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Register a new userLeo IV & V - A possible dwarf galaxy pair ?
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The last few years have seen the discovery of many faint and ultra-faint dwarf spheroidal galaxies around the Milky Way. Among these is a pair of satellites called Leo IV and Leo V. This pair is found at large distances from the Milky Way (154 and 175 kpc respectively). The rather small difference in radial distance, and the fact that they also show a close projected distance on the sky, has led to the idea that we might be seeing a new pair of bound galaxies - like the Magellanic Clouds. In this paper we investigate this speculation by means of a simple integration code (confirming the results with full N-body simulations). As the luminous mass of both faint dwarfs is far too low to allow them to be bound, we simulate the pair assuming extended dark matter haloes. Our results show that the minimum dark matter mass required for the pair to be bound is rather high - ranging from 1.6 x 10^10 Msun to 5.4 x 10^10 Msun (within the virial radii). Computing the mass of dark matter within a commonly adopted radius of 300 pc shows that our models are well within the predicted range of dark matter content for satellites so faint. We therefore conclude that it could be possible that the two galaxies constitute a bound pair.
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We have obtained deep photometry in two 1x1 degree fields covering the close pair of dwarf spheroidal galaxies (dSph) Leo IV and Leo V and part of the area in between. We find that both systems are significantly larger than indicated by previous measurements based on shallower data and also significantly elongated. With half-light radii of r_h=4.6 +- 0.8 (206 +- 36 pc) and r_h=2.6 +- 0.6 (133 +- 31 pc), respectively, they are now well within the physical size bracket of typical Milky Way dSph satellites. Their ellipticities of epsilon ~0.5 are shared by many faint (M_V>-8) Milky Way dSphs. The large spatial extent of our survey allows us to search for extra-tidal features with unprecedented sensitivity. The spatial distribution of candidate red giant branch and horizontal branch stars is found to be non-uniform at the ~3 sigma level. This substructure is aligned along the direction connecting the two systems, indicative of a possible `bridge of extra-tidal material. Fitting the stellar distribution with a linear Gaussian model yields a significance of 4 sigma for this overdensity, a most likely FWHM of ~16 arcmin and a central surface brightness of ~32 mag arcsec^{-2}. We investigate different scenarios to explain the close proximity of Leo IV and Leo V and the possible tidal bridge between them. Orbit calculations demonstrate that they are unlikely to be remnants of a single disrupted progenitor, while a comparison with cosmological simulations shows that a chance collision between unrelated subhalos is negligibly small. Leo IV and Leo V could, however, be a bound `tumbling pair if their combined mass exceeds 8 +- 4 x 10^9 M_sun. The scenario of an internally interacting pair appears to be the most viable explanation for this close celestial companionship. (abridged)
We present a revised analysis of a speculated stellar bridge between the Milky Way dwarf galaxies Leo IV and Leo V. Using data acquired with Subaru/Suprime-Cam over a 1deg x 4deg field encompassing the two satellites and the region in between, we confirm our previous detection of a stellar overdensity between Leo IV and Leo V (de Jong et al. 2010). The larger area coverage and improved depth of our current dataset allow for an improved analysis of the stellar overdensity that had previously appeared to bridge the two galaxies. A main-sequence turn-off feature visible in the stacked colour-magnitude diagram of the contiguously observed Subaru fields reveals an extended stellar structure at a distance of approximately 20 kpc. Its angular proximity to the Virgo overdensity, as well as a good correspondence in distance and metallicity, suggests that the smaller structure we detect may be associated with the much larger Virgo stellar overdensity.
We analysed a population of bright-red (BR) stars in the dwarf irregular galaxy Leo A by using multicolour photometry data obtained with the Subaru/Suprime-Cam ($B$, $V$, $R$, $I$, $Halpha$) and HST/ACS ($F475W$ & $F814W$) instruments. In order to separate the Milky Way (MW) and Leo A populations of red stars, we developed a photometric method, which enabled us to study the spatial distribution of BR stars within the Leo A galaxy. We found a significant difference in the scale-length (S-L) of radial distributions of the young and old red giant branch (RGB) stars -- $0.82 pm 0.04$ and $1.53 pm 0.03$, respectively. Also, we determined the S-L of BR stars of $0.85 pm 0.05$, which closely matches that of the young RGB stars. Additionally, we found a sequence of peculiar RGB stars and 8 dust-enshrouded stars in the Leo A galaxy.
We have surveyed a complete extent of Leo A - an apparently isolated gas-rich low-mass dwarf irregular galaxy in the Local Group. The $B$, $V$, and $I$ passband CCD images (typical seeing $sim$0.8) were obtained with Subaru Telescope equipped with Suprime-Cam mosaic camera. The wide-field ($20 times 24$) photometry catalog of 38,856 objects ($V sim 16-26$ mag) is presented. This survey is also intended to serve as a finding chart for future imaging and spectroscopic observation programs of Leo A.
We have studied Leo A - the isolated and extremely gas rich dwarf irregular galaxy of very low stellar mass and metallicity. Ages of the stellar populations in Leo A are ranging from ~10 Myr to ~10 Gyr. Here we report the discovery of an old stellar halo and a sharp stellar edge. Also we find the distribution of stars extending beyond the gaseous envelope of the galaxy. Therefore, Leo A by its structure as well as stellar and gaseous content is found to resemble massive disk galaxies. This implies that galaxies of very low stellar mass are also able to develop complex structures, challenging contemporary understanding of galaxy evolution.
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