(Abridged) Low-luminosity, active star-forming blue compact galaxies (BCGs) are excellent laboratories for investigating the process of star formation on galactic scales and probing the interplay between massive stars and the surrounding interstellar (or intergalactic) medium. We investigated the morphology, structure, and stellar content of the Blue Compact Galaxy Mrk 900, combining optical integral field observations obtained with VIMOS at the VLT and deep broad-band photometry taken at the 2.5 m NOT telescope. From the integral field data, we built continuum, emission, and diagnostic line ratio maps and produced velocity and velocity dispersion maps. We also generated the integrated spectrum of the major HII regions and the nuclear area to determine reliable physical parameters and oxygen abundances. The broad-band data, tracing the galaxy up to radius 4 kpc, allowed us to investigate the properties of the low surface brightness underlying stellar host. We disentangle two different stellar components in Mrk 900: a young population, which resolves into individual stellar clusters with ages ~5.5-6.6 Myr and extends about 1 kpc along the galaxy minor axis, is placed on top of a rather red and regular shaped underlying stellar host, several Gyr old. We find evidence of a substantial amount of dust and an inhomogeneous extinction pattern, with a dust lane crossing the central starburst. Mrk 900 displays overall rotation, although distorted in the central, starburst regions; the dispersion velocity map is highly inhomogeneous, with values increasing up to 60 kms$^{-1}$ at the periphery of the SF regions, where we also find hints of the presence of shocks. Our observational results point to an interaction or merger with a low-mass object or infalling gas as plausible trigger mechanisms for the present starburst event.
Nearby, low-metallicity dwarf starburst galaxies hosting active galactic nuclei (AGNs) offer the best local analogs to study the early evolution of galaxies and their supermassive black holes (BHs). Here we present a detailed multi-wavelength investigation of star formation and BH activity in the low-metallicity dwarf-dwarf galaxy merger Mrk 709. Using Hubble Space Telescope H$α$ and continuum imaging combined with Keck spectroscopy, we determine that the two dwarf galaxies are likely in the early stages of a merger (i.e., their first pass) and discover a spectacular $\sim 10$ kpc-long string …
We present an integral field spectroscopic study with the Gemini Multi-Object Spectrograph (GMOS) of the unusual blue compact dwarf (BCD) galaxy Mrk 996. We show through velocity and dispersion maps, emission-line intensity and ratio maps, and by a new technique of electron density limit imaging that the ionization properties of different regions in Mrk 996 are correlated with their kinematic properties. From the maps, we can spatially distinguish a very dense high-ionization zone with broad lines in the nuclear region, and a less dense low-ionization zone with narrow lines in the circumnuclea…
The optical spectroscopic and radio interferometric HI 21 cm-line observations of the blue compact dwarf galaxy Mrk 22 are presented. The Wolf-Rayet (WR) emission line features corresponding to high ionization lines of HeII $λ$4686 and CIV $λ$5808 from young massive stars are detected. The ages of two prominent star forming regions in the galaxy are estimated as $\sim$10 Myr and $\sim$ 4 Myr. The galaxy has non-thermal radio deficiency, which also indicates a young star-burst and lack of supernovae events from the current star formation activities, consistent with the detection of WR emission …
In this work we characterise the properties of the object SDSS J020536.84-081424.7, an extended nebular region with projected extension of $14 \times 14$ kpc$^{2}$ in the line of sight of the ETG Mrk 1172, using unprecedented spectroscopic data from MUSE. We perform a spatially resolved stellar population synthesis and estimate the stellar mass for both Mrk 1172 ($1 \times 10^{11} M_{\odot}$) and our object of study ($3 \times 10^{9} M_{\odot}$). While the stellar content of Mrk 1172 is dominated by an old ($\sim 10$ Gyr) stellar population, the extended nebular emission has its light dominate…
The incidence and properties of present-day dwarf galaxies hosting massive black holes (BHs) can provide important constraints on the origin of high-redshift BH seeds. Here we present high-resolution X-ray and radio observations of the low-metallicity, star-forming, dwarf-galaxy system Mrk 709 with the Chandra X-ray Observatory and the Karl G. Jansky Very Large Array. These data reveal spatially coincident hard X-ray and radio point sources with luminosities suggesting the presence of an accreting massive BH (M_BH ~ 10^5-10^7 Msun). Based on imaging from the Sloan Digital Sky Survey (SDSS), we…
