Evidence Of Recent Star Formation Research Articles

Herbig–Haro flows around the BBWo 192E (GM 1–23) nebula

ABSTRACT Looking for evidence of recent star formation, we have studied a small comet-shaped reflection nebula, known as BBWo 192E (GM 1–23), which is located in the dark cloud SL 4 in the Vela Molecular Ridge cloud C, and a young infrared cluster embedded into the nebula. We obtained the images of BBWo 192E in Hα and [S ii] lines and in a Sloan Digital Sky Survey i ′ filter with the Blanco telescope at the Cerro Tololo Inter-American Observatory in order to discover new Herbig–Haro (HH) flows. We used the Two-Micron All-Sky Survey and the Wide-field Infrared Survey Explorer to search for additional member stars of the cluster. We also studied the proper motions and parallaxes of the cluster members using GAIA Data Release 2. Five new groups containing at least nine HH objects, tracing several distinct outflows, were revealed. A previously unreported reflection nebula and a number of probable outflow sources were found in the infrared range. The proper motions allowed us to select eight probable member stars in the visual range. Their parallaxes correspond to a mean distance 800 ± 100 pc for this cluster. The bolometric luminosities of the brightest cluster members are 1010 L⊙ (IRAS 08513−4201, the strong source in the centre of the cluster) and 2–6 L⊙ for the five other stars. The existence of optical HH flows around the infrared cluster of young stellar objects suggests that star formation in this cloud is ongoing around the more massive Herbig Ae/Be star. Considering its morphology and other features, this star-forming region is similar to the zone of star formation near CPM 19.

Dark-ages reionization and galaxy formation simulation – XIII. AGN quenching of high-redshift star formation in ZF-COSMOS-20115

Massive quiescent galaxies (MQGs) are thought to have formed stars rapidly at early times followed by a long period of quiescence. The recent discovery of a MQG, ZF-COSMOS-20115 at $z\sim4$, only 1.5 Gyr after the big bang, places new constraints on galaxy growth and the role of feedback in early star formation. Spectroscopic follow-up confirmed ZF-COSMOS-20115 as a MQG at $z=3.717$ with an estimated stellar mass of ${\sim}10^{11}\mathrm{M}_\odot$, showing no evidence of recent star formation. We use the Meraxes semi-analytic model to investigate how ZF-COSMOS-20115 analogues build stellar mass, and why they become quiescent. We identify three analogue galaxies with similar properties to ZF-COSMOS-20115. We find that ZF-COSMOS-20115 is likely hosted by a massive halo with virial mass of ${\sim}10^{13}\mathrm{M}_\odot$, having been through significant mergers at early times. These merger events drove intense growth of the nucleus, which later prevented cooling and quenched star formation. ZF-COSMOS-20115 likely remained quiescent at $z<3.7$. We find that the analogues host the most massive black holes in our simulation and were luminous quasars at $z\sim5$, indicating that ZF-COSMOS-20115 and other MQGs may be the descendants of high-redshift quasars. In addition, the model suggests that ZF-COSMOS-20115 formed in a region of intergalactic medium that was reionized early.

Massive stars dying alone: the extremely remote environment of SN 2009ip

We present late-time HST images of the site of supernova (SN) 2009ip taken almost 3 yr after its bright 2012 luminosity peak. SN 2009ip is now slightly fainter in broad filters than the progenitor candidate detected by HST in 1999. The current source continues to be dominated by ongoing late-time CSM interaction that produces strong H-alpha emission and a weak pseudo-continuum, as found previously for 1-2 yr after explosion. The intent of these observations was to search for evidence of recent star formation in the local (1kpc; 10 arcsec) environment around SN 2009ip, in the remote outskirts of its host spiral galaxy NGC 7259. We can rule out the presence of any massive star-forming complexes like 30 Dor or the Carina Nebula at the SN site or within a few kpc. If the progenitor of SN 2009ip was really a 50-80 Msun star as archival HST images suggested, then it is strange that there is no sign of this type of massive star formation anywhere in the vicinity. A possible explanation is that the progenitor was the product of a merger or binary mass transfer, rejuvenated after a lifetime that was much longer than 4-5 Myr, allowing its natal H II region to have faded. A smaller region like the Orion Nebula would be an unresolved but easily detected point source. This is ruled out within 1.5 kpc around SN 2009ip, but a small H II region could be hiding in the glare of SN 2009ip itself. Later images after a few more years have passed are needed to confirm that the progenitor candidate is truly gone and to test for the presence of a small H II region or cluster at the SN position.

