Dwarf Spheroidal Galaxies Research Articles

Searching for light in the darkness: Bounds on ALP dark matter with the optical MUSE-faint survey

We use MUSE spectroscopic observations of the dwarf spheroidal galaxy Leo T between 470 and 935 nm to search for radiative decays of axion like particles (ALPs). Under the assumption that ALPs constitute the dark matter component of the Leo T halo, we derive bounds on the effective ALP-two-photon coupling. We improve existing limits by more than one order of magnitude in the ALP mass range 2.7-5.3 eV.

Detailed chemical evolution models of the Tucana dSph galaxy

ABSTRACT We use semi-analytical methods to obtain detailed chemical evolution models (CEMs) for the dwarf spheroidal (dSph) galaxy Tucana. Published star formation rates and the age–metallicity relationship are used to constrain the observables. The results show that Tucana: (i) behaved like a closed box for 75 per cent of its life, (ii) had either a primordial-gas accretion or a metal-rich wind during 15 per cent of its life (between 0.5 and 2.0 Gyr), and (iii) lost 95 per cent of its gas through a well-mixed wind at t ∼ 4.5 Gyr. Specifically, we find two CEMs: the metal-dilution model and the metal-loss model, which differ mainly during the range 0.5–2.0 Gyr. In order to discriminate between these CEMs, we compare the predicted [Xi/Fe]–[Fe/H] trends, which differ less than the average error of the observed trends for other dSphs of the Local Group. Furthermore, the models predict very different metallicity distribution functions. Therefore, an observational metallicity distribution function for Tucana is essential in order to discriminate between the metal-dilution and the metal-loss scenarios. In addition, because the difference of [Xi/Fe] between the two models is less than the average of the errors observed for others dSph and ultra-faint dwarf galaxies of the Local Group, greater precision is required in future observations, so that the errors are less than the difference between the models, thus enabling discrimination between them.

The bursty star formation history of the Fornax dwarf spheroidal galaxy revealed with the HST

ABSTRACT We present a new derivation of the star formation history (SFH) of the dSph galaxy Fornax in two central regions, characterized by unprecedented precision and age resolution. It reveals that star formation has proceeded in sharp bursts separated by periods of low level or quiescent activity. The SFH was derived through colour–magnitude diagram (CMD) fitting of two extremely deep Hubble Space Telescope CMDs, sampling the centre and one core radius. The attained age resolution allowed us to single out a major star formation episode at early times, a second strong burst 4.6 ± 0.4 Gyr ago and recent intermittent episodes ∼2–0.2 Gyr ago. Detailed testing with mock stellar populations was used to estimate the duration of the main bursts and study the occurrence of low-level star formation between them. The SFHs in both regions show common features, with activity at the same epochs and similar age–metallicity relationship. However, clear indications of a spatial gradient were also found, with mean age increasing with radius and star formation episodes being more prolonged in the centre. While some galaxy evolution models predict bursty SFHs in dwarf galaxies and thus a secular origin of the observed SFH cannot be excluded in Fornax, other evidence points to possible mergers or interactions as the cause of its bursty SFH. In particular, we calculated the Fornax orbit relative to the closest dwarfs and the Milky Way and observed a correspondence between the main intermediate-age and young events and peri-passages of Fornax around the Milky Way, possibly indicating tidally induced star formation.

