Cetus Dwarf Research Articles

Stellar chemo-kinematics of the Cetus dwarf spheroidal galaxy

Context. The great majority of early-type dwarf galaxies, in the Local Group as well as in other galaxy groups, are found in the vicinity of much larger galaxies, making it hard to disentangle the role of internal versus external effects in driving their evolution. Aims. In order to minimize environmental effects and gain an insight into the internal mechanisms that shape the properties of these systems, we study one of the few dwarf spheroidal galaxies of the Local Group found in isolation: Cetus. This system is of particular interest since it does not follow the Local Group morphology-density relation. Methods. We obtained Very Large Telescope (VLT) FORS2 spectra (R ∼ 2600) in the region of the nIR CaII triplet lines for 80 candidate red giant branch (RGB) stars. The analysis yielded line-of-sight velocities and metallicities ([Fe/H]) for 54 bona fide member stars. Results. The kinematic analysis shows that Cetus is a mainly pressure-supported (σv = 110−1.3+1.6 km s−1), dark-matter-dominated system (M1/2/LV = 23.9−8.9+9.7 M⊙/L⊙) with no significant signs of internal rotation. We find Cetus to be a metal-poor system with a significant metallicity spread (median [Fe/H]= − 1.71 dex, median-absolute-deviation = 0.49 dex), as expected for its stellar mass. We report the presence of a mild metallicity gradient compatible with those found in other dwarf spheroidals of the same luminosity; we trace the presence of a stellar population gradient also in the spatial distribution of stars in different evolutionary phases in ancillary SuprimeCam photometric data. There are tentative indications of two chemo-kinematically distinct sub-populations, with the more metal-poor stars showing a hotter kinematics than the metal-richer ones. Furthermore, the photometric dataset reveals the presence of a foreground population that most likely belongs to the Sagittarius stream. Conclusions. This study represents an important step forward in assessing the internal kinematics of the Cetus dwarf spheroidal galaxy as well as the first wide-area spectroscopic determination of its metallicity properties. With our analysis, Cetus adds to the growing scatter in stellar-dark matter halo properties in low-mass galactic systems. The presence of a metallicity gradient akin to those found in similarly luminous and pressure-supported systems inhabiting very different environments may hint at metallicity gradients in Local Group early-type dwarfs being driven by internal mechanisms.

Variable stars in the Cetus dwarf spheroidal galaxy: population gradients and connections with the star formation history★

We investigate the variable star content of the isolated, Local Group, dwarf spheroidal galaxy (dSph) Cetus. Multi-epoch, wide-field images collected with the VLT/VIMOS camera allowed us to detect 638 variable stars (630 RR Lyrae stars and 8 Anomalous Cepheids), 475 of which are new detections. We present a full catalogue of periods, amplitudes, and mean magnitudes. Motivated by the recent discovery that the pulsational properties of the RR Lyrae stars in the Tucana dSph revealed the presence of a metallicity gradient within the oldest (>10 Gyr old) stellar populations, we investigated the possibility of an analogous effect in Cetus. We found that, despite the obvious radial gradient in the Horizontal Branch (HB) and Red Giant Branch (RGB) morphologies, both becoming bluer on average for increasing distance from the center of Cetus, the properties of the RR Lyrae stars are homogeneous within the investigated area (out to r~15'), with no significant evidence of a radial gradient. We discuss this in connection with the star formation history (SFH) previously derived for the two galaxies. The observed differences between these two systems show that even systems this small show a variety of early evolutionary histories. These differences could be due to different merger or accretion histories.

THE ACS LCID PROJECT. III. THE STAR FORMATION HISTORY OF THE CETUS dSph GALAXY: A POST-REIONIZATION FOSSIL

We use deep HST/ACS observations to calculate the star formation history (SFH) of the Cetus dwarf spheroidal (dSph) galaxy. Our photometry reaches below the oldest main sequence turn-offs, which allows us to estimate the age and duration of the main episode of star formation in Cetus. This is well approximated by a single episode that peaked roughly 12+/-0.5 Gyr ago and lasted no longer than about 1.9+/-0.5 Gyr (FWHM). Our solution also suggests that essentially no stars formed in Cetus during the past 8 Gyrs. This makes Cetus' SFH comparable to that of the oldest Milky Way dSphs. Given the current isolation of Cetus in the outer fringes of the Local Group, this implies that Cetus is a clear outlier in the morphology-Galactocentric distance relation that holds for the majority of Milky Way dwarf satellites. Our results also show that Cetus continued forming stars through z ~ 1, long after the Universe was reionized, and that there is no clear signature of the epoch of reionization in Cetus' SFH. We discuss briefly the implications of these results for dwarf galaxy evolution models. Finally, we present a comprehensive account of the data reduction and analysis strategy adopted for all galaxies targeted by the LCID (Local Cosmology from Isolated Dwarfs) project. We employ two different photometry codes (DAOPHOT/ALLFRAME and DOLPHOT), three different SFH reconstruction codes (IAC-pop/MinnIAC, MATCH, COLE), and two stellar evolution libraries (BaSTI and Padova/Girardi), allowing for a detailed assessment of the modeling and observational uncertainties.

