Local Group Dwarf Galaxies Research Articles

Kinematics of the faintest gas-rich galaxy in the Local Group: DDO210

We present deep, high velocity resolution (~1.6 km s-1) Giant Meterwave Radio Telescope (GMRT) HI 21 cm synthesis images of the faint () local group dwarf galaxy DDO210. We find that the HI distribution in the galaxy is not axi-symmetric, but shows density enhancements in the eastern and southern halves of the galaxy. The optical emission is lopsided with respect to the HI, most of the bright optical emission arises from the eastern half of the HI disk. The velocity field of the galaxy is however quite regular and shows a systematic large scale pattern, consistent with the rotational motion. The rotation curve for the galaxy shows a peak (inclination corrected) rotation velocity of only ~8 km s-1, comparable to the random motions in the HI gas. After correcting for the dynamical support provided by random motions (the “asymmetric drift” correction), we find the corrected peak rotation velocity of ~16.0 km s-1. Mass modeling of the corrected rotation curve shows that the kinematics of DDO210 can be well fit with either a modified isothermal halo (with a central density pc-3 for a stellar mass to light ratio of 3.4) or an NFW halo. In the case of the NFW halo however, a good fit is obtained for a wide range of parameters; the halo parameters could not be uniquely determined from the fit. Density profiles with inner slope steeper than ~1.2 however provide a poor fit to the data. Finally, the rotation curve derived using MOND also provides a reasonable fit to the observed rotation curve.

Andromeda VIII: A New Tidally Distorted Satellite of M31

We report the detection of a new satellite of M31, projected close to M32. Andromeda VIII is tidally distorted, with length ~10 kpc and width a few kiloparsecs. It contains 5-12 planetary nebulae and 1-3 globular clusters and has a velocity of -204 km s-1 with respect to M31, some 350 km s-1 away from M32's velocity. There are also ~4 × 105 M☉ of H I, well-separated from the disk, at the same position and velocity. The satellite has luminosity of 1.2-2.4 × 108 L☉ and a central surface brightness of the order of μV = 24. Both these values are typical of Local Group dwarf galaxies. Its surface brightness is some 6 mag brighter than any of the stellar streams found in the Milky Way or M31. The three associated globular clusters have reddening consistent with foreground reddening from the Milky Way only, making it likely that the satellite is in front of M31, unlike the giant tidal stream of Ibata et al., which is behind M31 in the southeast quadrant. However, the major axis of And VIII is aligned with the western edge of this giant stream, and we suggest that its unusual fan shape is caused by superposition of two streams, the westernmost of which was tidally stripped from And VIII.

Gemini observations of Wolf-Rayet stars in the Local Group starburst galaxy IC 10

We present Gemini-N GMOS and CFHT MOS spectroscopy of Wolf-Rayet candidates in the Local Group dwarf galaxy IC10 that were previously identified by Massey et al. and Royer et al. From the present spectroscopic survey, the WC/WN ratio for IC10 remains unusually high, given its low metallicity, although none of the WC9 stars suspected from narrow-band imaging are confirmed. Our spectroscopy confirms 9 newly discovered Wolf-Rayet candidates from Royer et al., whilst spectral types of 14 Wolf-Rayet stars previously observed by Massey & Armandroff are refined here. In total, there are 26 spectroscopically confirmed Wolf-Rayet stars in IC~10. All but one of the fourteen WC stars are WC4-6 stars, the exception being # 10 from Massey et al., a broad-lined, apparently single WC7 star. There are a total of eleven WN stars, which are predominantly early WN3-4 stars, but include a rare WN10 star, #8 from Royer et al. #5 from Massey et al. is newly identified as a transition WN/C star. Consequently, the WC/WN ratio for IC10 is 14/11=1.3, unusually high for a metal-poor galaxy. Re-evaluating recent photometric data of Massey & Holmes, we suggest that the true WC/WN ratio may not be as low as 0.3. Finally, we present ground-based finding charts for all confirmed WR stars, plus HST/WFPC2 charts for twelve cases.