EVOLUTION OF THERMALLY PULSING ASYMPTOTIC GIANT BRANCH STARS. V. CONSTRAINING THE MASS LOSS AND LIFETIMES OF INTERMEDIATE-MASS, LOW-METALLICITY AGB STARS*

ABSTRACT Thermally pulsing asymptotic giant branch (TP-AGB) stars are relatively short lived (less than a few Myr), yet their cool effective temperatures, high luminosities, efficient mass loss, and dust production can dramatically affect the chemical enrichment histories and the spectral energy distributions of their host galaxies. The ability to accurately model TP-AGB stars is critical to the interpretation of the integrated light of distant galaxies, especially in redder wavelengths. We continue previous efforts to constrain the evolution and lifetimes of TP-AGB stars by modeling their underlying stellar populations. Using Hubble Space Telescope (HST) optical and near-infrared photometry taken of 12 fields of 10 nearby galaxies imaged via the Advanced Camera for Surveys Nearby Galaxy Survey Treasury and the near-infrared HST/SNAP follow-up campaign, we compare the model and observed TP-AGB luminosity functions as well as the ratio of TP-AGB to red giant branch stars. We confirm the best-fitting mass-loss prescription, introduced by Rosenfield et al., in which two different wind regimes are active during the TP-AGB, significantly improves models of many galaxies that show evidence of recent star formation. This study extends previous efforts to constrain TP-AGB lifetimes to metallicities ranging −1.59 ≲ [Fe/H] ≲ −0.56 and initial TP-AGB masses up to ∼4 M ⊙, which include TP-AGB stars that undergo hot-bottom burning.

The regulation of star formation in cool-core clusters: imprints on the stellar populations of brightest cluster galaxies

A fraction of brightest cluster galaxies (BCGs) shows bright emission in the UV and the blue part of the optical spectrum, which has been interpreted as evidence of recent star formation. Most of these results are based on the analysis of broadband photometric data. Here, we study the optical spectra of a sample of 19 BCGs hosted by X-ray luminous galaxy clusters at 0.15 < z < 0.3, a subset from the Canadian Cluster Comparison Project (CCCP) sample. We identify plausible star formation histories of the galaxies by fitting Simple Stellar Populations (SSPs) as well as composite populations, consisting of a young stellar component superimposed on an intermediate/old stellar component, to accurately constrain their star formation histories. We detect prominent young (~200 Myr) stellar populations in 4 of the 19 galaxies. Of the four, the BCG in Abell 1835 shows remarkable A-type stellar features indicating a relatively large population of young stars, which is extremely unusual even amongst star forming BCGs. We constrain the mass contribution of these young components to the total stellar mass to be typically between 1% to 3%, but rising to 7% in Abell 1835. We find that the four of the BCGs with strong evidence for recent star formation (and only these four galaxies) are found within a projected distance of 5 kpc of their host cluster's X-ray peak, and the diffuse, X-ray gas surrounding the BCGs exhibit a ratio of the radiative cooling-to-free-fall time ($t_{c}/t_{ff}$) of < 10. These are also some of the clusters with the lowest central entropy. Our results are consistent with the predictions of the precipitation-driven star formation and AGN feedback model, in which the radiatively cooling diffuse gas is subject to local thermal instabilities once the instability parameter $t_{c}/t_{ff}$ falls below ~10, leading to the condensation and precipitation of cold gas. [Abridged]