Inside the whale: the structure and dynamics of the isolated Cetus dwarf spheroidal

This paper presents a study of the Cetus dwarf, an isolated dwarf galaxy within the Local Group. A matched-filter analysis of the INT/WFC imaging of this system reveals no evidence for significant tidal debris that could have been torn off the galaxy, bolstering the hypothesis that Cetus has never significantly interacted with either the Milky Way or M31. Additionally, Keck/Deimos spectroscopic observations identify this galaxy as a distinct kinematic population possessing a systematic velocity of −87 ± 2 km s^(−1) and with a velocity dispersion of 17 ± 2 km s^(−1); while tentative, these data also suggest that Cetus possesses a moderate rotational velocity of ~8 km s^(−1). The population is confirmed to be relatively metal-poor, consistent with [Fe/H]~−1.9, and, assuming virial equilibrium, implies that the Cetus dwarf galaxy possesses a mass-to-light ratio of ~70. It appears, therefore, that Cetus may represent a primordial dwarf galaxy, retaining the kinematic and structural properties lost by other members of the dwarf population of the Local Group in their interactions with the large galaxies. An analysis of Cetus' orbit through the Local Group indicates that it is at apocentre; taken in conjunction with the general dwarf population, this shows the mass of the Local Group to be ≳ 2 × 10^(12) M_⊙.

Structural properties of the M31 dwarf spheroidal galaxies

The projected structures and integrated properties of the Andromeda I, II, III, V, VI, VII and Cetus dwarf spheroidal galaxies are analysed based upon resolved counts of red giant branch stars. The observations were taken as part of the Isaac Newton Telescope Wide Field Survey of M31 and its environs. For each object, we have derived isopleth maps, surface brightness profiles, intensity-weighted centres, position angles, ellipticities, tidal radii, core radii, concentration parameters, exponential scalelengths, Plummer scalelengths, half-light radii, absolute magnitudes and central surface brightnesses. Our analysis probes into larger radius and fainter surface brightnesses than most previous studies, and as a result we find that the galaxies are generally larger and brighter than has previously been recognized. In particular, the luminosity of Andromeda V is found to be consistent with the higher metallicity value which has been derived for it. We find that exponential and Plummer profiles provide adequate fits to the surface brightness profiles, although the more general King models provide the best formal fits. Andromeda I shows strong evidence of tidal disruption and S-shaped tidal tails are clearly visible. On the other hand, Cetus does not show any evidence of tidal truncation, let alone disruption, which is perhaps unsurprising given its isolated location. Andromeda II shows compelling evidence of a large excess of stars at small radius and suggests that this galaxy consists of a secondary core component, in analogy with recent results for Sculptor and Sextans. Comparing the M31 dwarf spheroidal population with the Galactic population, we find that the scaleradii of the M31 population are larger than those for the Galactic population by at least a factor of 2, for all absolute magnitudes. This difference is either due to environmental factors or due to orbital properties, suggesting that the ensemble average tidal field experienced by the M31 dwarf spheroidals is weaker than that experienced by the Galactic dwarf spheroidals. We find that the two populations are offset from one another in the central surface brightness – luminosity relation, which is probably related to this difference in their scale sizes. Finally, we find that the M31 dwarf spheroidals show the same correlation with distance from host as shown by the Galactic population, such that dwarf spheroidals with a higher central surface brightness are found further from their host. This again suggests that environment plays a significant role in dwarf galaxy evolution, and requires detailed modelling to explain the origin of this result.

