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

We wish to clarify whether strong magnetic fields can be effectively generated in typically low-mass dwarf galaxies and to assess the role of dwarf galaxies in the magnetization of the Universe. We performed a search for radio emission and magnetic fields in an unbiased sample of 12 Local Group (LG) irregular and dwarf irregular galaxies with the 100m Effelsberg telescope at 2.64 and 4.85GHz. Magnetic fields in LG dwarfs are three times weaker than in the normal spirals (<4.2+-1.8muG). The production of total magnetic fields appears to be regulated mainly by the star-formation surface density, with the power-law exponent of 0.30+-0.04, or by the gas surface density (with the exponent 0.47+-0.09). In addition, we find systematically stronger fields in objects of higher global star-formation rate. The dwarf galaxies follow a similar far-infrared relationship (with a slope of 0.91+-0.08) to that determined for high surface brightness spiral galaxies. The magnetic field strength in dwarf galaxies does not correlate with their maximum rotational velocity, indicating a small-scale rather than a large-scale dynamo process. If magnetization of the Universe by galactic outflows is coeval with its metal enrichment, we show that more massive objects (such as Lyman Break Galaxies) can efficiently magnetize the intergalactic medium with a magnetic field strength of about 0.8nG out to a distance of 160-530kpc at redshifts 5-3, respectively. Several times weaker fields and shorter magnetization distances are expected from primordial dwarf galaxies. We also predict that most star-forming local dwarfs might have magnetized their surroundings up to about 0.1muG within about 5kpc distance. Strong magnetic fields (>6muG) are observed only in dwarfs of extreme characteristics while typical LG dwarfs are unsuitable objects for the efficient supply of magnetic fields to the intergalactic medium.

We compare the cumulative star formation histories (SFHs) of Local Group (LG) dwarf galaxies with those in the volume-limited ACS Nearby Galaxy Survey Treasury (ANGST) sample (D < 4 Mpc), in order to understand how typical the LG dwarf galaxies are relative to those in the nearby universe. The SFHs were derived in a uniform manner from high quality optical color-magnitude diagrams constructed from Hubble Space Telescope imaging. We find that the {\it mean} cumulative SFHs of the LG dwarfs are comparable to the mean cumulative SFHs of the ANGST sample for the three different morphological types (dwarf spheroidals/ellipticals: dSph/dE; dwarf irregulars: dI; transition dwarfs: dTrans). We also discuss effects such as population gradients and systematic uncertainties in the stellar models that may influence the derived SFHs. Both the ANGST and Local Group dwarf galaxies show a consistent and strong morphology-density relationship, emphasizing the importance of environment in the evolution of dwarf galaxies. Specifically, we confirm that dIs are found at lower densities and higher luminosities than dSphs, within this large sample. We also find that dTrans are located in similar environments to those occupied by dwarf irregular galaxies, but have systematically lower luminosities that are more comparable to those of dwarf spheroidals. The similarity of the SFHs and morphology-density relationships of the LG and ANGST dwarf galaxies suggests that the LG dwarfs are a good representation of dwarf galaxies in the local universe.

Using dark matter halos traced by galaxy groups selected from the Sloan Digital Sky Survey Data Release 4, we find that about 1/4 of the faint galaxies ($\rmag >-17.05$, hereafter dwarfs) that are the central galaxies in their own halo are not blue and star forming, as expected in standard models of galaxy formation, but are red. In contrast, this fraction is about 1/2 for dwarf satellite galaxies. Many red dwarf galaxies are physically associated with more massive halos. In total, about $\sim 45$% of red dwarf galaxies reside in massive halos as satellites, while another $\sim 25$% have a spatial distribution that is much more concentrated towards their nearest massive haloes than other dwarf galaxies. We use mock catalogs to show that the reddest population of non-satellite dwarf galaxies are distributed within about 3 times the virial radii of their nearest massive halos. We suggest that this population of dwarf galaxies are hosted by low-mass halos that have passed through their massive neighbors, and that the same environmental effects that cause satellite galaxies to become red are also responsible for the red colors of this population of galaxies. We do not find any significant radial dependence of the population of dwarf galaxies with the highest concentrations, suggesting that the mechanisms operating on these galaxies affect color more than structure. However, over 30% of dwarf galaxies are red and isolated and their origin remains unknown.

