Local Group Dwarf Research Articles

The Origin of the 300 km s−1 Stream near Segue 1

Abstract We present a search for new members of the 300 km s−1 stream (300S) near the dwarf galaxy Segue 1 using wide-field survey data. We identify 11 previously unknown bright stream members in the APOGEE-2 and SEGUE-1 and 2 spectroscopic surveys. Based on the spatial distribution of the high-velocity stars, we confirm for the first time that this kinematic structure is associated with a 24°-long stream seen in SDSS and Pan-STARRS imaging data. The 300S stars display a metallicity range of −2.17 < [Fe/H] < −1.24, with an intrinsic dispersion of 0.21 − 0.09 + 0.12 dex. They also have chemical abundance patterns similar to those of Local Group dwarf galaxies, as well as that of the Milky Way halo. Using the open-source code galpy to model the orbit of the stream, we find that the progenitor of the stream passed perigalacticon about 70 Myr ago, with a closest approach to the Galactic center of about 4.1 kpc. Using Pan-STARRS DR1 data, we obtain an integrated stream luminosity of 4 × 103 L ⊙. We conclude that the progenitor of the stream was a dwarf galaxy that is probably similar to the satellites that were accreted to build the present-day Milky Way halo.

Integrated-light analyses vs. colour-magnitude diagrams

Context. Most of our knowledge of the stellar component of galaxies is based on the analysis of distant systems and comes from integrated light data. It is important to test whether the results of the star formation histories (SFH) obtained with standard full-spectrum fitting methods are in agreement with those obtained through colour-magnitude diagram (CMD) fitting (usually considered the most reliable approach). Aims. We compare SFHs recovered from the two techniques in Leo A, a Local Group dwarf galaxy most of whose stars formed during the last 8 Gyrs. This complements our previous findings in a field in the Large Magellanic Cloud bar, where star formation has been in progress since early epochs at varying rates. Methods. We have used GTC/OSIRIS in long-slit mode to obtain a high-quality integrated light spectrum by scanning a selected region within Leo A, for which a CMD reaching the old main sequence turn-off (oMSTO) is available from HST. We compared the SFH obtained from the two datasets, using state-of-art methods of integrated light (STECKMAP) and resolved stellar population analysis. In the case of the CMD, we computed the SFH both from a deep CMD (observed with HST/ACS) and from a shallower CMD (archival data from HST/WFPC2). Results. The agreement between the SFHs recovered from the oMSTO CMD and from full spectrum fitting is remarkable, particularly regarding the time evolution of the star formation rate. The overall extremely low metallicity of Leo A is recovered up to the last 2 Gyrs when some discrepancies appear. A relatively high metallicity found for the youngest stars from the integrated data is a recurring feature that might indicate that the current models or synthesis codes should be revised, but that can be significantly mitigated using a more restrictive metallicity range. We thoroughly inspect the robustness of both approaches separately, finding that the subtle differences between them are inherent to the methods themselves. The SFH recovered from the shallow CMD also presents differences with the other two. Conclusions. Modern full-spectral fitting codes are able to recover both average constant SFHs (LMC case) and SFHs with a dominant fraction of young stellar populations. The analysis of high S/N spectra seems to provide more reliable SFH estimates than that of CMDs not reaching the oMSTO. The comparison presented in this paper needs to be repeated for predominantly old systems, thus assessing the performance of full-spectrum fitting for a full range of SFHs.

SMHASH: a new mid-infrared RR Lyrae distance determination for the Local Group dwarf spheroidal galaxy Sculptor