Using VIMOS-IFU observations, we study the interstellar medium (ISM) of two star-forming dwarf galaxies, UM 461 and Mrk 600. Our aim was to search for the existence of metallicity inhomogeneities that might arise from infall of nearly pristine gas feeding ongoing localized star-formation. The IFU data allowed us to study the impact of external gas accretion on the chemical evolution as well as the ionised gas kinematics and morphologies of these galaxies. Both systems show signs of morphological distortions, including cometary-like morphologies. We analysed the spatial variation of 12 + log(O/…
Mrk\,622 is a Compton Thick AGN and a double-peaked narrow emission line galaxy, thus a dual AGN candidate. In this work, new optical long-slit spectroscopic observations clearly show that this object is rather a triple peaked narrow emission line galaxy, with both blue and red shifted narrow emission lines, as well as a much narrower emission line centred at the host galaxy systemic velocity. The average velocity offset between the blue and red shifted components is $\sim$500 km\,s$^{-1}$, which is producing the apparent double-peaked emission lines. These two components are in the loci of AG…
In most blue compact dwarf (BCD) galaxies, it remains unclear what triggers their bursts of star formation. We study the HI of three relatively isolated BCDs, Mrk 178, VII Zw 403, and NGC 3738, in detail to look for signatures of star formation triggers, such as gas cloud consumption, dwarf-dwarf mergers, and interactions with companions. High angular and velocity resolution atomic hydrogen (H I) data from the Very Large Array (VLA) dwarf galaxy HI survey, Local Irregulars That Trace Luminosity Extremes, The HI Nearby Galaxy Survey (LITTLE THINGS), allows us to study the detailed kinematics an…
Search for and study of extended emission-line regions (EELRs) related to AGN in early-type galaxies is interesting to probe the history of nuclear ionization activity and also to understand the process of external gas accretion. In this work, we present observations of the EELR in Mrk 78 obtained at the 6-m Russian telescope using the long-slit and 3D spectroscopy methods. We show that ionized-gas clouds at the 12-16 kpc projected distances from the nucleus are ionized by the AGN radiation. Also we have checked if the galaxy appearing in the optical images in the immediate neighbourhood of Mr…
We report the results of reverberation mapping three bright Seyfert galaxies, Mrk 79, NGC 3227, and Mrk 841, from a campaign conducted from December 2016 to May 2017 with the Wyoming Infrared Observatory (WIRO) 2.3-meter telescope. All three of these targets have shown asymmetric broad H$β$ emission lines in the past, although their emission lines were relatively symmetric during our observations. We measured Hβ time lags for all three targets and estimated masses of their black holes -- for the first time in the case of Mrk 841. For Mrk 79 and NGC 3227, the data are of sufficient quality to r…
We propose that there is an evolutionary link between ultra-compact blue dwarf galaxies (UCBDs) with active star formation and nucleated dwarfs based on the results of numerical simulations of dwarf-dwarf merging. We consider the observational fact that low-mass dwarfs can be very gas-rich, and thereby investigate the dynamical and chemical evolution of very gas-rich, dissipative dwarf-dwarf mergers. We find that the remnants of dwarf-dwarf mergers can be dominated by new stellar populations formed from the triggered starbursts and consequently can have blue colors and higher metallicities (Z~…
We present two-dimensional maps of emission-line fluxes and kinematics, as well as of the stellar kinematics of the central few kpc of five bright nearby Seyfert galaxies -- Mrk\,6, Mrk\,79, Mrk\,348, Mrk\,607 and Mrk\,1058 -- obtained from observations with the Gemini Multi-Object Spectrograph (GMOS) Integral Field Unit (IFU) on the Gemini North Telescope. The data cover the inner 3\farcs5$\times$5\farcs0 -- corresponding to physical scales in the range 0.6$\times$0.9 to 1.5$\times$2.2\,kpc$^2$ -- at a spatial resolution ranging from 110 to 280 pc with a spectral coverage of 4300 -- 7100\,Å a…