THE BLACK HOLE MASS SCALE OF CLASSICAL AND PSEUDO BULGES IN ACTIVE GALAXIES

The mass estimator used to calculate black hole (BH) masses in broad-line active galactic nuclei (AGNs) relies on a virial coefficient (the "$f$ factor") that is determined by comparing reverberation-mapped (RM) AGNs with measured bulge stellar velocity dispersions against the $M_{\rm BH}-\sigma_*$ relation of inactive galaxies. It has recently been recognized that only classical bulges and ellipticals obey a tight $M_{\rm BH}-\sigma_*$ relation; pseudobulges have a different zero point and much larger scatter. Motivated by these developments, we reevaluate the $f$ factor for RM AGNs with available $\sigma_*$ measurements, updated H$\beta$ RM lags, and new bulge classifications based on detailed decomposition of high-resolution ground-based and space-based images. Separate calibrations are provided for the two bulge types, whose virial coefficients differ by a factor of $\sim 2$: $f=6.3\pm1.5$ for classical bulges and ellipticals and $f = 3.2\pm0.7$ for pseudobulges. The structure and kinematics of the broad-line region, at least as crudely encoded in the $f$ factor, seems to related to the large-scale properties or formation history of the bulge. Lastly, we investigate the bulge stellar masses of the RM AGNs, show evidence for recent star formation in the AGN hosts that correlates with Eddington ratio, and discuss the potential utility of the $M_{\rm BH}-M_{\rm bulge}$ relation as a more promising alternative to the conventionally used $M_{\rm BH}-\sigma_*$ relation for future refinement of the virial mass estimator for AGNs.

Star-forming galactic contrails as a source of metal enrichment and ionizing radiation at high redshift★

A spectroscopically detected Lyman alpha emitting halo at redshift 3.216 in the GOODS-N field is found to reside at the convergence of several Lyman alpha filaments. HST images show that some of the filaments are inhabited by galaxies. Several of the galaxies in the field have pronounced head-tail structures, which are partly aligned with each other. The blue colors of most tails suggest the presence of young stars, with the emission from at least one of the galaxies apparently dominated by high equivalent width Lyman alpha. Faint, more diffuse, and similarly elongated, apparently stellar features, can be seen over an area with a linear extent of at least 90 kpc. The region within several arcseconds of the brightest galaxy exhibits spatially extended emission by HeII, NV and various lower ionization metal lines. The gas-dynamical features present are strongly reminiscent of ram-pressure stripped galaxies, including evidence for recent star formation in the stripped contrails. Spatial gradients in the appearance of several galaxies may represent a stream of galaxies passing from a colder to a hotter intergalactic medium. The stripping of gas from the in-falling galaxies, in conjunction with the occurrence of star formation and stellar feedback in the galactic contrails suggests a mechanism for the metal enrichment of the high redshift intergalactic medium that does not depend on long-range galactic winds, at the same time opening a path for the escape of ionizing radiation from galaxies.

GEMINI SPECTROSCOPY OF THE SHORT-HARD GAMMA-RAY BURST GRB 130603B AFTERGLOW AND HOST GALAXY

We present early optical photometry and spectroscopy of the afterglow and host galaxy of the bright short-duration gamma-ray burst GRB 130603B discovered by the Swift satellite. Using the Gemini South telescope, our prompt optical spectra reveal a strong trace from the afterglow superimposed on continuum and emission lines from the $z = 0.3568 \pm 0.0005$ host galaxy. The combination of a relatively bright optical afterglow (r' = 21.52 at $\Delta_t $= 8.4hr), together with an observed offset of 0\farcs9 from the host nucleus (4.8kpc projected distance at z=0.3568), allow us to extract a relatively clean spectrum dominated by afterglow light . The spatially resolved spectrum allows us to constrain the properties of the explosion site directly, and compare these with the host galaxy nucleus, as well as other short-duration GRB host galaxies. We find that while the host is a relatively luminous ($L \approx 0.8 L^{*}_{B}$), star-forming galaxy with solar metallicity, the spectrum of the afterglow exhibits weak CaII absorption features but negligible emission features. The explosion site therefore lacks evidence of recent star formation, consistent with the relatively long delay time distribution expected in a compact binary merger scenario. The star formation rate (both in an absolute sense and normalized to the luminosity) and metallicity of the host are both consistent with the known sample of short-duration GRB hosts and with recent results which suggest GRB130603B emission to be the product of the decay of radioactive species produced during the merging process of a NS-NS binary ("kilonova"). Ultimately, the discovery of more events similar to GRB130603B and their rapid follow-up from 8-m class telescopes will open new opportunities for our understanding of the final stages of compact-objects binary systems.

