A Critical Review of the Evidence for M32 being a Compact Dwarf Satellite of M31 rather than a More Distant Normal Galaxy

Since Baade's photographic study of M32 in the mid 1940s, it has been accepted as an established fact that M32 is a compact dwarf satellite of M31. The purpose of this paper is to report on the findings of our investigation into the nature of the existing evidence. We find that the case for M32 being a satellite of M31 rests upon Hubble Space Telescope (HST) based stellar population studies which have resolved red-giant branch (RGB) and red clump stars in M32 as well as other nearby galaxies. Taken in isolation, this recent evidence could be considered to be conclusive in favour of the existing view. However, the conventional scenario does not explain M32's anomalously high central velocity dispersion for a dwarf galaxy (several times that of either NGC 147, NGC 185 or NGC 205) or existing planetary nebula observations (which suggest that M32 is more than twice as distant as M31) and also requires an elaborate physical explanation for M32's inferred compactness. Conversely, we find that the case for M32 being a normal galaxy, of the order of three times as distant as M31, is supported by: (1) a central velocity dispersion typical of intermediate galaxies, (2) the published planetary nebula observations, and (3) known scaling relationships for normal early-type galaxies. However, this novel scenario cannot account for the high apparent luminosities of the RGB stars resolved in the M32 direction by HST observations. We are therefore left with two apparently irreconcilable scenarios, only one of which can be correct, but both of which suffer from potentially fatal evidence to the contrary. This suggests that current understanding of some relevant fields is still very far from adequate.     

Dwarf Galaxies in the M 81/82 Group. New Photometric Data

We present a list of 39 dwarf galaxies of the M 81/82 group. Three different methods are applied to determine their apparent integral magnitude: digital processing, equidensitometry, and calculation according to the formulae of FISHER and TULLY . The plates used were obtained with the Tautenburg Schmidt telescope. For a distance of the group of 3.5 Mpc the absolute magnitudes of the dwarf galaxies investigated are -15^m < M < -9^m.     

Prospects of the “WSO-UV” Project for Star Formation Study in Nearby Dwarf Galaxies

In the present work we consider the questions of star formation and evolution of nearby dwarf galaxies. We describe the method of star formation history determination based on multicolor photometry of resolved stars and models of color-magnitude diagrams of the galaxies. We present the results of star formation rate determination and its dependence on age and metallicity for dwarf irregular and dwarf spheroidal galaxies in the two nearby galaxy groups M81 and Cen A. Similar age of the last episode of star formation in the central part of the M81 group and also unusually high level of metal enrichment in the several galaxies of the Cen A group are mentioned. We pay special attention to the consideration of perspectives of star formation study in nearby dwarf galaxies with he new WSO-UV observatory.     

M dwarfs at large heliocentric distances

We present an analysis of the faint M star population seen as foreground contaminants in deep extragalactic surveys. We use space-based data to separate such stars from high-redshift galaxies in a publicly available data set, and consider the photometric properties of the resulting sample in the optical and infrared. The inferred distances place these stars well beyond the scaleheight of the thick disc. We find strong similarities between this faint sample (reaching   i '_AB= 25  ) and the brighter disc M dwarf population studied by other authors. The optical–infrared properties of the bulk of our sources spanning 6000 Å-4.5 μm are consistent with those 5–10 mag brighter. We also present deep spectroscopy of faint M dwarf stars reaching continuum limits of i '_AB≈ 26, and measure absorption-line strengths in the CaH2 and TiO5 bands. Both photometrically and spectroscopically, our sources are consistent with metallicities as low as a tenth solar: metal-rich compared with halo stars at similar heliocentric distances. We comment on the possible massive astrophysical compact halo object (MACHO) identification of M stars at faint magnitudes.     

On the mass of M31

Recent work by several groups has established the properties of the dwarf satellites to M31. We reexamine the reported kinematics of this group employing a fresh technique we have developed previously. By calculating the distribution of a χ statistic (which we define in the paper) for the M31 system, we conclude that the total mass (disc plus halo) of the primary is unlikely to be as great as that of our own Milky Way. In fact the χ distribution for M31 indicates that, like NGC 3992, it does not have a massive halo. In contrast, the analysis of the satellites of NGC 1961 and NGC 5084 provides strong evidence for massive haloes surrounding both spiral galaxies.     

On the structure of the low surface brightness dwarf galaxies in the M81 group

A detailed photometry of spheroidal dwarf galaxies in the M81 group has been carried out. The integral characteristics and the structural parameters of the spheroidal dwarfs has been determined. Their luminosity profiles are well fitted to a King law. The investigated spheroidal dwarfs together with the prototypes of the Local Group form a common sequence according to their main parameters. The observational data presented show that dSphs are not linked evolutionary with normal E and dE galaxies, but probably form a separate branch together with irregular low surface brightness dwarfs.     

