Portrait of a Dark Horse: A Photometric and Spectroscopic Study of the Ultra-Faint Milky Way Satellite Pegasus III (original) (raw)

Stellar systems in the direction of Pegasus I

Astronomy and Astrophysics, 2018

Context. In spite of the numerous studies of low-luminosity galaxies in different environments, there is still no consensus about their formation scenario. In particular, a large number of galaxies displaying extremely low surface brightnesses have been detected in recent years and the nature of these objects is still under discussion. Aims. In order to enlarge the sample of known low-surface-brightness (LSB) galaxies and to try to provide clues about their nature, we report the detection of eight such objects (µ eff,g 27 mag arcsec −2) towards the group of galaxies Pegasus I. They are located, in projection, within a radius of ∼200 kpc in the very center of Pegasus I, close to the dominant elliptical galaxies NGC 7619 and NGC 7626. Methods. We analyzed deep, high-quality GEMINI-GMOS images with ELLIPSE within IRAF in order to obtain their brightness profiles and structural parameters. We also fit Sérsic functions to these profiles in order to compare their properties with those of typical early-type galaxies. Results. Assuming that these galaxies are at the distance of Pegasus I, we have found that their sizes are intermediate among similar objects reported in the literature. In particular, we found that three of these galaxies can be classified as ultra-diffuse galaxies and a fourth one displays a nucleus. The eight new LSB galaxies show significant color dispersion around the extrapolation towards faint luminosities of the color-magnitude relation defined by typical early-type galaxies. In addition, they display values of the Sérsic index below 1 (concave brightness profiles in linear scale), in agreement with values obtained for LSB galaxies in other environments. Conclusions. We show that there seems to be a bias effect in the size distributions of the detected LSBs in different environments, in the sense that more distant groups/clusters lack small r eff objects, while large systems are not found in the Local Group and nearby environments. While there may be an actual shortage of large LSB galaxies in low-density environments like the Local Group, the non-detection of small (and faint) systems at large distances is clearly a selection effect. As an example, LSB galaxies with similar sizes to those of the satellites of Andromeda in the Local Group will be certainly missed in a visual identification at the distance of Pegasus I.

Stellar systems in the direction of Pegasus I - I. Low surface brightness galaxies

LA Referencia (Red Federada de Repositorios Institucionales de Publicaciones Científicas), 2018

Context. In spite of the numerous studies of low-luminosity galaxies in different environments, there is still no consensus about their formation scenario. In particular, a large number of galaxies displaying extremely low-surface brightnesses have been detected in the last years, and the nature of these objects is still under discussion. Aims. To enlarge the sample of known low-surface brightness (LSB) galaxies and to try to provide clues about their nature, we report the detection of eight of this type of objects (µ eff,g 27 mag arcsec −2) towards the group of galaxies Pegasus I. They are located, in projection, within a radius of ∼ 200 kpc in the very center of Pegasus I, close to the dominant elliptical galaxies NGC 7619 and NGC 7626. Methods. We analyzed deep, high-quality, GEMINI-GMOS images with ELLIPSE within IRAF in order to obtain their brightness profiles and structural parameters. We also fit Sérsic functions to these profiles in order to compare their properties with those of typical early-type galaxies. Results. Assuming that these galaxies are at the distance of Pegasus I, we have found that their sizes are intermediate among similar objects reported in the literature. In particular, we found that three of these galaxies can be classified as ultra-diffuse galaxies and a fourth one displays a nucleus. The eight new LSB galaxies show a significant color dispersion around the extrapolation towards faint luminosities of the color-magnitude relation defined by typical early-type galaxies. In addition, they display values of the Sérsic index below 1 (concave brightness profiles in linear scale), in agreement with values obtained for LSB galaxies in other environments. Conclusions. We also show that there seems to be a bias effect in the size distributions of the detected LSBs in different environments, in the sense that more distant groups/clusters lack small r eff objects, while large systems are not found in the Local Group and nearby environments. While there may be an actual shortage of large LSB galaxies in low-density environments like the Local Group, the non-detection of small (and faint) systems at large distances is clearly a selection effect. As an example, LSB galaxies with similar sizes to those of the satellites of Andromeda in the Local Group, will be certainly missed in a visual identification at the distance of Pegasus I.