We present a new distance estimation for the Milky Way dwarf spheroidal satellite Sculptor obtained from multi-epoch mid-infrared observations of RR Lyrae stars. The 3.6 {\mu}m observations have been acquired with the Infrared Array Camera on board the Spitzer Space Telescope as part of the SMHASH Program. Mid-infrared light curves for 42 RRL were obtained, from which we measured Sculptor's distance modulus to be {\mu} = 19.60 $\pm$ 0.02 (statistical) $\pm$ 0.04 (photometric) mag (with $\sigma_{sys}=$ = 0.09 mag), using the 3.6 {\mu}m empirical period-luminosity relations derived from the Galactic globular cluster M4, or {\mu} = 19.57 $\pm$ 0.02 (statistical) $\pm$ 0.04 (photometric) mag (with $\sigma_{sys}=$ = 0.11 mag) using empirical relations in the same passband recently derived from the Large Magellanic Cloud globular cluster Reticulum. Both these measurements are in good agreement with values presented in previous works with Sculptor RR Lyrae stars in optical bands, and are also consistent with recent near-infrared RR Lyrae results. Best agreement with the literature is found for the latter modulus which is equivalent to a distance of d = 82 $\pm$ 1 (statistical) $\pm$ 2 (photometric) kpc (with $\sigma_{sys}=$ = 4 kpc). Finally, using a subsample of RR Lyrae stars with spectroscopic metallicities, we demonstrate that these distance estimates are not affected by metallicity effects.

A Deficit of Dark Matter from Jeans Modeling of the Ultra-diffuse Galaxy NGC 1052-DF2

Abstract The discovery of the ultra-diffuse galaxy NGC 1052-DF2 and its peculiar population of star clusters has raised new questions about the connections between galaxies and dark matter (DM) halos at the extremes of galaxy formation. In light of debates over the measured velocity dispersion of its star clusters and the associated mass estimate, we constrain mass models of DF2 using its observed kinematics with a range of priors on the halo mass. Models in which the galaxy obeys a standard stellar-halo mass relation are in tension with the data and also require a large central density core. Better fits are obtained when the halo mass is left free, even after accounting for increased model complexity. The dynamical mass-to-light ratio for our model with a weak prior on the halo mass is , consistent with the stellar population estimate for DF2. We use tidal analysis to find that the low-mass models are consistent with the undisturbed isophotes of DF2. Finally, we compare with Local Group dwarf galaxies and demonstrate that DF2 is an outlier in both its spatial extent and its relative DM deficit.

Local Group Dwarf Galaxies

AbstractLocal Group dwarf galaxies are a unique astrophysical laboratory because they are the only objects in which we can reliably and precisely characterize the star formation histories of low-mass galaxies going back to the epoch of reionization. There are of order 100 known galaxies less massive than the Small Magellanic Cloud within ~1 Megaparsec of the Milky Way, with a vide variety of star formation history, gas content, and mass to light ratios. In this overview the current understanding of the formation and evolution of low-mass galaxies across cosmic time will be presented, and the possibility of drawing links between the properties of individual systems and the broader Local Group and cosmological context will be discussed. Local Group dwarfs will remain a uniquely powerful testbed to constrain the properties of dark matter and to evaluate the performance of simulations for the foreseeable future.

Pushing back the limits: detailed properties of dwarf galaxies in a ΛCDM universe

We present the results of a set of high-resolution chemo-dynamical simulations of dwarf galaxies in a ΛCDM cosmology. Out of an original (3.4 Mpc/h)3 cosmological box, a sample of 27 systems are re-simulated from z = 70 to z = 0 using a zoom-in technique. Gas and stellar properties are confronted to the observations in the greatest details: in addition to the galaxy global properties, we investigated the model galaxy velocity dispersion profiles, half-light radii, star formation histories, stellar metallicity distributions, and [Mg/Fe] abundance ratios. The formation and sustainability of the metallicity gradients and kinematically distinct stellar populations are also tackled. We show how the properties of six Local Group dwarf galaxies, NGC 6622, Andromeda II, Sculptor, Sextans, Ursa Minor and Draco are reproduced, and how they pertain to three main galaxy build-up modes. Our results indicate that the interaction with a massive central galaxy could be needed for a handful of Local Group dwarf spheroidal galaxies only, the vast majority of the systems and their variety of star formation histories arising naturally from a ΛCDM framework. We find that models fitting well the local Group dwarf galaxies are embedded in dark haloes of mass between 5 × 108 to a few 109 M⊙, without any missing satellite problem. We confirm the failure of the abundance matching approach at the mass scale of dwarf galaxies. Some of the observed faint however gas-rich galaxies with residual star formation, such as Leo T and Leo P, remain challenging. They point out the need of a better understanding of the UV-background heating.

