A new model is proposed in which typical galaxies form most of their stellar mass in a phase with an intrinsically red stellar population. In the standard picture, galaxies with intrinsically red stellar populations are believed to have old stellar populations, so that only galaxies with blue stellar populations have significant star formation, and subsequent changes to the stellar population come from predominantly from aging and merging populations which have already formed. However, several observational puzzles have developed which are difficult to reconcile with this standard scenario. The most massive blue star-forming galaxies, presumed to be at the end of their stellar mass growth, are $\sim 1$ dex less massive, have a $\sim 1$ dex lower $M_*/M_{BH}$ ratio, and have a bottom-lighter IMF than local quiescent galaxies. Here, a new solution is proposed: at low temperature and high metallicity, galaxies can continue to form stars efficiently without being able to form O and B stars. These red star-forming galaxies would have many of the same properties of the population currently described as post-starburst galaxies, allowing a new interpretation of their origin. Finally, additional falsifiable observational predictions of this model are also discussed.
Context: Star forming early-type galaxies with blue optical colours at low redshift can be used to test our current understanding of galaxy formation and evolution. Aims: We want to reveal the fuel and triggering mechanism for star formation in these otherwise passively evolving red and dead stellar systems. Methods: We undertook an optical and ultraviolet study of 55 star forming blue early-type galaxies, searching for signatures of recent interactions that could be driving the molecular gas into the galaxy and potentially triggering the star formation. Results: We report here our results on …
We employ the very large cosmological hydrodynamical simulation BLUETIDES to investigate the predicted properties of the galaxy population during the epoch of reionisation ($z>8$). BLUETIDES has a resolution and volume ($(400/h\approx 577)^{3}\,{\rm cMpc^3}$) providing a population of galaxies which is well matched to depth and area of current observational surveys targeting the high-redshift Universe. At $z=8$ BLUETIDES includes almost 160,000 galaxies with stellar masses $>10^{8}\,{\rm M_{\odot}}$. The population of galaxies predicted by BLUETIDES closely matches observational constraints on…
Star-forming blue early-type galaxies at low redshift can give insight to the stellar mass growth of L$*$ elliptical galaxies in the local Universe. We wish to understand the reason for star formation in these otherwise passively evolving red and dead stellar systems. The fuel for star formation can be acquired through recent accretion events such as mergers or flyby. The signatures of such events should be evident from a structural analysis of the galaxy image. We carried out structural analysis on SDSS $r$-band imaging data of 55 star-forming blue elliptical galaxies, derived the structural …
We identify and characterize compact dwarf starburst (CDS) galaxies in the RESOLVE survey, a volume-limited census of galaxies in the local universe, to probe whether this population contains any residual ``blue nuggets,'' a class of intensely star-forming compact galaxies first identified at high redshift $z$. Our 50 low-$z$ CDS galaxies are defined by dwarf masses (stellar mass $M_* < 10^{9.5}$ M$_{\odot}$), compact bulged-disk or spheroid-dominated morphologies (using a quantitative criterion, $μ_Δ> 8.6$), and specific star formation rates above the defining threshold for high-$z$ blue nugg…
We present the analysis of the spin vector orientation of 5$\,$987 SDSS galaxies having negative redshift from $-$87.6 to $-$0.3 km$\,$s$^{-1}$. Two dimensional observed parameters are used to compute three dimensional galaxy rotation axes by applying `position angle--inclination' method. We aim to examine the non-random effects in the spatial orientation of blue-shifted galaxies. We generate 5$\times$10$^6$ virtual galaxies to find expected isotropic distributions by performing numerical simulations. We have written MATLAB program to facilitate the simulation process and eliminate the manual …
The star formation rates (SFRs) in weak emission line (WEL) galaxies in a volume-limited ($0.02 < z < 0.05$) sample of blue early-type galaxies (ETGs) identified from SDSS, are constrained here using 1.4 GHz radio continuum emission. The direct detection of 1.4 GHz radio continuum emission is made in 8 WEL galaxies and a median stacking is performed on 57 WEL galaxies using VLA FIRST images. The median stacked 1.4 GHz flux density and luminosity are estimated as 79 $\pm$ 19 $μ$Jy and 0.20 $\pm$ 0.05 $\times$ 10$^{21}$ W Hz$^{-1}$ respectively. The radio far-infrared correlation in 4 WEL galaxi…