The properties of dwarf spheroidal galaxies in the Cen A group

Dwarf spheroidal galaxies (dSphs) have been extensively investigated in the Local Group, but their low luminosity and surface brightness make similar work in more distant galaxy groups challenging. Modern instrumentation unlocks the possibility of scrutinizing these faint systems in other environments, expanding the parameter space of group properties. We use MUSE spectroscopy to study the properties of 14 known or suspected dSph satellites of Cen A. Twelve targets are confirmed to be group members based on their radial velocities. Two targets are background galaxies at ∼50 Mpc: KK 198 is a face-on spiral galaxy, and dw1315−45 is an ultra-diffuse galaxy with an effective radius of ∼2300 pc. The 12 confirmed dSph members of the Cen A group have old and metal-poor stellar populations and follow the stellar metallicity-luminosity relation defined by the dwarf galaxies in the Local Group. In the three brightest dwarf galaxies (KK 197, KKs 55, and KKs 58), we identify globular clusters, as well as a planetary nebula in KK 197, although its association with this galaxy and/or the extended halo of Cen A is uncertain. Using four discrete tracers, we measure the velocity dispersion and dynamical mass of KK 197. This dSph appears dark matter dominated and lies on the radial acceleration relation of star-forming galaxies within the uncertainties. It also is consistent with predictions stemming from modified Newtonian dynamics. Surprisingly, in the dwarf KK 203 we find an extended Hα ring. Careful examination of Hubble Space Telescope photometry reveals a very low level of star formation at ages between 30 and 300 Myr. The Hα emission is most likely linked to a ∼40 Myr old supernova remnant, although other possibilities for its origin cannot be entirely ruled out.

Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and Hamburg/ESO Surveys

Abstract We analyze the dynamical properties of ∼1500 very metal-poor (VMP; [Fe/H] ≲ −2.0) halo stars, based primarily on medium-resolution spectroscopic data from the HK and Hamburg/ESO surveys. These data, collected over the past 30 yr, are supplemented by a number of calibration stars and other small samples, along with astrometric information from Gaia DR2. We apply a clustering algorithm to the 4D energy-action space of the sample, and identify a set of 38 dynamically tagged groups (DTGs), containing between 5 and 30 member stars. Many of these DTGs can be associated with previously known prominent substructures such as Gaia-Sausage/Enceladus (GSE), Sequoia, Helmi Stream (HStr), and Thamnos. Others are associated with previously identified smaller dynamical groups of stars and streams. We identify 10 new DTGs as well, many of which have strongly retrograde orbits. We also investigate possible connections between our DTGs and ∼300 individual r-process-enhanced (RPE) stars from a recent literature compilation. We find that several of these objects have similar dynamical properties to GSE (5), the HStr (4), Sequoia (1), and Rg5 (1), indicating that their progenitors might have been important sources of RPE stars in the Galaxy. Additionally, a number of our newly identified DTGs are shown to be associated with at least two RPE stars each (DTG-2: 3, DTG-7: 2; DTG-27: 2). Taken as a whole, these results are consistent with ultra-faint and/or dwarf spheroidal galaxies as birth environments in which r-process nucleosynthesis took place, and then were disrupted by the Milky Way.

Multiwavelength analysis of low surface brightness galaxies to study possible dark matter signature

ABSTRACTLow surface brightness (LSB) galaxies have very diffuse, low surface density stellar discs that appear faint in optical images. They are very rich in neutral hydrogen (H i) gas, which extends well beyond the stellar discs. Their extended H i rotation curves and stellar discs indicate that they have very massive dark matter (DM) haloes compared to normal bright galaxies. Hence, LSB galaxies may represent valuable laboratories for the indirect detection of DM. In this paper, we search for weakly interacting massive particle annihilation signatures in four LSB galaxies and present an analysis of nearly 9 yr of data from the Fermi Large Area Telescope (Fermi-LAT). Above 500 MeV, no excess emission was detected from the LSB galaxies. We obtain constraints on the DM cross-section for different annihilation channels, for both individual and stacked targets. In addition to this, we use radio data from the Very Large Array radio telescope in order to derive DM constraints, following a multiwavelength approach. The constraints obtained from the four considered LSB galaxies are nearly three orders of magnitude weaker than the predicted limits for the thermal relic abundances and the combined limits achieved from Fermi-LAT observations of dwarf spheroidal galaxies. Finally, we discuss the possibility of detecting emission from LSB galaxies using the upcoming ground-based γ-ray and radio observatories, namely the Cherenkov Telescope Array and the Square Kilometre Array.

