Dwarf Objects Research Articles

Exploring Celestial Object Characteristics: An In-depth Analysis of Quasars, Stars, and White Dwarfs Using the Sloan Digital Sky Survey (SDSS) Dataset

This research utilizes the Sloan Digital Sky Survey (SDSS) dataset, examining 12,884 observations to explore quasars, stars, and white dwarf objects. Magnitude data and coordinates across five filter bands are analyzed, revealing unique features through statistical methods. The identification of 77,429 quasars with 15 dimensions enhances the dataset. Thorough analyses of stellar and white dwarf classes, coupled with visualization techniques, unveil variable relationships. Residual validation and Gaussian kernel density plots confirm significant class differences. Non-linear regression and a normal distribution mixture model depict complex variable relationships. A parallel coordinates plot aids in interpreting data patterns, while predictive modeling via regression exposes meaningful coefficients. Logistic regression effectively classifies astronomical objects in the SDSS training data. This research contributes to understanding celestial object characteristics, offering valuable insights for astronomers and astrophysicists in analyzing large-scale astronomical datasets.

Examining the Rotation Period Distribution of the 40 Myr Tucana–Horologium Association with TESS

The Tucana–Horologium association (Tuc-Hor) is a 40 Myr old moving group in the southern sky. In this work, we measure the rotation periods of 313 Tuc-Hor objects with TESS light curves derived from TESS full-frame images and membership lists driven by Gaia EDR3 kinematics and known youth indicators. We recover a period for 81.4% of the sample and report 255 rotation periods for Tuc-Hor objects. From these objects we identify 11 candidate binaries based on multiple periodic signals or outlier Gaia DR2 and EDR3 renormalized unit weight error values. We also identify three new complex rotators (rapidly rotating M dwarf objects with intricate light-curve morphology) within our sample. Along with the six previously known complex rotators that belong to Tuc-Hor, we compare their light-curve morphology between TESS Cycle 1 and Cycle 3 and find that they change substantially. Furthermore, we provide context for the entire Tuc-Hor rotation sample by describing the rotation period distributions alongside other youth indicators such as Hα and Li equivalent width, as well as near-ultraviolet and X-ray flux. We find that measuring rotation periods with TESS is a fast and effective means to confirm members in young moving groups.

MORPHOLOGICAL STUDY OF A SAMPLE OF DWARF TIDAL GALAXIES USING THE C-A PLANE

In this investigation, we determined the Concentration (C) and Asymmetry (A) parameters in a sample of tidal dwarf galaxies (TDG) or candidate galaxies. Most of the galaxies in the sample were found to be in a very precise region of the C-A plane, which clearly separates them from other galaxies. In addition, the stellar mass (Mstar) and the star formation rate (SFR) in the sample were determined using optical images and GALEX observations. The main results are: the Mstar and the SFR in the TDG sample do not follow a linear correlation with the C and A respectively, as observed in the rest of galaxies, and the Mstar and the SFR have a linear correlation similar to that followed by galaxies at high redshift. Then, we can conclude that the C-A plane can be a useful method for the morphological identification of candidates for TDG or dwarf objects from very turbulent environments.

Looking into the cradle of the grave: J22564–5910, a potential young post-merger hot subdwarf

Context. We present the discovery of J22564–5910, a new type of hot subdwarf (sdB) which shows evidence of gas present in the system and it has shallow, multi-peaked hydrogen and helium lines which vary in shape over time. All observational evidence points towards J22564–5910 being observed very shortly after the merger phase that formed it. Aims. Using high-resolution, high signal-to-noise spectroscopy, combined with multi-band photometry, Gaia astrometry, and TESS light curves, we aim to interpret these unusual spectral features. Methods. The photometry, spectra, and light curves were all analysed, and their results were combined in order to support our interpretation of the observations: the likely presence of a magnetic field combined with gas features around the sdB. Based on the triple-peaked H lines, the magnetic field strength was estimated and, by using the SHELLSPEC code, qualitative models of gas configurations were fitted to the observations. Results. All observations can either be explained by a magnetic field of ∼650 kG, which enables the formation of a centrifugal magnetosphere, or a non-magnetic hot subdwarf surrounded by a circumstellar gas disc or torus. Both scenarios are not mutually exclusive and both can be explained by a recent merger. Conclusions. J22564–5910 is the first object of its kind. It is a rapidly spinning sdB with gas still present in the system. It is the first post-merger star observed this early after the merger event, and as such it is very valuable system to test merger theories. If the magnetic field can be confirmed, it is not only the first magnetic sdB, but it hosts the strongest magnetic field ever found in a pre-white dwarf object. Thus, it could represent the long sought-after immediate ancestor of strongly magnetic white dwarfs.

