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Register a new userFull-Fledged Dwarf Irregular Galaxy Leo A
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Authors
Vladas Vansevicius
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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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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 present deep Hubble Space Telescope single-star photometry of Leo A in B, V, and I. Our new field of view is offset from the centrally located field observed by Tolstoy et al. (1998) in order to expose the halo population of this galaxy. We report the detection of metal-poor red horizontal branch stars, which demonstrate that Leo A is not a young galaxy. In fact, Leo A is as least as old as metal-poor Galactic Globular Clusters which exhibit red horizontal branches, and are considered to have a minimum age of about 9 Gyr. We discuss the distance to Leo A, and perform an extensive comparison of the data with stellar isochrones. For a distance modulus of 24.5, the data are better than 50% complete down to absolute magnitudes of 2 or more. We can easily identify stars with metallicities between 0.0001 and 0.0004, and ages between about 5 and 10 Gyr, in their post-main-sequence phases, but lack the detection of main-sequence turnoffs which would provide unambiguous proof of ancient (>10 Gyr) stellar generations. Blue horizontal branch stars are above the detection limits, but difficult to distinguish from young stars with similar colors and magnitudes. Synthetic color-magnitude diagrams show it is possible to populate the blue horizontal branch in the halo of Leo A. The models also suggest ~50% of the total astrated mass in our pointing to be attributed to an ancient (>10 Gyr) stellar population. We conclude that Leo A started to form stars at least about 9 Gyr ago. Leo A exhibits an extremely low oxygen abundance, of only 3% of Solar, in its ionized interstellar medium. The existence of old stars in this very oxygen-deficient galaxy illustrates that a low oxygen abundance does not preclude a history of early star formation.
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.
Leo I is considered one of the youngest dwarf spheroidals (dSph) in the Local Group. Its isolation, extended star formation history (SFH), and recent perigalacticon passage (~1 Gyr ago) make Leo~I one of the most interesting nearby stellar systems. Here, we analyse deep photometric Hubble Space Telescope data via colour-magnitude diagram fitting techniques to study its global and radially-resolved SFH. We find global star formation enhancements in Leo I ~13, 5.5, 2.0, and 1.0 Gyr ago, after which it was substantially quenched. Within the context of previous works focused on Leo I, we interpret the most ancient and the youngest ones as being linked to an early formation (surviving reionisation) and the latest perigalacticon passage (transition from dIrr to dSph), respectively. We clearly identify the presence of very metal poor stars ([Fe/H]~-2) ageing ~5-6 and ~13 Gyr old. We speculate with the possibility that this metal-poor population in Leo I is related to the merging with a low mass system (possibly an ultra-faint dwarf). This event would have triggered star formation (peak of star formation ~5.5 Gyr ago) and accumulated old, metal poor stars from the accreted system in LeoI. Some of the stars born during this event would also form from accreted gas of low-metallicity (giving rise to the 5-6 Gyr low-metallicity tail). Given the intensity and extension of the 2.0 Gyr burst, we hypothesise that this enhancement could also have an external origin. Despite the quenching of star formation around 1 Gyr ago (most probably induced by ram pressure stripping with the Milky Way halo at pericentre), we report the existence of stars as young as 300-500 Myr. We also distinguish two clear spatial regions: the inner ~190 pc presents an homogeneous stellar content (size of the gaseous star forming disc in LeoI from ~4.5 to 1 Gyr ago), whereas the outer regions display a clear positive age gradient.
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