arXiv subjects

J. B. Jones

Publications and source records attributed to J. B. Jones.

At least 19 recordsLinked to original sources

arXiv2008

Structural Properties of Ultra-Compact Dwarf Galaxies in the Fornax and Virgo Clusters

We present a detailed analysis of two-band HST/ACS imaging of 21 ultra-compact dwarf (UCD) galaxies in the Virgo and Fornax Clusters. The aim of this work is to test two formation hypotheses for UCDs--whether they are bright globular clusters (GCs) or "threshed'' early-type dwarf galaxies--by comparison of UCD structural parameters and colors with GCs and galaxy nuclei. We find that the UCD surface brightness profiles can be described by a range of models and that the luminous UCDs can not be described by standard King models with tidal cutoffs as they have extended outer halos. This is not expected from traditional King models of GCs, but is consistent with recent results for massive GCs. The total luminosities, colors and sizes of the UCDs are consistent with them being either luminous GCs or threshed nuclei of both early-type and late-type galaxies (not just early-type dwarfs). For the most luminous UCDs we estimate color gradients over a limited range of radius. These are systematically positive in the sense of getting redder outwards: mean Delta(F606W-F814W)=0.14 mag per 100 pc with rms=0.06 mag per 100 pc. The positive gradients found in the bright UCDs are consistent with them being either bright GCs or threshed early-type dwarf galaxies (except VUCD3). In contrast to the above results we find a very significant difference in the sizes of UCDs and early-type galaxy nuclei: the effective radii of UCDs are 2.2 times larger than those of early-type galaxy nuclei at the same luminosity. This result suggests an important test can be made of the threshing hypothesis by simulating the process and predicting what size increase is expected.

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arXiv2007

2MASS Galaxies in the Fornax Cluster Spectroscopic Survey

The Fornax Cluster Spectroscopic Survey (FCSS) is an all-object survey of a region around the Fornax Cluster of galaxies undertaken using the 2dF multi-object spectrograph on the Anglo-Australian Telescope. Its aim was to obtain spectra for a complete sample of all objects with 16.5 < b_j < 19.7 irrespective of their morphology (i.e. including `stars', `galaxies' and `merged' images). We explore the extent to which (nearby) cluster galaxies are present in 2MASS. We consider the reasons for the omission of 2MASS galaxies from the FCSS and vice versa. We consider the intersection (2.9 square degrees on the sky) of our data set with the infra-red 2 Micron All-Sky Survey (2MASS), using both the 2MASS Extended Source Catalogue (XSC) and the Point Source Catalogue (PSC). We match all the XSC objects to FCSS counterparts by position and also extract a sample of galaxies, selected by their FCSS redshifts, from the PSC. We confirm that all 114 XSC objects in the overlap sample are galaxies, on the basis of their FCSS velocities. A total of 23 Fornax Cluster galaxies appear in the matched data, while, as expected, the remainder of the sample lie at redshifts out to z = 0.2 (the spectra show that 61% are early type galaxies, 18% are intermediate types and 21% are strongly star forming).The PSC sample turns out to contain twice as many galaxies as does the XSC. However, only one of these 225 galaxies is a (dwarf) cluster member. On the other hand, galaxies which are unresolved in the 2MASS data (though almost all are resolved in the optical) amount to 71% of the non-cluster galaxies with 2MASS detections and have redshifts out to z=0.32.

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arXiv2007

Compact Stellar Systems around NGC 1399

We have obtained spectroscopic redshifts of colour-selected point sources in four wide area VLT-FLAMES fields around the Fornax Cluster giant elliptical galaxy NGC 1399, identifying as cluster members 30 previously unknown faint (-10.5<M_g'<-8.8) compact stellar systems (CSS), and improving redshift accuracy for 23 previously catalogued CSS. By amalgamating our results with CSS from previous 2dF observations and excluding CSS dynamically associated with prominent (non-dwarf) galaxies surrounding NGC 1399, we have isolated 80 `unbound' systems that are either part of NGC 1399's globular cluster (GC) system or intracluster GCs. For these unbound systems, we find (i) they are mostly located off the main stellar locus in colour-colour space; (ii) their projected distribution about NGC 1399 is anisotropic, following the Fornax Cluster galaxy distribution, and there is weak evidence for group rotation about NGC 1399; (iii) their completeness-adjusted radial surface density profile has a slope similar to that of NGC 1399's inner GC system; (iv) their mean heliocentric recessional velocity is between that of NGC 1399's inner GCs and that of the surrounding dwarf galaxies, but their velocity dispersion is significantly lower; (v) bright CSS (M_V<-11) are slightly redder than the fainter systems, suggesting they have higher metallicity; (vi) CSS show no significant trend in $g' - i'$ colour index with radial distance from NGC 1399.