Red Supergiants as Chemical Abundance Probes: The Local Group dwarf NGC6822

AbstractRed Supergiant Stars (RSGs) are important probes of stellar and chemical evolution in star-forming environments. They represent the brightest near-IR stellar components of external galaxies and probe the most recent stellar population to provide robust, independent abundance estimates. The Local Group dwarf irregular galaxy, NGC6822, is a reasonably isolated galaxy with an interesting structure and turbulent history. Using RSGs as chemical abundance probes, we estimate metallicities in the central region of NGC6822, finding a suggestion of a metallicity gradient (in broad agreement with nebular tracers), however, this requires further study for confirmation. With intermediate resolution Multi-object spectroscopy (from e.g. KMOS, EMIR, MOSFIRE) combined with state-of-the-art stellar model atmospheres, we demonstrate how RSGs can be used to estimate stellar abundances in external galaxies. In this context, we compare stellar and nebular abundance tracers in NGC 6822 and by combining stellar and nebular tracers we estimate an abundance gradient of −0.18 ± 0.05 dex/kpc.

Long Period Variables in Local Group dwarf Irregular Galaxies

AbstractThe long-term SAAO survey of Local Group galaxies in the near-infrared (JHKs) has included five dwarf irregulars (dIrr), namely, NGC 6822, IC 1613, WLM, Sgr dIG and NGC 3109. We have found long-period (Mira) variables in all of them. Most of the Miras, which follow a linear LMC period-luminosity (PL) relation well, are carbon-rich. A small group of oxygen-rich Miras are brighter than the linear PL relation predicts, presumably because they are undergoing hot-bottom burning (HBB).

The Local Group Dwarf Galaxies. The Star Formation Histories derived using the Long Period Variable Stars

AbstractDwarf galaxies in the Local Group (LG) represent a distinct as well as diverse family of tracers of the earliest phases of galaxy assembly and the processing resulting from galactic harrassment. Their stellar populations can be resolved and used as probes of the evolution of their host galaxy. In this regard, we present the first reconstruction of the star formation history (SFH) of them using the most evolved AGB stars that are long period variable (LPV). LPV stars trace stellar populations as young as ∼ 30 Myr to as old as the oldest globular clusters. For the nearby, relatively massive and interacting gas-rich dwarf galaxies, the Magellanic Clouds, we found that the bulk of the stars formed ∼ 10 Gyr ago for the LMC, while the strongest episode of star formation in the SMC occurred a few Gyr later. A peak in star formation around 0.7 Gyr ago in both Clouds is likely linked to their recent interaction. The Andromeda satellite pair NGC147/185 show different histories; the main epoch of star formation for NGC 185 occurred 8.3 Gyr ago, followed by a much lower, but relatively constant star formation rate (SFR). In the case of NGC 147, the SFR peaked only 6.9 Gyr ago, staying intense until ∼ 3 Gyr ago. Star formation in the isolated gas-rich dwarf galaxy IC 1613 has proceeded at a steady rate over the past 5 Gyr, without any particular dominant epoch. Due to lack of sufficient data, we have conducted an optical monitoring survey at the Isaac Newton Telescope (INT) of 55 dwarf galaxies in the LG to reconstruct the SFH of them uniformly. The observations are made over ten epochs, spaced approximately three months apart, as the luminosity of LPV stars varies on timescales of months to years. The system of galactic satellites of the large Andromeda spiral galaxy (M31) forms one of the key targets of our monitoring survey. We present the first results in the And I dwarf galaxy, where we discovered 116 LPVs among over 10,000 stars.

What are the implications of stellar chemical abundances in dwarf galaxies?

AbstractWe analyse the chemical abundances of stars in the local group dwarf galaxies using the SAGA database. The inspection of the relationship between Eu and Ba abundances confirms an anomalously Ba-rich population in Fornax, which indicates a pre-enrichment of interstellar gas with r-process elements.