Velocity dispersion measurements of dwarf galaxies in the Coma cluster – implications for the structure of the fundamental plane

We present intermediate-resolution spectroscopic data for a set of dwarf and giant galaxies in the Coma cluster, with  −20.6 < M_R < −15.7.  The photometric and kinematic properties of the brighter galaxies can be cast in terms of parameters which present little scatter with respect to a set of scaling relations known as the fundamental plane. To determine the form of these fundamental scaling relations at lower luminosities, we have measured velocity dispersions for a sample comprising 69 galaxies on the border of the dwarf and giant regime. Combining these data with our photometric survey, we find a tight correlation of luminosity and velocity dispersion,   L ∝σ^2.0  , substantially flatter than the Faber–Jackson relation characterizing giant elliptical galaxies. In addition, the variation of mass-to-light ( M / L ) ratio with velocity dispersion is quite weak in our dwarf sample:   M / L ∝σ^0.2.  Our overall results are consistent with theoretical models invoking large-scale mass removal and subsequent structural readjustment, e.g. as a result of galactic winds.     

Distances and metallicities for 17 Local Group galaxies

We have obtained Johnson V and Gunn  i photometry for a large number of Local Group galaxies using the Isaac Newton Telescope Wide Field Camera (INT WFC). The majority of these galaxies are members of the M31 subgroup and the observations are deep enough to study the top few magnitudes of the red giant branch in each system. We previously measured the location of the tip of the red giant branch (TRGB) for Andromeda I, Andromeda II and M33 to within systematic uncertainties of typically <0.05 mag. As the TRGB acts as a standard candle in old, metal-poor stellar populations, we were able to derive distances to each of these galaxies. Here we derive TRGB distances to the giant spiral galaxy M31 and 13 additional dwarf galaxies – NGC 205, 185, 147, Pegasus, WLM, LGS3, Cetus, Aquarius, And III, V, VI, VII and the newly discovered dwarf spheroidal And IX. The observations for each of the dwarf galaxies were intentionally taken in photometric conditions. In addition to the distances, we also self-consistently derive the median metallicity of each system from the colour of their red giant branches. This allows us to take into account the small metallicity variation of the absolute I magnitude of the TRGB. The homogeneous nature of our data and the identical analysis applied to each of the 17 Local Group galaxies ensures that these estimates form a reliable set of distance and metallicity determinations that are ideal for comparative studies of Local Group galaxy properties.     

Globular cluster systems in nearby dwarf galaxies – I. HST/ACS observations and dynamical properties of globular clusters at low environmental density

We investigate the old globular cluster (GC) population of 68 faint  ( M _V > −16 mag)  dwarf galaxies located in the halo regions of nearby (≲12 Mpc) loose galaxy groups and in the field environment based on archival Hubble Space Telescope ( HST )/Advanced Camera for Surveys (ACS) images in F606W and F814W filters. The combined colour distribution of 175 GC candidates peaks at  ( V − I ) = 0.96 ± 0.07 mag  and the GC luminosity function turnover for the entire sample is found at   M _{V ,TO}=−7.6 ± 0.11 mag  , similar to the old metal-poor Large Magellanic Cloud (LMC) GC population. Our data reveal a tentative trend of   M _{V ,TO}  becoming fainter from late- to early-type galaxies. The luminosity and colour distributions of GCs in dIrrs show a lack of faint blue GCs (bGCs). Our analysis reveals that this might reflect a relatively younger GC system than typically found in luminous early-type galaxies. If verified by spectroscopy, this would suggest a later formation epoch of the first metal-poor star clusters in dwarf galaxies. We find several bright (massive) GCs which reside in the nuclear regions of their host galaxies. These nuclear clusters have similar luminosities and structural parameters as the peculiar Galactic clusters suspected of being the remnant nuclei of accreted dwarf galaxies, such as M54 and ωCen. Except for these nuclear clusters, the distribution of GCs in dIrrs in the half-light radius versus cluster mass plane is very similar to that of Galactic young halo clusters, which suggests comparable formation and dynamical evolution histories. A comparison with theoretical models of cluster disruption indicates that GCs in low-mass galaxies evolve dynamically as self-gravitating systems in a benign tidal environment.     

Search for dwarf galaxies in the IC 342 complex of galaxies

A survey of dwarf galaxies was carried out in the inner part of the IC 342 complex of galaxies, which is a subgroup of the M81 group, in an area of about 42 sq. degrees by means of Tautenburg 52-in. Schmidt plates. Fifteen new dwarf candidates and four extended “cloudy” objects of very low surface brightness not yet classified were discovered. Coordinates, linear dimensions as well as remarks on the nature of the cloudy objects are given. A deeper analysis with 6-meter plates is planned to study a part of the new objects more exactly.     