Young stars in the outer H I disc of NGC 6822

We present wide-field optical imaging covering the entire neutral hydrogen disc of the Local Group dwarf galaxy NGC 6822. These observations reveal the presence of numerous blue, young stars at large galactocentric radii well beyond R25. Blue stars are also found that are associated with NGC 6822's companion H i cloud, indicating that star formation was triggered in the companion in the last 108 yr. In general, blue stars are present where the H i surface densities reach values ≳ 5 × 1020 cm−2. However, over one-third of the blue stars detected within the H i disc are found at lower surface densities. The young stars trace the distribution of the neutral hydrogen in the inner disc, but seem to be avoiding the supergiant H i shell in NGC 6822, setting a lower limit for its age of 108 yr. The extended distribution of young stars implies that stars can form at large galactocentric radii in dwarf galaxies; the H i is therefore not necessarily much more extended than the stellar population. This finding has important consequences for the chemical enrichment of the interstellar medium throughout (dwarf) galaxies.

The dwarf LSB galaxy population of the Virgo cluster -- I. The faint-end slope of the luminosity function

The widely varying dwarf galaxy counts in different environments provide a strong challenge to standard hierarchical clustering models. The luminosity function is not universal, but seems to be strongly dependent upon environment. In this paper we describe an automated procedure for detecting and measuring very low surface brightness (LSB) features in deep CCD data. We apply this procedure to large area CCD survey fields of the Virgo cluster. We show that there are many more faint ( 10 � MB � 14) low surface brightness galaxies than what would be predicted from extrapolation of the Virgo cluster catalogue luminosity function. Over our limited range of measurement the faint end slope of the luminosity function becomes � = 1.6. The luminosity function is flatter in the inner regions of the cluster than it is in the outer regions. Although these galaxies contribute a small fraction of the total stellar light of the cluster, they may contribute significantly to the mass in galaxies if they have large mass-to-light ratios similar to those recently measured for Local Group dwarf galaxies.

Wolf-Rayet stars at 1 - 2 Mpc

We present new narrow-band imaging and optical spectroscopy of Wolf-Rayet stars in the Local Group dwarf galaxy IC 10 (Gemini-N- gmos) and Sculptor group spiral galaxy NGC 300 (VLT fors2).

Dwarf LSB galaxies and their environment: The Virgo Cluster, the Ursa Major Cluster, isolated galaxies and voids

The widely varying dwarf galaxy counts in different environments provide a strong challenge to standard hierarchical clustering models. The luminosity function is not universal, but seems to be strongly dependent upon environment. In this paper we describe an automated procedure for detecting and measuring very Low Surface Brightness (LSB) features in deep CCD data. We apply this procedure to large area CCD survey fields of the Virgo cluster and find 105 dwarf LSB galaxies previously uncatalogued over an area of ∼14 sq deg. We show that there are many more faint (14≤MB≤–10) LSB galaxies than would be predicted from extrapolation of the Virgo cluster catalogue luminosity function. Over our limited range of measurement the faint end slope of the luminosity function becomesα=–1.6. Although these galaxies contribute a small fraction of the total stellar light of the cluster, they may contribute significantly to the mass in galaxies if they have large mass-to-light ratios similar to those recently measured for Local Group dwarf galaxies. By a comparison with different environments, the Virgo Cluster seems to be highly `dwarf rich' if considering the dwarf to giant ratio or it's luminosity function faint end slope.

H [CSC]i[/CSC] Imaging of LGS 3 and an Apparently Interacting High-Velocity Cloud

We present a 93' × 93' map of the area near the Local Group dwarf galaxy LGS 3, centered on an H I cloud 30' away from the galaxy. Previous authors associated this cloud with LGS 3 but relied on observations made with a 36' beam. Our high-resolution (34), wide-field Arecibo observations of the region reveal that the H I cloud is distinct from the galaxy and suggest an interaction between the two. We point out faint emission features in the map that may be gas that has been tidally removed from the H I cloud by LGS 3. We also derive the rotation curve of the cloud and find that it is in solid-body rotation out to a radius of 10', beyond which the rotation velocity begins to decline. Assuming a spherical geometry for the cloud, the implied mass is 2.8 × 107(d/Mpc) M☉, where d is the distance in Mpc. The observed H I mass is 5.5 × 106(d/Mpc)2 M☉, implying that the cloud is dark matter dominated unless its distance is at least 1.9 Mpc. We propose that the cloud is a high-velocity cloud that is undergoing a tidal interaction with LGS 3 and therefore is located roughly 700 kpc away from the Milky Way. The cloud then contains a total mass of ~2.0 × 107 M☉, 82% of which consists of dark matter.

