Your search for "Sextans A" returned 463 results

We present a comprehensive study of young stellar populations in six dwarf galaxies in or near the Local Group: Phoenix, Pegasus, Sextans A, Sextans B, WLM, and NGC 6822. Their star-forming regions, selected from GALEX wide-field far-UV imaging, were imaged (at sub-pc resolution) with the WFPC2 camera on board the Hubble Space Telescope (HST) in six bandpasses from far-UV to I to detect and characterize their hot massive star content. This study is part of HST treasury survey program HST-GO-11079; the general data characteristics and reduction procedures are detailed in this paper and results are presented for the first six galaxies. From a total of 180 HST images, we provide catalogs of the multi-band stellar photometry and derive the physical parameters of massive stars by analyzing it with model-atmosphere colors. We use the results to infer ages, number of massive stars, extinction, and spatial characteristics of the young stellar populations. The hot massive star content varies largely across our galaxy sample, from an inconspicuous presence in Phoenix and Pegasus to the highest relative abundance of young massive stars in Sextans A and WLM. Albeit to a largely varying extent, most galaxies show a very young population (a few Myrs, except for Phoenix), and older ones (a few 10⁷ years in Sextans A, Sextans B, NGC 6822, and WLM, ∼10⁸yr in Phoenix and Pegasus), suggesting discrete bursts of recent star formation in the mapped regions. The hot massive star content (indicative of the young populations) broadly correlates with the total galaxy stellar mass represented by the integrated optical magnitude, although it varies by a factor of ∼3 between Sextans A, WLM, and Sextans B, which have similar M_V. Extinction properties are also derived.

A large extension of the Sextans dwarf spheroidal galaxy, 7 deg², has been surveyed for variable stars using the Dark Energy Camera at the Blanco Telescope at Cerro Tololo Inter-American Observatory, Chile. We report seven anomalous Cepheids, 199 RR Lyrae stars, and 16 dwarf Cepheids in the field. This is only the fifth extragalactic system in which dwarf Cepheids have been systematically searched. Henceforth, the new stars increase the census of stars coming from different environments that can be used to asses the advantages and limitations of using dwarf Cepheids as standard candles in populations for which the metallicity is not necessarily known. The dwarf Cepheids found in Sextans have a mean period of 0.066 day and a mean g amplitude of 0.87 mag. They are located below the horizontal branch, spanning a range of 0.8 mag: 21.9 < g < 22.7. The number of dwarf Cepheids in Sextans is low compared with other galaxies such as Carina, which has a strong intermediate-age population. On the other hand, the number and ratio of RR Lyrae stars to dwarf Cepheids are quite similar to those of Sculptor, a galaxy which, as Sextans, is dominated by an old stellar population. The dwarf Cepheid stars found in Sextans follow a well-constrained period–luminosity relationship with an rms = 0.05 mag in the g band, which was set up by anchoring to the distance modulus given by the RR Lyrae stars. Although the majority of the variable stars in Sextans are located toward the center of the galaxy, we have found two RR Lyrae stars and one anomalous Cepheid in the outskirts of the galaxy that may be extratidal stars and suggest that this galaxy may be undergoing tidal destruction. These possible extratidal variable stars share the same proper motions as Sextans, as seen by recent Gaia measurements. Two additional stars that we initially classified as foreground RR Lyrae stars may actually be other examples of Sextans extratidal anomalous Cepheids, although radial velocities are needed to prove that scenario.

We present a radio interferometer mosaic of H I line emission within a radius of 45' of the small irregular galaxy Sextans A at a resolution of about 1'. Sextans A lies on the edge of the Local Group and was targeted because single-dish observations showed it to have an unusually extended disk of H I. However, we detected H I emission only to a radius of 9' at 8 × 10¹⁸ cm^-2, whereas single-dish observations detected H I to a radius 3 times that at approximately the same column density. We also recover only 62% of the flux detected in the single-dish maps. We conclude that the missing H I is real and is most likely to be found in a very smooth, low column density distribution whose scale is too large to be resolved with the interferometer. We put limits on the size of density fluctuations in this extended gas on the order of 10'–15'. We argue that this unusually smooth structure implies that Sextans A has not been perturbed by another galaxy for at least 6–10 Gyr. The fact that the current star formation rate is 4–5 times higher than the rate averaged over the galaxy's lifetime implies that internal processes are driving the modest fluctuation in the current star formation rate.