TIDAL TAILS OF MINOR MERGERS. II. COMPARING STAR FORMATION IN THE TIDAL TAILS OF NGC 2782

The peculiar spiral NGC 2782 is the result of a minor merger with a mass ratio ~4:1 occurring ~200 Myr ago. This merger produced a molecular and HI rich, optically bright Eastern tail and an HI-rich, optically faint Western tail. Non-detection of CO in the Western Tail by Braine et al. (2001) suggested that star formation had not yet begun. However, deep UBVR and H-alpha narrowband images show evidence of recent star formation in the Western tail, though it lacks massive star clusters and cluster complexes. Using Herschel PACS spectroscopy, we discover 158 micron [CII] emission at the location of the three most luminous H-alpha sources in the Eastern tail, but not at the location of the even brighter H-alpha source in the Western tail. The Western tail is found to have a normal star formation efficiency (SFE), but the Eastern tail has a low SFE. The lack of CO and [CII] emission suggests the Western tail HII region may have a low carbon abundance and be undergoing its first star formation. The Western tail is more efficient at forming stars, but lacks massive clusters. We propose that the low SFE in the Eastern tail may be due to its formation as a splash region where gas heating is important even though it has sufficient molecular and neutral gas to make massive star clusters. The Western tail, which has lower gas surface density and does not form high mass star clusters, is a tidally formed region where gravitational compression likely enhances star formation.

A STELLAR MASS THRESHOLD FOR QUENCHING OF FIELD GALAXIES

We demonstrate that dwarf galaxies (10^7 < M_stellar < 10^9 Msun) with no active star formation are extremely rare (<0.06%) in the field. Our sample is based on the NASA-Sloan Atlas which is a re-analysis of the Sloan Digital Sky Survey Data Release 8. We examine the relative number of quenched versus star forming dwarf galaxies, defining quenched galaxies as having no Halpha emission (EW_Halpha < 2 AA) and a strong 4000AA-break. The fraction of quenched dwarf galaxies decreases rapidly with increasing distance from a massive host, leveling off for distances beyond 1.5 Mpc. We define galaxies beyond 1.5 Mpc of a massive host galaxy to be in the field. We demonstrate that there is a stellar mass threshold of M_stellar < 1.0x10^9 Msun below which quenched galaxies do not exist in the field. Below this threshold, we find that none of the 2951 field dwarf galaxies are quenched; all field dwarf galaxies show evidence for recent star formation. Correcting for volume effects, this corresponds to a 1-sigma upper limit on the quenched fraction of 0.06%. In more dense environments, quenched galaxies account for 23% of the dwarf population over the same stellar mass range. The majority of quenched dwarf galaxies (often classified as dwarf elliptical galaxies) are within 2 virial radii of a massive galaxy, and only a few percent of quenched dwarf galaxies exist beyond 4 virial radii. Thus, for galaxies with stellar mass less than 1.0x10^9 Msun, ending star-formation requires the presence of a more massive neighbor, providing a stringent constraint on models of star formation feedback.

THE TYPE II SUPERNOVA RATE INz∼ 0.1 GALAXY CLUSTERS FROM THE MULTI-EPOCH NEARBY CLUSTER SURVEY

We present 7 spectroscopically confirmed Type II cluster supernovae (SNeII) discovered in the Multi-Epoch Nearby Cluster Survey, a supernova survey targeting 57 low redshift 0.05 < z < 0.15 galaxy clusters with the Canada-France-Hawaii Telescope. We find the rate of Type II supernovae within the virial radius of these galaxy clusters to be 0.026 (+0.085 -0.018 stat; +0.003 -0.001 sys) SNe per century per 1e10 solar masses. Surprisingly, one SNII is in a red sequence host galaxy that shows no clear evidence of recent star formation. This is unambiguous evidence in support of ongoing, low-level star formation in at least some cluster elliptical galaxies, and illustrates that galaxies that appear to be quiescent cannot be assumed to host only Type Ia SNe. Based on this single SNII we make the first measurement of the SNII rate in red sequence galaxies, and find it to be 0.007 (+0.014 -0.007 stat; +0.009 -0.001 sys) SNe per century per 1e10 solar masses. We also make the first derivation of cluster specific star formation rates (sSFR) from cluster SNII rates. We find that for all galaxy types, sSFR is 5.1 (+15.8 -3.1 stat; +0.9 -0.9 sys) solar masses per year per 1e12 solar masses, and for red sequence galaxies only, it is 2.0 (+4.2 -0.9 stat; +0.4 -0.4 sys) solar masses per year per 1e12 solar masses. These values agree with SFRs measured from infrared and ultraviolet photometry, and H-alpha emission from optical spectroscopy. Additionally, we use the SFR derived from our SNII rate to show that although a small fraction of cluster Type Ia SNe may originate in the young stellar population and experience a short delay time, these results do not preclude the use of cluster SNIa rates to derive the late-time delay time distribution for SNeIa.