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arXiv2007

Searches for Ultra-Compact Dwarf Galaxies in Galaxy Groups

We present the results of a search for ultra-compact dwarf galaxies (UCDs) in six different galaxy groups: Dorado, NGC1400, NGC0681, NGC4038, NGC4697 and NGC5084. We searched in the apparent magnitude range 17.5 < b_j < 20.5 (except NGC5084: 19.2 < b_j < 21.0). We found 1 definite plus 2 possible UCD candidates in the Dorado group and 2 possible UCD candidates in the NGC1400 group. No UCDs were found in the other groups. We compared these results with predicted luminosities of UCDs in the groups according to the hypothesis that UCDs are globular clusters formed in galaxies. The theoretical predictions broadly agree with the observational results, but deeper surveys are needed to fully test the predictions.

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arXiv2006

Kinematics, Substructure and Luminosity-weighted Dynamics of Six Nearby Galaxy Groups

We have redefined group membership of six southern galaxy groups in the local universe (mean cz < 2000 km/s) based on new redshift measurements from our recently acquired Anglo-Australian Telescope 2dF spectra. For each group, we investigate member galaxy kinematics, substructure, luminosity functions and luminosity-weighted dynamics. Our calculations confirm that the group sizes, virial masses and luminosities cover the range expected for galaxy groups, except that the luminosity of NGC 4038 is boosted by the central starburst merger pair. We find that a combination of kinematical, substructural and dynamical techniques can reliably distinguish loose, unvirialised groups from compact, dynamically relaxed groups. Applying these techniques, we find that Dorado, NGC 4038 and NGC 4697 are unvirialised, whereas NGC 681, NGC 1400 and NGC 5084 are dynamically relaxed.

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arXiv2006

Ultra-Compact Stellar Systems in the Fornax Galaxy Cluster

Our VLT (FLAMES) observations near NGC1399 investigate the connection between ultra-compact dwarf galaxies (UCDs), NGC1399 globular clusters and intra-cluster globulars. We have uncovered 30 faint compact stellar systems in the Fornax galaxy cluster, adding to 62 bright UCDs previously reported. The magnitude limit of these stellar systems extends down to the globular cluster domain. We detect a filament of UCDs and globular clusters stretching across NGC1399 and find weak evidence for its rotation. These compact stellar systems not only congregate around several cluster galaxies but are also widely distributed through intra-cluster space.

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arXiv2005

Discovery of Ultra-Compact Dwarf Galaxies in the Virgo Cluster

We have discovered nine ultra-compact dwarf galaxies (UCDs) in the Virgo Cluster, extending samples of these objects outside the Fornax Cluster. Using the 2dF multi-fiber spectrograph on the Anglo-Australian Telescope, the new Virgo members were found among 1500 color-selected, star-like targets with 16.0 < b_j < 20.2 in a two-degree diameter field centered on M87 (NGC4486). The newly-found UCDs are comparable to the UCDs in the Fornax Cluster, with sizes <~ 100 pc, -12.9 < M_B < -10.7, and exhibiting red, absorption-line spectra, indicative of an older stellar population. The properties of these objects remain consistent with the tidal threshing model for the origin of UCDs from the surviving nuclei of nucleated dwarf ellipticals disrupted in the cluster core, but can also be explained as objects that were formed by mergers of star clusters created in galaxy interactions. The discovery that UCDs exist in Virgo shows that this galaxy type is probably a ubiquitous phenomenon in clusters of galaxies; coupled with their possible origin by tidal threshing, the UCD population is a potential indicator and probe of the formation history of a given cluster. We also describe one additional bright UCD with M_B = -12.0 in the core of the Fornax Cluster. We find no further UCDs in our Fornax Cluster Spectroscopic Survey down to b_j = 19.5 in two additional 2dF fields extending as far as 3 degrees from the center of the cluster. All six Fornax bright UCDs identified with 2dF lie within 0.5 degree (projected distance of 170 kpc) of the central elliptical galaxy NGC1399.

