Recent Star Formation Activity Research Articles

JWST MIRI and NIRCam observations of NGC 891 and its circumgalactic medium

We present new JWST observations of the nearby, prototypical edge-on, spiral galaxy NGC 891. The northern half of the disk was observed with NIRCam in its F150W and F277W filters. Absorption is clearly visible in the mid-plane of the F150W image, along with vertical dusty plumes that closely resemble the ones seen in the optical. A $ kpc ^2$ area of the lower circumgalactic medium (CGM) was mapped with MIRI F770W at 12 pc scales. Thanks to the sensitivity and resolution of JWST, we detect dust emission out to $ 4$ kpc from the disk, in the form of filaments, arcs, and super-bubbles. Some of these filaments can be traced back to regions with recent star formation activity, suggesting that feedback-driven galactic winds play an important role in regulating baryonic cycling. The presence of dust at these altitudes raises questions about the transport mechanisms at play and suggests that small dust grains are able to survive for several tens of million years after having been ejected by galactic winds in the disk-halo interface. We lay out several scenarios that could explain this emission: dust grains may be shielded in the outer layers of cool dense clouds expelled from the galaxy disk, and/or the emission comes from the mixing layers around these cool clumps where material from the hot gas is able to cool down and mix with these cool cloudlets. This first set of data and upcoming spectroscopy will be very helpful to understand the survival of dust grains in energetic environments, and their contribution to recycling baryonic material in the mid-plane of galaxies.

Molecular Gas Kinematics in Local Early-Type Galaxies with ALMA

Local early-type galaxies (ETGs) are mostly populated by old stars, with little or no recent star formation activity. For this reason, they have historically been believed to be essentially devoid of cold gas, which is the fuel for the formation of new stars. Over the past two decades, however, increasingly-sensitive instrumentation observing the sky at (sub-)millimetre wavelengths has revealed the presence of significant amounts of cold molecular gas in the hearts of nearby ETGs. The unprecedented capabilities offered by the Atacama Large Millimeter/submillimeter Array (ALMA), in particular, have allowed us to obtain snapshots of the central regions of these ETGs with unprecedented detail, mapping this gas with higher sensitivity and resolution than ever before possible. Studies of the kinematics of the observed cold gas reservoirs are crucial for galaxy formation and evolution theories, providing, e.g., constraints on the fundamental properties and fuelling/feedback processes of super-massive black holes (SMBHs) at the centre of these galaxies. In this brief review, we summarise what the first 10 years of ALMA observations have taught us about the distribution and kinematics of the cold molecular gas component in nearby ellipticals and lenticulars.

Optical- and UV-continuum Morphologies of Compact Radio Source Hosts

We present the first systematic search for UV signatures from radio source-driven active galactic nuclei (AGN) feedback in Compact Steep Spectrum (CSS) radio galaxies. Owing to their characteristic sub-galactic jets (1–20 kpc projected linear sizes), CSS hosts are excellent laboratories for probing galaxy scale feedback via jet-triggered star formation. The sample consists of seven powerful CSS galaxies, and two galaxies host to radio sources >20 kpc as the control, at low to intermediate redshifts (z < 0.6). Our new Hubble Space Telescope images show extended UV continuum emission in six out of seven CSS galaxies, with five CSS hosts exhibiting UV knots cospatial and aligned along the radio-jet axis. Young (≲ 10 Myr), massive (≳ 5 M ⊙) stellar populations are likely to be the dominant source of the blue excess emission in radio galaxies at these redshifts. Hence, the radio-aligned UV regions could be attributed to jet-induced starbursts. Lower near-UV star formation rates compared to other indicators suggest low scattered AGN light contribution to the observed UV. Dust attenuation of UV emission appears unlikely from high internal extinction correction estimates in most sources. Comparison with evolutionary synthesis models shows that our observations are consistent with recent (∼1−8 Myr old) star-forming activity likely triggered by current or an earlier episode of radio emission, or by a confined radio source that has frustrated growth, due to a dense environment. While follow-up spectroscopic and polarized light observations are needed to constrain the activity-related components in the observed UV, the detection of jet-induced star formation is a confirmation of an important prediction of the jet feedback paradigm.