The Absence of Stars in Compact High‐Velocity Clouds

We present the results of our search for faint Local Group dwarf galaxies in compact high-velocity clouds (HVCs). We used digitized Palomar Observatory Sky Survey (POSS) data to examine 1 deg2 of sky around each of ~250 northern hemisphere HVCs. The POSS images were processed to remove foreground stars and large-scale backgrounds, smoothed to enhance arcminute-sized low surface brightness features, and then compared with the original plates. Using this technique, we located 60 candidate dwarf galaxies in the ~250 deg2 that we surveyed. Follow-up observations of these candidates have revealed several distant clusters of galaxies and a number of possible Galactic cirrus clouds, but no Local Group dwarfs. It appears that many of the low surface brightness features in the sky survey data are plate flaws. The second-generation red POSS plates are sensitive down to surface brightness levels of 25-26 mag arcsec-2, and our follow-up images reached 10 σ limiting magnitudes of R = 21-23 for point sources. Given these limits, all known Local Group galaxies except four of the very diffuse, extended dwarf spheroidals located within 100 kpc of the Milky Way would have been detected had they been in our survey. Therefore, we can rule out the possibility that these HVCs are associated with normal but faint dwarf galaxies. If compact HVCs do contain stars, they must have surface brightnesses 1 mag arcsec-2 fainter than most known Local Group galaxies.

Atomic Hydrogen Gas in Dark Matter Minihalos and the Compact High‐Velocity Clouds

We calculate the coupled hydrostatic and ionization structures of pressure-supported gas clouds that are confined by gravitationally dominant dark-matter (DM) mini-halos and by an external bounding pressure provided by a hot medium. We focus on clouds that are photoionized and heated by the present-day background metagalactic field and determine the conditions for the formation of warm (WNM), and multi-phased (CNM/WNM) neutral atomic hydrogen (HI) cores in the DM-dominated clouds. We consider LCDM dark-matter halos, and we compute models for a wide range of halo masses, total cloud gas masses, and external bounding pressures. We present models for the pressure-supported HI structures observed in the Local Group dwarf galaxies Leo A and Sag DIG. We then construct minihalo models for the multi-phased (and low-metallicity) compact high-velocity HI clouds (CHVCs). If the CHVCs are drawn from the same family of halos that successfully reproduce the dwarf galaxy observations, then the CHVCs must be "circumgalactic objects" with characteristic distances of 150 kpc. For such systems we find that multi-phased behavior occurs for peak WNM HI column densities between 2e19 and 1e20 cm^-2, consistent with observations. If the large population of CHVCs represent "missing low-mass satellites" of the Galaxy, then these clouds must be pressure-confined to keep the gas neutral. For an implied CHVC minihalo scale velocity of v_s=12 km s^-1, the confining pressure must exceed ~50 cm^-3 K. A hot (2e6 K) Galactic corona could provide the required pressure at 150 kpc.

The Southern Dwarf Hunt: Local Group Dwarf Candidates in the Southern Sky

We present observations of 82 Local Group dwarf galaxy candidates, of which 62 were chosen visually from ESO-SRC survey plates of the southern sky (32 of which were not previously cataloged) and the rest suggested by various sources in the literature. Two are the Local Group galaxies Antlia and Cetus; nine are more distant galaxies, though still within a few megaparsecs; 45 are background galaxies; seven are planetary (or other emission) nebulae; 15 are reflection or other Galactic nebulae; two are galaxy clusters; one is a Galactic star cluster; and one is a misidentified star. We conclude that there is no large population of faint Local Group dwarf galaxies of any familiar type awaiting discovery. We point out the danger of relying on a single type of data to reach conclusions about an object.

C Star Survey of Local Group Dwarf Galaxies. III. The Sagittarius Dwarf Irregular and the Leo I Dwarf Spheroidal Galaxies

We present the latest results of our ongoing homogeneous cool C star survey of Local Group dwarf galaxies. We apply our two-color photometric technique to the study of two small galaxies: the Sagittarius dwarf (SagDIG) and Leo I. We identify 16 C stars in SagDIG and 13 C stars in Leo I. Even though both galaxies have a known C star population, we identify seven previously unknown C stars in each galaxy. The photometric properties of all the known C stars in each galaxy are presented. It is shown that our definition of a C star, based on our photometric criteria, produces a subset of carbon stars with homogeneous properties useful for population comparison.