Stellar profile independent determination of the dark matter distribution of the Fornax Local Group dwarf spheroidal galaxy

ABSTRACT We detail a method to measure the correspondence between dark matter (DM) models and observations of stellar populations within Local Group dwarf spheroidal galaxies (LG dSphs) that assumes no parametric stellar distribution. Solving the spherical or cylindrical Jeans equations, we calculate the consistency of DM and stellar kinematic models with stellar positions and line-of-sight velocities. Our method can be used to search for signals of standard and exotic DM distributions. Applying our methodology to the Fornax LG dSph and using statistical bootstrapping, we find: (i) that oblate or prolate cored DM haloes match the stellar data, respectively, ≃60 or ≃370 times better than oblate or prolate cusped DM haloes for isotropic and isothermal stellar velocity dispersions, (ii) that cusped spherical DM haloes and cored spherical DM haloes match the Fornax data similarly well for isotropic stellar velocity dispersions, (iii) that the semiminor to semimajor axial ratio of spheroidal DM haloes are more extreme than 80 per cent of those predicted by Lambda cold dark matter with baryon simulations, (iv) that oblate cored or cusped DM haloes are, respectively, ≃5 or ≃30 times better matches to Fornax than prolate cored or cusped DM haloes, and (v) that Fornax shows no evidence of a disc-like structure with more than two per cent of the total DM mass. We further note that the best-fitting cusped haloes universally favour the largest mass and size fit parameters. If these extreme limits are decreased, the cusped halo likelihoods decrease relative to those of cored haloes.

Homogeneity in the early chemical evolution of the Sextans dwarf spheroidal galaxy

We present the high-resolution spectroscopic analysis of two new extremely metal-poor star (EMPS) candidates in the dwarf spheroidal galaxy Sextans. These targets were preselected from medium-resolution spectra centered around the Ca II triplet in the near-infrared and were followed-up at higher resolution with VLT/UVES. We confirm their low metallicities with [Fe/H] = −2.95 and [Fe/H] = −3.01, which place them among the most metal-poor stars known in Sextans. The abundances of 18 elements, including C, Na, the α, Fe-peak, and neutron-capture elements, are determined. In particular, we present the first unambiguous detection of Zn in a classical dwarf at extremely low metallicity. Previous indications were made of a large scatter in the abundance ratios of the Sextans stellar population around [Fe/H] ∼ − 3 when compared to other galaxies, particularly with very low observed [α/Fe] ratios. We took the opportunity of reanalyzing the full sample of EMPS in Sextans and find a [α/Fe] Milky Way-like plateau and a ∼0.2 dex dispersion at fixed metallicity.

Maximally self-interacting dark matter: models and predictions

We study self-interacting dark matter (SIDM) scenarios, where the s-wave self-scattering cross section almost saturates the Unitarity bound. Such self-scattering cross sections are singly parameterized by the dark matter mass, and are featured by strong velocity dependence in a wide range of velocities. They may be indicated by observations of dark matter halos in a wide range of masses, from Milky Way’s dwarf spheroidal galaxies to galaxy clusters. We pin down the model parameters that saturates the Unitarity bound in well-motivated SIDM models: the gauged Lμ− Lτ model and composite asymmetric dark matter model. We discuss implications and predictions of such model parameters for cosmology like the H0 tension and dark-matter direct-detection experiments, and particle phenomenology like the beam-dump experiments.

Long-term Orbital Evolution of Galactic Satellites and the Effects on Their Star Formation Histories

Abstract We investigate the orbital motions of dwarf spheroidal galaxies (dSphs) in the halo of the Milky Way (MW) to understand their possible effects on the diversity of the star formation histories seen in these MW satellites. In this work, we explicitly consider a time-varying gravitational potential due to the growth of the MW’s dark halo mass to calculate the long-term orbital evolutions of the dSphs, guided with Gaia DR2 proper motions, over the past 13.5 billion years. We find that the infall time of a satellite, defined as when the galaxy first crosses within the growing virial radius of the MW’s halo, coincides well with the time when the star formation rate (SFR) is peaked for the sample of classical dSphs. On the other hand, ultra-faint dSphs already finished their SF activity prior to their infall times as already suggested in previous works, but there is a signature that their earlier SF histories are affected by interaction with the growing MW’s halo to some extent. We also find, for classical dSphs, that the relative fraction of stars formed after the peak of the SFR to the current stellar mass is smaller for the smaller pericentric radius of the galaxy at its first infall. These results suggest that the infalling properties of the dSphs into the MW and the resultant environmental effects, such as ram pressure stripping and/or tidal disturbance in the MW’s dark halo containing hot gas, play important roles in their star formation histories.