What drives the wheels of evolution in NGC 1512?

Context. Environmental and secular processes play a pivotal role in the evolution of galaxies. These can be external processes such as interactions or internal processes linked to the action of bar, bulge, and spiral structures. Ongoing star formation in spiral galaxies can be affected by these processes. By studying the star formation progression in the galaxy, we can gain insights into the role of different processes that regulate the overall evolution of a galaxy. Aims. The ongoing interaction between the barred-spiral galaxy NGC 1512 and its satellite NGC 1510 offers an opportunity to investigate how galactic interactions and the presence of a galactic bar influence the evolution of NGC 1512. We aim to understand the recent star formation activity in the galaxy pair and thus gain insight into the evolution of NGC 1512. Methods. The UltraViolet Imaging Telescope (UVIT) on board AstroSat enables us to characterise the star-forming regions in the galaxy with a superior spatial resolution of ∼85 pc in the galaxy rest frame. We identified and characterised 175 star-forming regions in the UVIT far-ultraviolet (FUV) image of NGC 1512 and correlated with the neutral hydrogen (H I) distribution. Extinction correction was applied to the estimated photometric magnitude. We traced the star-forming spiral arms of the galaxy and studied the star formation properties across the galaxy in detail. Results. We detect localised regions of star-formation enhancement and distortions in the galactic disc. We find this to be consistent with the distribution of H I in the galaxy. This is evidence of past and ongoing interactions affecting the star formation properties of the galaxy. We studied the properties of the inner ring. We find that the regions of the inner ring show maximum star-formation-rate density (log(SFRDmean[M⊙ yr−1 kpc−2]) ∼ −1.7) near the major axis of the bar, hinting at a possible crowding effect in these regions. The region of the bar in the galaxy is also depleted of UV emission. This absence suggests that the galactic bar may have played an active role in the redistribution of gas and quenching of star formation inside the identified bar region. We therefore suggest that both secular and environmental factors might be influencing the evolution of NGC 1512.

Multiscale stellar associations across the star formation hierarchy in PHANGS–HST nearby galaxies: methodology and properties

ABSTRACT We develop a method to identify and determine the physical properties of stellar associations using Hubble Space Telescope (HST) NUV−U−B−V−I imaging of nearby galaxies from the Physics at High Angular Resolution in Nearby GalaxieS with the Hubble Space Telescope (PHANGS–HST) survey. We apply a watershed algorithm to density maps constructed from point source catalogues Gaussian smoothed to multiple physical scales from 8 to 64 pc. We develop our method on two galaxies that span the distance range in the PHANGS–HST sample: NGC 3351 (10 Mpc) and NGC 1566 (18 Mpc). We test our algorithm with different parameters such as the choice of detection band for the point source catalogue (NUV or V), source density image filtering methods, and absolute magnitude limits. We characterize the properties of the resulting multiscale associations, including sizes, number of tracer stars, number of associations, and photometry, as well as ages, masses, and reddening from spectral energy distribution fitting. Our method successfully identifies structures that occupy loci in the UBVI colour–colour diagram consistent with previously published catalogues of clusters and associations. The median ages of the associations increase from log(age/yr) = 6.6 to log(age/yr) = 6.9 as the spatial scale increases from 8 to 64 pc for both galaxies. We find that the youngest stellar associations, with ages &amp;lt;3 Myr, indeed closely trace H ii regions in H α imaging, and that older associations are increasingly anticorrelated with the H α emission. Owing to our new method, the PHANGS–HST multiscale associations provide a far more complete census of recent star formation activity than found with previous cluster and compact association catalogues.