We have selected a sample of nearby galaxies from Sloan Digital Sky Survey Data Release 7 (SDSS DR7) to investigate the physical properties variation from blue cloud to green valley to red sequence. The sample is limited in a narrow range in color-stellar mass diagram. After splitting green valley galaxies into two parts---a bluer green valley (green 1) and a redder one (green 2) and three stellar mass bins, we investigate the physical properties variation across the green valley region. Our main results are as following: (i) The percentages of pure bulge and bulge-dominated/elliptical galaxie…
To properly understand the evolution of high-redshift galaxy clusters, both passive and star-forming galaxies have to be considered. Here we study the clustering environment of 21 radio galaxies and quasars at 1<z<2.5 from the third Cambridge catalog (3C). We use optical and near-infrared Hubble Space Telescope images with a 2' field-of-view, where the filters encompass the rest-frame 4000 Angstroem break. Passive red and star-forming blue galaxies were separated in the color--magnitude diagram using a redshift-dependent cut derived from galaxy evolution models. We find that about 16 of 21 rad…
It is well known from the Butcher-Oemler effect that galaxies in dense environment are mostly red with little star formation and the fraction of blue galaxies in galaxy groups/clusters also declines rapidly with redshifts. A recent work by Hashimoto et al. reported a local 'blue cluster' with high fraction of blue galaxies ($\sim 0.57$), higher than the model predictions. They ascribed this blue cluster to the feeding of gas along a filamentary structure around the cluster. In this work we use group catalog from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) and the state-of-art of sem…
Extremely metal poor (XMP) galaxies are known to be very rare, despite the large numbers of low-mass galaxies predicted by the local galaxy luminosity function. This paper presents a sub-sample of galaxies that were selected via a morphology-based search on SDSS images with the aim of finding these elusive XMP galaxies. By using the recently discovered extremely metal-poor galaxy, Leo P, as a guide, we obtained a collection of faint, blue systems, each with isolated HII regions embedded in a diffuse continuum, that have remained optically undetected until now. Here we show the first results fr…
We examine the ultraviolet and optical colours of 906 cluster galaxies from the KASI-Yonsei Deep Imaging Survey of Clusters (KYDISC). Galaxies have been divided into two categories, morphologically-disturbed and undisturbed galaxies, based on the visual signatures related to recent mergers. We find that galaxies with signatures of recent mergers show significantly bluer colours than undisturbed galaxies. Disturbed galaxies populate more on the cluster outskirts, suggesting recent accretion into the cluster environment, which implies that disturbed galaxies can be less influenced by the environ…
Elliptical galaxies with blue optical colours and significant star formation are hypothesised to be major merger remnants of gas-rich spiral galaxies or normal elliptical galaxies with a sudden burst of star formation. We present here a scenario in which blue elliptical galaxies identified in shallow imaging surveys may fail to recover faint features that are indicative of past merger activity using a nearby major merger remnant. Based on deep optical imaging data of the post-merger galaxy, NGC 7252, we demonstrate that the galaxy can appear as an elliptical galaxy if it is observed at higher …
We derive the stellar-to-halo mass relations, SHMR, of local blue and red central galaxies separately, as well as the fraction of halos hosting blue/red central galaxies. We find that: 1) the SHMR of central galaxies is segregated by color, with blue centrals having a SHMR above the one of red centrals; at logMh~12, the Ms/Mh ratio of the blue centrals is ~0.05, which is ~1.7 times larger than the value of red centrals. 2) The intrinsic scatters of the SHMRs of red and blue centrals are ~0.14 and ~0.11dex, respectively. The intrinsic scatter of the average SHMR of all central galaxies changes …
In this paper we test if nearby blue spheroid (BSphs) galaxies may become the progenitors of star-forming spiral galaxies or passively-evolving elliptical galaxies. Our sample comprises 428 galaxies of various morphologies in the redshift range 0.002<z<0.02 (8-87 Mpc) with panchromatic data from the Galaxy and Mass Assembly survey. We find that BSph galaxies are structurally (mean effective surface brightness, effective radius) very similar to their passively-evolving red counterparts. However, their star-formation and other properties such as colour, age and metallicity are more like star-for…