New constraints on the mass of fermionic dark matter from dwarf spheroidal galaxies

ABSTRACT Dwarf spheroidal galaxies are excellent systems to probe the nature of fermionic dark matter due to their high observed dark matter phase-space density. In this work, we review, revise, and improve upon previous phase-space considerations to obtain lower bounds on the mass of fermionic dark matter particles. The refinement in the results compared to previous works is realized particularly due to a significantly improved Jeans analysis of the galaxies. We discuss two methods to obtain phase-space bounds on the dark matter mass, one model-independent bound based on Pauli’s principle, and the other derived from an application of Liouville’s theorem. As benchmark examples for the latter case, we derive constraints for thermally decoupled particles and (non-)resonantly produced sterile neutrinos. Using the Pauli principle, we report a model-independent lower bound of $m \ge 0.18\, \mathrm{keV}$ at 68 per cent CL and $m \ge 0.13\, \mathrm{keV}$ at 95 per cent CL. For relativistically decoupled thermal relics, this bound is strengthened to $m \ge 0.59\, \mathrm{keV}$ at 68 per cent CL and $m \ge 0.41\, \mathrm{keV}$ at 95 per cent CL, while for non-resonantly produced sterile neutrinos the constraint is $m \ge 2.80\, \mathrm{keV}$ at 68 per cent CL and $m \ge 1.74\, \mathrm{keV}$ at 95 per cent CL. Finally, the phase-space bounds on resonantly produced sterile neutrinos are compared with complementary limits from X-ray, Lyman α, and big bang nucleosynthesis observations.

Investigating the detection of dark matter subhalos as extended sources with Fermi -LAT

Cold dark matter (DM) models for structure formation predict that DM subhalos are present in the Galaxy. In the standard paradigm of DM as weakly interacting massive particle, subhalos are expected to shine in gamma rays and to provide a signal detectable with current instruments, notably with the Large Area Telescope (LAT) aboard the Fermi~satellite. This is the main motivation behind searches for DM signals towards dwarf spheroidal galaxies and unidentified Fermi-LAT sources. A significant angular extension detected from unassociated sources located at relatively high latitudes is considered a "smoking gun" signature for identifying DM subhalos. In the present work, we systematically explore, by means of state-of-the-art models of cold DM halos in the Galaxy, the detectability of extended subhalos with Fermi-LAT. We simulate a DM signal exploring different assumptions of subhalos distribution in the Galaxy and DM profile, and reconstruct its flux through a realistic Fermi-LAT analysis pipeline. In the most optimistic case, we show that a detection of extended DM subhalos can be made for annihilation cross sections higher than $3 \times 10^{-26}$ cm$^3$/s (for a 100 GeV DM mass), still compatible with existing gamma-ray constraints, and that, in this case, the preference for extension of the source (vs point-like hypothesis) is significant. For fainter signals, instead, halos not only do not show significant extension, but they are not even detectable significantly as point-like sources.

Diversity of Dark Matter Density Profiles in the Galactic Dwarf Spheroidal Satellites

Abstract The core–cusp problem is one of the controversial issues in the standard paradigm of Λ cold dark matter (ΛCDM) theory. However, under the assumption of conventional spherical symmetry, the strong degeneracy among model parameters makes it unclear whether dwarf spheroidal (dSph) galaxies indeed have cored dark matter density profiles at their centers. In this work, we revisit this problem using nonspherical mass models, which have the advantage of being able to alleviate the degeneracy. Applying our mass models to the currently available kinematic data of the eight classical dSphs, we find that within finite uncertainties, most of these dSphs favor cusped central profiles rather than cored ones. In particular, Draco has a cusped dark matter halo with high probability even considering a prior bias. We also find the diversity of the inner slopes in their dark matter halos. To clarify the origin of this diversity, we investigate the relation between the inner dark matter density slope and stellar-to-halo mass ratio for the sample dSphs and find that this relation is generally in agreement with the predictions from recent ΛCDM and hydrodynamical simulations. We also find that the simulated subhalos have an anticorrelation between the dark matter density at 150 pc and pericenter distance, which is consistent with the observed one. We estimate their astrophysical factors for dark matter indirect searches and circular velocity profiles associated with huge uncertainties. To more precisely estimate their dark matter profiles, wide-field spectroscopic surveys for the dSphs are essential.