WIYN Survey for Carbon Stars in the M31 and Cetus Dwarf Spheroidal Galaxies: Evolutionary Implications

We report results of a photometric survey with the WIYN telescope for carbon stars in the M 31 dwarf spheroidal (dSph) companions And III, And V, And VI, and And VII, as well as in the relatively isolated Local Group dSph Cetus. We find three carbon-star candidates in And VII and one carbon star in each And VI and Cetus. Comparing the carbon star content with other Local Group dwarf galaxies, we argue against the presence of substantial intermediate-age stellar populations in the all of the galaxies surveyed with the exception of And VII. We discuss these results in the context of the origin of the Andromeda dSphs and conclude that these are ancient galaxies, most of which ceased star formation long before the main merger events in M31. The M31 dSphs therefore show less diversity in star formation histories than the Galactic dSph companions, or the M31 dE companions, as illustrated by NGC147 which was surveyed as a calibration object. All of our dSph targets except And V have candidate carbon stars below the tip of the RGB, which resemble CH stars found in globular clusters. We estimate that 0.3% of stars in the dSphs are CH stars, presumably as a result of C pollution from a binary companion. Comparisons with CH star frequencies in globular clusters could constrain the impact of dense environments on the frequency of this form of binary star evolution.

The Stellar Populations of the Cetus Dwarf Spheroidal Galaxy

We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry in the V and I passbands of the recently discovered Local Group dwarf spheroidal galaxy in Cetus. Our color-magnitude diagram extends from above the first-ascent red giant branch (RGB) tip to approximately half a magnitude below the horizontal branch (HB). Given a reddening of E(B-V) = 0.03, the magnitude of the RGB tip yields a distance modulus of (m-M)0 = 24.46 ± 0.14. After applying the reddening and distance modulus, we have utilized the color distribution of RGB stars to determine a mean metal abundance of [Fe/H] = -1.7 on the Zinn & West scale, with an intrinsic internal abundance dispersion of ~0.2 dex. An indirect calculation of the HB morphology of Cetus based on the mean dereddened HB color yields (B-R)/(B+V+R) = -0.91 ± 0.09, which represents an HB that is redder than what can be attributed solely to Cetus's metal abundance. Thus, Cetus is affected by the in which another parameter besides metallicity is controlling the HB morphology. If we adopt the conventional age hypothesis explanation for the second-parameter effect, then this implies that Cetus is 2-3 Gyr younger than Galactic globular clusters at its metallicity.

Updated Information on the Local Group

ABSTRACTThe present note updates the information published in my recent monograph on The Galaxies of the Local Group. Highlights include (1) the addition of the newly discovered Cetus dwarf spheroidal as a certain member of the Local Group; (2) an improved distance for the Sagittarius dwarf irregular galaxy (SagDIG), which now places this object very close to the edge of the Local Group zero‐velocity surface; (3) more information on the evolutionary histories of some individual Local Group members; and (4) improved distance determinations to, and luminosities for, a number of Local Group members. These data increase the number of certain (or probable) Local Group members to 36. The spatial distribution of these galaxies supports Hubble’s claim that the Local Group “is isolated in the general field.” Currently available evidence suggests that star formation continued much longer in many dwarf spheroidals than it did in the main body of the Galactic halo. It is suggested that “young” globular clusters, such as Ruprecht 106, might have formed in now defunct dwarf spheroidals. Assuming SagDIG, which is the most remote Local Group galaxy, to lie on, or just inside, the zero‐velocity surface of the Local Group yields a dynamical age ≳ 17.9 ± 2.7 Gyr. However, this value is meaningful only if the outer regions of the local Group are in virial equilibrium.

A New Local Group Galaxy in Cetus

We report the discovery of a previously uncataloged Local Group galaxy in the constellation Cetus. Faintly visible on UKST survey plates, it has a smooth, diffuse appearance and appears to be a dwarf spheroidal of type dE3.5. A color-magnitude diagram in V, V-I shows a clear giant branch but no sign of recent star formation. From the position of the tip of the giant branch, we derive a reddening-corrected distance modulus of 24.45 ± 0.15 and a metallicity of -1.9 ± 0.2. With an implied heliocentric distance of 775 ± 50 kpc, and a corresponding Local Group barycentric distance of 615 kpc, the Cetus dwarf lies well within the boundaries of the Local Group, and although it currently lacks a radial velocity measurement, it is undoubtedly a member of the Local Group. The nearest Local Group galaxies are WLM and IC 1613 at angular separations of 73 and 163 and roughly 175 and 230 kpc total distance, respectively. Although the Cetus dwarf is unlikely to be directly associated with any other Local Group galaxy, it does lie in the general direction of the extension of the Local Group toward the Sculptor Group.