The evolution of Lithium: implications of a universal Spite plateau

ABSTRACT The cosmological 7Li problem consists in explaining why the primordial Li abundance, as predicted by the standard Big Bang nucleosynthesis theory with constraints from WMAP and Planck, is a factor of 3 larger than the Li abundance measured in the stars of the Spite plateau defined by old, warm dwarf stars of the Milky Way halo. Several explanations have been proposed to explain this difference, including various Li depletion processes as well as non-standard Big Bang nucleosynthesis, but the main question remains unanswered. In this paper, we present detailed chemical evolution models for dwarf spheroidal and ultra faint galaxies, compute the galactic evolution of 7Li abundance in these objects, and compare it with observations of similar objects. In our models, Li is mainly produced by novae and cosmic rays, and to a minor extent, by low and intermediate mass stars. We adopt the yield combination that best fits the Li abundances in the Milky Way stars. It is evident that the observations of dwarf objects define a Spite plateau, identical to that observed in the Milky Way, thus suggesting that the Spite plateau could be a universal feature and its meaning should be discussed. The predictions of our models for dwarf galaxies are obtained by assuming as Li primordial abundance either the one detected in the atmospheres of the oldest halo stars (Spite plateau; A(Li) ∼ 2.2 dex), or the one from cosmological observations (WMAP; A(Li) ∼ 2.66 dex). Finally, we discuss the implications of the universality of the Spite plateau results.

Evidence of galaxy interaction in the narrow-line Seyfert 1 galaxy IRAS 17020+4544 seen by NOEMA

ABSTRACT The narrow-line Seyfert 1 galaxy IRAS 17020+4544 is one of the few sources where both an X-ray ultrafast outflow and a molecular outflow were observed to be consistent with energy conservation. However, IRAS 17020+4544 is less massive and has a much more modest active galactic nucleus (AGN) luminosity than the other examples. Using recent CO(1–0) observations with the NOrthern Extended Millimeter Array, we characterized the molecular gas content of the host galaxy for the first time. We found that the molecular gas is distributed into an apparent central disc of 1.1 × 109 M⊙, and a northern extension located up to 8 kpc from the centre with a molecular gas mass $M_{\mathrm{ H}_2}\sim 10^8\, \mathrm{ M}_\odot$. The molecular gas mass and the CO dynamics in the northern extension reveal that IRAS 17020+4544 is not a standard spiral galaxy, instead it is interacting with a dwarf object corresponding to the northern extension. This interaction possibly triggers the high accretion rate on to the supermassive black hole. Within the main galaxy, which hosts the AGN, a simple analytical model predicts that the molecular gas may lie in a ring, with less molecular gas in the nuclear region. Such distribution may be the result of the AGN activity that removes or photodissociates the molecular gas in the nuclear region (AGN feedback). Finally, we have detected a molecular outflow of mass $M_{\mathrm{ H}_2}=(0.7\!-\!1.2)\times 10^7\, \mathrm{ M}_\odot$ in projection at the location of the northern galaxy, with a similar velocity to that of the massive outflow reported in previous millimetre data obtained by the Large Millimeter Telescope.

The beam pulse amplifier in space and laboratory plasmas

We discuss the physical background of the a new mechanism in which short electromagnetic pulses can be strongly amplified by interaction with suprathermal electrons in a near-equilibrium near-uniform magnetised plasma layer. This occurs under the special condition that the wave phase and group speeds along the field lines are equal, a condition met by extraordinary mode electromagnetic waves propagating in the medium. In this case the pulse interacts strongly via the Čerenkov resonance with suprathermal electrons moving along the field lines at the same speed as the pulse, leading to rapid wave growth, the instability within the pulse being shown to be equivalent to that of a monoenergetic electron beam moving along the field lines in an infinite uniform plasma. This effect accounts for basic features of powerful pulsed electromagnetic emissions in planetary magnetospheres, specifically whistler-mode chorus emissions in space plasmas, the remarkable radio emissions observed from brown dwarf objects, as well as in laboratory plasma.