The ultraviolet (UV) continuum slope ($β~$where$~f_λ\proptoλ^β$) of galaxies is sensitive to a variety of properties, from the metallicity and age of the stellar population to dust attenuation throughout the galaxy. Considerable attention has focused on identifying reionization-era galaxies with very blue UV slopes ($β<-3$). Not only do such systems provide a signpost of low-metallicity stars, but they also identify galaxies likely to leak ionizing photons from their HII regions as such blue UV slopes require the reddening effect of nebular continuum to be diminished. In this paper we present …
The morphological types of 5840 galaxies were classified by a visual inspection of color images using the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) to produce a morphology catalog of a representative sample of local galaxies with $z<0.01$. The sample galaxies are almost complete for galaxies brighter than $r_{pet}=17.77$. Our classification system is basically the same as that of the Third Reference Catalog of Bright Galaxies with some simplification for giant galaxies. On the other hand, we distinguish fine features of dwarf elliptical-like galaxies to classify 5 subtypes: dwarf el…
As part of the Bluedisk survey we analyse the radial gas-phase metallicity profiles of 50 late-type galaxies We compare the metallicity profiles of a sample of HI-rich galaxies against a control sample of HI-'normal' galaxies. We find the metallicity gradient of a galaxy to be strongly correlated with its HI mass fraction (M(HI) / Mstar). We note that some galaxies exhibit a steeper metallicity profile in the outer disc than in the inner disc. These galaxies are found in both the HI-rich and control samples. This contradicts a previous indication that these outer drops are exclusive to HI-rich…
We obtained GALEX FUV, NUV, and Spitzer/IRAC 3.6$μ$m photometry for > 2000 galaxies, available for 90% of the S4G sample. We find a very tight "GALEX Blue Sequence (GBS)" in the (FUV-NUV) versus (NUV-[3.6]) color-color diagram which is populated by irregular and spiral galaxies, and is mainly driven by changes in the formation timescale ($τ$) and a degeneracy between $τ$ and dust reddening. The tightness of the GBS provides an unprecedented way of identifying star-forming galaxies and objects that are just evolving to (or from) what we call the "GALEX Green Valley (GGV)". At the red end of the…
We measured the X-ray fluxes from an optically-selected sample of blue compact dwarf galaxies (BCDs) with metallicities <0.07 and solar distances less than 15 Mpc. Four X-ray point sources were observed in three galaxies, with five galaxies having no detectable X-ray emission. Comparing X-ray luminosity and star formation rate, we find that the total X-ray luminosity of the sample is more than 10 times greater than expected if X-ray luminosity scales with star formation rate according to the relation found for normal-metallicity star-forming galaxies. However, due to the low number of sources …
With the aim of exploring the fast evolutionary path from the blue cloud of star-forming galaxies to the red sequence of quiescent galaxies in the local universe, we select a local advanced merging infrared luminous and ultraluminous galaxy (adv-merger (U)LIRGs) sample and perform careful dust extinction corrections to investigate their positions in the SFR-$M_{\ast}$, u-r and NUV-r color-mass diagrams. The sample consists of 89 (U)LIRGs at the late merger stage, obtained from cross-correlating the IRAS Point Source Catalog Redshift Survey and 1 Jy ULIRGs samples with the Sloan Digital Sky Sur…
How elliptical galaxies form is a key question in observational cosmology. While the formation of massive ellipticals is strongly linked to mergers, the low mass (Mstar < 10^9.5 MSun) regime remains less well explored. In particular, studying elliptical populations when they are blue, and therefore rapidly building stellar mass, offers strong constraints on their formation. Here, we study 108 blue, low-mass ellipticals (which have a median stellar mass of 10^8.7 MSun) at z < 0.3 in the COSMOS field. Visual inspection of extremely deep optical HSC images indicates that less than 3 per cent of t…
We use the full VIPERS redshift survey in combination with SDSS-DR7 to explore the relationships between star-formation history (using d4000), stellar mass and galaxy structure, and how these relationships have evolved since z~1. We trace the extents and evolutions of both the blue cloud and red sequence, by fitting double Gaussians to the d4000 distribution of galaxies in narrow stellar mass bins, for four redshift intervals over 0<z<1. This reveals downsizing in star formation, as the high-mass limit of the blue cloud retreats steadily with time from M*~10^11.2 M_sun at z~0.9 to M*~10^10.7 M…