Multiply lensed star forming clumps in the A521-sys1 galaxy at redshift 1

ABSTRACT We study the population of star-forming clumps in A521-sys1, a $z=1.04$ system gravitationally lensed by the foreground ($z=0.25$) cluster Abell 0521. The galaxy presents one complete counter-image with a mean magnification of $\rm \mu \sim 4$ and a wide arc containing two partial images of A521-sys1 with magnifications reaching $\mu &amp;gt; 20$, allowing the investigations of clumps down to scales of ${\rm R}_{\rm eff} &amp;lt; 50$ pc. We identify 18 unique clumps with a total of 45 multiple images. Intrinsic sizes and UV magnitudes reveal clumps with elevated surface brightnesses comparable to similar systems at redshifts $z\gtrsim 1.0$. Such clumps account for ${\sim}40\,\rm{per\ cent}$ of the galaxy UV luminosity implying a significant fraction of the recent star-formation activity is taking place there. Clump masses range from 106 to $10^9\,{\rm M}_\odot$ and sizes from tens to hundreds of parsec resulting in mass surface densities from 10 to $10^3\,{\rm M}_\odot\,{\rm pc}^{-2}$ with a median of ${\sim}10^2\,{\rm M}_\odot\,{\rm pc}^{-2}$. These properties suggest that we detect star formation taking place across a wide range of scale from cluster aggregates to giant star-forming complexes. We find ages of less than 100 Myr consistent with clumps being observed close to their natal region. The lack of galactocentric trends with mass, mass density, or age and the lack of old migrated clumps can be explained either by dissolution of clumps after few ∼100 Myr or by stellar evolution making them fall below the detectability limits of our data.

Recent star formation history of the dwarf irregular galaxy Leo A

Context.Leo A is a gas-rich dwarf irregular galaxy of low stellar mass and metallicity. Its star formation history extends up to ∼10 Gyr. A few prominent H II regions indicate that star formation processes are active in the galaxy to this day. In the present epoch, Leo A is a highly isolated galaxy, which allows for it to be studied in a self-propagating star formation regime. Aims. Our aim is to study the recent star formation history of the Leo A galaxy over the last ∼300 Myr. Methods. We analysed populations of main sequence (MS) and blue helium-burning (BHeB – ‘blue loop’) stars using multi-colour photometry data obtained with the Subaru/Suprime-Cam (B, V, R, I, Hα) and the Hubble Space Telescope (HST) Advanced Camera for Surveys (F475W and F814W) cameras. We made use of colour-magnitude diagrams and stellar isochrones to determine individual ages of the BHeB stars, which enabled us to study the spatial distribution of these stars in different age bins. In addition, we used Hα emission and H I column density maps to study the relationship between young stellar populations and the surrounding interstellar medium. Results. We found that significant differences are visible in the morphology of stellar distributions represented by the BHeB and MS stars of different ages. We also studied a well-known H I hole in the Leo A galaxy and found that there is a noticeable difference in the surface number density of stars in the western and eastern parts of the hole. The bright young (&lt; 20 Myr) MS stars residing in the western part of the hole indicate the recent star-forming activity in this region after the quiescent period of ∼300 Myr. Furthermore, there is a shock front (prominent in Hα) that closely resembles the shape of the western edge of the H I hole. This shock front could have been formed by the combined stellar feedback from the young MS stars or a Type II supernova located within the H I hole. Additionally, an analysis of the Suprime-Cam photometry in Hα and R passbands enabled us to identify 17 stars with a prominent Hα emission (Hα − R ≤ −0.06) which indicate the presence of early-type (Be, B[e], or A-shell) emission-line stars in the Leo A galaxy.

Observing correlations between dark matter accretion and galaxy growth: II. testing the impact of galaxy mass, star formation indicator, and neighbour colours