We present the results from a spectroscopic monitoring campaign to obtain reverberation-mapping measurements and investigate the broad-line region kinematics for active galactic nuclei (AGN) of Mrk~817 and NGC~7469. This campaign was undertaken with the Lijiang 2.4-meter telescope, the median spectroscopic sampling is 2.0 days for Mrk~817 and 1.0 days for NGC~7469. We detect time lags of the broad emission lines including H$β$, H$γ$, He~{\sc ii} and He~{\sc i} for both AGNs, and including Fe~{\sc ii} for Mrk~817 with respect to the varying AGN continuum at 5100~Å. Investigating the relationshi…
We present the results of our spatially resolved investigation into the interplay between the ages of the stellar populations and the kinematics of the warm ionised outflows in the well-studied type II quasar Markarian 34. Utilising IFS data, we determine the spatial distribution of the young stellar population (YSP; $t_{ysp} < 100 \mbox{ Myr}$) using spectral synthesis modelling. We also employ the $\mbox{[OIII]}λ5007$ emission line as a tracer of the warm ionised gas kinematics. We demonstrate a spatial correlation between the outer edges of the blue-side of the outflow and an enhancement in…
The quasi-stellar object (QSO)/merger Mrk 231 is arguably the nearest and best laboratory for studying QSO feedback. It hosts several outflows, including broad-line winds, radio jets, and a poorly-understood kpc scale outflow. In this Letter, we present integral field spectroscopy from the Gemini telescope that represents the first unambiguous detection of a wide-angle, kpc scale outflow from a powerful QSO. Using neutral gas absorption, we show that the nuclear region hosts an outflow with blueshifted velocities reaching 1100 km/s, extending 2-3 kpc from the nucleus in all directions in the p…
We present Gemini Near-infrared Integral Field Spectrograph (NIFS) observations of the inner $1.5\times1.5$ kpc$^2$ of the narrow-line Seyfert 1 galaxy Mrk 42 at a spatial resolution of 60 pc and spectral resolution of 40 km s$^{-1}$. The emission-line flux and equivalent width maps clearly show a ring of circumnuclear star formation regions (CNSFRs) surrounding the nucleus with radius of $\sim$500 pc. The spectra of some of these regions show molecular absorption features which are probably of CN, TiO or VO, indicating the presence of massive evolved stars in the thermally pulsing asymptotic …
We aim to quantify the molecular gas reservoir in a subset of 6 low-metallicity galaxies from the Herschel Dwarf Galaxy Survey with newly acquired CO data, and link this reservoir to the observed star formation activity. We present CO(1-0), CO(2-1), and CO(3-2) observations obtained at the ATNF Mopra 22-m, APEX, and IRAM 30-m telescopes, as well as [CII] 157um and [OI] 63um observations obtained with the Herschel/PACS spectrometer in the 6 galaxies: Haro11, Mrk1089, Mrk930, NGC4861, NGC625, and UM311. We derive molecular gas mass from several methods including the use of the CO-to-H2 conversio…
To elucidate the location, physical conditions, mass outflow rate, and kinetic luminosity of the outflow from the active nucleus of the Seyfert 1 galaxy Mrk 509 we used coordinated ultraviolet and X-ray spectral observations in 2012 to follow up our lengthier campaign conducted in 2009. We observed Mrk 509 with the Cosmic Origins Spectrograph (COS) on the Hubble Space Telescope (HST) on 2012-09-03 and 2012-10-11 coordinated with X-ray observations using the High Energy Transmission Grating on the Chandra X-ray Observatory. Our far-ultraviolet spectra used grating G140L on COS to cover waveleng…
We present the remarkable discovery that the dwarf irregular galaxy NGC 2366 is an excellent analog of the Green Pea (GP) galaxies, which are characterized by extremely high ionization parameters. The similarities are driven predominantly by the giant H II region Markarian 71 (Mrk 71). We compare the system with GPs in terms of morphology, excitation properties, specific star-formation rate, kinematics, absorption of low-ionization species, reddening, and chemical abundance, and find consistencies throughout. Since extreme GPs are associated with both candidate and confirmed Lyman continuum (L…
We report here our finding of two new blueberry galaxies using optical IFU spectroscopic data from the MaNGA survey. Both the blueberries are found to be compact ($\leq$ $1 - 2$ kpc) starburst systems located at the outskirts of Low Surface Brightness (LSB) galaxies. Our blueberries have the lowest stellar masses $\sim$ 10$^{5}$ M$_{\odot}$ amongst the locally known blueberry galaxies. We find a significantly large mean metallicity difference ($\sim$ 0.5 dex) between the blueberry sources and their associated LSBs. Moreover, the radial metallicity gradients in our blueberries are also differen…
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…