Self-gravitating strange dark matter halos around galaxies

A new family of nonrelativistic, Newtonian, non-quantum equilibrium configurations describing galactic halos is introduced, by considering strange quark matter conglomerates with masses larger than about 8 GeV as new possible components of the dark matter. Originally introduced to explain the state of matter in neutron stars, such conglomerates may also form in the high-density and temperature conditions of the primordial Universe and then decouple from ordinary baryonic matter, providing the fundamental components of dark matter for the formation of pristine gravitational potential wells and the subsequent evolution of cosmic structures. The obtained results for halo mass and radius are consistent with the rotational velocity curve observed in the Galaxy. Additionally, the average density of such dark matter halos is similar to that derived for halos of dwarf spheroidal galaxies, which can therefore be interpreted as downscaled versions of larger dark matter distributions around Milky Way-sized galaxies and hint for a common origin of the two families of cosmic structures.

Cherenkov Telescope Array sensitivity to branon dark matter models

In the absence of a clear hint of dark matter (DM) signals in the GeV regime so far, heavy, \U0001d4aa(TeV) DM candidates are gradually earning more and more attention within the community. Among others, extra-dimensional brane-world models may produce thermal DM candidates with masses up to 100 TeV . These heavy DM candidates could be detected with the next generation of very-high-energy gamma-ray observatories such as the Cherenkov Telescope Array (CTA) . In this work, we study the sensitivity of CTA to branon DM via the observation of representative astrophysical DM targets, namely dwarf spheroidal galaxies. In particular, we focus on Draco and Sculptor, two well-known dwarfs visible from the Northern and Southern Hemisphere, respectively. For each of these targets, we simulated 300 h of CTA observations and studied the sensitivity of both CTA-North and CTA-South to branon annihilations using the latest publicly available instrument response functions and most recent analysis tools. We computed annihilation cross section values needed to reach a 5σ detection as a function of the branon mass. Additionally, in the absence of a predicted DM signal, we obtained 2σ upper limits on the annihilation cross section. These limits lie 1.5–2 orders of magnitude above the thermal relic cross section value, depending on the considered branon mass. Yet, CTA will allow to exclude a significant portion of the brane tension-mass parameter space in the 0.1–60 TeV branon mass range, and up to tensions of ∼ 10 TeV . More importantly, CTA will significantly enlarge the region already excluded by AMS and CMS, and will provide valuable complementary information to future SKA radio observations. We conclude that CTA will possess potential to constrain brane-world models and, more in general, TeV DM candidates.

Globular Clusters Lost by the Sagittarius Dwarf Spheroidal Galaxy

In this work a search was carried out for globular clusters belonging to the Sagittarius (Sgr) tidal stream using the analysis of spatial positions, radial velocities relative to the Galactic Standard of Rest (V_{GSR}),proper motions and ratio of "age -- metallicity" ([Fe/H]) for globular clusters and for stars in the tidal stream. As a result, three categories of globular clusters were obtained: A -- most certainly in the stream: Terzan 8, Whiting 1, Arp 2, NGC 6715, Terzan 7, Pal 12; B -- kinematic outliers: Pal 5, NGC 5904, NGC 5024, NGC 5053, NGC 5272, NGC 288; C -- lowest rank candidates: NGC 6864, NGC 5466, NGC 5897, NGC 7492, NGC 4147.