ABSTRACT A crucial question in galaxy formation is what role new accretion has in star formation. Theoretical models have predicted a wide range of correlation strengths between halo accretion and galaxy star formation. Previously, we presented a technique to observationally constrain this correlation strength for isolated Milky Way mass galaxies at z ∼ 0.12, based on the correlation between halo accretion and the density profile of neighbouring galaxies. By applying this technique to both observational data from the Sloan Digital Sky Survey and simulation data from the UniverseMachine, where we can test different correlation strengths, we ruled out positive correlations between dark matter accretion and recent star formation activity. In this work, we expand our analysis by (1) applying our technique separately to red and blue neighbouring galaxies, which trace different infall populations, (2) correlating dark matter accretion rates with Dn 4000 measurements as a longer-term quiescence indicator than instantaneous star-formation rates, and (3) analysing higher-mass isolated central galaxies with 1011.0 &amp;lt; M*/M⊙ &amp;lt; 1011.5 out to z ∼ 0.18. In all cases, our results are consistent with non-positive correlation strengths with ≳ 85 per cent confidence, which is most consistent with models where processes such as gas recycling dominate star formation in massive z = 0 galaxies.

Canis Major OB1 stellar group contents revealed by Gaia

ABSTRACT Canis Major OB1 (CMa OB1) is a Galactic stellar association with a very intriguing star-formation scenario. There are more than two dozen known star clusters in its line of sight, but it is not clear which ones are physically associated with CMa OB1. We use a clustering code that employs five-dimensional data from the Gaia DR2 catalogue to identify physical groups and obtain their astrometric parameters and, in addition, we use two different isochrone-fitting methods to estimate the ages of these groups. We find 15 stellar groups with distances between 570 and 1650 pc, including 10 previously known and five new open cluster candidates. Four groups, precisely the youngest ones (&amp;lt; 20 Myr), CMa05, CMa06, CMa07, and CMa08, are confirmed to be part of CMa OB1. We find that CMa08, a new cluster candidate, may be the progenitor cluster of runaway stars. CMa06 coincides with the well-studied CMa R1 star-forming region. While CMa06 is still forming stars, due to the remaining material of the molecular cloud associated with the Sh 2-262 nebula, CMa05, CMa07, and CMa08 seem to be in more evolved stages of evolution, with no recent star-forming activity. The properties of these CMa OB1 physical groups fit well in a monolithic scenario of star formation, with a common formation mechanism, and having suffered multiple episodes of star formation. This suggests that the hierarchical model alone, which explains the populations of other parts of the same association, is not sufficient to explain its whole formation history.

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.

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.

Observing correlations between dark matter accretion and galaxy growth – I. Recent star formation activity in isolated Milky Way-mass galaxies

ABSTRACT The correlation between fresh gas accretion on to haloes and galaxy star formation is critical to understanding galaxy formation. Different theoretical models have predicted different correlation strengths between halo accretion rates and galaxy star formation rates, ranging from strong positive correlations to little or no correlation. Here, we present a technique to observationally measure this correlation strength for isolated Milky Way-mass galaxies with z &amp;lt; 0.123. This technique is based on correlations between dark matter accretion rates and the projected density profile of neighbouring galaxies; these correlations also underlie past work with splashback radii. We apply our technique to both observed galaxies in the Sloan Digital Sky Survey as well as simulated galaxies in the UniverseMachine where we can test any desired correlation strength. We find that positive correlations between dark matter accretion and recent star formation activity are ruled out with $\gtrsim 85{{\ \rm per\ cent}}$ confidence. Our results suggest that star formation activity may not be correlated with fresh accretion for isolated Milky Way-mass galaxies at z = 0 and that other processes, such as gas recycling, dominate further galaxy growth.

H i gas content of SDSS galaxies revealed by ALFALFA: implications for the mass–metallicity relation and the environmental dependence of H i in the local Universe