Galaxies grow through both internal and external processes. In about 10% of nearby red galaxies with little star formation, gas and stars are counter-rotating, demonstrating the importance of external gas acquisition in these galaxies. However, systematic studies of such phenomena in blue, star-forming galaxies are rare, leaving uncertain the role of external gas acquisition in driving evolution of blue galaxies. Based on new measurements with integral field spectroscopy of a large representative galaxy sample, we find an appreciable fraction of counter-rotators among blue galaxies (9 out of 4…
We discuss and test possible evolutionary connections between Blue Compact Dwarf galaxies (BCDs) and other types of dwarf galaxies. BCDs provide ideal laboratories to study intense star formation episodes in low mass dwarf galaxies, and have sometimes been considered a short-lived evolutionary stage between types of dwarf galaxies. To test these connections, we consider a sample of BCDs as well as a comparison sample of nearby galaxies from the Local Volume Legacy (LVL) survey for context. We fit the multi-wavelength spectral energy distributions (SED, far-ultra-violet to far-infrared) of each…
The search for chemically unevolved galaxies remains prevalent in the nearby Universe, mostly because these systems provide excellent proxies for exploring in detail the physics of high-z systems. The most promising candidates are extremely metal-poor galaxies (XMPs), i.e., galaxies with <1/10 solar metallicity. However, due to the bright emission line based search criteria traditionally used to find XMPs, we may not be sampling the full XMP population. In 2014 we reoriented this search using only morphological properties and uncovered a population of ~150 `blue diffuse dwarf (BDD) galaxies', …
The geometry and physics of the spiral structure of the giant Hubble type Sc galaxy NGC 309 is studied. A schematic of two patterns with three arms in each is suggested for the blue spiral. The red and blue patterns form together a grand design with two-fold symmetry. A possible gas-dynamics explanation of the phenomenon is suggested which shows how the two-arm red spiral may induce the formation of the six-arm coherent blue spiral. Key words: galaxies: individual (NGC 309) -- galaxies: spiral
Luminous Compact Blue Galaxies (LCBGs) are compact, star-forming galaxies that are rarely observed in the local universe but abundant at z=1. This increase in LCBG number density over cosmic lookback time roughly follows the increase in the star formation rate density of the universe over the same period. We use publicly available data in the COSMOS field to study the evolution of the largest homogeneous sample of LCBGs to date by deriving their luminosity function in four redshift bins over the range $0.1\leq~z\leq1$. We find that over this redshift range, the characteristic luminosity (M$^{*…
In a programme of observations of local luminous blue compact galaxies (BCGs), we are investigating kinematics by using tracers of both stars and ionized gas. Here we summarise our program and present new data on the local Lyman break galaxy analogue Haro 11. From spatially-resolved spectroscopy around the near-infrared Ca II triplet, we find that its stars and ionized gas have similar velocity fields. Our programme so far indicates however that emission line velocities can differ locally by a few tens of km/s from the Ca II values. Comparing our data to simple stellar population models, we as…
FIR observation of BCD galaxies with Herschel has revealed a wealth of new insights in these objects which are thought to resemble high-redshift forming galaxies. Dust and cold gas showed to be colder, in more or less quantities than expected and of uncertain origin. However, not unlike in the local universe, not all the dust or the cold gas is accounted for, making it more challenging. SPICA and its factor 10 to 100 in sensitivity will allow to image the faint extended cold gas/dusty disks in BCDGs in addition to detect faint C and O lines only marginally or not at all detected by Herschel/
Large surveys have shown that red galaxies are preferentially aligned with their halos while blue galaxies have a more isotropic distribution. Since halos generally align with their filaments this introduces a bias in the measurement of the cosmic shear from weak lensing. It is therefore vitally important to understand why this difference arises. We explore the stability of different disc orientations within triaxial halos. We show that, in the absence of gas, the disc orientation is most stable when its spin is along the minor axis of the halo. Instead when gas cools onto a disc it is able to…