TIDAL TAILS OF MINOR MERGERS: STAR FORMATION EFFICIENCY IN THE WESTERN TAIL OF NGC 2782

While major mergers and their tidal debris are well studied, they are less common than minor mergers (mass ratios < 0.3). The peculiar spiral NGC 2782 is the result of a merger between two disk galaxies with a mass ratio of ~4:1 occurring ~200 Myr ago. This merger produced a molecular and H I-rich, optically bright eastern tail and an H I-rich, optically faint western tail. Non-detection of CO in the western tail by Braine et al. suggested that star formation had not yet begun to occur in that tidal tail. However, deep H{\alpha} narrowband images show evidence of recent star formation in the western tail. Across the entire western tail, we find the global star formation rate per unit area ({\Sigma}SFR) to be several orders of magnitude less than expected from the total gas density. Together with extended FUV+NUV emission from Galaxy Evolution Explorer along the tail, this indicates a low global star formation efficiency in the tidal tail producing lower mass star clusters. The H II region that we observed has a local (few-kiloparsec scale) {\Sigma}SFR from H{\alpha} that is less than that expected from the total gas density, which is consistent with other observations of tidal debris. The star formation efficiency of this H II region inferred from the total gas density is low, but normal when inferred from the molecular gas density. These results suggest the presence of a very small, locally dense region in the western tail of NGC 2782 or of a low-metallicity and/or low-pressure star-forming region.

DELTA SCUTI, SX PHOENICIS, AND RR LYRAE STARS IN GALAXIES AND GLOBULAR CLUSTERS

The distances to four galaxies and two globular clusters which are derived with the aid of period-luminosity and period-color relations of δ Scuti and SX Phe stars are compared to the distances derived by other methods, in particular RR Lyrae stars. We examine the luminosities of horizontal branch or RR Lyrae stars in Oosterhoff I and II globular clusters. Observational data from a variety of sources indicate a discontinuous jump of ~0.2 mag in the luminosities of RR Lyrae variables at [Fe/H] ≈ –1.5 as we transition from Oosterhoff I to Oosterhoff II clusters. If Oosterhoff I clusters have RR Lyrae variables with average M V values of M V = 0.53 mag at [Fe/H] = –1.5, it implies that RR Lyrae stars in Oosterhoff II clusters average M V values are ~0.34 mag. Unlike the Oosterhoff I clusters which show an increase in the V luminosity of RR Lyrae stars as [Fe/H] becomes smaller, little or no change in the V luminosity of RR Lyrae variables is evident in Oosterhoff II clusters in the interval of [Fe/H] from –1.5 to –2.2. We find distance moduli found with RR Lyrae variables agree to ≤0.04 mag with those found with the δ Scuti and/or SX Phe variables if the M V values of RR Lyrae stars above are adopted. We find evidence of recent star formation (presence of near solar-metallicity δ Scuti stars with ages of 150 Myr to 1 Gyr) in the Large Magellanic Cloud (LMC), Small Magellanic Cloud, and the central region of the Fornax (dSph) galaxies. We also find an older population of metal-poor δ Scuti variables (SX Phe stars) in the LMC and Fornax galaxies. The Carina dSph is unique in that only an old population of metal-poor δ Scuti variables is evident. No evidence of recent δ Scuti star formation is found. The minimum periods observed for the SX Phe variables (blue stragglers) in the globular clusters M55 and ω Cen indicate that they could have been formed in a burst of metal-poor single star formation in the last 2.9-6 Gyr. If formed by the more acceptable scenario of stellar mergers, it is likely that the merged remnant resembles a normal star in a relatively advanced stage of main-sequence evolution with an enriched He core and ordinary He envelope. We present equations to calculate intrinsic-color indices for δ Scuti, SX Phe, and RR Lyrae stars at mean light. Finally, we show that the fundamental-radial-pulsating stars (δ Sct and SX Phe variables) have larger average light amplitudes than the first-overtone pulsating variables. The fundamental metal-poor variables (SX Phe stars) have the largest average and individual amplitudes.