Survey of galaxies in the local volume in Hα: Faint companions of M31

Observations on the 6-m telescope in the Hα line and in the continuum are reported for 10 dwarf companions of the galaxy M31: And I, And II, And III, And V, And IX, And X, Cass dSph, Peg DSph, NGC147, NGC221, and one irregular dwarf galaxy in the background, And IV. All the observed companions of M31 have current star formation rates (SFR) on the order of or less than 10⁻⁶ M_⊙ /yr. On a “star formation rate-neutral hydrogen mass” diagram for galaxies in the local volume, the dwarf spheroidal companions of Andromeda lie in the region of extremely low values for these parameters. __________ Translated from Astrofizika, Vol. 49, No. 3, pp. 337–350 (August 2006).     

Structural parameters of Mayall II = G1 in M31

Mayall II = G1 is one of the most luminous globular clusters (GCs) known in M31. New deep, high-resolution observations with the Advanced Camera for Surveys on the Hubble Space Telescope are used to provide accurate photometric data to the smallest radii yet. In particular, we present the precise variation of ellipticity and position angle, and of surface brightness for the core of the object. Based on these accurate photometric data, we redetermine the structural parameters of G1 by fitting a single-mass isotropic King model. We derive a core radius,   r _c= 0.21 ± 0.01  arcsec (= 0.78 ± 0.04  pc)  , a tidal radius,   r _t= 21.8 ± 1.1  arcsec (= 80.7 ± 3.9  pc)  , and a concentration index   c = log ( r _t/ r _c) = 2.01 ± 0.02  . The central surface brightness is 13.510 mag arcsec⁻². We also calculate the half-light radius, at   r _h= 1.73 ± 0.07  arcsec (= 6.5 ± 0.3  pc)  . The results show that, within 10 core radii, a King model fits the surface brightness distribution well. We find that this object falls in the same region of the   M_V   versus  log   R _h  diagram as ω Centauri, M54 and NGC 2419 in the Milky Way. All three of these objects have been claimed to be the stripped cores of now defunct dwarf galaxies. We discuss in detail whether GCs, stripped cores of dwarf spheroidals and normal dwarf galaxies form a continuous distribution in the   M_V   versus  log   R _h  plane, or if GCs and dwarf spheroidals constitute distinct classes of objects; we present arguments in favour of this latter view.     

Searching for tidal tails – investigating galaxy harassment

Galaxy harassment has been proposed as a physical process that morphologically transforms low surface density disc galaxies into dwarf elliptical galaxies in clusters. It has been used to link the observed very different morphology of distant cluster galaxies (relatively more blue galaxies with 'disturbed' morphologies) with the relatively large numbers of dwarf elliptical galaxies found in nearby clusters. One prediction of the harassment model is that the remnant galaxies should lie on low surface brightness tidal streams or arcs. We demonstrate in this paper that we have an analysis method that is sensitive to the detection of arcs down to a surface brightness of 29 B μ and we then use this method to search for arcs around 46 Virgo cluster dwarf elliptical galaxies. We find no evidence for tidal streams or arcs and consequently no evidence for galaxy harassment as a viable explanation for the relatively large numbers of dwarf galaxies found in the Virgo cluster.     

Tidal dwarfs in the M81 group: the second generation?

We present a quantitative star formation history derivation of the four suspected tidal dwarf galaxies in the M 81 group: Holmberg IX, BK3N,Arp-loop (A0952+69) and Garland using HST/WFPC2 images of these galaxies. We construct a library of synthetic Colour-Magnitude Diagrams(CMDs) based on theoretical isochrones and data-derived determinations of photometric errors. These synthetic CMDs were combined linearly andχ²-compared to observed photometry. All the galaxies show continuous star formation between about 20 and 200 Myr ago with star formation rates between 7.5⋅10^-3 M_⊙/yr and 7.67⋅10^-4 M_⊙/yr. The metallicity of the detected stars is spanning rather a wide range, being lower than solar abundance. We suppose, that all the galaxies were formed out of material from metal-poor outer part of the giant spiral galaxy M81after tidal interaction about 200 Myr ago. However, this suggestion requires significantly more deep color-magnitude diagrams to be sure with the scenario of the galaxy evolution. This revised version was published online in July 2006 with corrections to the Cover Date.     

The central density cusp of the Sagittarius dwarf spheroidal galaxy

We present an analysis of the density profile in the central region of the Sagittarius dwarf spheroidal galaxy. A strong density enhancement of Sgr stars is observed. The position of the peak of the detected cusp is indistinguishable from the centre of M54. The photometric properties of the cusp are fully compatible with those observed in the nuclei of dwarf elliptical galaxies, indicating that the Sgr dSph would appear as a nucleated galaxy independently of the presence of M54 at its centre.     