We compare and contrast the stellar structures of isolated Local Group dwarf galaxies, as traced by their oldest stellar populations, with the satellite dwarf galaxies of the Milky Way and M 31. All Local Group dwarfs with Mv ≤ −6 and μo &amp;lt; 26.5 mag arcsec−2 are considered, taking advantage of measurements from surveys that use similar observations and analysis techniques. For the isolated dwarfs, we use the results from Solitary Local (Solo) Dwarf Galaxy Survey. We begin by confirming that the structural and dynamical properties of the two satellite populations are not obviously statistically different from each other, but we note that there many more satellites around M 31 than around the Milky Way down to equivalent magnitude and surface brightness limits. We find that dwarfs in close proximity to a massive galaxy generally show more scatter in their Kormendy relations than those in isolation. Specifically, isolated Local Group dwarf galaxies show a tighter trend of half-light radius versus magnitude than the satellite populations, and similar effects are also seen for related parameters. There appears to be a transition in the structural and dynamical properties of the dwarf galaxy population around ∼400 kpc from the Milky Way and M 31, such that the smallest, faintest, and most circular dwarf galaxies are found closer than this separation. We discuss the impact of selection effects on our analysis, and we argue that our results point to the significance of tidal interactions on the population of systems within approximately 400 kpc from the Milky Way and M 31.

We have observed the center of the Local Group dwarf irregular galaxy IC 1613 with WFPC2 aboard the Hubble Space Telescope in the F439W, F555W, and F814W filters. We find a dominant old stellar population (aged ~7 Gyr), identifiable by the strong red giant branch (RGB) and red clump populations. From the (V-I) color of the RGB, we estimate a mean metallicity of the intermediate-age stellar population [Fe/H] = -1.38 +/- 0.31. We confirm a distance of 715 +/- 40 kpc using the I-magnitude of the RGB tip. The main-sequence luminosity function down to I ~25 provides evidence for a roughly constant SFR of approximately 0.00035 solar masses per year across the WFPC2 field of view (0.22 square kpc) during the past 250-350 Myr. Structure in the blue loop luminosity function implies that the SFR was ~50% higher 400-900 Myr ago than today. The mean heavy element abundance of these young stars is 1/10th solar. The best explanation for a red spur on the main-sequence at I = 24.7 is the blue horizontal branch component of a very old stellar population at the center of IC 1613. We have also imaged a broader area of IC 1613 using the 3.5-meter WIYN telescope under excellent seeing conditions. The AGB-star luminosity function is consistent with a period of continuous star formation over at least the age range 2-10 Gyr. We present an approximate age-metallicity relation for IC 1613, which appears similar to that of the Small Magellanic Cloud. We compare the Hess diagram of IC 1613 to similar data for three other Local Group dwarf galaxies, and find that it most closely resembles the nearby, transition-type dwarf galaxy Pegasus (DDO 216).

This review aims to give an overview of the contribution of the Hubble Space Telescope to our understanding of the detailed properties of Local Group dwarf galaxies and their older stellar populations. The exquisite stable high spatial resolution combined with photometric accuracy of images from the Hubble Space Telescope have allowed us to probe further back into the history of star formation of a large variety of different galaxy types with widely differing star formation properties. It has allowed us to extend our studies out to the edges of the Local Group and beyond with greater accuracy than ever before. We have learned several important things about dwarf galaxy evolution from these studies. Firstly we have found that no two galaxies have identical star formation histories; some galaxies may superficially look the same today, but they have invariably followed different paths to this point. Now that we have managed to probe deep into the star formation history of dwarf irregular galaxies in the Local Group it is obvious that there are a number of similarities with the global properties of dwarf elliptical/spheroidal type galaxies, which were previously thought to be quite distinct. The elliptical/spheroidals tend to have one or more discrete episodes of star formation through-out their history and dwarf irregulars are characterized by quasi-continuous star-formation.