ABSTRACT The neutral hydrogen (H i) gas is an important barometer of recent star formation and metal enrichment activities in galaxies. I develop a novel statistical method for predicting the H i-to-stellar mass ratio, $f_{\mathrm{H\,{\small I}}}$, of galaxies from their stellar mass and optical colour, and apply it to a volume-limited galaxy sample jointly observed by the Sloan Digital Sky Survey and the Arecibo Legacy Fast ALFA survey. I eliminate the impact of the Malmquist bias against H i-deficient systems on the $f_{\mathrm{H\,{\small I}}}$ predictor by properly accounting for the H i detection probability of each galaxy in the analysis. The best-fitting $f_{\mathrm{H\,{\small I}}}$ predictor, with an estimated scatter of 0.272 dex, provides excellent description to the observed H i mass function. After defining an H i excess parameter as the deviation of the observed $f_{\mathrm{H\,{\small I}}}$ from the expected value, I confirm that there exists a strong secondary dependence of the mass–metallicity relation on H i excess. By further examining the 2D metallicity distribution on the specific star formation rate (sSFR) versus H i excess plane, I show that the metallicity dependence on H i is likely more fundamental than that on sSFR. In addition, I find that the environmental dependence of H i in the local Universe can be effectively described by the cross-correlation coefficient between H i excess and the red galaxy overdensity ρcc = − 0.18. This weak anticorrelation also successfully explains the observed dependence of H i clustering on $f_{\mathrm{H\,{\small I}}}$. My method provides a useful framework for learning H i gas evolution from the synergy between future H i and optical galaxy surveys.

Recovering the origin of the lenticular galaxy NGC3115 using multi-band photometry

AbstractWe perform simultaneous multi-band fitting, using the routine GALFITM, of the galaxy NGC3115, in order to recover the stellar populations of its main components (a bulge, a thin disc and a thick disc). We model 11 bands, from ultraviolet to infrared, in order to take into account the galaxy younger stellar population and the presence of the Active Galactic Nuclei (AGN). We find that the majority of the galaxy baryonic mass belongs to the thick disc, which is also the oldest galaxy component, consistent with results from the literature. Differently from previous works, we find that the bulge has the bluest colour and it is younger than the thick disc, either as a result of recent star formation activity, or AGN feedback, or white dwarf emission in an old stellar population. Finally, we propose that NGC3115 was formed either through a two-phase formation scenario, or via an outside-in quenching of an isolated spiral galaxy, whose thick disc had been heated-up via minor mergers with dwarf satellites.

The VIMOS Public Extragalactic Redshift Survey (VIPERS)

Aims. We analyse the properties of the host galaxies of a [NeV]-selected sample to investigate whether and how they are affected by the AGN. Methods. We have selected a sample of galaxies at 0.62 &lt; z &lt; 1.2 from the VIMOS Public Extragalactic Redshift Survey (VIPERS) and divided it in blue cloud galaxies, red passive galaxies and green valley galaxies using the NUVrK diagram. Within each category, galaxies with AGN activity were identified based on the detection of the high-ionisation [NeV]λ3426 emission line. For each galaxy we derived several properties (stellar age and mass, the (r−K) colour, the [OII] luminosity) and compared them between active and inactive galaxies matched in stellar mass and redshift. Results. We find statistically significant differences in the properties between active and inactive galaxies. These differences imply that the AGN is more often found in galaxies with younger stellar populations and more recent star-forming activity than their parent samples. Interestingly, the AGN identified through the [NeV]λ3426 emission line is not commonly found by traditional AGN-selection techniques based on shallow X-ray data, mid-IR colours, and classical line diagnostic diagrams, and might thus reveal a specific evolutionary phase. The spectral analysis reveals a sub-set of AGN within the blue cloud that has spectral signatures implying a sudden suppression of star formation activity similar to post-starburst galaxies. Conclusion. Using the rich dataset of the large VIPERS sample we identify a novel class of active post-starburst galaxies that would be missed by traditional selection techniques. These galaxies belong to the blue cloud, but their star-formation activity has been recently suppressed, possibly by the AGN identified through the presence of the [NeV]λ3426 emission line in their spectra. Our results support the idea that AGN feedback may be responsible for halting star-formation in active blue galaxies and for their transition into the red sequence, at least in the 0.6–1.2 redshift range and for stellar masses greater than 5 × 1010 ℳ⊙. Our results are based on a complete spectroscopic sample and limited by the [NeV] observability, and the AGN can be variable and with a relatively short duty cycle. Considering this, AGN feedback that makes blue galaxies quickly transition to the red sequence may be even more common than previously believed.

