample; this is explained by a combination of observational and astrophysical biases in the selection of stars for radial-velocity planet searches. We demonstrate that UV activity-rotation relation in the full F - M star sample is characterized by a power-law decline (with index $\alpha$ ~ -1.1), starting at rotation periods >~3.5 days. Using N V or Si IV spectra and a knowledge of the star's bolometric flux, we present a new analytic relationship to estimate the intrinsic stellar EUV irradiance in the 90 - 360 Ang band with an accuracy of roughly a factor of ~2. Finally, we study the correlation between SPI strength and UV activity in the context of a principal component analysis that controls for the sample biases. We find that SPIs are not a statistically significant contributor to the observed UV activity levels.

The Evolution of Protoplanetary Disks: Probing the Inner Disk of Very Low Accretors

Abstract We report FUV, optical, and NIR observations of three T Tauri stars in the Orion OB1b subassociation with Hα equivalent widths consistent with low or absent accretion and various degrees of excess flux in the mid-infrared. We aim to search for evidence of gas in the inner disk in HST Advanced Camera for Surveys/Solar Blind Channel spectra, and to probe the accretion flows onto the star using Hα and He i λ10830 in spectra obtained at the Magellan and SOAR telescopes. At the critical age of 5 Myr, the targets are at different stages of disk evolution. One of our targets is clearly accreting, as shown by redshifted absorption at freefall velocities in the He i line and wide wings in Hα; however, a marginal detection of FUV H2 suggests that little gas is present in the inner disk, although the spectral energy distribution indicates that small dust still remains close to the star. Another target is surrounded by a transitional disk, with an inner cavity in which little sub-micron dust remains. Still, the inner disk shows substantial amounts of gas, accreting onto the star at a probably low but uncertain rate. The third target lacks both a He i line or FUV emission, consistent with no accretion or inner gas disk; its very weak IR excess is consistent with a debris disk. Different processes occurring in targets with ages close to the disk dispersal time suggest that the end of the accretion phase is reached in diverse ways.

UVIT view of ram-pressure stripping in action: star formation in the stripped gas of the GASP jellyfish galaxy JO201 in Abell 85

Jellyfish are cluster galaxies that experience strong ram-pressure effects that strip their gas. Their H$\alpha$ images reveal ionized gas tails up to 100 kpc, which could be hosting ongoing star formation. Here we report the ultraviolet (UV) imaging observation of the jellyfish galaxy JO201 obtained at a spatial resolution $\sim$ 1.3 kpc. The intense burst of star formation happening in the tentacles is the focus of the present study. JO201 is the "UV-brightest cluster galaxy" in Abell 85 ($z \sim$ 0.056) with knots and streams of star formation in the ultraviolet. We identify star forming knots both in the stripped gas and in the galaxy disk and compare the UV features with the ones traced by H$\alpha$ emission. Overall, the two emissions remarkably correlate, both in the main body and along the tentacles. Similarly, also the star formation rates of individual knots derived from the extinction-corrected FUV emission agree with those derived from the H$\alpha$ emission and range from $\sim$ 0.01 -to- 2.07 $M_{\odot} \, yr^{-1}$. The integrated star formation rate from FUV flux is $\sim$ 15 $M_{\odot} \, yr^{-1}$. The unprecedented deep UV imaging study of the jellyfish galaxy JO201 shows clear signs of extraplanar star-formation activity due to a recent/ongoing gas stripping event.

Star formation associated with neutral hydrogen in the outskirts of early-type galaxies

About 20 percent of all nearby early-type galaxies ($M_{\star} \gtrsim 6 \times 10^{9}$ M$_{\odot}$) outside the Virgo cluster are surrounded by a disc or ring of low-column-density neutral hydrogen (HI) gas with typical radii of tens of kpc, much larger than the stellar body. In order to understand the impact of these gas reservoirs on the host galaxies, we analyse the distribution of star formation out to large radii as a function of HI properties using GALEX UV and SDSS optical images. Our sample consists of 18 HI-rich galaxies as well as 55 control galaxies where no HI has been detected. In half of the HI-rich galaxies the radial UV profile changes slope at the position of the HI radial profile peak. To study the stellar populations, we calculate the FUV-NUV and UV-optical colours in two apertures, 1-3 and 3-10 R$_{eff}$ . We find that HI -rich galaxies are on average 0.5 and 0.8 mag bluer than the HI-poor ones, respectively. This indicates that a significant fraction of the UV emission traces recent star formation and is associated with the HI gas. Using FUV emission as a proxy for star formation, we estimate the integrated star formation rate in the outer regions (R > 1R$_{eff}$) to be on average $6 \times 10^{-3}$ M$_{\odot}$ yr$^{-1}$ for the HI-rich galaxies. This rate is too low to build a substantial stellar disc and, therefore, change the morphology of the host. We find that the star formation efficiency and the gas depletion time are similar to those at the outskirts of spirals.

