Properties Of Dwarf Spheroidal Galaxies Research Articles

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.

DISCOVERY OF AN ULTRA-DIFFUSE GALAXY IN THE PISCES-PERSEUS SUPERCLUSTER

ABSTRACT We report the discovery of DGSAT I, an ultra-diffuse, quenched galaxy located 10.°4 in projection from the Andromeda galaxy (M31). This low-surface brightness galaxy (μ V = 24.8 mag arcsec−2), found with a small amateur telescope, appears unresolved in sub-arcsecond archival Subaru/Suprime-Cam images, and hence has been missed by optical surveys relying on resolved star counts, in spite of its relatively large effective radius (R e (V) = 12″) and proximity (15′) to the well-known dwarf spheroidal galaxy And II. Its red color (V − I = 1.0), shallow Sérsic index (n V = 0.68), and the absence of detectable Hα emission are typical properties of dwarf spheroidal galaxies and suggest that it is mainly composed of old stars. Initially interpreted as an interesting case of an isolated dwarf spheroidal galaxy in the local universe, our radial velocity measurement obtained with the BTA 6 m telescope (V h = 5450 ± 40 km s−1) shows that this system is an M31-background galaxy associated with the filament of the Pisces-Perseus supercluster. At the distance of this cluster (∼78 Mpc), DGSAT I would have an R e ∼ 4.7 kpc and M V ∼ −16.3. Its properties resemble those of the ultra-diffuse galaxies (UDGs) recently discovered in the Coma cluster. DGSAT I is the first case of these rare UDGs found in this galaxy cluster. Unlike the UDGs associated with the Coma and Virgo clusters, DGSAT I is found in a much lower density environment, which provides a fresh constraint on the formation mechanisms for this intriguing class of galaxy.

THE STRUCTURE AND DARK HALO CORE PROPERTIES OF DWARF SPHEROIDAL GALAXIES

The structure and dark matter halo core properties of dwarf spheroidal galaxies (dSphs) are investigated. A double-isothermal (DIS) model of an isothermal, non selfgravitating stellar system, in gravitational equilibrium and embedded in an isothermal dark halo core provides an excellent fit to the various observed stellar surface density distributions �∗(r). Despite its constant velocity dispersion, the stellar system can be well characterised by King profiles (King 1966) with a broad distribution of concentration parameters c = log(r∗,t/r∗,c), with r∗,t and r∗,c the stellar tidal and core radius, respectively. The DIS model confirms the suggestion of Kormendy & Freeman (2014) that the core scale length of the stellar system, defined as a∗ = −(dln�∗/dr 2 ) −1/2 , is sensitive to the central dark matter density �0. In contrast to single-component systems, r∗,t however does not trace the tidal radius of the galaxy but the core radius rc of its dark matter halo. c is therefore sensitive to the ratio �∗/�0 with �∗ and �0 the stellar and dark matter velocity dispersion, respectively. Simple empirical relationships are derived that allow to calculate the dark halo core parameters �0, rc and �0, given the observable quantities �∗, a∗ and c. The DIS model is applied to the Milky Way’s dSphs. Their halo velocity dispersions lie in a narrow range of 10 km/s ≤ �0 ≤ 18 km/s with halo core radii of 280 pc ≤ rc ≤ 1.3 kpc and rc ≈ 2a∗. All dSphs follow closely the same universal scaling relations h�0rci ≡ �0 × rc = 75 +8545 M⊙ pc −2 and � 2 0 ×r −1 c = 0.45 +0.51 −0.27 (km/s) 2 pc −1 that characterise the cores of more massive galaxies over a range of 18 magnitudes in blue magnitude MB. For given h�0rcithe core mass is a strong function of core radius, Mc ∼ r 2 . Inside a fixed radius ru, with ru the logarithmic mean of the dSph’s core radii, the total mass Mu = 2.17h�0rcir 2 u is however roughly constant. Outliers with smaller masses are expected for dSphs with core radii that are much larger or smaller than ru. For the Milky Way’s dSphs we find ru = 400 ± 100 pc and Mu = 2.6 ± 1.4 × 10 7 M⊙, in agreement with Strigari et al. (2008). Due to their small rc, the core densities of the Galaxy’s dSphs are very higher, with �0 = 0.03 - 0.3 M⊙ pc −3 . The dSphs would have to be on galactic orbits with pericenters smaller than a few kpc in order for their stellar systems to be affected by Galactic tides which is very unlikely. dSphs should therefore be tidally undisturbed. Observational evidence for tidal effects might then provide a serious challenge for the cold dark matter scenario.