Discovery of 18 Stars with −3.10 < [Fe/H] < −1.45 in the Sagittarius Dwarf Galaxy*

Abstract Studies of the early chemical evolution of some larger dwarf galaxies (>107 M ⊙) are limited by the small number of stars known at low metallicities in these systems. Here we present metallicities and carbon abundances for eighteen stars with metallicities between −3.08 ≤ [Fe/H] ≤ −1.47 in the Sagittarius dwarf spheroidal galaxy, using medium-resolution spectra from the MagE spectrograph on the Magellan–Baade Telescope. This sample more than doubles the number of known very metal-poor stars ([Fe/H] ≤ −2.0) in the Sagittarius dwarf galaxy, and identifies one of the first known extremely metal-poor stars ([Fe/H] ≤ −3.0) in the system. These stars were identified as likely metal-poor members of Sagittarius using public, metallicity-sensitive photometry from SkyMapper DR1.1 and proper motion data from Gaia DR2, demonstrating that this dearth of metal-poor stars in some dwarf galaxies can be addressed with targeted searches using public data. We find that none of the stars in our sample are enhanced in carbon, in contrast to the relative prevalence of such stars in the Milky Way halo. Subsequent high-resolution spectroscopy of these stars would be key in detailing the early chemical evolution of the system.

Ejection of supermassive black holes and implications for merger rates in fuzzy dark matter haloes

ABSTRACT Fuzzy dark matter (FDM) consisting of ultralight axions has been invoked to alleviate galactic-scale problems in the cold dark matter scenario. FDM fluctuations, created via the superposition of waves, can impact the motion of a central supermassive black hole (SMBH) immersed in an FDM halo. The SMBH will undergo a random walk, induced by FDM fluctuations, that can result in its ejection from the central region. This effect is strongest in dwarf galaxies, accounting for wandering SMBHs and the low detection rate of active galactic nuclei in dwarf spheroidal galaxies. In addition, a lower bound on the allowed axion masses is inferred both for Sagitarius A* and heavier SMBH; to avoid ejection from the galactic centres, axion masses of the order of 10−22 eV or lighter are excluded. Stronger limits are inferred for merging galaxies. We find that the event rate of SMBH mergers in FDM haloes and the associated SMBH growth rates can be reduced by at least an order of magnitude.

A fresh look at the RR Lyrae population in the Draco dwarf spheroidal galaxy with Gaia

ABSTRACT We present a catalogue of 285 RR Lyrae stars (RRLs) in the Draco dwarf spheroidal (dSph) galaxy, obtained by combining data from a number of different surveys including the Data Release 2 (DR2) of the European Space Agency cornerstone mission Gaia. We have determined individual distances to the RRLs in our sample using for the first time a Gaia G-band luminosity–metallicity relation ($M_{G}\small{--}{\rm [Fe/H]}$) and study the structure of the Draco dSph as traced by its RRL population. We find that the RRLs located in the western/south-western region of Draco appear to be closer to us, which may be a clue of interaction between Draco and the Milky Way. The average distance modulus of Draco measured with the RRLs is $\mu =19.53\pm 0.07$ mag, corresponding to a distance of $80.5\pm 2.6$ kpc, in good agreement with previous determinations in the literature. Based on the pulsation properties of the RRLs, we confirm the Oosterhoff-intermediate nature of Draco. We present an additional sample of 41 candidate RRLs in Draco, which we selected from the Gaia DR2 catalogue based on the uncertainty of their G-band magnitudes. Additional epoch data that will become available in the Gaia Data Release 3 will help to confirm whether these candidates are bona fide Draco RRLs.

XMM–Newton study of X-ray sources in the field of Willman 1 dwarf spheroidal galaxy★

ABSTRACT We present the results of the analysis of three XMM–Newton observations of the Willman 1 dwarf spheroidal galaxy (Wil 1). X-ray sources are classified on the basis of spectral analysis, hardness ratios, X-ray-to-optical flux ratio, X-ray variability, and cross-correlation with available catalogues in optical and infrared wavelengths. We catalogued 97 sources in the field of Wil 1. Our classification shows the presence of a β-type symbiotic star in Wil 1. We classified one M dwarf foreground star in the field of Wil 1. Moreover, 54 sources are classified as background active galactic nuclei and galaxies. Our study shows that the luminosity of the X-ray sources of Wil 1 does not exceed ∼1034 erg s−1 in the energy range of 0.2–12.0 keV, which is similar to observed luminosities of sources in nearby dwarf spheroidal galaxies.