The Star Formation History of LGS 3

We have determined the distance and star formation history of the Local Group dwarf galaxy LGS 3 from deep Hubble Space Telescope WFPC2 observations. LGS 3 is intriguing because ground-based observations showed that, while its stellar population is dominated by old, metal-poor stars, there is a handful of young, blue stars. Also, the presence of H I gas makes this a possible transition object between dwarf spheroidal and dwarf irregular galaxies. The HST data are deep enough to detect the horizontal branch and young main sequence for the first time. A new distance of D = 620 ± 20 kpc has been measured from the positions of the tip of the red giant branch, the red clump, and the horizontal branch. The mean metallicity of the stars older than 8 Gyr is [Fe/H] = -1.5 ± 0.3. The most recent generation of stars has [Fe/H] ≈ -1. For the first few Gyr the global star formation rate was several times higher than the historical average and has been fairly constant since then. However, we do see significant changes in stellar populations and star formation history with radial position in the galaxy. Most of the young stars are found in the central 63 pc (21''), where the star formation rate has been relatively constant, while the outer parts have had a declining star formation rate.

Linear Nonadiabatic Properties of SX Phoenicis Variables

We present a detailed linear, nonadiabatic pulsational scenario for oscillating Blue Stragglers (BSs)/SX Phoenicis (SX Phe) in Galactic Globular Clusters (GGCs) and in Local Group (LG) dwarf galaxies. The sequences of models were constructed by adopting a wide range of input parameters and properly cover the region of the HR diagram in which these objects are expected to be pulsationally unstable. Current calculations together with more metal-rich models already presented by Gilliland et al. suggest that the pulsation properties of SX Phe are partially affected by metal content. Our calculations also suggest that the secular period change seems to be a good observable to identify the pulsation mode of cooler SX Phe variables. The dependence of pulsation properties on the helium envelope content is discussed and we find that the secular period change for He-enhanced models is approximately a factor of two larger than for canonical ones. According to this evidence we suggest that the pulsation properties of SX Phe can be soundly adopted to constrain the evolutionary history of BSs, and in turn to single out the physical mechanisms which trigger their formation.

Radial Velocity of the Phoenix Dwarf Galaxy: Linking Stars and H [CSC]i[/CSC] Gas

We present the first radial velocity measurement of the stellar component of the Local Group dwarf galaxy Phoenix, using the FORS1 instrument at the VLT's Unit Telescope 1 (Antu). From the spectra of 31 red giant branch stars, we derive a heliocentric optical radial velocity for Phoenix of V⊙ = -52 ± 6 km s-1. On the basis of this velocity, and taking into account the results of a series of semianalytical and numerical simulations, we discuss the possible association of the H I clouds observed in the Phoenix vicinity. We conclude that the characteristics of the H I cloud with heliocentric velocity -23 km s-1 are consistent with this gas having been associated with Phoenix in the past and being lost by the galaxy after the last event of star formation in the galaxy, about 100 Myr ago. Two possible scenarios are discussed: the ejection of the gas by the energy released by the supernovae (SNe) produced in that last event of star formation and a ram pressure stripping scenario. We derive that the kinetic energy necessary to eject the gas is ESNe ~ 2 × 1051 ergs and that the number of SNe necessary to transfer this amount of kinetic energy to the gas cloud is ~20. This is consistent with the number of SNe expected for the last event of star formation in Phoenix, according to the star formation history derived by Martinez-Delgado, Gallart, & Aparicio. The drawback of this scenario is the regular appearance of the H I cloud and its anisotropic distribution with respect to the stellar component. Another possibility is that the H I gas was stripped as a consequence of ram pressure by the intergalactic medium. In our simulations, the structure of the gas remains quite smooth as it is stripped from Phoenix, keeping a distribution similar to that of the observed H I cloud. Both in the SNe ejection case and in the ram pressure sweeping scenario, the distances and relative velocities imply that the H I cloud is not gravitationally bound to Phoenix, since this would require a Phoenix total mass about an order of magnitude larger than its total estimated mass. Finally, we discuss the possibility that Phoenix may be a bound Milky Way satellite. The minimum required mass of the Milky Way for Phoenix to be bound is MMW(<450 kpc) ≥ 1.2 × 1012 M⊙, which comfortably fits within most current estimates.