Full-spectral fitting techniques to characterise the stellar content of ultra diffuse galaxies

AbstractUnderstanding the peculiar properties of Ultra Diffuse Galaxies (UDGs) via spectroscopic analysis is a challenging task that is now becoming feasible. The advent of 10m-class telescopes and high sensitivity instruments is enabling the gathering of high quality spectra even for the faintest systems. In addition, advances in the modelling of stellar populations, stellar libraries, and full-spectral fitting codes are allowing the recovery of the stellar content shaping those spectra with unprecedented reliability. In this contribution we report on the extensive tests we have carried out using the inversion code STECKMAP. The similarities between the Star Formation Histories (SFH) recovered from STECKMAP (applied to high-quality spectra) and deep Colour-Magnitude diagrams fitting (resolved stars) in two Local Group dwarf galaxies (LMC and LeoA) are remarkable, demonstrating the impressive performance of STECKMAP. We exploit the capabilities of STECKMAP and perform one of the most complete and reliable characterisations of the stellar component of UDGs to date using deep spectroscopic data. We measure radial and rotation velocities, SFHs and mean population parameters, such as ages and metallicities, for a sample of five UDG candidates in the Coma cluster. From the radial velocities, we confirm the Coma membership of these galaxies. We find that their rotation properties, if detected at all, are compatible with dwarf-like galaxies. The SFHs of the UDG are dominated by old (∼ 7 Gyr), metal-poor ([M/H] ∼ -1.1) and alpha-enhanced ([Mg/Fe]∼ 0.4) populations followed by a smooth or episodic decline which halted ∼ 2 Gyr ago, possibly a sign of cluster-induced quenching. We find no obvious correlation between individual SFH shapes and any UDG morphological properties. The recovered stellar properties for UDGs are similar to those found for DDO 44, a local UDG analogue resolved into stars. We conclude that the UDGs in our sample are extended dwarfs whose properties are likely the outcome of both internal processes, such as bursty SFHs and/or high-spin haloes, as well as environmental effects within the Coma cluster.

Chemical evolution model to derive metallicity distributions for each stellar population

AbstractWe have constructed a chemical evolution model in order to reproduce the both metallicity distribution functions (MDFs) of red giant branch stars (RGBs) and RR Lyrae stars (RRLs) of a dwarf galaxy, simultaneously. The detailed chemical abundances of RGBs of the Local Group dwarf galaxies have been measured by spectroscopic observations. Moreover, the metallicity of RRLs of a dwarf galaxy are estimated by using the theoretical period-luminosity relations in the previous study and it is found that the mean metallicity of RRLs are lower than that of RGBs. In order to investigate the MDFs of RGBs and RRLs, we combine our chemical evolution model with the stellar evolutionally isochrones and calculate the metallicity of RGBs and RRLs, respectively. As a result, our chemical evolution model reproduces the peak metallicity of both MDFs of RGBs and RRLs of Sculptor and Fornax dwarf spheroidal galaxies (dSphs), simultaneously. Therefore, it is found that the difference of the mean metallicity between RGBs and RRLs are caused by the effects of stellar evolution. Moreover, by using the theoretical period-luminosity-metallicity relation of the RRLs, our chemical evolution model determines that the distance modulus of Sculptor and Fornax dSphs are 19.68 ± 0.09 and ${20.81^{+0.13}_{-0.11}}$, respectively. However, our model underestimates the number of metal-rich RRLs ([Fe/H] > −1.5) of Fornax dSph. This result suggests that the mass-loss rate of metal-rich RGBs would be larger than that of metal-poor RGBs.

The major merger origin of the Andromeda II kinematics

AbstractProlate rotation (i.e. rotation around the long axis) has been reported for two Local Group dwarf galaxies: Andromeda II, a dwarf spheroidal satellite of M31, and Phoenix, a transition type dwarf galaxy. The prolate rotation may be an exceptional indicator of a past major merger between dwarf galaxies. We showed that this type of rotation cannot be obtained in the tidal stirring scenario, in which the satellite is transformed from disky to spheroidal by tidal forces of the host galaxy. However, we successfully reproduced the observed Andromeda II kinematics in controlled, self-consistent simulations of mergers between equal-mass disky dwarf galaxies on a radial or close-to-radial orbit. In simulations including gas dynamics, star formation and ram pressure stripping, we are able to reproduce more of the observed properties of Andromeda II: the unusual rotation, the bimodal star formation history and the spatial distribution of the two stellar populations, as well as the lack of gas. We support this scenario by demonstrating the merger origin of prolate rotation in the cosmological context for sufficiently resolved galaxies in the Illustris large-scale cosmological hydrodynamical simulation.