We investigate the stellar masses of the class of star-forming objects known as Luminous Compact Blue Galaxies (LCBGs) by studying a sample of galaxies in the distant cluster MS$~$0451.6-0305 at $z\approx0.54$ with ground-based multicolor imaging and spectroscopy. For a sample of 16 spectroscopically-confirmed cluster LCBGs (colour $B-V < 0.5$, surface brightness $μ_B < 21$ mag arcsec$^{-2}$, and magnitude $M_B < -18.5$), we measure stellar masses by fitting spectral energy distribution (SED) models to multiband photometry, and compare with dynamical masses (determined from velocity dispersion…
One of the key goals of the Bluedisk survey is to characterize the impact of gas accretion in disc galaxies in the context of galaxy evolution. It contains 50 disc galaxies in the stellar mass range 10^10-10^11 Msun, of which half are bluer and more HI-rich galaxies than their HI-normal (control) counterparts. In this paper, we investigate how ongoing disc growth affects the molecular gas distribution and the star-formation efficiency in these galaxies. We present 12CO observations from the IRAM 30-m telescope in 26 galaxies of the Bluedisk survey. We compare the amount and spatial distributio…
(Abridged) The formation of massive spheroidal galaxies is studied on a visually classified sample of 910 galaxies extracted from the ACS/HST images of the GOODS North and South fields (0.4<z<.5). Three key observables are considered: comoving number density, internal colour distribution; and the Kormendy relation. The comoving number density of the most massive galaxies is found not to change significantly with redshift. One quarter of the whole sample of early-types are photometrically classified as blue galaxies. On a volume-limited subset out to z<0.7, the average stellar mass of the blue …
Ram pressure stripping is one of the most efficient mechanisms able to affect the gas reservoir in cluster galaxies and in the last decades many studies have characterized the properties of stripped galaxies. A definite census of the importance of this process in local clusters is though still missing. Here we characterize the fraction of galaxies showing signs of stripping at optical wavelengths, using the data of 66 clusters from the WINGS and OMEGAWINGS surveys. We focus on the infalling galaxy population and hence only consider blue, bright (B<18.2) late-type spectroscopically confirmed cl…
We present the deepest constraints yet on the median rest-UV+optical SED of $z\sim10$ galaxies, prior to JWST science operations. We constructed stacks based on four robust $J_{125}$-dropouts, previously identified across the GOODS fields. We used archival HST/WFC3 data and the full depth Spitzer/IRAC mosaics from the GREATS program, the deepest coverage at $\sim3-5μ$m to date. The most remarkable feature of the SED is a blue IRAC $[3.6]-[4.5]=-0.18\pm0.25$ mag color. We also find a nearly flat $H_{160}-[3.6]=0.07\pm0.22$ mag color, corresponding to a UV slope $β= -1.92\pm0.25$. This is consis…
Low-resolution LAMOST and Keck spectra of blue supergiant stars distributed over the disks of the Local Group spiral galaxies M 31 and M 33 are analyzed to determine stellar effective temperatures, gravities, metallicities, and reddening. Logarithmic metallicities at the center of the galaxies (in solar units) of $0.30\pm0.09$ and $0.11\pm0.04$ and metallicity gradients of $-0.37\pm0.13$ dex/$R_{25}$ and $-0.36\pm0.16$ dex/$R_{25}$ are measured for M 31 and M 33, respectively. For M 33 the 2-dimensional distribution of metallicity indicates a deviation from azimutal symmetry with an off-centre…
While bright, blue, compact galaxies are common at $\rm z \sim 1$, they are relatively rare in the local universe, and their evolutionary paths are uncertain. We have obtained resolved H I observations of nine $\rm z \sim 0$ luminous compact blue galaxies (LCBGs) using the Giant Metrewave Radio Telescope and Very Large Array in order to measure their kinematic and dynamical properties and better constrain their evolutionary possibilities. We find that the LCBGs in our sample are rotating galaxies that tend to have nearby companions, relatively high central velocity dispersions, and can have di…
The nature of possible evolutionary pathways between various types of dwarf galaxies is still not fully understood. Blue compact dwarf galaxies (BCDs) provide a unique window into dwarf galaxy formation and evolution and are often thought of as an evolutionary stage between different classes of dwarf galaxies. In this study we use deep optical and near-infrared observations of the underlying hosts of BCDs in order to study the structural differences between different types of dwarf galaxies. When compared with dwarf irregular galaxies of similar luminosities, we find that the underlying hosts …