Color Dispersion as an Indicator of Stellar Population Complexity: Insights from the Pixel Color–Magnitude Diagrams of 32 Bright Galaxies in Abell 1139 and Abell 2589

Abstract We investigate the properties of bright galaxies of various morphological types in Abell 1139 and Abell 2589, using pixel color–magnitude diagram (pCMD) analysis. The sample contains 32 galaxies brighter than M r = −21.3 mag with spectroscopic redshifts, which are deeply imaged in the g and r bands using the MegaCam mounted on the Canada–France–Hawaii Telescope. After masking contaminants with two-step procedures, we examine how the detailed properties in the pCMDs depend on galaxy morphology and infrared color. The mean g − r color as a function of surface brightness (μ r ) in the pCMD of a galaxy shows good performance in distinguishing between early- and late-type galaxies, but it is not perfect because of the similarity between elliptical galaxies and bulge-dominated spiral galaxies. On the other hand, the g − r color dispersion as a function of μ r works better. We find that the best set of parameters for galaxy classification is a combination of the minimum color dispersion at μ r ≤ 21.2 mag arcsec−2 and the maximum color dispersion at 20.0 ≤ μ r ≤ 21.0 mag arcsec−2; the latter reflects the complexity of stellar populations at the disk component in a typical spiral galaxy. Finally, the color dispersion measurements of an elliptical galaxy appear to be correlated with the Wide-field Infrared Survey Explorer infrared color ([4.6]–[12]). This indicates that the complexity of stellar populations in an elliptical galaxy is related to its recent star formation activities. From this observational evidence, we infer that gas-rich minor mergers or gas interactions may have usually occurred during the recent growth of massive elliptical galaxies.

The host of the Type I SLSN 2017egm

Context. Type I superluminous supernova (SLSN) host galaxies are predominantly low-metallicity, highly star-forming (SF) dwarfs. One of the current key questions is whether Type I SLSNe can only occur in such environments and hosts. Aims. Here we present an integral-field study of the massive, high-metallicity spiral NGC 3191, the host of SN 2017egm, the closest Type I SLSN known to date. We use data from PMAS/CAHA and the public MaNGA survey to shed light on the properties of the SLSN site and the origin of star formation in this non-starburst spiral galaxy. Methods. We map the physical properties of different H ii regions throughout the galaxy and characterise their stellar populations using the STARLIGHT fitting code. Kinematical information allows us to study a possible interaction with its neighbouring galaxy as the origin of recent star formation activity which could have caused the SLSN. Results. NGC 3191 shows intense star formation in the western part with three large SF regions of low metallicity. Taking only the properties of emitting gas, the central regions of the host have a higher metallicity, a lower specific star formation rate, and lower ionisation. Modelling the stellar populations gives a different picture: the SLSN region has two dominant stellar populations with different ages, the younger one with an age of 2–10 Myr and lower metallicity, likely the population from which the SN progenitor originated. Emission line kinematics of NGC 3191 show indications of interaction with its neighbour MCG+08-19-017 at ~45 kpc, which might be responsible for the recent starburst. In fact, this galaxy pair has hosted a total of four SNe, 1988B (Type Ia), SN 2003ds (Type Ic in MCG+08-19-017), PTF10bgl (Type II), and 2017egm, underlying the enhanced SF in both galaxies due to interaction. Conclusions. Our study shows that care should be taken when interpreting global host and even gas properties without looking at the stellar population history of the region. The SLSNe seem to be consistent with massive stars (&gt;20 M⊙) requiring low metallicity (&lt;0.6 Z⊙), environments that can also occur in massive late-type galaxies, but not necessarily with starbursts.