Synergies between Massive Stars and Metal-poor Dwarf Irregular Galaxies

AbstractThe community of massive stars is working intensively on Local Group dwarf irregular galaxies (dIrr). They are a reservoir of metal-poor massive stars that serve to understand the physics of their higher redshift siblings and population III stars, interpret the farthest, most energetic SNe and GRBs, and compute feedback through Cosmic History. Along the way, we became interested in the recent star-formation history and initial mass-function of the host dIrr’s, their chemical evolution, and gas and dust content. Our team is working to unveil and characterize with spectroscopy the OB-stars in IC 1613, Sextans A and SagDIG, that form a sequence of decreasing metal content. We showcase some results to stimulate synergies between both communities.

Subaru Hyper Suprime-Cam Survey for the Local Group Dwarf Galaxies: Ursa Minor

AbstractWe have carried out a wide and deep imaging survey for the Local Group dwarf spheroidal galaxy Ursa Minor (UMi) using Hyper Suprime-Cam (HSC). The data cover out beyond the nominal tidal radius down to ~25 mag in i band, which is ~2 mag below the main sequence turn-off point. The structural parameters of UMi are derived using red giant branch (RGB) stars and sub-giant branch (SGB) stars, and the tidal radius is suggested to be larger than those estimated by the previous studies. It is also found that the distribution of bluer RGB/SGB stars is more extended than that of redder RGB/SGB stars. The fraction of binary systems is estimated to be ~0.4 from the morphology of the main sequences.

The chemical connection between damped Lyman-α systems and Local Group dwarf galaxies

Abundances of the volatile elements S and Zn have now been measured in around 80 individual stars in the Sculptor dwarf spheroidal galaxy, covering the metallicity range − 2.4 ≤ [Fe/H] ≤−0.9. These two elements are of particular interest as they are not depleted onto dust in gas, and their ratio, [S/Zn], has thus commonly been used as a proxy for [α/Fe] in Damped Lyman-α systems (DLAs). The S abundances in Sculptor are similar to other α-elements in this galaxy, consistent with S being mainly created in core-collapse supernovae, but also having some contribution from type Ia supernovae. However, our results show that Zn and Fe do not trace all the same nucleosynthetic production channels. In particular, (contrary to Fe) Zn is not significantly produced by type Ia supernovae. Thus, [S/Zn] cannot be reliably used as a proxy for [α/Fe]. We propose [O/S] as a function of [S/H] as a possible alternative. At higher metallicities, the values of [S/Zn] measured in DLAs are inconsistent with those in local dwarf galaxies, and are more compatible with the Milky Way disk. Low-metallicity DLAs are, however, consistent with the most metal-poor stars in Local Group dwarf spheroidal galaxies. Assuming that the dust depletions of S and Zn are negligible, our comparison indicates that the star formation histories of DLAs are on average different from both the Milky Way and the Sculptor dwarf spheroidal galaxy.

On the early evolution of Local Group dwarf galaxy types: star formation and supernova feedback

According to star formation histories (SFHs), Local Group dwarf galaxies can be broadly classified in two types: those forming most of their stars before $z=2$ (${\it fast}$) and those with more extended SFHs (${\it slow}$). The most precise SFHs are usually derived from deep but not very spatially extended photometric data; this might alter the ratio of old to young stars when age gradients are present. Here we correct for this effect and derive the mass formed in stars by $z=2$ for a sample of 16 Local Group dwarf galaxies. We explore early differences between ${\it fast}$ and ${\it slow}$ dwarfs, and evaluate the impact of internal feedback by supernovae (SN) on the baryonic and dark matter (DM) component of the dwarfs. ${\it Fast}$ dwarfs assembled more stellar mass at early times and have larger amounts of DM within the half-light radius than ${\it slow}$ dwarfs. By imposing that ${\it slow}$ dwarfs cannot have lost their gas by $z=2$, we constrain the maximum coupling efficiency of SN feedback to the gas and to the DM to be $\sim$10%. We find that internal feedback alone appears insufficient to quench the SFH of ${\it fast}$ dwarfs by gas deprivation, in particular for the fainter systems. Nonetheless, SN feedback can core the DM halo density profiles relatively easily, producing cores of the sizes of the half-light radius in ${\it fast}$ dwarfs by $z=2$ with very low efficiencies. Amongst the "classical" Milky Way satellites, we predict that the smallest cores should be found in Draco and Ursa Minor, while Sculptor and Fornax should host the largest ones.