Gaia Data Release 2

Context. Gaia Data Release 2 provides high-precision astrometry and three-band photometry for about 1.3 billion sources over the full sky. The precision, accuracy, and homogeneity of both astrometry and photometry are unprecedented. Aims. We highlight the power of the Gaia DR2 in studying many fine structures of the Hertzsprung-Russell diagram (HRD). Gaia allows us to present many different HRDs, depending in particular on stellar population selections. We do not aim here for completeness in terms of types of stars or stellar evolutionary aspects. Instead, we have chosen several illustrative examples. Methods. We describe some of the selections that can be made in Gaia DR2 to highlight the main structures of the Gaia HRDs. We select both field and cluster (open and globular) stars, compare the observations with previous classifications and with stellar evolutionary tracks, and we present variations of the Gaia HRD with age, metallicity, and kinematics. Late stages of stellar evolution such as hot subdwarfs, post-AGB stars, planetary nebulae, and white dwarfs are also analysed, as well as low-mass brown dwarf objects. Results. The Gaia HRDs are unprecedented in both precision and coverage of the various Milky Way stellar populations and stellar evolutionary phases. Many fine structures of the HRDs are presented. The clear split of the white dwarf sequence into hydrogen and helium white dwarfs is presented for the first time in an HRD. The relation between kinematics and the HRD is nicely illustrated. Two different populations in a classical kinematic selection of the halo are unambiguously identified in the HRD. Membership and mean parameters for a selected list of open clusters are provided. They allow drawing very detailed cluster sequences, highlighting fine structures, and providing extremely precise empirical isochrones that will lead to more insight in stellar physics. Conclusions. Gaia DR2 demonstrates the potential of combining precise astrometry and photometry for large samples for studies in stellar evolution and stellar population and opens an entire new area for HRD-based studies.

Seeking large-scale magnetic fields in a pure-disk dwarf galaxy NGC 2976

It is still unknown how magnetic field-generation mechanisms could operate in low-mass dwarf galaxies. Here, we present a detailed study of a nearby pure-disk dwarf galaxy NGC 2976. Unlike previously observed dwarf objects, this galaxy possesses a clearly defined disk. For the purpose of our studies, we performed deep multi-frequency polarimetric observations of NGC 2976 with the VLA and Effelsberg radio telescopes. Additionally, we supplement them with re-imaged data from the WSRT-SINGS survey. The magnetic field morphology discovered in NGC 2976 consists of a southern polarized ridge. This structure does not seem to be due to just a pure large-scale dynamo process (possibly cosmic-ray driven) at work in this object, as indicated by the RM data and dynamo number calculations. Instead, the field of NGC 2976 is modified by past gravitational interactions and possibly also by ram pressure inside the M 81 galaxy group environment. The estimates of total (7 muG) and ordered (3 muG) magnetic field strengths, as well as degree of field order (0.46), which is similar to those observed in spirals, suggest that tidally generated magnetized gas flows can further enhance dynamo action in the object. NGC 2976 is apparently a good candidate for the efficient magnetization of its neighbourhood. It is able to provide an ordered (perhaps also regular) magnetic field into the intergalactic space up to a distance of about 5 kpc. Tidal interactions (and possibly also ram pressure) can lead to the formation of unusual magnetic field morphologies (like polarized ridges) in galaxies out of the star-forming disks, which do not follow any observed component of the interstellar medium (ISM), as observed in NGC 2976. These galaxies are able to provide ordered magnetic fields far out of their main disks.