Evidence of different star formation histories for high- and low-luminosity radio galaxies

We present the results of our investigation into the stellar populations of 24 radio galaxies at z≃ 0.5 drawn from four complete, low-frequency-selected radio surveys. We use the strength of the 4000-Å break as an indicator of recent star formation and compare this with radio luminosity, optical spectral classification and morphological classification. We find evidence of different star formation histories for high- and low-luminosity radio sources; our group of low radio luminosity sources (typically Fanaroff–Riley type I sources) has systematically older stellar populations than the higher radio luminosity group. Our sample is also fairly well divided by optical spectral classification. We find that galaxies classified as having low excitation spectra (LEGs) possess older stellar populations than high excitation line objects (HEGs), with the HEGs showing evidence for recent star formation. We also investigate the link between radio morphology, as used by Owen and Laing, and the stellar populations. We find that there is a preference for the ‘fat-double’ sources to have older stellar populations than the ‘classical double’ sources, although this is also linked to these sources lying predominantly in the LEG and HEG categories, respectively. These results are consistent with the hypothesis that HEGs are powered by accretion of cold gas, which could be supplied, for example, by recent mergers, secular instabilities or filamentary cold flows. These processes could also trigger star formation in the host galaxy. The host galaxies of the LEGs do not show evidence for recent star formation and an influx of cold gas and are consistent with being powered by the accretion of the hot phase of the interstellar medium.

SCG0018-4854: a young and dynamic compact group

Compact groups of galaxies are in particular good laboratories for studying galaxy interactions and their effects on the evolution of galaxies due to their high density and low velocity dispersion. SCG0018-4854 is a remarkably high galaxy density and low velocity dispersion group with evidence of a recent interaction. We obtained VLT FORS2 optical observations and we present spectroscopic and photometric evidence of how dramatically galaxy interactions have affected each of the four member galaxies. We found peculiar kinematics for each galaxy and evidence of recent star formation. In particular, the gas and stellar radial velocity curves of two galaxies are irregular with a level of asymmetry similar to that of other interacting galaxies. We discovered the presence of a bar for NGC 92 therefore revising a previous morphological classification and we obtained spectroscopic confirmation of a galactic-scale outflow of NGC 89. Peculiar kinematics and dynamic consideration lead to a rough estimate of the age of the latest interaction: 0.2-0.7 Gyr, suggesting that SCG0018-4854 is a young and dynamical group.

The UV-optical colours of brightest cluster galaxies in optically and X-ray selected clusters

Many brightest cluster galaxies (BCGs) at the centers of X-ray selected clusters exhibit clear evidence for recent star formation. However, studies of BCGs in optically-selected clusters show that star formation is not enhanced when compared to control samples of non-BCGs of similar stellar mass. Here we analyze a sample of 113 BCGs in low redshift (z<0.1), optically-selected clusters, a matched control sample of non-BCGs, and a smaller sample of BCGs in X-ray selected clusters. We convolve the SDSS images of the BCGs to match the resolution of the GALEX data and we measure UV-optical colours in their inner and outer regions. We find that optically-selected BCGs exhibit smaller scatter in optical colours and redder inner NUV-r colours than the control galaxies, indicating that they are a homogenous population with very little ongoing star formation. The BCGs in the X-ray selected cluster sample span a similar range in optical colours, but have bluer NUV-r colours. Among X-ray selected BCGs, those located in clusters with central cooling times of less than 1 Gyr are significantly bluer than those located in clusters where the central gas cooling times are long. Our main conclusion is that the location of a galaxy at the centre of its halo is not sufficient to determine whether or not it is currently forming stars. One must also have information about the thermodynamic state of the gas in the core of the halo.

The evolution of spheroidal galaxies in different environments

AbstractWe analyse the kinematic and chemical evolution of 203 distant spheroidal (elliptical and S0) galaxies at 0.2 &lt; z &lt; 0.8 which are located in different environments (rich clusters, low‐mass clusters and in the field). VLT/FORS and CAHA/MOSCA spectra with intermediate‐resolution have been acquired to measure the internal kinematics and stellar populations of the galaxies. From HST/ACS and WFPC2 imaging, surface brightness profiles and structural parameters were derived for half of the galaxy sample. The scaling relations of the Faber‐Jackson relation and Kormendy relation as well as the Fundamental Plane indicate a moderate evolution for the whole galaxy population in each density regime. In all environments, S0 galaxies show a faster evolution than elliptical galaxies. For the cluster galaxies a slight radial dependence of the evolution out to one virial radius is found. Dividing the samples with respect to their mass, a mass dependent evolution with a stronger evolution of lower‐mass galaxies (M &lt; 2 × 1011 M⊙) is detected. Evidence for recent star formation is provided by blue colours and weak [OII] emission or strong Hδ absorption features in the spectra. The results are consistent with a down‐sizing formation scenario which is independent from the environment of the galaxies (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Blue straggler stars in dwarf spheroidal galaxies - II. Sculptor and Fornax