We present the abundance analyses of three isolated A-type supergiant stars in the dwarf irregular galaxy Sextans A (=DDO 75) from high-resolution spectra obtained with the Ultraviolet-Visual Echelle Spectrograph (UVES) on the Kueyen telescope (UT2) of the ESO Very Large Telescope (VLT). Detailed model atmosphere analyses have been used to determine the stellar atmospheric parameters and the elemental abundances of the stars. The mean iron-group abundance was determined from these three stars to be [(Fe II, Cr II)/H] = -0.99 ± 0.04 ± 0.06. This is the first determination of the present-day iron-group abundances in Sextans A. These three stars now represent the most metal-poor massive stars for which detailed abundance analyses have been carried out. The mean stellar α-element abundance was determined from the α-element magnesium as [α(Mg I)/H] = -1.09 ± 0.02 ± 0.19. This is in excellent agreement with the nebular α-element abundances as determined from oxygen in the H II regions. These results are consistent from star to star, with no significant spatial variations over a length of 0.8 kpc in Sextans A. This supports the nebular abundance studies of dwarf irregular galaxies, where homogeneous oxygen abundances are found throughout, and argues against in situ (on the spot) enrichment. The α/Fe abundance ratio is [α(Mg I)/Fe II, Cr II] = -0.11 ± 0.02 ± 0.10, which is slightly lower but consistent with the solar ratio. This is consistent with the results from A-supergiant analyses in other Local Group dwarf irregular galaxies, NGC 6822 and WLM. The results of near-solar [α/Fe] ratios in dwarf galaxies is in stark contrast with the high [α/Fe] results from metal-poor stars in the Galaxy (which plateau at values near +0.4 dex) and is most clearly seen from these three stars in Sextans A because of their lower metallicities. The low [α/Fe] ratios are consistent with the slow chemical evolution expected for dwarf galaxies from analyses of their stellar populations.

view Abstract Citations (88) References (56) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Mass Distribution and the Law of Gravity in the Local Group Dwarf Irregular Galaxy IC 1613 Lake, George ; Skillman, Evan D. Abstract IC 1613 is a dwarf galaxy in the Local Group with an absolute magnitude M_B_ = 14.3 (L_B_ = 8.3 x 10^7^ L_sun_). The distance (725 kpc) is well determined from its Cepheids. The galaxy is gas rich, with M_Gas_/L~0.75. We imaged the H I distribution and velocity field using the VLA. The H I is lumpy in the inner parts, but the contours are regular at larger radii. The velocity field is regular and the line of nodes is coincident with the major axis of the gas distribution. The mean streaming velocity as a function of radius is corrected for asymmetric drift to determine the rotation curve. The peak velocity of ~25 km s^-1^ occurs at the outermost radius measured (~2.6 kpc). The acceleration at this point is ~5 x 10^-10^ cm s^-1^. At such low accelerations, modification of Newtonian gravity has been proposed by Milgrom (1983a,b), but we find no evidence for any such effect. With generous allowances for uncertainties in the inclination, velocity dispersion, gas mass, stellar M/L, and distance, the critical acceleration in Milgrom's theory must be a_0_ <~ 10^-9^ cm s^-1^. With all the uncertainties stretched to extremes, a_0_ <~ 3 x 10^-9^ cm s^-1^. Analysis of the rotation curves of bright galaxies has determined that this parameter should be ~1 x 10^-8^ h_50_ cm s^-1^, and the dynamics of the Coma Cluster limit the value to be a_0_ >~ 4 x 10^-8^ h^2^_50_ cm s^-1^. These differences are significant and the parameter constraints clearly depend on the mass of the system, subverting Milgrom's proposal. The rotation curve of IC 1613 shows evidence for dark matter in the outer parts. The maximum density of a dark halo that can be tolerated, p <~ 6 x 10^-26^ g cm^-3^, is an order of magnitude lower than that needed to explain the velocity dispersions of the extreme dwarf spheroidals. Publication: The Astronomical Journal Pub Date: October 1989 DOI: 10.1086/115215 Bibcode: 1989AJ.....98.1274L Keywords: Dwarf Galaxies; Gravitation Theory; Irregular Galaxies; Local Group (Astronomy); Mass Distribution; Acceleration (Physics); Astronomical Maps; H Lines; Halos; Velocity Distribution; Astrophysics; GALAXIES: INDIVIDUAL; GALAXIES: LOCAL GROUP; GALAXIES: INTERNAL MOTIONS; GRAVITATION full text sources ADS | data products SIMBAD (13) NED (12)