A population of so-called "blueberry" starbursting dwarf galaxies with extremely blue colors, low-metallicities, and enormous ionization ratios, has recently been found by Yang et al. (2017). Yet we still do not know their detailed properties, such as morphologies, AGN occupations, massive star contents, infrared emission, dust properties, etc. As a pilot study of the blueberries, we investigate the spectroscopic and photometric properties of two blueberry candidates, RGG B and RGG 5, for which Hubble Space Telescope high-resolution images are available. We find that RGG B and RGG 5 perhaps ar…
Establishing the spatial extents and the nature of the outer stellar populations of dwarf galaxies are necessary ingredients for the determination of their total masses, current dynamical states and past evolution. We here describe our investigation of the outer stellar content of the Boötes I ultra-faint dwarf galaxy, a satellite of the Milky Way Galaxy. We identify candidate member blue horizontal branch and blue straggler stars of Boötes I, both tracers of the underlying ancient stellar population, using a combination of multi-band Pan-STARRS photometry and Gaia astrometry. We find a total …
Blue Compact Dwarf (BCD) Galaxies in the nearby Universe provide a means for studying feedback mechanisms and star-formation processes in low-metallicity environments in great detail. Due to their vicinity, these local analogues to young galaxies are well suited for high-resolution studies that would be unfeasible for primordial galaxies in the high-redshift universe. Here we present HST-WFC3 observations of one such BCD, Mrk 71, one of the most powerful local starbursts known, in the light of [O II], He II, Hb, [O III], Ha, and [S II]. At D=3.44 Mpc, this extensive suite of emission line imag…
Half-dozen of extreme representatives of void dwarf galaxy population were found in our study of evolutionary status of a hundred galaxies in the nearby Lynx-Cancer void. They are very gas-rich, extremely low-metallicity [7.0 < 12+log(O/H)< ~7.3] objects, with blue colours of outer parts. The colours indicate the ages of the oldest visible stellar population of one to a few Gyr. They all are intrinsically faint, mostly Low Surface Brightness dwarfs, with M_B range of -9.5 to -14 mag. Thus, their finding is a subject of the severe observational selection. The recent advancement in search for su…
The origin of the observed steep rotation curves of blue compact dwarf galaxies (BCDs) remains largely unexplained by theoretical models of BCD formation. We therefore investigate the rotation curves in BCDs formed from mergers between gas-rich dwarf irregular galaxies based on the results of numerical simulations for BCD formation. The principal results are as follows. The dark matter of merging dwarf irregulars undergoes a central concentration so that the central density can become up to six times higher than those of the initial dwarf irregulars. However, the more compact dark matter halo …
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 …
We developed a spectroscopic monitoring project to investigate the kinematics of the broad-line region (BLR) in active galactic nuclei (AGN) with ultra-fast outflows (UFOs). Mrk~79 is a radio-quiet AGN with UFOs and warm absorbers, had been monitored by three reverberation mapping (RM) campaigns, but its BLR kinematics is not understood yet. In this paper, we report the results from a new RM-campaign of Mrk~79, which was undertaken by Lijiang 2.4-m telescope. Mrk~79 is seeming to come out the faint state, the mean flux approximates a magnitude fainter than historical record. We successfully me…
We present BV photometry of four Sagittarius dwarf spheroidal galaxy globular clusters: Arp 2, NGC 5634, Palomar 12, and Terzan 8, obtained with the Danish telescope at ESO-La Silla. We measure the structural parameters of the clusters using a King profile fitting, obtaining the first reliable measurements of the tidal radius of Arp 2 and Terzan 8. These two clusters are remarkably extended and with low concentrations; with a concentration of only c = 0.41 +/- 0.02, Terzan 8 is less concentrated than any cluster in our Galaxy. Blue stragglers are identified in the four clusters, and their spat…
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…
We investigate the structural properties of the underlying hosts of 34 blue compact dwarf (BCD) galaxies with deep near-infrared (NIR) photometry. The BCD sample is selected from the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey in the Great observatories origins Deep Survey North and South fields. We extract the surface brightness profile (SBP) in the optical F 435W and NIR F 160W bands. The SBPs of BCDs in the H band reach 26 mag arcsec^-2 at the 3σlevel, which is so far the deepest NIR imaging of BCDs. Then we fit the SBPs with one- and two- component Sersic models. About half of…