The existence of blue straggler stars (BSSs) in dwarf spheroidal galaxies (dSphs) is still an open question. In fact, many BSS candidates have been observed in the Local Group dSphs, but it is unclear whether they are real BSSs or young stars. Shedding light on the nature of these BSS candidates is crucial, in order to understand the star formation history of dSphs. In this paper, we consider BSS candidates in Sculptor and Fornax. In Fornax there are strong hints that the BSS population is contaminated by young stars, whereas in Sculptor there is no clear evidence of recent star formation. We derive the radial and luminosity distribution of BSS candidates from wide field imaging data extending beyond the nominal tidal radius of these galaxies. The observations are compared with the radial distribution of BSSs expected from dynamical simulations. In Sculptor the radial distribution of BSS candidates is consistent with that of red horizontal branch (RHB) stars and is in agreement with theoretical expectations for BSSs generated via mass transfer in binaries. On the contrary, in Fornax the radial distribution of BSS candidates is more concentrated than that of all the considered stellar populations. This result supports the hypothesis that most of BSS candidates in Fornax are young stars and is consistent with previous studies.

Evidence for recent star formation in BCGs: a correspondence between blue cores and UV excess

We present a joint analysis of near-ultraviolet (NUV) data from the GALEX mission and (optical) colour profiles for a sample of 7 Brightest Cluster Galaxies (BCGs) in the Canadian Cluster Comparison Project. We find that every BCG which has a blue rest-frame UV colour also shows a blue-core in its optical colour profile. Conversely, BCGs that lack blue cores and show monotonic colour gradients typical of old elliptical galaxies, are red in the UV. We interpret this as evidence that the NUV enhancement in the blue BCGs is driven by recent star formation and not from old evolved stellar populations such as horizontal branch stars. Furthermore, the UV enhancement cannot be from an AGN because the spatial extent of the blue cores is significantly larger than the possible contamination region due to a massive black hole. The recent star formation in the blue BCGs typically has an age less than 200 Myrs and contributes mass fractions of less than a percent. Although the sample studied here is small, we demonstrate, for the first time, a one-to-one correspondence between blue cores in elliptical galaxies (in particular BCGs) and a NUV-enhancement observed using GALEX. The combination of this one-to-one correspondence and the consistently young age of recent star formation, coupled with additional correlations with the host cluster’s X-ray properties, strongly suggests that the star formation is fueled by gas cooling out of the intracluster medium. In turn, this implies that any AGN heating of the intracluster medium in massive clusters only acts to reduce the magnitude of the cooling flow and that once this flow starts, it is nearly always active. Collectively, these results suggest that AGN feedback in present-day BCGs, while important, cannot be as efficient as suggested by the recent theoretical model by proposed by De Lucia et al. (2006).

Resurrecting the red from the dead: optical properties of BCGs in X-ray luminous clusters★

We present measurements of surface brightness and colour profiles for the brightest cluster galaxies (BCGs) in a sample of 48 X-ray luminous galaxy clusters. These data were obtained as part of the Canadian Cluster Comparison Project (CCCP). The Kormendy relation of our BCGs is steeper than that of the local ellipticals, suggesting differences in the assembly history of these types of systems. We also find that while most BCGs show monotonic colour gradients consistent with a decrease in metallicity with radius, 25% of the BCGs show colour profiles that turn bluer towards the centre (blue-cores). We interpret this bluing trend as evidence for recent star formation. The excess blue light leads to a typical offset from the red sequence of 0.5 to 1.0 mag in (g'-r'), thus affecting optical cluster studies that may reject the BCG based on colour. All of the blue-core BCGs are located within ~10 kpc of the peak in the cluster X-ray emission. Furthermore, virtually all of the BCGs with recent star formation are in clusters that lie above the Lx-Tx relation. Based on photometry alone, these findings suggest that central star formation is a ubiquitous feature of BCGs in dynamically relaxed cool-core clusters. This implies that while AGNs and other heating mechanisms are effective at tempering cooling, they do not full compensate for the energy lost via radiation.