NGC 5011C: an overlooked dwarf galaxy in the Centaurus A group

(abridged) We report the discovery of a previously unnoticed member of the Centaurus A Group, NGC 5011C. While the galaxy is a well known stellar system listed with a NGC number its true identity remained hidden because of coordinate confusion and wrong redshifts in the literature. NGC 5011C attracted our attention since, at a putative distance of 45.3 Mpc, it would be a peculiar object having a very low surface brightness typical of a dwarf galaxy, and at the same time having the size of an early-type spiral or S0 galaxy. To confirm or reject this peculiarity, our immediate objective was to have the first reliable measurement of its recession velocity. The observations were carried out with EFOSC2 at the 3.6m ESO telescope. We found that NGC 5011C has indeed a low redshift of v_sun=647+/-96 km/sec and thus is a nearby dwarf galaxy rather than a member of the distant Centaurus cluster as believed for the past 23 years. Rough distance estimates based on photometric parameters also favor this scenario. As a byproduct of our study we update the redshift for NGC 5011B at v_sun=3227+/-50 km/sec. Applying population synthesis techniques, we find that NGC 5011B has a luminosity-weighted age of 4+/-1 Gyr and a solar metallicity, and that the luminosity-weighted age and metallicity of NGC 5011C are 0.9+/-0.1 Gyr and 1/5 solar. Finally we estimate a stellar mass of NGC 5011C comparable to that of dwarf spheroidal galaxies in the Local Group.Comment: 13 pages, 6 figures, accepted by the Astronomical Journa

Horologium II: a Second Ultra-faint Milky Way Satellite in the Horologium Constellation

We report the discovery of a new ultra-faint Milky Way satellite candidate, Horologium II, detected in the Dark Energy Survey Y1A1 public data. Horologium II features a half light radius of $r_{h}=47\pm10$ pc and a total luminosity of $M_{V}=-2.6^{+0.2}_{-0.3}$ that place it in the realm of ultra-faint dwarf galaxies on the size-luminosity plane. The stellar population of the new satellite is consistent with an old ($\sim13.5$ Gyr) and metal-poor ([Fe/H]$\sim-2.1$) isochrone at a distance modulus of $(m-M)=19.46\pm0.20$, or a heliocentric distance of $78\pm8$ kpc, in the color-magnitude diagram. Horologium II has a distance similar to the Sculptor dwarf spheroidal galaxy ($\sim82$ kpc) and the recently reported ultra-faint satellites Eridanus III ($87\pm8$ kpc) and Horologium I ($79\pm8$ kpc). All four satellites are well aligned on the sky, which suggests a possible common origin. As Sculptor is moving on a retrograde orbit within the Vast Polar Structure when compared to the other classical MW satellite galaxies including the Magellanic Clouds, this hypothesis can be tested once proper motion measurements become available.Comment: 5 pages, 3 figures, 1 table. Accepted for publication in ApJL. (w.r.t. v1: figures updated; minor changes throughout the text

The impact of accurate distances to dwarf elliptical galaxies on cosmology

The Surface Brightness Fluctuation method has been shown to be a powerful distance indicator for dwarf elliptical galaxies to very low surface brightness levels. It is applicable to stellar systems that are out of reach for classical indicators requiring resolved stellar populations such as the tip magnitude of the red giant branch. I briefly discuss a few results from recent SBF studies of dEs to demonstrate the significance of the SBF method to address long-standing issues related to cosmography, dark matter in galaxy groups, substructures in clusters, and the discrepancy between the mass function of collapsed objects and the faint end of the galaxy luminosity function. For the analysis of the large number of galaxy images that need to be processed as part of such SBF studies we are currently developing a fast, semi-automatic reduction pipeline that will be made readily available to the astronomical community.Comment: 9 pages, 3 figures, to appear in the proceedings of the IAU Colloquium No. 198 "Near-Field Cosmology with Dwarf Elliptical Galaxies

New dwarf galaxy candidates in the Centaurus group

Recent studies of the distribution and kinematics of the Milky Way and Andromeda satellite galaxy systems have confirmed the existence of coplanar, corotating structures of galaxies. In addition to the 'missing satellite problem', these structures pose a major challenge to the standard $\Lambda$CDM scenario of structure formation. We complement the efforts made by the dwarf galaxy community to extend these studies to other nearby galaxy groups by systematically searching for faint, unresolved dwarf members with a low surface brightness in the Southern Centaurus group of galaxies. The aim is to determine whether these coplanar, corotating structures are a universal phenomenon. We imaged an area of 60 square degrees (0.3 Mpc$^2$) around the M83 subgroup with the wide-field Dark Energy Camera (DECam) at the CTIO 4 m Blanco telescope in $g$ and $r$ down to a limiting surface brightness of $\mu_r\approx 30$ mag arcsec$^{-2}$. Various image-filtering techniques were applied to the DECam data to enhance the visibility of extremely low-surface brightness objects. We report the discovery of 16 new dwarf galaxy candidates in the direction of the M83 subgroup, roughly doubling the number of known dwarfs in that region. The photometric properties of the candidates, when compared to those of the Local Group, suggest membership in the M83 subgroup. The faintest objects have a central star density of $\approx1.3 L_\odot$ pc$^{-2}$ and a total magnitude of $g = 20.25$, corresponding to $M_g = -9.55$ at the nominal distance of 4.9 Mpc. The sky distribution of the new objects is significantly prolonged toward Cen A, suggesting that many of them belong to the Cen A subgroup or a common halo. We also provide updated surface photometry for the brighter, known dwarf members in the surveyed area