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.

We present the results of high-quality long-slit spectroscopy of planetary nebulae (PNe) and H II regions in the two dwarf irregular (dIrr) galaxies Sextans A and B, which belong to a small group of galaxies just outside the Local Group. The observations were obtained with the New Technology Telescope ESO Multi-Mode Instrument. In Sextans A we obtained the element abundances in its only known PN and in three H II regions with the classical T_e method. The oxygen abundances in these three H II regions of Sextans A are all consistent within the individual rms uncertainties, with an average 12 + log(O/H) = 7.54 ± 0.06. The oxygen abundance of the PN in Sextans A is, however, significantly higher: 12 + log(O/H) = 8.02 ± 0.05. This PN is even more enriched in nitrogen and helium, suggesting a classification as a PN of type I. The PN abundances of S and Ar, which are presumably unaffected by nucleosynthesis in the progenitor star, are well below those in the H II regions, indicating lower metallicity at the epoch of the PN progenitor formation (∼1.5 Gyr ago, according to our estimates based on the PN parameters). In Sextans B we obtained spectra of one PN and six H II regions. Element abundances with the T_e method could be derived for the PN and three of the H II regions. For two of these H II regions, which have a separation of only ∼70 pc in projection, the oxygen abundances do not differ within the rms uncertainties, with a mean of 12 + log(O/H) = 7.53 ± 0.05. The third H II region, which is about 0.6 kpc northeast of the first two, is twice as metal-rich, with 12 + log(O/H) = 7.84 ± 0.05. This suggests considerable inhomogeneity in the present-day metallicity distribution in Sextans B. Whether this implies a general chemical inhomogeneity among populations of comparable age in Sextans B, and thus a metallicity spread at a given age, or whether we happen to see the short-lived effects of freshly ejected nucleosynthesis products prior to their dispersal and mixing with the ambient interstellar medium will require further study. For the PN we measured an O/H ratio of 12 + log(O/H) = 7.47 ± 0.16, consistent with that of the low-metallicity H II regions. We discuss the new metallicity data for the H II regions and PNe in the context of the published star formation histories and published abundances of the two dIrr galaxies. Both dIrrs show generally similar star formation histories in the sense of continuous star formation with amplitude variations but differ in their detailed enrichment timescales and star formation rates as a function of time. If we combine the photometrically derived estimates for the mean metallicity of the old red giant branch population in both dIrrs with the present-day metallicity of the H II regions, both dIrrs have experienced chemical enrichment by at least 0.8 dex (lower limit) throughout their history.

We present color-magnitude diagrams and luminosity functions of stellar populations in the nearby dwarf galaxy Sextans B, based on VI photometry acquired with the Carnegie Observatories spectroscopic multislit and imaging camera CCD prime-focus camera on the Palomar 5 m telescope. Combining previously published Cepheid observations with our new data, we obtain a distance to Sextans B based on the multiwavelength period-luminosity (PL) relations; BVRI data for eight Cepheids having periods ranging from 4 to 30 days were used. This Population I true distance modulus for Sextans B is found to be (m - M)₀ = 25.69 ± 0.27 mag (1.37 ± 0.18 Mpc) with an extinction value of A_V = 0.05 mag. The Cepheid distance is then compared with the Population II distance, determined by applying the tip of the red giant branch (TRGB) method. A discontinuity in the I-band luminosity function is detected at I = 21.60 ± 0.10 mag. Adopting M_I = -4.04 ± 0.02 mag as the absolute magnitude of the TRGB, and taking into account the Galactic extinction value of A_I = 0.02 mag, we obtain a Population II distance modulus of (m - M)₀ = 25.56 ± 0.10 mag (1.29 ± 0.07 Mpc). Additional systematic uncertainties in the TRGB distance scale probably exist at the 5%-10% level. The agreement between the two distances and their uncertainties derived from distinctly independent methods further demonstrates that the accuracy and precision of the TRGB method as an extragalactic distance indicator are comparable to those of the Cepheid PL relation.