SAGITTARIUS II, DRACO II AND LAEVENS 3: THREE NEW MILKY WAY SATELLITES DISCOVERED IN THE PAN-STARRS 1 3πSURVEY

The Astrophysical Journal, 2015

We present the discovery of three new Milky Way satellites from our search for compact stellar overdensities in the photometric catalog of the Panoramic Survey Telescope and Rapid Response System 1 (Pan-STARRS 1, or PS1) 3π survey. The first satellite, Laevens 3, is located at a heliocentric distance of d = 67 ± 3 kpc. With a total magnitude of M V = −4.4 ± 0.3 and a half-light radius r h = 7 ± 2 pc, its properties resemble those of outer halo globular clusters. The second system, Draco II/Laevens 4 (Dra II), is a closer and fainter satellite (d ∼ 20 kpc, M V = −2.9 ± 0.8), whose uncertain size (r h = 19 +8 −6 pc) renders its classification difficult without kinematic information; it could either be a faint and extended globular cluster or a faint and compact dwarf galaxy. The third satellite, Sagittarius II/Laevens 5 (Sgr II), has an ambiguous nature as it is either the most compact dwarf galaxy or the most extended globular cluster in its luminosity range (r h = 37 +9 −8 pc and M V = −5.2 ± 0.4). At a heliocentric distance of 67 ± 5 kpc, this satellite lies intriguingly close to the expected location of the trailing arm of the Sagittarius stellar stream behind the Sagittarius dwarf spheroidal galaxy (Sgr dSph). If confirmed through spectroscopic follow up, this connection would locate this part of the trailing arm of the Sagittarius stellar stream that has so far gone undetected. It would further suggest that Sgr II was brought into the Milky Way halo as a satellite of the Sgr dSph.

A Wide Field Planetary Camera 2 Study of the Resolved Stellar Population of the Pegasus Dwarf Irregular Galaxy (DDO 216)

The Astronomical Journal, 1998

The stellar population of the Pegasus dwarf irregular galaxy is investigated in images taken in the F439W (B), F555W (V), and F814W (I) bands with the Wide Field Planetary Camera 2 (WFPC2) on the Hubble Space Telescope. With WFPC2 the Pegasus dwarf is highly resolved into individual stars to limiting magnitudes of about 25.5 in B and V and 25 in I. These and ground-based data are combined to produce color-magnitude diagrams that show the complex nature of the stellar population in this small galaxy. A young (\0.5 Gyr) main-sequence stellar component is present and clustered in two centrally located clumps, while older stars form a more extended disk or halo. The colors of the main sequence require a relatively large extinction of mag. The mean color of the well-populated red giant A V \ 0.47 branch (RGB) is relatively blue, consistent with a moderate-metallicity young, or older metal-poor, stellar population. The RGB also has signiÐcant width in color, implying a range of stellar ages and/or metallicities. A small number of extended asymptotic giant branch stars are found beyond the RGB tip. Near the faint limits of our data is a populous red clump superposed on the RGB. E †orts to Ðt selfconsistent stellar population models based on the Geneva stellar evolutionary tracks yield a revised distance of 760 kpc. Quantitative Ðts to the stellar population are explored as a means of constraining the star formation history. The numbers of main-sequence and core helium burning blue-loop stars require that the star formation rate was higher in the recent past, by a factor of 3È4 about 1 Gyr ago. Unique results cannot be obtained for the star formation history over longer time baselines without better information on stellar metallicities and deeper photometry. The youngest model consistent with the data contains stars with constant metallicity of Z \ 0.001 that mainly formed 2È4 Gyr ago. If stellar metallicity declines with increasing stellar age, then older ages are allowed of up to B8 Gyr. However, even at its peak of star-forming activity, the intermediate-ageÈdominated model for the Pegasus dwarf most likely remained relatively dim, with M V B [14.

SDSS-III: Massive Spectroscopic Surveys of the Distant Universe, the Milky Way, and ExtraSolar Planetary Systems

Astronomical Journal, 2011

Building on the legacy of the Sloan Digital Sky Survey (SDSS-I and II), SDSS-III is a program of four spectroscopic surveys on three scientific themes: dark energy and cosmological parameters, the history and structure of the Milky Way, and the population of giant planets around other stars. In keeping with SDSS tradition, SDSS-III will provide regular public releases of all its data, beginning with SDSS DR8 (which occurred in Jan 2011). This paper presents an overview of the four SDSS-III surveys. BOSS will measure redshifts of 1.5 million massive galaxies and Lya forest spectra of 150,000 quasars, using the BAO feature of large scale structure to obtain percent-level determinations of the distance scale and Hubble expansion rate at z<0.7 and at z~2.5. SEGUE-2, which is now completed, measured medium-resolution (R=1800) optical spectra of 118,000 stars in a variety of target categories, probing chemical evolution, stellar kinematics and substructure, and the mass profile of the dark matter halo from the solar neighborhood to distances of 100 kpc. APOGEE will obtain high-resolution (R~30,000), high signal-to-noise (S/N>100 per resolution element), H-band (1.51-1.70 micron) spectra of 10^5 evolved, late-type stars, measuring separate abundances for ~15 elements per star and creating the first high-precision spectroscopic survey of all Galactic stellar populations (bulge, bar, disks, halo) with a uniform set of stellar tracers and spectral diagnostics. MARVELS will monitor radial velocities of more than 8000 FGK stars with the sensitivity and cadence (10-40 m/s, ~24 visits per star) needed to detect giant planets with periods up to two years, providing an unprecedented data set for understanding the formation and dynamical evolution of giant planet systems. (Abridged)

MUSE Observations of NGC330 in the Small Magellanic Cloud: Helium Abundance of Bright Main-sequence Stars