THE STELLAR STRUCTURE AND KINEMATICS OF DWARF SPHEROIDAL GALAXIES FORMED BY TIDAL STIRRING

Using high-resolution N-body simulations we study the stellar properties of dwarf spheroidal galaxies resulting from the tidally induced morphological transformation of disky dwarfs on a cosmologically motivated eccentric orbit around the Milky Way. Dwarf galaxy models initially consist of an exponential stellar disk embedded in an extended spherical dark matter halo. Depending on the initial orientation of the disk with respect to the orbital plane, different final configurations are obtained. The least evolved dwarf is triaxial and retains a significant amount of rotation. The more evolved dwarfs are prolate spheroids with little rotation. We show that the final density distribution of stars can be approximated by a simple modification of the Plummer law. The kinematics of the dwarfs is significantly different depending on the line of sight which has important implications for mapping the observed stellar velocity dispersions of dwarfs to subhalo circular velocities. When the dwarfs are observed along the long axis, the measured velocity dispersion is higher and decreases faster with radius. In the case where rotation is significant, when viewed perpendicular to the long axis, the effect of minor axis rotation is detected, as expected for triaxial systems. We model the velocity dispersion profiles and rotation curves of the dwarfs by solving the Jeans equations for spherical and axisymmetric systems and adjusting different sets of free parameters. We find that the mass is typically overestimated when the dwarf is seen along the long axis and underestimated when the observation is along the short or intermediate axis. The effect of non-sphericity cannot however bias the inferred mass by more than 60 percent in either direction, even for the most strongly stripped dwarf which is close to disruption.

The evolution of the photometric properties of Local Group dwarf spheroidal galaxies

We investigate the present-day photometric properties of the dwarf spheroidal galaxies in the Local Group. From the analysis of their integrated colours, we consider a possible link between dwarf spheroidals and giant ellipticals. From the analysis of the V vs (B-V) plot, we search for a possible evolutionary link between dwarf spheroidal galaxies (dSphs) and dwarf irregular galaxies (dIrrs). By means of chemical evolution models combined with a spectro-photometric model, we study the evolution of six Local Group dwarf spheroidal galaxies (Carina, Draco, Sagittarius, Sculptor, Sextans and Ursa Minor). The chemical evolution models, which adopt up-to-date nucleosynthesis from low and intermediate mass stars as well as nucleosynthesis and energetic feedback from supernovae type Ia and II, reproduce several observational constraints of these galaxies, such as abundance ratios versus metallicity and the metallicity distributions. The proposed scenario for the evolution of these galaxies is characterised by low star formation rates and high galactic wind efficiencies. Such a scenario allows us to predict integrated colours and magnitudes which agree with observations. Our results strongly suggest that the first few Gyrs of evolution, when the star formation is most active, are crucial to define the luminosities, colours, and other photometric properties as observed today. After the star formation epoch, the galactic wind sweeps away a large fraction of the gas of each galaxy, which then evolves passively. Our results indicate that it is likely that at a certain stage of their evolution, dSphs and dIrrs presented similar photometric properties. However, after that phase, they evolved along different paths, leading them to their currently disparate properties.

The Local Group Dwarf Spheroidal Galaxies: A Key to Building Blocks in the Universe

Recent studies have been revealing the properties of dwarf spheroidal galaxies (dSphs). Their low mass indicates that the dSphs may provide a clue to physical properties of the building blocks in the hierarchical structure formation. We select the Local Group dSphs as a sample. To obtain the information on the star formation history of dSphs, we investigate the relation between their metallicity and virial mass. According to our scenario, the star formation efficiency of the dSphs is low because of strong regulation. This is consistent with their high mass-to-light ratios. We also comment on the environmental effects on the dSphs.