Tip of the red giant branch distances to the dwarf galaxies dw1335-29 and dw1340-30 in the Centaurus group

The abundance and spatial distribution of dwarf galaxies are excellent empirical benchmarks to test models of structure formation on small scales. The nearby Centaurus group, with its two subgroups centered on CenA and M83, stands out as an important alternative to the Local Group for scrutinizing cosmological model predictions in a group of galaxies context. We have obtained deep optical images of three recently discovered M83 satellite galaxy candidates with the FORS2 instrument mounted on the Very Large Telescope. We aim to confirm their group membership and study their stellar population. Deep VI-band photometry is used to resolve the brightest stars in our targets. Artificial star tests are performed to estimate the completeness and uncertainties of the photometry. The color-magnitude diagrams reveal the red giant branch (RGB) stars allowing to use the Sobel edge detection method to measure the magnitude of the RGB tip and thus derive distances and group membership for our targets. The mean metallicity of the dwarf galaxies are further determined by fitting BASTI model isochrones to the mean RGB locus. We confirm the two candidates, dw1335-29 and dw1340-30, to be dwarf satellites of the M83 subgroup, with estimated distances of 5.03 +- 0.24 Mpc and 5.06 +- 0.24 Mpc, respectively. Their respective mean metallicities of = -1.79 +- 0.4 and = -2.27 +- 0.4 are consistent with the metallicity-luminosity relation for dwarf galaxies. The third candidate, dw1325-33, could not be resolved into stars due to insufficiently deep images, implying its distance must be larger than 5.3 Mpc. Using the two newly derived distances we assess the spatial distribution of the galaxies in the M83 subgroup and discuss a potential plane-ofsatellites around M83.Comment: 10 pages, 9 figures, accepted for publication in A&

Discs of Satellites: the new dwarf spheroidals

The spatial distributions of the most recently discovered ultra faint dwarf satellites around the Milky Way and the Andromeda galaxy are compared to the previously reported discs-of-satellites (DoS) of their host galaxies. In our investigation we pay special attention to the selection bias introduced due to the limited sky coverage of SDSS. We find that the new Milky Way satellite galaxies follow closely the DoS defined by the more luminous dwarfs, thereby further emphasizing the statistical significance of this feature in the Galactic halo. We also notice a deficit of satellite galaxies with Galactocentric distances larger than 100 kpc that are away from the disc-of-satellites of the Milky Way. In the case of Andromeda, we obtain similar results, naturally complementing our previous finding and strengthening the notion that the discs-of-satellites are optical manifestations of a phase-space correlation of satellite galaxies.Comment: acc. for pub. in MNRA

New low surface brightness dwarf galaxies in the Centaurus group

We conducted an extensive CCD search for faint, unresolved dwarf galaxies of very low surface brightness in the whole Centaurus group region encompassing the Cen A and M 83 subgroups lying at a distance of roughly 4 and 5 Mpc, respectively. The aim is to significantly increase the sample of known Centaurus group members down to a fainter level of completeness, serving as a basis for future studies of the 3D structure of the group. Following our previous survey of 60 square degrees covering the M 83 subgroup, we extended and completed our survey of the Centaurus group region by imaging another 500 square degrees area in the g and r bands with the wide-field Dark Energy Survey Camera at the 4m Blanco telescope at CTIO. The limiting central surface brightness reached for suspected Centaurus members is $\mu_r \approx 29$ mag arcsec$^{-2}$, corresponding to an absolute magnitude $M_r \approx -9.5$. The images were enhanced using different filtering techniques. We found 41 new dwarf galaxy candidates, which together with the previously discovered 16 dwarf candidates in the M 83 subgroup amounts to almost a doubling of the number of known galaxies in the Centaurus complex, if the candidates are confirmed. We carried out surface photometry in g and r, and report the photometric parameters derived therefrom, for all new candidates as well as previously known members in the surveyed area. The photometric properties of the candidates, when compared to those of LG dwarfs and previously known Centaurus dwarfs, suggest membership in the Centaurus group. The sky distribution of the new objects is generally following a common envelope around the Cen A and M 83 subgroups. How the new dwarfs are connected to the intriguing double-planar feature recently reported by Tully et al. (2015) must await distance information for the candidates.Comment: 14 pages, 12 figures, accepted for publication in A&