Quantitative Spectroscopy of Supergiants in the Local Group Dwarf Galaxy IC 1613: Metallicity and Distance

Abstract We present a spectral analysis of 21 blue supergiant stars of spectral types late B to early A within the Local Group dwarf galaxy IC 1613, based on VLT Focal Reducer and Low Dispersion Spectrograph 2 low-resolution spectra. Combining our results with studies of early B-type blue supergiants, we report a wide bimodal distribution of metallicities with two peaks around [Z] ∼ −0.50 dex and [Z] ∼ −0.85 dex. The bimodal distribution correlates with spatial location, when compared with column densities of neutral hydrogen in IC 1613. While the low [Z] objects appear in regions of relatively high ISM H i column densities or close to them, the high [Z] supergiants are found in the central H i hole that is almost devoid of hydrogen. This suggests there are varied chemical evolution histories for the young stellar populations in IC 1613. Utilizing the flux-weighted gravity–luminosity relation, we determine IC 1613's distance modulus as m − M = 24.39 ± 0.11 mag. This value is in agreement within previous distance measurements using the near-infrared period–luminosity relationship of Cepheids and the tip of the red giant branch.

Current Velocity Data on Dwarf Galaxy NGC 1052-DF2 do not Constrain it to Lack Dark Matter

Abstract It was recently proposed that the globular cluster system of the very low surface brightness galaxy NGC 1052-DF2 is dynamically very cold, leading to the conclusion that this dwarf galaxy has little or no dark matter. Here, we show that a robust statistical measure of the velocity dispersion of the tracer globular clusters implies a mundane velocity dispersion and a poorly constrained mass-to-light ratio. Models that include the possibility that some of the tracers are field contaminants do not yield a more constraining inference. We derive only a weak constraint on the mass-to-light ratio of the system within the half-light radius ( at the 90% confidence level) or within the radius of the furthest tracer ( at the 90% confidence level). This limit may imply a mass-to-light ratio on the low end for a dwarf galaxy, but many Local Group dwarf galaxies fall well within this contraint. With this study, we emphasize the need to reliably account for measurement uncertainties and to stay as close as possible to the data when determining dynamical masses from very small data sets of tracers.

A Chandra Survey of Milky Way Globular Clusters. I. Emissivity and Abundance of Weak X-Ray Sources

Abstract Based on archival Chandra data, we have carried out an X-ray survey of 69, or nearly half the known population of, Milky Way globular clusters (GCs), focusing on weak X-ray sources, mainly cataclysmic variables (CVs) and coronally active binaries (ABs). Using the cumulative X-ray luminosity per unit stellar mass (i.e., X-ray emissivity) as a proxy of the source abundance, we demonstrate a paucity (lower by 41% ± 27% on average) of weak X-ray sources in most GCs relative to the field, which is represented by the Solar Neighborhood and Local Group dwarf elliptical galaxies. We also revisit the mutual correlations among the cumulative X-ray luminosity (L X), cluster mass (M), and stellar encounter rate (Γ), finding , and . The three quantities can further be expressed as , which indicates that the dynamical formation of CVs and ABs through stellar encounters in GCs is less dominant than previously suggested, and that the primordial formation channel has a substantial contribution. Taking these aspects together, we suggest that a large fraction of primordial, soft binaries have been disrupted in binary–single or binary–binary stellar interactions before they could otherwise evolve into X-ray-emitting close binaries, whereas the same interactions also have led to the formation of new close binaries. No significant correlations between and cluster properties, including dynamical age, metallicity, and structural parameters, are found.