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Register a new user6-meter telescope observations of three dwarf spheroidal galaxies with very low surface brightness
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Dwarf spheroidal galaxies (dSphs) are mostly investigated in the Local Group. DSphs are difficult targets for observations because of their small size and very low surface brightness. Here we measure spectroscopic and photometric parameters of three candidates for isolated dSphs, KKH65=BTS23, KK180, and KK227, outside the Local Group. The galaxies are found to be of low metallicity and low velocity dispersion. They are among the lowest surface brightness objects in the Local Universe. According to the measured radial velocities, metallicities, and structural and photometric parameters, KKH65 and KK227 are representatives of the ultra-diffuse quenched galaxies. KKH65 and KK227 belong to the outer parts of the groups NGC3414 and NGC5371, respectively. KK180 is located in the Virgo cluster infall region.
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I consider a sample of eight pressure-supported low-surface brightness galaxies in terms of Milgroms modified Newtonian dynamics (MOND). These objects include seven nearby dwarf spheroidal galaxies -- Sextans, Carina, Leo II, Sculptor, Draco, Leo I, Fornax, and the ultra-diffuse galaxy DF44. The objects are modelled as Milgromian isotropic isothermal spheres characterised by two parameters that are constrained by observations: the constant line-of-sight velocity dispersion and the central surface density. The velocity dispersion determines the total mass, and, with the implied mass-to-light ratio, the central surface brightness. This then specifies the radial run of surface brightness over the entire isothermal sphere. For these objects the predicted radial distribution of surface brightness is shown to be entirely consistent with observations. This constitutes a success for MOND that is independent of the reduced dynamical mass.
Low-surface-brightness galaxies (LSBGs) -- defined as systems that are fainter than the surface-brightness limits of past wide-area surveys -- form the overwhelming majority of galaxies in the dwarf regime (M* < 10^9 MSun). Using NewHorizon, a high-resolution cosmological simulation, we study the origin of LSBGs and explain why LSBGs at similar stellar mass show the large observed spread in surface brightness. New Horizon galaxies populate a well-defined locus in the surface brightness -- stellar mass plane, with a spread of ~3 mag arcsec^-2, in agreement with deep SDSS Stripe data. Galaxies with fainter surface brightnesses today are born in regions of higher dark-matter density. This results in faster gas accretion and more intense star formation at early epochs. The stronger resultant supernova feedback flattens gas profiles at a faster rate which, in turn, creates shallower stellar profiles (i.e. more diffuse systems) more rapidly. As star formation declines towards late epochs (z<1), the larger tidal perturbations and ram pressure experienced by these systems (due to their denser local environments) accelerate the divergence in surface brightness, by increasing their effective radii and reducing star formation respectively. A small minority of dwarfs depart from the main locus towards high surface brightnesses, making them detectable in past wide surveys. These systems have anomalously high star-formation rates, triggered by recent, fly-by or merger-driven starbursts. We note that objects considered extreme/anomalous at the depth of current datasets, e.g. `ultra-diffuse galaxies, actually dominate the predicted dwarf population and will be routinely visible in future surveys like LSST.
We report the automatic detection of a new sample of very low surface brightness (LSB) galaxies, likely members of the Virgo cluster. We introduce our new software, {tt DeepScan}, that has been designed specifically to detect extended LSB features automatically using the DBSCAN algorithm. We demonstrate the technique by applying it over a 5 degree$^2$ portion of the Next-Generation Virgo Survey (NGVS) data to reveal 53 low surface brightness galaxies that are candidate cluster members based on their sizes and colours. 30 of these sources are new detections despite the region being searched specifically for LSB galaxies previously. Our final sample contains galaxies with $26.0leqlangle mu_{e}rangleleq28.5$ and $19leq m_{g}leq21$, making them some of the faintest known in Virgo. The majority of them have colours consistent with the red sequence, and have a mean stellar mass of $10^{6.3pm0.5} M_{odot}$ assuming cluster membership. After using {tt ProFit} to fit Sersic profiles to our detections, none of the new sources have effective radii larger than 1.5 Kpc and do not meet the criteria for ultra-diffuse galaxy (UDG) classification, so we classify them as ultra-faint dwarfs.
We have obtained low resolution spectra of nineteen red and blue low surface brightness galaxies, using the Marcario Low Resolution Spectrograph on the 9.2m Hobby-Eberly Telescope. These galaxies form a very heterogeneous class, whose spectra qualitatively resemble those of high surface brightness galaxies covering the full range of spectra seen in galaxies of Hubble types from E to Irr. We use a combination of emission line (EW(Halpha), NII/Halpha) and absorption line (Mgb, Hbeta, <Fe>) based diagnostics to investigate the star-formation and chemical enrichment histories of these galaxies. These are diverse, with some galaxies having low metallicity and very young mean stellar ages, and other galaxies showing old, super-solar metallicity stellar populations. In contrast with some previous studies which found a strong trend of decreasing metallicity with decreasing central surface brightness, we find a population of galaxies with low surface brightness and near-solar metallicity. Correlations between several of the gas phase and stellar population age and metallicity indicators are used to place contraints on plausible evolutionary scenarios for LSB galaxies. The redshift range spanned by these galaxies is broad, with radial velocities from 3400 km/s to more than 65000 km/s. A subset of the sample galaxies have published HI redshifts and gas masses based on observations with the Arecibo 305m single-dish radio telescope, which place these galaxies far off of the mean Tully-Fisher relation. Our new optical redshifts do not agree with the published HI redshifts for these galaxies. Most of the discrepancies can be explained by beam confusion in the Arecibo observations, causing erroneous HI detections for some of the galaxies.
We present Very Large Array ({sc vla}) and Westerbork Synthesis Radio Telescope ({sc wsrt}) 21-cm H{sc i} observations of 19 late-type low surface brightness (LSB) galaxies. Our main findings are that these galaxies, as well as having low surface brightnesses, have low H{sc i} surface densities, about a factor of $sim 3$ lower than in normal late-type galaxies. We show that LSB galaxies in some respects resemble the outer parts of late-type normal galaxies, but may be less evolved. LSB galaxies are more gas-rich than their high surface brightness counterparts. The rotation curves of LSB galaxies rise more slowly than those of HSB galaxies of the same luminosity, with amplitudes between 50 and 120~km~s$^{-1}$, and are often still increasing at the outermost measured point. The shape of the rotation curves suggests that LSB galaxies have low matter surface densities. We use the average total mass surface density of a galaxy as a measure for the evolutionary state, and show that LSB galaxies are among the least compact, least evolved galaxies. We show that both $M_{rm HI}/L_B$ and $M_{rm dyn}/L_B$ depend strongly on central surface brightness, consistent with the surface brightness--mass-to-light ratio relation required by the Tully-Fisher relation. LSB galaxies are therefore slowly evolving galaxies, and may well be low surface density systems in all respects.
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