The Stripe 82 Massive Galaxy Project. III. A Lack of Growth among Massive Galaxies

Abstract The average stellar mass (M *) of high-mass galaxies ( ) is expected to grow by ∼30% since , largely through ongoing mergers that are also invoked to explain the observed increase in galaxy sizes. Direct evidence for the corresponding growth in stellar mass has been elusive, however, in part because the volumes sampled by previous redshift surveys have been too small to yield reliable statistics. In this work, we make use of the Stripe 82 Massive Galaxy Catalog (s82-mgc) to build a mass-limited sample of 41,770 galaxies ( ) with optical–to–near-IR photometry and a large fraction (&gt;55%) of spectroscopic redshifts. Our sample spans 139 deg2, significantly larger than most previous efforts. After accounting for a number of potential systematic errors, including the effects of M * scatter, we measure galaxy stellar mass functions over and detect no growth in the typical M * of massive galaxies with an uncertainty of 9%. This confidence level is dominated by uncertainties in the star formation (SF) history assumed for M * estimates, although our inability to characterize low-surface-brightness outskirts may be the most important limitation of our study. Even among these high-mass galaxies, we find evidence for differential evolution when splitting the sample by recent SF activity. While low-SF systems appear to become completely passive, we find a mostly subdominant population of galaxies with residual, but low rates of SF (∼1 M ⊙ yr−1) whose number density does not evolve. Interestingly, these galaxies become more prominent at higher M *, representing ∼10% of all galaxies at and perhaps dominating at even larger masses.

HI properties and star formation history of a fly-by pair of blue compact dwarf galaxies

A fly-by interaction has been suggested to be one of the major explanations for enhanced star formation in blue compact dwarf (BCD) galaxies, yet no direct evidence for this scenario has been found to date. In the HI Parkes all-sky survey (HIPASS), ESO 435-IG 020 and ESO 435- G 016, a BCD pair were found in a common, extended gas envelope of atomic hydrogen, providing an ideal case to test the hypothesis that the starburst in BCDs can be indeed triggered by a fly-by interaction. Using high-resolution data from the Australia Telescope Compact Array (ATCA), we investigated HI properties and the spectral energy distribution (SED) of the BCD pair to study their interaction and star formation histories. The high-resolution HI data of both BCDs reveal a number of peculiarities, which are suggestive of tidal perturbation. Meanwhile, 40% of the HIPASS flux is not accounted for in the ATCA observations with no HI gas bridge found between the two BCDs. Intriguingly, in the residual of the HIPASS and the ATCA data, 10% of the missing flux appears to be located between the two BCDs. While the SED-based age of the most dominant young stellar population is old enough to have originated from the interaction with any neighbors (including the other of the two BCDs), the most recent star formation activity traced by strong H$\alpha$ emission in ESO 435-IG 020 and the shear motion of gas in ESO 435- G 016, suggest a more recent or current tidal interaction. Based on these and the residual emission between the HIPASS and the ATCA data, we propose an interaction between the two BCDs as the origin of their recently enhanced star formation activity. The shear motion on the gas disk, potentially with re-accretion of the stripped gas, could be responsible for the active star formation in this BCD pair.

CO-Dark Star Formation and Black Hole Activity in 3C 368 at z = 1.131: Coeval Growth of Stellar and Supermassive Black Hole Masses∗ †

Abstract We present the detection of four far-infrared fine-structure oxygen lines, as well as strong upper limits for the CO(2–1) and [N ii] 205 μm lines, in 3C 368, a well-studied radio-loud galaxy at z = 1.131. These new oxygen lines, taken in conjunction with previously observed neon and carbon fine-structure lines, suggest a powerful active galactic nucleus (AGN), accompanied by vigorous and extended star formation. A starburst dominated by O8 stars, with an age of ∼6.5 Myr, provides a good fit to the fine-structure line data. This estimated age of the starburst makes it nearly concurrent with the latest episode of AGN activity, suggesting a link between the growth of the supermassive black hole and stellar population in this source. We do not detect the CO(2–1) line, down to a level twelve times lower than the expected value for star-forming galaxies. This lack of CO line emission is consistent with recent star formation activity if the star-forming molecular gas has low metallicity, is highly fractionated (such that CO is photodissociated throughout much of the clouds), or is chemically very young (such that CO has not yet had time to form). It is also possible, although we argue it is unlikely, that the ensemble of fine-structure lines is emitted from the region heated by the AGN.