Isolated dwarf galaxies in the local supercluster and its surroundings

We present a list of 75 isolated dwarf galaxies of later types which have no neighbors with a relative radial velocity difference of less than 500 km/s or projected distances of less than 500 kpc. These were selected from ~2000 dwarf galaxies with radial velocities V_LG<3500 km/s within the volume of the local supercluster. In terms of their sizes, luminosities, and the amplitudes of their internal motions, the isolated dwarf galaxies do not differ significantly from gas-rich dwarf galaxies in groups and clusters. However, the median mass of neutral hydrogen per unit luminosity for the isolated galaxies is a factor of two greater than for the galaxies of later types in groups. We have also identified 10 presumably isolated spheroidal dwarf galaxies. The discovery of isolated dwarf galaxies populated exclusively by old stars is of great interest for modern cosmological scenarios of galaxy formation.     

The relation of dwarf galaxies with bright barred galaxies in virgo cluster

On the basis of extensive observational material it is statistically demonstrated that dwarf galaxies appear more often around barred galaxies than around normal spiral galaxies. Moreover, the more barred spiral galaxies there are in a given region, the more dwarf galaxies are found there as well.Translated from Astrofizika, Vol. 38, No. 1, pp. 45–53, January–March, 1995.     

The Morphologies of Dwarf Markarian Galaxies

The morphologies of the 96 dwarf (M(B) -17^m) galaxies in the Markarian catalog are determined from the digitized Schmidt plates obtained for the construction of the Hubble Space Telescope Guide Star Catalog. The fraction of double nucleus galaxies within the dwarf Markarian galaxies is determined to be twice that found for all galaxies in the Markarian catalog. In addition to the 12 previously known cases, four definite and two probable galaxies with double nuclei are identified. The fraction of dwarf Markarian galaxies with bright star forming regions is found to be twice that of Virgo cluster dwarf galaxies. No Elliptical galaxies are found in the sample. Galaxies with blue compact dwarf and S0 morphologies are more often found to contain unresolved regions of UV excess emission. Dwarf Markarian galaxies with different morphological structures and spectral classes are found to have similar FIR properties.     

Multivariate study of dynamically hot stellar systems: Clues to the origin of ultra compact and ultra faint dwarfs

A multivariate classification has been performed for a large sample of dynamically hot stellar systems comprising globular clusters to giant ellipticals, in quest of the formation theory of ultra compact dwarf galaxies (UCDs). For this K means cluster analysis is carried out together with the optimum criterion (Sugar et al., 2003) with respect to three parameters, logarithm of stellar mass, logarithm of effective radius and stellar mass to light ratio. The present data set has been taken from Misgeld and Hilker (2011). We found five groups MK1–MK5. These are predominated by giant ellipticals (gEs), faint dwarf ellipticals (dEs), globular clusters (GCs), massive compact objects (UCDs and nuclei of dE,Ns) and bright dwarf ellipticals respectively. Almost all UCDs are found either in MK3 or MK4. The fraction is roughly 50%–50% between MK3 and MK4. Comparable fraction of UCDs share properties either with normal GCs or with nuclei of dE,N. This adds a quantitative constraint to the long discussed hypothesis that UCDs may be formed either as massive globular clusters or have an origin similar to nuclei of dwarf galaxies. We finally find that for our clustering test in mass-size-stellar M/L ratios, ultra faint dwarf galaxies are attributed to globular cluster group (MK3) and not to the dwarf galaxy group (MK2). This highlights that there is no clear cut morphological distinction between extended star clusters and ultra faint dwarfs. These groups are highly consistent with the groups found in a previous classification for a smaller sample and completely different set of parameters.     

Neighbourhoods of isolated star forming dwarf galaxies

We report the discovery of small groups of uncatalogued, compact, star forming (SF) dwarf galaxies (DGs) by Hα mapping of the neighbourhoods of apparently isolated, catalogued, SF DGs. Our sample consists of dwarf  ( M ≥−18 mag)  galaxies at least 2 Mpc away from any other catalogued galaxy. The galaxies were selected to exhibit Hα emission of any intensity, i.e. not selecting only strong starbursts, as an indicator of recent or on-going star formation with the goal of understanding why are they presently forming stars. We identified possible neighbours by imaging the galaxies and their surroundings through Hα filters centred at or near the redshift of the galaxy, and searching for localized Hα emission with the characteristics of the line emission from the sample galaxies.
We identified 20 possible SF neighbour galaxies, 17 of them not previously catalogued, in three of the five search fields where we had good quality data, and present here their positions and, images and morphology, as well as some indications of binarity. The relatively large number of possible neighbour candidates, combined with their relative faintness, argue that it would be virtually impossible to identify truly isolated galaxies. It seems that the objects we selected as extremely isolated are probably the brightest members of sparse groups of galaxies, where the other members are also DGs that are presently forming stars. In order to enhance the confidence of this statement regular redshifts are required for our candidate neighbours.