Luminous Compact Blue Galaxies (LCBGs) are an extreme star-bursting population of galaxies that were far more common at earlier epochs than today. Based on spectroscopic and photometric measurements of LCBGs in massive (M >10^15 M_sun), intermediate redshift (0.5 < z < 0.9) galaxy clusters, we present their rest-frame properties including star-formation rate, dynamical mass, size, luminosity, and metallicity. The appearance of these small, compact galaxies in clusters at intermediate redshift helps explain the observed redshift evolution in the size-luminosity relationship among cluster galaxi…
Cosmological constraints from galaxy clusters rely on accurate measurements of the mass and internal structure of clusters. An important source of systematic uncertainty in cluster mass and structure measurements is the secure selection of background galaxies that are gravitationally lensed by clusters. This issue has been shown to be particular severe for faint blue galaxies. We therefore explore the selection of faint blue background galaxies, by reference to photometric redshift catalogs derived from the COSMOS survey and our own observations of massive galaxy clusters at z~0.2. We show tha…
The recent JWST tentative discovery of a population of super-early (redshift $z> 10$), relatively massive (stellar mass $M_* = 10^{8-9} M_{\odot}$) and evolved (metallicity $Z \approx 0.1 Z_{\odot}$) galaxies, which nevertheless show blue ($β\simeq -2.6$) spectra, and very small dust attenuation ($A_{\rm V} \leq 0.02$), challenges our interpretation of these systems. To solve the puzzle we propose two solutions in which dust is either (a) ejected by radiation pressure, or (b) segregated with respect to UV-emitting regions. We clarify the conditions for which the two scenarios apply, and show t…
We present a research of morphologies, spectra and environments of $\approx$ 2350 "green valley" galaxies at $0.2<z<1.0$ in the COSMOS field. The bimodality of dust-corrected \nuvr\ color is used to define "green valley" (thereafter, GV), which removes dusty star-forming galaxies from truly transiting galaxies between blue cloud and red sequence. Morphological parameters of green galaxies are intermediate between those of blue and red galaxy populations, both on the Gini--Asymmetry and the Gini--M$_{\rm 20}$ planes. Approximately 60% to 70% green disk galaxies have intermediate or big bulges, …
In this chapter I review the recent developments regarding the study of Blue Stragglers (BSS) in dwarf galaxies. The loose density environment of dwarf galaxies resembles that of the Galactic Halo, hence it is natural to compare their common BSS properties. At the same time, it is unescapable to compare with the BSS properties in Galactic Globular clusters, which constitute the reference point for BSS studies. Admittedly, the literature on BSS in dwarf galaxies is not plentiful. The limitation is mostly due to the large distance to even the closest dwarf galaxies. Nevertheless, recent studies …
Red and blue galaxies are traditionally classified using some specific cuts in colour or other galaxy properties, which are supported by empirical arguments. The vagueness associated with such cuts are likely to introduce a significant contamination in these samples. Fuzzy sets are vague boundary sets which can efficiently capture the classification uncertainty in the absence of any precise boundary. We propose a method for classification of galaxies according to their colours using fuzzy set theory. We use data from the SDSS to construct a fuzzy set for red galaxies with its members having di…
We analyse the relation between AGN host properties and large scale environment for a representative red and blue AGN host galaxy sample selected from the DR4 SDSS. A comparison is made with two carefully constructed control samples of non-active galaxies, covering the same redshift range and color baseline. The cross-correlation functions show that the density distribution of neighbours is almost identical for blue galaxies, either active, or non-active. Although active red galaxies inhabit environments less dense compared to non-active red galaxies, both reside in environments considerably d…
I introduce a simple empirical measure of average galaxy cluster sizes based on a comparison of the correlation lengths of red galaxies with blue that can provide a more accurate and bias-free measure of the average size and number density of galaxy clusters. Using 269,000 galaxies from the SDSS DR6 survey, I show that this 3D correlation length, averaged over many clusters, remains very nearly constant at L0= 4.797 +/- 0.024 Mpc/h from small redshifts out to redshifts of 0.5. This serves as a new measure of cosmic length scales and provides a means of testing the standard cosmological model t…