Visiting two objects in the field of the ring galaxy HRG 2302

Aims. We investigate the nature of two galaxies that are located in the field of the ring galaxy HRG 2302. Methods. This study is based on direct BVRI imaging and long-slit spectrophotometric data in the range of 4000−9500 A obtained with the 1.6 m telescope of the Observatorio do Pico dos Dias, Brazil. The spectra were used to determine the radial velocity. Results. The primary objective of the retrieval of the photometric data was to identify the fine structures of objects H and I. In addition, we performed image processing and made a photometric analysis to obtain the integrated standard BVRI magnitudes. The contour maps show evidence of material connecting both galaxies, suggesting that they might be interacting close companions. We estimated redshifts of z = 0.0689 and z = 0.0692. The spectra of the two galaxies resemble those of an early-type galaxy. The fact that the objects have a small radial-velocity difference and the structures around object I suggest an ongoing tidal interaction between the two galaxies. Conclusions. The H-I system seems to be composed of two early-type spiral galaxies (S0/Sa). Galaxy I shows evidence of tidal perturbation: an off-centered bulge, some material extending along the NE direction, and structures that have been enhanced by image filtering procedures. There are some dwarf objects around it. Neither object shows evidence of nuclear activity.

Extended star clusters in NGC 1023 from HST/ACS mosaic imaging

Faint fuzzies are a relatively new class of star cluster, first found in the nearby S0 galaxy NGC 1023 by Larsen & Brodie using WFPC2 images from the Hubble Space Telescope (HST). Here we investigate the star cluster system of NGC 1023 using an eight pointing mosaic of ACS images from HST. We identify blue and red normal globular clusters (two of which are particularly luminous and dense) and two ultra compact dwarf objects (with effective radius ~10 pc). With our more complete spatial coverage, we also find 81 red and 27 blue faint fuzzies (FFs). We confirm the association of the red FFs with the disk of NGC 1023, consistent with them being long-lived open clusters. Half of the blue FFs appear to be associated with the dwarf satellite galaxy NGC 1023A (which was largely absent from the original HST/WFPC2 coverage), while the remainder are spatially coincident with the densest HI gas that surrounds NGC 1023. The blue FFs have colours that are consistent with young (few 100 Myr old) star clusters that formed during the most recent interaction between NGC 1023 and its satellite NGC 1023A.

Bridging the gap between low- and high-mass dwarf galaxies

While the dark matter content within the most massive giant and smallest dwarf galaxies has been probed -- spanning a range of over one million in mass -- an important observational gap remains for galaxies of intermediate mass. This gap covers K band magnitudes of approximately -16 > M_K > -18 (for which dwarf galaxies have B--K ~ 2). On the high mass side of the gap are dwarf elliptical (dE) galaxies, that are dominated by stars in their inner regions. While the low mass side includes dwarf spheroidal (dSph) galaxies that are dark matter-dominated and ultra compact dwarf (UCD) objects that are star-dominated. Evolutionary pathways across the gap have been suggested but remain largely untested because the `gap' galaxies are faint, making dynamical measurements very challenging. With long exposures on the Keck telescope using the ESI instrument we have succeeded in bridging this gap by measuring the dynamical mass for five dwarf galaxies with M_K ~ -17.5 (M_B ~ --15.5). With the exception of our brightest dwarf galaxy, they possess relatively flat velocity dispersion profiles of around 20 km/s. By examining their 2D scaling relations and 3D fundamental manifold, we found that the sizes and velocity dispersions of these gap galaxies reveal continuous trends from dE to dSph galaxies. We conclude that low-luminosity dwarf elliptical galaxies are dominated by stars, not by dark matter, within their half light radii. This finding can be understood if internal feedback processes are operating most efficiently in gap galaxies, gravitationally heating the centrally-located dark matter to larger radii. Whereas external environmental processes, which can strip away stars, have a greater influence on dSph galaxies resulting in their higher dark matter fractions. Abridged.