It has long been speculated that Blue Compact Dwarf galaxies (BCDs) are formed through the interaction between low-mass gas-rich galaxies, but a few candidates of such systems have been studied in detail. We study a sample of compact star-forming dwarf galaxies that are selected from a merging dwarf galaxy catalog. We present a detailed study of their spectroscopic and structural properties. We find that these BCDs looking galaxies host extended stellar shells and thus are confirmed to be a dwarf-dwarf merger. Their stellar masses range between $8\times10^7$~M$_{\sun}$ and $2\times10^9$~M$_{\s…
The presence of blue cores in some dwarf early-type galaxies (dEs) in galaxy clusters suggests the scenario of late-type galaxy infall and subsequent transformation into red, quiescent dEs. We present Gemini Multi-Object Spectrographs long-slit spectroscopy of two dEs with blue cores (dE(bc)s), EVCC 591 and EVCC 516, located at the core and outskirt of the Virgo cluster, respectively. We obtained their internal kinematics along the major axis out to, at least, ~ 1 effective radius. EVCC 591 shows evidence of a kinematically decoupled core (KDC) with a size of 2" (160 pc), exhibiting an inverte…
X-shooter and ISIS WHT spectra of the starforming galaxy PHL 293B also known as A2228-00 and SDSS J223036.79-000636.9 are presented in this paper. We find broad (FWHM = 1000km/s) and very broad (FWZI = 4000km/s) components in the Balmer lines, narrow absorption components in the Balmer series blueshifted by 800km/s, previously undetected FeII multiplet (42) absorptions also blueshifted by 800km/s, IR CaII triplet stellar absorptions consistent with [Fe/H] < -2.0 and no broad components or blushifted absorptions in the HeI lines. Based on historical records, we found no optical variability at t…
A study of the blue compact dwarf (BCD) galaxy Mrk 996 based on high resolution optical VLT VIMOS integral field unit spectroscopy is presented. Mrk 996 displays multi-component line emission, with most line profiles consisting of a narrow, central Gaussian with an underlying broad component. The broad HI Balmer component splits into two separate broad components inside a 1".5 radius from the nucleus; these are attributed to a two-armed mini-spiral. The rotation curve of Mrk 996 derived from the Hαnarrow component yields a total mass of 5x10^8 Msol within a radius of 3 kpc. The high excitation…
Blue compact dwarf galaxies (BCDs) form stars at, for their sizes, extraordinarily high rates. In this paper, we study what triggers this starburst and what is the fate of the galaxy once its gas fuel is exhausted. We select four BCDs with smooth outer regions, indicating them as possible progenitors of dwarf elliptical galaxies. We have obtained photometric and spectroscopic data with the FORS and ISAAC instruments on the VLT. We analyse their infra-red spectra using a full spectrum fitting technique which yields the kinematics of their stars and ionized gas together with their stellar popula…
The presence of blue-cored dwarf early-type galaxies (dE(bc)s) in high-density environments supports the scenario of the transformation of infalling late-type galaxies into quiescent dwarf early-type galaxies by environmental effects. While low-density environments lacking environmental processes could not be relevant to the formation of dE(bc)s, we discovered a large sample of rare dE(bc)s in isolated environments at z < 0.01 using the NASA-Sloan Atlas catalog. Thirty-two isolated dE(bc)s were identified by visual inspection of the Sloan Digital Sky Survey images and g - r color profiles. We …
We present and analyse the photometric properties of a nearly complete sample of blue compact dwarf (BCD) and irregular galaxies in the Virgo cluster from multi-band SDSS images. Our study intends to shed light on the ongoing debate of whether a structural evolution from present-day star-forming dwarf galaxies in a cluster environment into ordinary early-type dwarf galaxies is possible based on the structural properties. For this purpose, we decompose the surface brightness profiles of the BCDs into the luminosity contribution of the starburst component and that of their underlying low surface…
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/
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 …