The Astronomical Journal

We present observations of the most bright main sequence stars in the Small Magellanic Cloud stellar cluster NGC 330 obtained with the integral field spectrograph MUSE@VLT. The use of this valuable instrument allows us to study both photometric and spectroscopic properties of stellar populations of this young star cluster. The photometric data provide us a precise color magnitude diagram, which seems to support the presence of two stellar populations of ages of ≈ 18 Myr and ≈ 30 Myr assuming a metallicity of Z = 0.002. Thanks to the spectroscopic data, we derive helium abundance of 10 main sequence stars within the effective radius R eff = 20 of NGC 330, thus leading to an estimation of (He) = 10.93 ± 0.05 (1σ). The helium elemental abundances of stars likely belonging to the two possible stellar populations, do not show differences or dichotomy within the uncertainties. Thus, our results suggest that the two stellar populations of NGC 330, if they exist, share similar original He abundances. If we consider stellar rotation velocity in our analysis, a coeval (30 Myr) stellar population, experiencing different values of rotation, cannot be excluded. In this case, the mean helium abundance < (He) > rot obtained in our analysis is 11.00 ± 0.05 dex. We also verified that possible NLTE effects cannot be identified with our analysis because of the spectral resolution and they are within our derived abundance He uncertainties. Moreover, the analysis of the He abundance as a function of the distance from the cluster center of the observed stars do not show any correlation.

Simultaneous CCD Photometry of Two Eclipsing Binary Stars in Pegasus - Part 1: KW Pegasi

The coincidental location of BX Peg and KW Peg in the same fieldof-view captured by the primary imaging system at UnderOak Observatory (UO) provided an opportunity to study both variable stars from the same exposures. Herein new findings for the eclipsing binary KW Peg will be presented while those from BX Peg will be discussed in a separate paper (Part 2). KW Peg, described as an "Algol type" eclipsing variable (P = 0.816402 d), is only reported in a single work published over twenty years ago. Photometric data collected in three bandpasses (B, V, and I c), produced eight new times of minimum for KW Peg. These were used to update the linear ephemeris and further analyze potential changes in orbital periodicity by examining the available history of eclipse timings. In addition, synthetic fitting of light curves by Roche modeling was accomplished with programs employing the Wilson-Devinney code. Results from the present study provide a reasonable case for classifying KW Peg as a short-period RS CVn eclipsing binary rather than Algol-like. The primary star in KW Peg would appear to be a late stage G9V-K0V dwarf whereas the secondary is a slightly cooler K0-K1 companion. The eclipse-timing diagram for KW Peg is quite simple and indicates that, on average, the orbital period for this system has remained fairly constant over the past two decades.

Fundamental properties of the new dwarf galaxy And VI - alias Pegasus Dwarf - another companion of M31

Cornell University - arXiv, 1998

We present medium deep CCD imaging in B, V, and I of the Pegasus Dwarf galaxy (And VI) which was recently found by Karachentsev & Karachentseva (1998), and independently also by Armandroff et al. (1999). The Calar Alto 2.2m images show a low surface brightness galaxy. Its structure resembles that of the other known dSph companions of M31 And I, II, III, and V. The brightest stars are resolved in all three colors. Color-magnitude diagrams in either B−V or V−I show the tip of the red giant branch which allows us to estimate a true distance modulus of 24.5 ± 0.2. The color-magnitude diagrams and the structure show no evidence for recent star formation, thus, a classification as spheroidal dwarf galaxy with a rather old population seems appropriate. The total absolute magnitude of this dwarf is M V,0 = −10.4 ± 0.2.

Deep SOAR follow-up photometry of two Milky Way outer-halo companions discovered with Dark Energy Survey

Monthly Notices of the Royal Astronomical Society, 2018

We report the discovery of a new star cluster, DES 3, in the constellation of Indus, and deeper observations of the previously identified satellite DES J0222.7−5217 (Eridanus III). DES 3 was detected as a stellar overdensity in first-year Dark Energy Survey data, and confirmed with deeper photometry from the 4.1-m Southern Astrophysical Research (SOAR) telescope. The new system was detected with a relatively high significance and appears in the DES images as a compact concentration of faint blue point sources. We determine that DES 3 is located at a heliocentric distance of 76.2 kpc and it is dominated by an old (9.8 Gyr) and metal-poor ([Fe/H] −1.84) population. While the age and metallicity values of DES 3 are comparable to typical globular clusters (objects with a high stellar density, stellar mass of ∼10 5 M and luminosity M V ∼ −7.3), its half-light radius (r h ∼ 6.87 pc) and luminosity (M V ∼ −1.7) are more indicative of faint star cluster. Based on the angular size, DES 3, with a value of r h ∼ 0. 31, is among the smallest faint star clusters known to date. Furthermore, using deeper imaging of DES J0222.7−5217 taken with the SOAR telescope, we update structural parameters and perform the first isochrone modelling. Our analysis yields the first age (12.6 Gyr) and metallicity ([Fe/H] −2.01) estimates for this object. The half-light radius (r h 11.24 pc) and luminosity (M V −2.4) of DES J0222.7−5217 suggest that it is likely a faint star cluster. The discovery of DES 3 indicates that the census of stellar systems in the Milky Way is still far from complete, and demonstrates the power of modern wide-field imaging surveys to improve our knowledge of the Galaxy's satellite population.