Transition type dwarf galaxies are thought to be systems undergoing the process of transformation from a star-forming into a passively evolving dwarf, which makes them particularly suitable to study ...evolutionary processes driving the existence of different dwarf morphological types. Here we present results from a spectroscopic survey of ~200 individual red giant branch stars in the Phoenix dwarf, the closest transition type with a comparable luminosity to 'classical' dwarf galaxies. We measure a systemic heliocentric velocity Vhelio = -21.2 plus or minus 1.0 km s super( -1). Our survey reveals the clear presence of prolate rotation that is aligned with the peculiar spatial distribution of the youngest stars in Phoenix. We speculate that both features might have arisen from the same event, possibly an accretion of a smaller system. The evolved stellar population of Phoenix is relatively metal-poor (<Fe/H > = -1.49 plus or minus 0.04 dex) and shows a large metallicity spread (sFe/H = 0.51 plus or minus 0.04 dex), with a pronounced metallicity gradient of -0.13 plus or minus 0.01 dex arcmin super( -1) similar to luminous, passive dwarf galaxies. We also report a discovery of an extremely metal-poor star candidate in Phoenix and discuss the importance of correcting for spatial sampling when interpreting the chemical properties of galaxies with metallicity gradients. This study presents a major leap forward in our knowledge of the internal kinematics of the Phoenix transition type dwarf galaxy and the first wide area spectroscopic survey of its metallicity properties. A table containing the measured velocities, metallicities, and CaT equivalent widths of all spectroscopic targets is available online at the CDS.
ABSTRACT
The Milky Way underwent its last significant merger ten billion years ago, when the Gaia-Enceladus-Sausage (GES) was accreted. Accreted GES stars and progenitor stars born prior to the ...merger make up the bulk of the inner halo. Even though these two main populations of halo stars have similar durations of star formation prior to their merger, they differ in α/Fe-Fe/H space, with the GES population bending to lower α/Fe at a relatively low value of Fe/H. We use cosmological simulations of a ‘Milky Way’ to argue that the different tracks of the halo stars through the α/Fe-Fe/H plane are due to a difference in their star formation history and efficiency, with the lower mass GES having its low and constant star formation regulated by feedback whilst the higher mass main progenitor has a higher star formation rate prior to the merger. The lower star formation efficiency of GES leads to lower gas pollution levels, pushing α/Fe-Fe/H tracks to the left. In addition, the increasing star formation rate maintains a higher relative contribution of Type II SNe to Type Ia SNe for the main progenitor population that formed during the same time period, thus maintaining a relatively high α/Fe. Thus the different positions of the downturns in the α/Fe-Fe/H plane for the GES stars are not reflective of different star formation durations, but instead reflect different star formation efficiencies.
We discuss how knowledge of the whole evolutionary history of dwarf galaxies, including details on the early star formation events, can provide insight on the origin of the different dwarf galaxy ...types. We suggest that these types may be imprinted by the early conditions of formation rather than only being the result of a recent morphological transformation driven by environmental effects. We present precise star formation histories of a sample of Local Group dwarf galaxies, derived from color-magnitude diagrams reaching the oldest main-sequence turnoffs. We argue that these galaxies can be assigned to two basic types: fast dwarfs that started their evolution with a dominant and short star formation event and slow dwarfs that formed a small fraction of their stars early and have continued forming stars until the present time (or almost). These two different evolutionary paths do not map directly onto the present-day morphology (dwarf spheroidal versus dwarf irregular). Slow and fast dwarfs also differ in their inferred past location relative to the Milky Way and/or M31, which hints that slow dwarfs were generally assembled in lower-density environments than fast dwarfs. We propose that the distinction between a fast and slow dwarf galaxy primarily reflects the characteristic density of the environment where they form. At a later stage, interaction with a large host galaxy may play a role in the final gas removal and ultimate termination of star formation.
We discuss the role that dwarf galaxies may have played in the formation of the Galactic halo (Halo) using RR Lyrae stars (RRL) as tracers of their ancient stellar component. The comparison is ...performed using two observables (periods, luminosity amplitudes) that are reddening and distance independent. Fundamental mode RRL in 6 dwarf spheroidals (dSphs) and 11 ultra faint dwarf galaxies (~1300) show a Gaussian period distribution well peaked around a mean period of left angle bracketPabright angle bracket = 0.610 + or - 0.001 days (sigma = 0.03). The Halo RRL (~15,000) are characterized by a broader period distribution. The fundamental mode RRL in all the dSphs apart from Sagittarius are completely lacking in High Amplitude Short Period (HASP) variables, defined as those having P<, ~ 0.48 days and AV> or = 0.75 mag. Such variables are not uncommon in the Halo and among the globular clusters and massive dwarf irregulars. To further interpret this evidence, we considered 18 globulars covering a broad range in metallicity (-2.3 lesssim Fe/H <, ~ -1.1) and hosting more than 35 RRL each. The metallicity turns out to be the main parameter, since only globulars more metal-rich than Fe/H ~ -1.5 host RRL in the HASP region. This finding suggests that dSphs similar to the surviving ones do not appear to be the major building-blocks of the Halo. Leading physical arguments suggest an extreme upper limit of ~50% to their contribution. On the other hand, massive dwarfs hosting an old population with a broad metallicity distribution (Large Magellanic Cloud, Sagittarius) may have played a primary role in the formation of the Halo.