We present sensitive CO (J = 1 $\to$ 0) emission line observations of the three metal-poor dwarf irregular galaxies Leo P (Z ∼ 3% Z_⊙), Sextans A (Z ∼ 7.5% Z_⊙), and Sextans B (Z ∼ 7.5% Z_⊙), all obtained with the Combined Array for Millimeter-wave Astronomy interferometer. While no CO emission was detected, the proximity of the three systems allows us to place very stringent (4σ) upper limits on the CO luminosity (L_CO) in these metal-poor galaxies. We find the CO luminosities to be L_CO < 2900 K km s⁻¹ pc² for Leo P, L_CO < 12,400 K km s⁻¹ pc² for Sextans A, and L_CO < 9700 K km s⁻¹ pc² for Sextans B. Comparison of our results with recent observational estimates of the factor for converting between L_CO and the mass of molecular hydrogen, as well as theoretical models, provides further evidence that either the CO-to-H₂ conversion factor increases sharply as metallicity decreases, or that stars are forming in these three galaxies very efficiently, requiring little molecular hydrogen.

We present multifiber echelle radial velocity results for 551 stars in the Sextans dwarf spheroidal galaxy and identify 294 stars as probable Sextans members. The projected velocity dispersion profile of the binned data remains flat to a maximum angular radius of 30'. We introduce a nonparametric technique for estimating the projected velocity dispersion surface and use this to search for kinematic substructure. Our data do not confirm previous reports of a kinematically distinct stellar population at the Sextans center. Instead we detect a region near the Sextans core radius that is kinematically colder than the overall Sextans sample with 95% confidence.

We report a possible discovery of a relic star cluster in the Sextans dwarf spheroidal galaxy. Using the hk index (≡(Ca − b)−(b − y)) as a photometric metallicity indicator, we discriminate the metal-poor and metal-rich stars in the galaxy and find unexpected number density excess of metal-poor stars located 77 (∼190 pc in projected distance) away from the known galactic center. The V − I color−magnitude diagram (CMD) for stars around the density excess reveals that both the main sequence and the giant branch are considerably narrower and redder than the bulk of field stars in Sextans. Our stellar population models show (a) that the narrow CMD is best reproduced by a simple stellar population with an age of ∼13 Gyr and [Fe/H] of ∼−2.3 dex, and (b) that the redder V − I color of the hk-weak population is explained only if it is ∼2 Gyr older than the field stars. The results lead us to conclude that the off-centered density peak is likely associated with an old, metal-poor globular cluster. The larger spatial extent (>80 pc in radius) and the smaller number of stars (∼1000) than typical globular clusters point to a star cluster that is in the process of dissolution. The finding serves as the first detection of a surviving star cluster in Sextans, supporting previous suggestions of the presence of star cluster remnants in the galaxy. If confirmed, the survival of a relic star cluster until now implies a cored dark matter halo profile for this dwarf galaxy.

We have identified 82 short-period variable stars in Sextans A from deep Wide Field Planetary Camera 2 observations. All the periodic variables appear to be short-period Cepheids, with periods as small as 0.8 days for fundamental mode Cepheids and 0.5 days for first-overtone Cepheids. These objects have been used, along with measurements of the red giant branch (RGB) tip and red clump, to measure a true distance modulus to Sextans A of μ₀ = 25.61 ± 0.07, corresponding to a distance of d = 1.32 ± 0.04 Mpc. Comparing distances calculated by these techniques, we find that short-period Cepheids (P < 2 days) are accurate distance indicators for objects at or below the metallicity of the Small Magellanic Cloud. As these objects are quite numerous in low-metallicity star-forming galaxies, they have the potential for providing extremely precise distances throughout the Local Group. We have also compared the relative distances produced by other distance indicators. We conclude that calibrations of RR Lyrae stars, the RGB tip, and the red clump are self-consistent, but that there appears to be a small dependence of long-period Cepheid distances on metallicity. Finally, we present relative distances of Sextans A, Leo A, IC 1613, and the Magellanic Clouds.