Massive star formation in Wolf-Rayet galaxies

(Abridged) We have performed a comprehensive analysis of a sample of 20 starburst galaxies, most of them classified as Wolf-Rayet galaxies. In this paper, the last of the series, we analyze the global properties of our galaxy sample using multiwavelength data (X-ray, FUV, optical, NIR, FIR, and radio). The agreement between our Ha-based SFR and those provided by indicators at other wavelengths is remarkable, but we consider that the new Ha-based calibration provided by Calzetti et al. (2007) should be preferred over older calibrations. The FUV-based SFR provides a powerful tool to analyze the star-formation activity in both global and local scales independently to the Ha emission. We provide empirical relationships between the ionized gas mass, neutral gas mass, dust mass, stellar mass, and dynamical mass with the B-luminosity. Although all mass estimations increase with increasing luminosity, we find important deviations to the general trend in some objects, that seem to be consequence of their particular evolutionary histories. We investigate the mass-metallicity relations and conclude that both the nature and the star-formation history are needed to understand the relationships between both properties. The majority of the galaxies follow a Schmidt-Kennicutt scaling law of star-formation that agrees with that reported in individual star-forming regions within M~51 but not with that found in normal spiral galaxies. We found a relation between the reddening coefficient and the warm dust mass indicating that the extinction is mainly internal to the galaxies. Considering all data, we found that 17 up to 20 galaxies are clearly interacting or merging with low-luminosity dwarf objects or HI clouds. We conclude that interactions do play a fundamental role in the triggering mechanism of the strong star-formation activity observed in dwarf starburst galaxies.

Erratum: Infrared spectroscopy and analysis of brown dwarf and planetary mass objects in the Orion nebula cluster

Erratum: Infrared spectroscopy and analysis of brown dwarf and planetary mass objects in the Orion nebula cluster

Massive star formation in Wolf-Rayet galaxies

(Abridged) We have performed a comprehensive multiwavelength analysis of a sample of 20 starburst galaxies that show the presence of a substantial population of very young massive stars. In this paper, the second of the series, we present the results of the analysis of long-slit intermediate-resolution spectroscopy of star-formation bursts for 16 galaxies of our sample. We study the spatial localization of the WR stars in each galaxy. We analyze the excitation mechanism and derive the reddening coefficient, physical conditions and chemical abundances of the ionized gas. We study the kinematics of the ionized gas to check the rotation/turbulence pattern of each system. When possible, tentative estimates of the Keplerian mass of the galaxies have been calculated. Our analysis has revealed that a substantial fraction of the galaxies show evidences of perturbed kinematics. With respect to the results found in individual galaxies, we remark the detection of objects with different metallicity and decoupled kinematics in Haro 15 and Mkn 1199, the finding of evidences of tidal streams in IRAS 08208+2816, Tol 9 and perhaps in SBS 1319+579, and the development of a merging process in SBS 0926+606 A and in Tol 1457-262. All these results reinforce the hypothesis that interactions with or between dwarf objects is a very important mechanism in the triggering of massive star formation in starburst galaxies, specially in dwarf ones. It must be highlighted that only deep and very detailed observationscan provide clear evidences that these subtle interaction processes are taking place.

The VLT-FLAMES survey of massive stars: evolution of surface N abundances and effective temperature scales in the Galaxy and Magellanic Clouds

We present an analysis of high resolution VLT-FLAMES spectra of 61 B-type stars with relatively narrow-lined spectra lo cated in 4 fields centered on the Milky Way clusters; NGC3293 & NGC4755 and the Large and Small Magellanic cloud clusters; NGC2004 and NGC330. For each object a quantitative analysis was carried out using the non-LTE model atmosphere code TLUSTY; resulting in the determination of their atmospheric parameters and phot ospheric abundances of the dominant metal species (C, N, O, Mg, Si, Fe). The results are discussed in relation to our earlier work on 3 younger clusters in these galaxies; NGC6611, N11 and NGC346 paying particular attention to the nitrogen abundances which are a n important probe of the role of rotation in the evolution of s tars. This work along with that of the younger clusters provides a consistent dataset of abundances and atmospheric parameters for over 100 B-type stars in the three galaxies. We provide effective temperature scales for B-type dwarfs in all three gal axies and for giants and supergiants in the SMC and LMC. In each galaxy a dependence on luminosity is found between the three classes with the unevolved dwarf objects having significantly higher e ffective temperatures. A metallicity dependence is present b etween the SMC and Galactic dwarf objects, and whilst the LMC stars are only slightly cooler than the SMC stars, they are significantly hotter than the ir Galactic counterparts.