Based on Hubble Space Telescope observations from the Local Cosmology from Isolated Dwarfs project, we present the star formation histories, as a function of galactocentric radius, of four isolated ...Local Group dwarf galaxies: two dSph galaxies, Cetus and Tucana, and two transition galaxies (dTrs), LGS-3 and Phoenix. The oldest stellar populations of the dSphs and dTrs are, within the uncertainties, coeval (~13 Gyr) at all galactocentric radii. We find that there are no significative differences between the four galaxies in the fundamental properties (such as the normalized star formation rate or age-metallicity relation) of their outer regions (radii greater than four exponential scale lengths); at large radii, these galaxies consist exclusively of old (gap10.5 Gyr) metal-poor stars. The duration of star formation in the inner regions varies from galaxy to galaxy, and the extended central star formation in the dTrs produces the dichotomy between dSph and dTr galaxy types. The dTr galaxies show prominent radial stellar population gradients: The centers of these galaxies host young (lap1 Gyr) populations, while the age of the last formation event increases smoothly with increasing radius. This contrasts with the two dSph galaxies. Tucana shows a similar, but milder, gradient, but no gradient in age is detected Cetus. For the three galaxies with significant stellar population gradients, the exponential scale length decreases with time. These results are in agreement with outside-in scenarios of dwarf galaxy evolution, in which a quenching of the star formation toward the center occurs as the galaxy runs out of gas in the outskirts.
We present an analysis of the star formation history (SFH) of the transition-type (dIrr/dSph) Local Group galaxy LGS-3 (Pisces) based on deep photometry obtained with the Advanced Camera for Surveys ...onboard the Hubble Space Telescope. Our observations reach the oldest main-sequence turnoffs at high signal to noise, allowing a time resolution at the oldest ages of Delta *s ~ 1.1 Gyr. Our analysis, based on three different SFH codes, shows that the SFH of LGS-3 is dominated by a main episode ~11.7 Gyr ago with a duration of ~1.4 Gyr. Subsequently, LGS-3 continued forming stars until the present, although at a much lower rate. Roughly 90% of the stars in LGS-3 were formed in the initial episode of star formation. Extensive tests of self-consistency, uniqueness, and stability of the solution have been performed together with the IAC-star/IAC-pop/MinnIAC codes, and these results are found to be independent of the photometric reduction package, the stellar evolution library, and the SFH recovery method. There is little evidence of chemical enrichment during the initial episode of star formation, after which the metallicity increased more steeply reaching a present-day value of Z ~ 0.0025. This suggests a scenario in which LGS-3 first formed stars mainly from infalling fresh gas, and after about 9 Gyr ago, from a larger fraction of recycled gas. The lack of early chemical enrichment is in contrast to that observed in the isolated dSph galaxies of comparable luminosity, implying that the dSphs were more massive and subjected to more tidal stripping. We compare the SFH of LGS-3 with expectations from cosmological models. Most or all the star formation was produced in LGS-3 after the reionization epoch, assumed to be completed at z ~ 6 or ~12.7 Gyr ago. The total mass of the galaxy is estimated to be between 2 and 4 X 108 M corresponding to circular velocities between 28 km s--1 and 36 km s--1. These values are close to but somewhat above the limit of 30 km s--1 below which the UV background is expected to prevent any star formation after reionization. Feedback from supernovae (SNe) associated with the initial episode of star formation (mechanical luminosity from SNe Lw = 5.3 X 1038 erg s--1) is probably inadequate to completely blow away the gas. However, the combined effects of SN feedback and UV background heating might be expected to completely halt star formation at the reionization epoch for the low mass of LGS-3; this suggests that self-shielding is important to the early evolution of galaxies in this mass range.
ABSTRACT We report the discovery of one RR Lyrae star in the ultra-faint satellite galaxy Hydra II based on time series photometry in the g, r and i bands obtained with the Dark Energy Camera at ...Cerro Tololo Inter-American Observatory, Chile. The association of the RR Lyrae star discovered here with Hydra II is clear because is located at 42 ″ from the center of the dwarf, well within its half-light radius of 102 ″ . The RR Lyrae star has a mean magnitude of i = 21.30 0.04 which is too faint to be a field halo star. This magnitude translates to a heliocentric distance of 151 8 kpc for Hydra II; this value is ∼ 13 % larger than the estimate from the discovery paper based on the average magnitude of several blue horizontal branch star candidates. The new distance implies a slightly larger half-light radius of 76 − 10 + 12 pc and a brighter absolute magnitude of M V = − 5.1 0.3 , which keeps this object within the realm of the dwarf galaxies. A comparison with other RR Lyrae stars in ultra-faint systems indicates similar pulsational properties among them, which are different to those found among halo field stars and those in the largest of the Milky Way satellites. We also report the discovery of 31 additional short period variables in the field of view (RR Lyrae, SX Phe, eclipsing binaries, and a likely anomalous cepheid) which are likely not related with Hydra II.
PDF