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 present the emission Halpha line images for 40 galaxies of the Local Volume based on the observations at the 6-meter BTA telescope. Among them there are eight satellites of the Milky Way and Andromeda M31 as well as two companions to M51. The measured Halpha fluxes of the galaxies are used to determine their integral (SFR) and specific (sSFR) star formation rates. The values of Log(sSFR) for the observed galaxies lie in the range of (-9, -14)/yr. A comparison of SFR estimates derived from the Halpha flux and from the ultraviolet FUV flux yields evidence that two blue compact galaxies MRK 47…
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…
The 2MIG isolated AGNs. 3. Optical--IR variability and dust reverberation in the NLSy1 galaxies Mrk~42 and Mrk~493
Main Astronomical Observatory of the NAS of Ukraine（乌克兰国家科学院主天文台）；International Center for Astronomical, Medical and Ecological Research (ICAMER)（国际天文、医学和生态研究中心）；Instituto de Astrofísica de Canarias（加那利群岛天体物理研究所）；Universidad de la Laguna（拉古纳大学）
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 …
It has long been speculated that many starburst or compact dwarf galaxies are resulted from dwarf-dwarf galaxy merging, but unequivocal evidence for this possibility has rarely been reported in the literature. We present the first study of deep optical broadband images of a gas-dominated blue compact dwarf galaxy (BCD) VCC848 (Mstar=2e8Msun) which hosts extended stellar shells and thus is confirmed to be a dwarf-dwarf merger. VCC848 is located in the outskirts of the Virgo Cluster. By analyzing the stellar light distribution, we found that VCC848 is the result of a merging between two dwarf ga…
We present a census of the 12,060 spectra of blue objects ($(g-r)_0<-0.25$) in the Sloan Digital Sky Survey (SDSS) Data Release 8 (DR8). As part of the data release, all of the spectra were cross-correlated with 48 template spectra of stars, galaxies and QSOs to determine the best match. We compared the blue spectra by eye to the templates assigned in SDSS DR8. 10,856 of the objects matched their assigned template, 170 could not be classified due to low signal-to-noise (S/N), and 1034 were given new classifications. We identify 7458 DA white dwarfs, 1145 DB white dwarfs, 273 rarer white dwarfs…
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 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…
We present a homogeneous study of blue straggler stars across ten outer halo globular clusters, three classical dwarf spheroidal and nine ultra-faint galaxies based on deep and wide-field photometric data taken with MegaCam on the Canada-France-Hawaii Telescope. We find blue straggler stars to be ubiquitous among these Milky Way satellites. Based on these data, we can test the importance of primordial binaries or multiple systems on blue straggler star formation in low density environments. For the outer halo globular clusters we find an anti-correlation between the specific frequency of blue …
The Gaia catalogue of nearby stars (GCNS) divided all objects with parallaxes $>$10mas into GCNS-selected and GCNS-rejected 100pc samples. Below the white dwarf (WD) sequence in the complete GCNS color-magnitude diagram (CMD), at $Gabs>14.7+4.7(G-RP)$, there appear 60 GCNS-selected faint blue white dwarfs (FBWDs). However this CMD region is also populated by 411 GCNS-rejected objects, mainly from crowded regions towards the Galactic centre and the Magellanic Clouds. The WD catalog of Gentile Fusillo et al. (2021) lists only 47 GCNS-selected but also 8 GCNS-rejected objects. I confirm 59 of the…
A clear link between a dwarf-dwarf merger event and enhanced star formation (SF) in the recent past was recently identified in the gas-dominated merger remnant VCC 848, offering by far the clearest view of a gas-rich late-stage dwarf-dwarf merger. We present a joint analysis of JVLA HI emission-line mapping, optical imaging and numerical simulations of VCC 848, in order to examine the impact of the merger on the stellar and gaseous distributions. VCC 848 has less than 30% of its HI gas concentrated within the central high-surface-brightness star-forming region, while the remaining HI is entrai…
Ultra-long gamma-ray bursts (ulGRBs) are a new population of GRBs with extreme durations of $\sim 10^{4}$ s. Leading candidates for their origin are blue supergiant collapsars, magnetars, and white dwarf tidal disruption events (WD-TDEs) caused by massive black holes (BHs). Recent observations of supernova-like (SN-like) bumps associated with ulGRBs challenged both the WD-TDE and the blue supergiant models because of the detection of SNe and the absence of hydrogen lines, respectively. We propose that WD-TDEs can accommodate the observed SN-like bumps if the fallback WD matter releases energy …