Searching for FUV line emission from 107 K gas in massive elliptical galaxies and galaxy clusters as a tracer of turbulent velocities

Non-thermal pressure from turbulence and bulk flows is a fundamental ingredient in hot gaseous halos, and in the intracluster medium it will be measured through emission line kinematics with calorimeters on future X-ray spacecraft. In this paper we present a complementary method for measuring these effects, using forbidden FUV emission lines of highly ionized Iron which trace $10^7$ K gas. The brightest of these is [Fe XXI] $\lambda$1354.1. We search for these lines in archival HST-COS spectra from the well-known elliptical galaxies M87 and NGC4696, which harbor large reservoirs of $10^7$ K gas. We report a 2.2$\sigma$ feature which we attribute to [Fe XXI] from a filament in M87, and positive residuals in the nuclei of M87 and NGC4696, for which the 90\% upper limits on the line flux are close to the predicted fluxes based on X-ray observations. In a newer reduction of the data from the Hubble Spectroscopic Legacy Archive, these limits become tighter and the [Fe XXI] feature reaches a formal significance of 5.3$\sigma$, neglecting uncertainty in fitting the continuum. Using our constraints, we perform emission measure analysis, constraining the characteristic path length and column density of the $\sim10^7$ K gas. We also examine several sightlines towards filaments or cooling flows in other galaxy clusters, for which the fraction of gas at $10^7$ K is unknown, and place upper limits on its emission measure in each case. A medium-resolution HST-COS observation of the M87 filament for $\sim$10 orbits would confirm our detection of [Fe XXI] and measure its width.

Recent star formation in the Hi dominated outer regions of early-type galaxies

AbstractContextAccording to the ATLAS3D project, about 20 percent of all nearby early-type galaxies (D < 42 Mpc; MK < -21.5 mag; stellar mass Mstars ≳ 6 × 109 M⊙) outside clusters are surrounded by a disc or ring of low-column-density neutral hydrogen (Hi) gas with typical radii of tens of kpc, much larger than the stellar body.AimsOur aim is to understand the impact of these gas systems on the host galaxies, in particular, whether there is any recent star formation related to the Hi and effect of recent star formation on the host early-type galaxies.Methods and sampleWe analyse the distribution of star formation out to large radii by using resolved Hi images together with UV and optical images. We calculate the UV-UV and UV-optical colours in two apertures, 1-3 and 3-10 Reff. Using FUV emission as a proxy for star formation, we also estimate the integrated star formation rate in the outer regions. Our sample consists of 18 Hi-rich galaxies as well as 55 control galaxies where no Hi has been detected. We select the control sample galaxies to match the Hi-rich galaxies in stellar mass, environment, distance and stellar kinematics.ResultsIn half of the Hi-rich galaxies the radial UV profile changes slope at the position of the Hi radial profile peak. We find that the FUV-NUV and UV-optical colours in the first and second apertures of the Hi-rich galaxies are on average 0.5 and 0.8 mag bluer than the Hi-poor ones, respectively. We also find that the Hi-rich early-type galaxies have colour gradients that are almost 2 times stronger than the Hi-poor ones. we estimate the integrated star formation rate in the outer regions (R > 1 Reff) to be on average ~ 6.1×10−3 M⊙ yr−1 for the Hi-rich galaxies. We find that the gas depletion time in the outermost region (3-10 Reff) is ~ 80 Gyrs, which is similar to that estimated for the outskirts of spirals.ConclusionsStudying the stellar populations in early type galaxies with and without Hi, we find that galaxies with Hi generally show UV and UV-Optical colours in the outer parts that are bluer than those of early-type galaxies without Hi. This shows that the Hi is actively involved in recent star formation. The star formation rate in the outer regions is too low to build a stellar disc, and therefore change the morphology of the host even when integrated over several Gyrs. Star formation in outermost regions does not depend on the type of the galaxies.

THE EVOLUTION OF INNER DISK GAS IN TRANSITION DISKS

Investigating the molecular gas in the inner regions of protoplanetary disks provides insight into how the molecular disk environment changes during the transition from primordial to debris disk systems. We conduct a small survey of molecular hydrogen (H$_2$) fluorescent emission, using 14 well-studied Classical T Tauri stars at two distinct dust disk evolutionary stages, to explore how the structure of the inner molecular disk changes as the optically thick warm dust dissipates. We simulate the observed HI-Lyman $\alpha$-pumped H$_2$ disk fluorescence by creating a 2D radiative transfer model that describes the radial distributions of H$_{2}$ emission in the disk atmosphere and compare these to observations from the Hubble Space Telescope. We find the radial distributions that best describe the observed H$_2$ FUV emission arising in primordial disk targets (full dust disk) are demonstrably different than those of transition disks (little-to-no warm dust observed). For each best-fit model, we estimate inner and outer disk emission boundaries (r$_{in}$ and r$_{out}$), describing where the bulk of the observed H$_2$ emission arises in each disk, and we examine correlations between these and several observational disk evolution indicators, such as n$_{13-31}$, r$_{in,CO}$, and the mass accretion rate. We find strong, positive correlations between the H$_2$ radial distributions and the slope of the dust SED, implying the behavior of the molecular disk atmosphere changes as the inner dust clears in evolving protoplanetary disks. Overall, we find that H$_2$ inner radii are $\sim$4 times larger in transition systems, while the bulk of the H$_2$ emission originates inside the dust gap radius for all transitional sources.