We have investigated the radial g-r color gradients of early-type galaxies in the Sloan Digital Sky Survey (SDSS) DR6 in the redshift range 0.00<z<0.06. The majority of massive early-type galaxies show a negative color gradient (red-cored) as generally expected for early-type galaxies. On the other hand, roughly 30 per cent of the galaxies in this sample show a positive color gradient (blue-cored). These "blue-cored" galaxies often show strong H beta absorption line strengths and/or emission line ratios that are indicative of the presence of young stellar populations. Combining the optical dat…
We present a radio continuum study of a population of extremely young and starburst galaxies, termed as blueberries at ${\sim}$ 1 GHz using the upgraded Giant Metrewave Radio Telescope (uGMRT). We find that their radio-based star formation rate (SFR) is suppressed by a factor of ${\sim}$ 3.4 compared to the SFR based on optical emission lines. This might be due to (i) the young ages of these galaxies as a result of which a stable equilibrium via feedback from supernovae has not yet been established (ii) escape of cosmic ray electrons via diffusion or galactic scale outflows. The estimated non-…
We have used Hubble Space Telescope ACS images to generate color-magnitude diagrams that reach below the magnitude of the horizontal branch in the Sculptor Group dwarf galaxies ESO294-010 and ESO410-005. In both diagrams blue horizontal branch stars are unambiguously present, a signature of the existence of an ancient stellar population whose age is comparable to that of the Galactic halo globular clusters. The result is reinforced by the discovery of numerous RR Lyrae variables in both galaxies. The occurrence of these stars in the first direct confirmation of the existence of ancient stellar…
The Gaia HR diagram shows the presence of apparently young stars at high tangential velocities. Using a simple analytical model I show that these stars are likely to be blue stragglers. Once normalized to red giant stars, the fraction of nearby halo blue stragglers is of order 20 percent, and remarkably close to that measured in dwarf galaxies. Motivated by this similarity, I apply to field blue stragglers scaling relations inferred from blue stragglers in dwarf galaxies. Doing this for the Milky Way halo returns an average stellar density of 3.4 x 10^-5 Msun/pc^3 and a dark matter density of …
We investigated the typical environment and physical properties of "red discs" and "blue bulges", comparing those to the "normal" objects in the blue cloud and red sequence. Our sample is composed of cluster members and field galaxies at $z \le 0.1$, so that we can assess the impact of the local and global environment. We find that disc galaxies display a strong dependence on environment, becoming redder for higher densities. This effect is more pronounced for objects within the virial radius, being also strong related to the stellar mass. We find that local and global environment affect galax…
We construct a sample of 10,680 wall galaxies and 3,064 void galaxies with $M_R \lesssim -20$ by cross referencing a void catalog from literature with Baryon Oscillation Spectroscopic Survey (BOSS) CMASS and WiggleZ galaxies, where the CMASS survey targets redder galaxies and the WiggleZ survey targets bluer galaxies. Comparing the density profiles of the red and blue galaxies as a function of the void radius, we find that the number ratio of red-to-blue galaxies increases with distances from the void centers, suggesting a deficit of luminous and normal red galaxies in voids. We find a mean (g…
We present a statistical study of the redshift evolution of the cluster galaxy population over a wide redshift range from 0.1 to 1.1, using $\sim 1900$ optically-selected CAMIRA clusters from $\sim 232$~deg$^2$ of the Hyper Suprime-Cam (HSC) Wide S16A data. Our stacking technique with a statistical background subtraction reveals color-magnitude diagrams of red-sequence and blue cluster galaxies down to faint magnitudes of $m_z\sim 24$. We find that the linear relation of red-sequence galaxies in the color-magnitude diagram extends down to the faintest magnitudes we explore with a small intrins…
We combine ultraviolet to near-infrared photometry with HI 21cm line observations for a complete volume-limited sample of nearby galaxies in different environments (from isolated galaxies to Virgo cluster members), to study the migration of spirals from the blue to the red sequence. Although our analysis confirms that, in the transition region between the two sequences, a high fraction of spirals host active galactic nuclei (AGN), it clearly shows that late-types with quenched star formation are mainly HI deficient galaxies preferentially found in the Virgo cluster. This not only suggests that…