Advanced Camera for Surveys Imaging of 25 Galaxies in Nearby Groups and in the Field

We present Hubble Space Telescope Advanced Camera for Surveys images and color-magnitude diagrams for 25 nearby galaxies with radial velocities VLG < 500 km s-1. Distances are determined based on the luminosities of stars at the tip of the red giant branch that range from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, are found to be the nearest known representatives of blue compact dwarf objects. Using high-quality data on distances and radial velocities of 110 nearby field galaxies, we derive their mean Hubble ratio to be 68 km s-1 Mpc-1 with a standard deviation of 15 km s-1 Mpc-1. Peculiar velocities of most of the galaxies, Vpec = VLG - 68D, follow a Gaussian distribution with σv = 63 km s-1 but with a tail toward high negative values. Our data display the known correlation between peculiar velocity and galaxy elevation above the Local Supercluster plane. The small observed fraction of galaxies with high peculiar velocities, Vpec < -500 km s-1, may be understood as objects associated with nearby groups (Coma I, Eridanus) outside the local volume.

UV Properties of Early-Type Galaxies in the Virgo Cluster

We study the UV properties of a volume-limited sample of early-type galaxies in the Virgo Cluster combining new GALEX far-ultraviolet (1530 A) and near-ultraviolet (2310 A) data with spectrophotometric data available at other wavelengths. The sample includes 264 elliptical, lenticular, and dwarf galaxies spanning a large range in luminosity (M_B ≤ -15). While the NUV to optical or near-IR color-magnitude relations (CMRs) are similar to those observed at optical wavelengths, with a monotonic reddening of the color index with increasing luminosity, the (FUV - V) and (FUV - H) CMRs show a discontinuity between massive and dwarf objects. An even more pronounced dichotomy is observed in the (FUV - NUV) CMR. For elliptical galaxies, the (FUV - NUV) color becomes bluer with increasing luminosity and with increasing reddening of the optical or near-IR color indices. For the dwarfs, the opposite trend is observed. These observational evidences are consistent with the idea that the UV emission is dominated by hot, evolved stars in giant systems, while in dwarf ellipticals residual star formation activity is more common.

The Y Band at 1.035 Microns: Photometric Calibration and the DwarfStellar/Substellar Color Sequence

ABSTRACTWe define and characterize a photometric bandpass (called “Y”) that is centered near 1.035 μm, in between the traditionally classified “optical” and “infrared” spectral regimes. We present Y magnitudes and Y−H and Y−K colors for a sample consisting mostly of photometric and spectral standards, spanning the spectral type range sdO to T5 V. Deep molecular absorption features in the near‐infrared spectra of extremely cool objects are such that the Y−H and Y−K colors grow rapidly with advancing spectral type especially from late M through mid‐L, substantially more rapidly than J−H or H−K, which span a smaller total dynamic range. Consistent with other near‐infrared colors, however, Y−H and Y−K colors turn blueward in the L6–L8 temperature range, with later T‐type objects having colors similar to those of warmer M and L stars. Y−J colors remain constant at 1.0 ± 0.15 mag from early‐L through late‐T dwarfs. The slope of the interstellar reddening vector within this filter is AY = 0.38AV. Reddening moves stars nearly along the YHK dwarf color sequence, making it more difficult to distinguish unambiguously very low mass candidate brown dwarf objects from higher mass stars seen, e.g., through the Galactic plane or toward star‐forming regions. Other diagrams involving the Y band may be somewhat more discriminating.

L Dwarfs Found in Sloan Digital Sky Survey Commissioning Imaging Data

This paper describes the discovery of seven dwarf objects of spectral type L (objects cooler than the latest M dwarfs) in commissioning imaging data taken by the Sloan Digital Sky Survey (SDSS). Low-resolution spectroscopy shows that they have spectral types from L0 to L8. Comparison of the SDSS and the Two Micron All Sky Survey (2MASS) photometry for several of these objects indicates the presence of significant opacity at optical wavelengths. This comparison also demonstrates the high astrometric accuracy (better than 1'' for these faint sources) of both surveys. The L dwarfs are shown to occupy a distinctive region of color-color space as measured in the SDSS filters, which should enables their identification in a straightforward way. This should lead eventually to a complete sample of many hundreds of these low-mass objects, or about 1 per 15 deg2 to i' ≈ 20, in the complete SDSS data set.