The chemical abundances in a sample of dwarf irregular galaxies

Deep groundbased spectra of Hubble V and Hubble X, the two largest and brightest H ii regions in the Local Group dwarf galaxy, NGC 6822, are presented. The extinction, excitation and ionization are studied on scales as small as 4 pc. The chemical composition of some elements within the regions as well as possible small scale variations in the abundances of helium, nitrogen, oxygen and neon over the face of the two regions are investigated. The detection of the emission lines (OI) and (ClIII) are probably related to shock-heating from gas ejected from supernova explosions and planetary nebula ejections, respectively, although the contribution to the total ionization is very small. No signicant dierences in chemical composition was found between Hubble V and Hubble X. However, some small scale variations in the abundances of oxygen, and possibly nitrogen and helium, over the face of both H ii regions were noted, although dierent physical conditions could apply within the regions, such as local fluctuations in the temperature.

A Radio Telescope Search for Axions

The axion is a hypothetical elementary particle and a cold dark matter candidate. It could dominate the potential wells of most astrophysical objects. Axions spontaneously decay into nearly monochromatic microwave photons. We present results from a radio telescope search for these axion decay photons of mass ma = 298-363 μeV in Local Group dwarf galaxies. We report a limit on the axion-to-two-photon coupling constant gaγγ > 1.0 × 10-9 GeV-1.

A Carbon Star Survey of the Local Group Dwarf Galaxies. II. Pegasus, DDO 210, and Tucana

We present the latest results of our ongoing four-filter photometric survey of C stars in Local Group dwarf irregular galaxies. Observations of the two low-luminosity dwarf irregular galaxies, Pegasus and DDO 210, revealed 40 and three C stars, respectively, assuming that the reddening of Pegasus is negligible. No C stars were identified in Tucana. Our observations permit the estimation of the color-magnitude diagram contamination by foreground M dwarfs thus yielding reliable C/M ratios. Our R and I photometry of the C stars cannot be used to solve the extinction controversy toward Pegasus. The three C stars in DDO 210 are quite bright when compared with C star populations in other dwarf galaxies. A larger, fainter population in that galaxy seems improbable, however. The statistics of C stars currently on hand for dwarf galaxies show a well-defined trend with the absolute magnitude of dwarf galaxies.

Evidence for Tidal Interaction and a Supergiant H [CSC]i[/CSC] Shell in the Local Group Dwarf Galaxy NGC 6822

We present a wide-field, high spatial and velocity resolution map of the entire extended H I distribution of the Local Group dwarf galaxy NGC 6822. The observations were obtained with the Australia Telescope Compact Array in mosaicking mode. The interstellar medium of NGC 6822 is shaped by the presence of numerous H I holes and shells, including a supergiant shell, and the effects of tidal interaction in the form of a tidal arm and an infalling or interacting H I complex. The H I shell is situated outside the optical galaxy and occupies roughly a quarter of the area of the main H I disk. It measures 2.0 × 1.4 kpc, making it one of the largest supergiant H I shells ever found. The giant hole shows no signs of expansion, and no obvious creation mechanism is evident from our data. If star formation was the cause, an energy equivalent of ~100 supernovae (1053 ergs) is needed to create the hole. We derive an upper limit for the age of order 100 Myr. The presence of a possible tidal arm indicates that NGC 6822 may recently have undergone some interaction. An H I complex located in the northwest of the galaxy may be the interaction partner. We argue that it is likely that these features were created about 100 Myr ago in an event that also enhanced the star formation rate.

Local Group Dwarf Galaxies and the Star Formation Law at High Redshift

I show how the existing observational data on Local Group dwarf galaxies can be used to estimate the average star formation law during the first 3 Gyr of the history of the universe. I find that the observational data are consistent with the orthodox Schmidt law with a star formation efficiency of about 4% if the star formation is continuous (during the first 3 Gyr). The efficiency is proportionally higher if most of the gas in the dwarfs was consumed (and never replenished) in a short time interval well before the universe turned 3 Gyr.