view Abstract Citations (42) References (28) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The H i Content of the Local Group Dwarf (Spheroidal or Irregular?) Galaxy PHOENIX Carignan, Claude ; Demers, Serge ; Cote, Stephanie Abstract Neutral hydrogen was detected in the Local Group dwarf galaxy Phoenix. The 21 cm H I line emission, observed at a heliocentric radial velocity of 56 km s^-1^, is clearly separated from a much larger scale component at ~140 km s^-1^ which was previously associated with the Magellanic Stream. The profile width {DELTA}V_50_~ 21 km s^-1^, combined with the high inclination of the system, suggests that not much rotation is present in this system which would then be mainly supported by turbulent motions. Using a newly determined distance estimate, the integrated profile gives a total H I mass of ~1.0 x 10^5^ M_sun_ for an M_H I_/L_V_~0.07 M_sun_/L_sun_. Most of the known properties of Phoenix imply that it is intermediate between typical dwarf irregular and dwarf spheroidal galaxies. It is suggested that, in fact, the two types of galaxies may belong to the same class of objects having a different history of star formation. Publication: The Astrophysical Journal Pub Date: November 1991 DOI: 10.1086/186185 Bibcode: 1991ApJ...381L..13C Keywords: Dwarf Galaxies; Galactic Evolution; Galactic Structure; Irregular Galaxies; Local Group (Astronomy); Charge Coupled Devices; Emission Spectra; Hydrogen; Neutral Gases; Star Formation; Astrophysics; GALAXIES: INTERNAL MOTIONS; GALAXIES: INTERSTELLAR MATTER; RADIO SOURCES: 21 CM RADIATION full text sources ADS | data products SIMBAD (7) NED (1)

In the Local Group there are three main types of dwarf galaxies: Dwarf Irregulars, Dwarf Spheroidals, and Dwarf Ellipticals. Intermediate/transitional types are present as well. This contribution reviews the idea that the present day variety of dwarf galaxy morphologies in the Local Group might reveal the existence of a transformation chain of events, of which any particular dwarf galaxy represents a manifestation of a particular stage. In other words, all dwarf galaxies that now are part of the Local Group would have formed identically in the early universe, but then evolved differently because of morphological transformations induced by dynamical processes like galaxy harassment, ram pressure stripping, photo-evaporation, and so forth. We start describing the population of dwarf galaxies and their spatial distribution in the LG. Then, we describe those phenomena that can alter the morphology of a dwarf galaxies, essentially by removing, partially or completely, their gas content. Lastly, we discuss morphological signatures in the Local Group Dwarf Galaxies that can be attributed to different dynamical phenomena. While it is difficult to identify a unique and continuous transformation sequence, we have now a reasonable understanding of the basic evolutionary paths that lead to the various dwarf galaxy types.

Cold dark matter models for galaxy formation predict that low-mass systems will be the first sites of star formation. As these objects have shallow gravitational potential wells, the subsequent growth of their stellar populations may be halted by heating and gas loss due to reionization. This effect has been suggested to have profoundly influenced properties of present-day dwarf galaxies, including their stellar populations and even survival as visible galaxies. In this Letter we draw on results from quantitative studies of Local Group dwarf galaxy star formation histories, especially for Milky Way satellites, to show that no clear signature exists for a widespread evolutionary impact from reionization. All nearby dwarf galaxies studied in sufficient detail contain ancient populations indistinguishable in age from the oldest Galactic globular clusters. Ancient star formation activity proceeded over several gigayears, and some dwarf spheroidal galaxies even experienced fairly continuous star formation until just a few gigayears ago. Despite their uniformly low masses, their star formation histories differ considerably. The evolutionary histories of nearby dwarf galaxies appear to reflect influences from a variety of local processes rather than a dominant effect from reionization. Subject headings: cosmology: observations — early universe — galaxies: dwarf — galaxies: evolution — Local Group — stars: Population II

Dwarf galaxies are key objects for small-scale cosmological tests like the abundance problems or the planes-of-satellites problem. A crucial task is therefore to get accurate information for as many nearby dwarf galaxies as possible. Using extremely deep, ground-based V and i-band Subaru Suprime Cam photometry with a completeness of i = 27 mag, we measure the distance of the dwarf galaxy [TT2009] 25 using the tip of the red giant branch as a standard candle. This dwarf resides in the field around the Milky Way-analog NGC 891. Using a Bayesian approach, we measure a distance of 10.28−1.73+1.17 Mpc, which is consistent with the distance of NGC 891, and thus confirm it as a member of NGC 891. The dwarf galaxy follows the scaling relations defined by the Local Group dwarfs. We do not find an extended stellar halo around [TT2009] 25. In the small field of view of 100 kpc covered by the survey, only one bright dwarf galaxy and the giant stream are apparent. This is comparable to the Milky Way, where one bright dwarf resides in the same volume, as well as the Sagittarius stream – excluding satellites which are farther away but would be projected in the line-of-sight. It is thus imperative to survey for additional dwarf galaxies in a larger area around NGC 891 to test the abundance of dwarf galaxies and compare this to the number of satellites around the Milky Way.

Understanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations they are molecular, with H2 the dominant species and carbon monoxide (CO) the best available tracer. When the abundances of carbon and oxygen are low compared with that of hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies, CO forms slowly and is easily destroyed, so it is difficult for it to accumulate inside dense clouds. Here we report interferometric observations of CO clouds in the local group dwarf irregular galaxy Wolf-Lundmark-Melotte (WLM), which has a metallicity that is 13 per cent of the solar value and 50 per cent lower than the previous CO detection threshold. The clouds are tiny compared to the surrounding atomic and H2 envelopes, but they have typical densities and column densities for CO clouds in the Milky Way. The normal CO density explains why star clusters forming in dwarf irregulars have similar densities to star clusters in giant spiral galaxies. The low cloud masses suggest that these clusters will also be low mass, unless some galaxy-scale compression occurs, such as an impact from a cosmic cloud or other galaxy. If the massive metal-poor globular clusters in the halo of the Milky Way formed in dwarf galaxies, as is commonly believed, then they were probably triggered by such an impact.

Late one night in October 2015, Andrew Wetzel was fretting. For 15 days, his cosmological models had been swirling virtual dark matter around cybernetic gas and dust and slowly generating a synthetic galaxy approximately the size of our own Milky Way, and Wetzel was about to receive the results. “I finally got the plot up to compare our simulation with the Milky Way,” recalls Wetzel, an assistant professor of physics at the University of California, Davis. “I went to bed very happy that night.” Cosmological simulations such as this one have gotten better at reproducing dwarf galaxies, but questions remain. This simulation shows a Milky Way-like galaxy at the center surrounded by a realistic population of satellite dwarf galaxies and a stellar halo. Image credit: Andrew Wetzel (University of California, Davis, CA) and Phil Hopkins (California Institute of Technology, Pasadena, CA). That’s because, for the first time, a simulation had accurately reproduced a realistic population of dwarf galaxies near the Milky Way. These small companions orbit larger galaxies like planets around a star, and for a decade and a half, they had presented astronomers with a big problem. Simulations suggested that thousands of companion galaxies should surround the Milky Way, but telescopes had seen only a handful. Researchers seemed to be in the dark about some important aspect of the cosmos. With the latest simulations from Wetzel and his team, the mystery of the missing satellites might appear to be explained (1). But more questions are emerging, with newly discovered galaxies more diverse than anyone predicted, and simulations, as yet, unable to recreate their richness. There may even turn out to be too many dwarfs for simulations to contend with. Future supercomputer models and next-generation telescopes will address these conundrums, potentially illuminating a critical stage in the growth of the …

view Abstract Citations (68) References (74) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Imaging and Spectrophotometry of Virgo Cluster Irregular Galaxies Gallagher, John S., III ; Hunter, Deidre A. Abstract Large-aperture spectrophotometric observations and CCD images in the Hα and R bands are presented for Virgo Cluster Magellanic, dwarf and amorphous irregular galaxies. These data provide a basis for comparisons between star-forming properties, morphologies, and emission-line characteristics in field and cluster irregular galaxies. Most Virgo Cluster Magellanic and dwarf irregular galaxies are optically similar to field galaxies. Modest cluster-field differences may exist in H I gas content (cluster lower than field) and in star-formation rates per unit area (cluster lower than field). A minority of Virgo Cluster irregulars have optical and H I properties consistent with recent gas loss, but the rate of production of gas-poor irregulars is too low to account for the large population of Virgo Cluster dE galaxies or to explain the deficiency in the number of irregulars relative to spirals. Also puzzling are the properties of red amorphous or peculiar E/S0 galaxies in our sample. Radial brightness profiles have been measured for these galaxies, and only NGC 4641 follows the r^1/4^ law of classical ellipticals, four galaxies have exponential brightness distributions, and one system has an indeterminant profile form. The physical nature of the red amorphous galaxies thus remains unclear. We also suggest that the properties of Virgo Cluster irregular galaxies are best understood in terms of environmental modifications of normal systems in the circumcluster environment and in the cluster core. However, these processes do not appear to account for the large core region population of dE galaxies, and thus initial conditions may also have played a role in shaping the Virgo Cluster dwarf galaxy population. Publication: The Astronomical Journal Pub Date: September 1989 DOI: 10.1086/115180 Bibcode: 1989AJ.....98..806G Keywords: Astronomical Photometry; Dwarf Galaxies; Irregular Galaxies; Spectrophotometry; Virgo Galactic Cluster; Astronomical Catalogs; Brightness Distribution; Density Distribution; H I Regions; Magellanic Clouds; Visual Photometry; Astrophysics; GALAXIES: GENERAL full text sources ADS | data products SIMBAD (43) NED (41)