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arXiv2005

A Comparison of surface brightness profiles for ultra-compact dwarfs and dwarf elliptical nuclei: implications for the `threshing' scenario

Using imaging from the Hubble Space Telescope, we derive surface brightness profiles for ultra-compact dwarfs in the Fornax cluster and for the nuclei of dwarf elliptical galaxies in the Virgo cluster. Ultra-compact dwarfs are more extended and have higher surface brightnesses than typical dwarf nuclei, while the luminosities, colors, and sizes of the nuclei are closer to those of Galactic globular clusters. This calls into question the production of ultra-compact dwarfs via ``threshing'' whereby the lower surface brightness envelope of a dwarf elliptical is removed by tidal processes, leaving behind a bare nucleus. Threshing may still be a viable model if the relatively bright Fornax ultra compact dwarfs considered here are descended from dwarf ellipticals whose nuclei are at the upper end of their luminosity and size distributions.

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arXiv2004

A class of compact dwarf galaxies from disruptive processes in galaxy clusters

Dwarf galaxies have attracted increased attention in recent years, because of their susceptibility to galaxy transformation processes within rich galaxy clusters. Direct evidence for these processes, however, has been difficult to obtain, with a small number of diffuse light trails and intra-cluster stars being the only signs of galaxy disruption. Furthermore, our current knowledge of dwarf galaxy populations may be very incomplete, because traditional galaxy surveys are insensitive to extremely diffuse or compact galaxies. Aware of these concerns, we recently undertook an all-object survey of the Fornax galaxy cluster. This revealed a new population of compact members, overlooked in previous conventional surveys. Here we demonstrate that these 'ultra-compact' dwarf galaxies are structurally and dynamically distinct from both globular star clusters and known types of dwarf galaxy, and thus represent a new class of dwarf galaxy. Our data are consistent with the interpretation that these are the remnant nuclei of disrupted dwarf galaxies, making them an easily observed tracer of galaxy disruption.

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arXiv2002

Surface brightness measurements for APM galaxies

This paper considers some simple surface brightness (SB) estimates for galaxies in the Automated Plate Measuring Machine (APM) catalogue in order to derive homogeneous SB data for a very large sample of faint galaxies. The isophotal magnitude and area are used to estimate the central surface brightness and total magnitude based on the assumption of an exponential SB profile. The surface brightness measurements are corrected for field effects on each UK Schmidt plate and the zero-point of each plate is adjusted to give a uniform sample of SB and total magnitude estimates over the whole survey. Results are obtained for 2.4 million galaxies with blue photographic magnitudes brighter than b_J = 20.5 covering 4300 deg^2 in the region of the south galactic cap. Almost all galaxies in our sample have central surface brightness in the range 20 to 24 b_J mag per arcsec^2. The SB measurements we obtain are compared to previous SB measurements and we find an acceptable level of error of +/- 0.2 b_J mag per arcsec^2. The distribution of SB profiles is considered for different galaxy morphologies for the bright APM galaxies. We find that early-type galaxies have more centrally concentrated profiles.

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arXiv2002

The Fornax Cluster Spectroscopic Survey: A Sample of Confirmed Cluster Dwarfs

The Fornax Cluster Spectroscopic Survey (FCSS) project utilises the 2dF multi-object spectrograph on the AAT. Its aim is to obtain spectra for a complete sample of all 14000 objects with 16.5 3 arcsec (abut 300 pc). This apparent minimum scale size implies an equivalent minimum luminosity for a dwarf of a given surface brightness. This produces a limit on their distribution in the magnitude-surface brightness plane, such that we do not observe dEs with high surface brightnesses but faint absolute magnitudes. Above this observed minimum scale-size of 3 arcsec, the dEs and dS0s fill the whole area of the magnitude-surface brightness plane sampled by our selection limits. The observed correlation between magnitude and surface brightness noted by several recent studies of brighter galaxies is not seen with our fainter cluster sample. A comparison of our results with the Fornax Cluster Catalog (FCC) of Ferguson illustrates that attempts to determine cluster membership solely on the basis of observed morphology can produce significant errors.

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arXiv2001

Ultra-Compact Dwarf Galaxies in the Fornax Cluster

By utilising the large multi-plexing advantage of the 2dF spectrograph on the Anglo-Australian Telescope, we have been able to obtain a complete spectroscopic sample of all objects in a predefined magnitude range, 16.5<Bj<19.7, regardless of morphology, in an area towards the centre of the Fornax Cluster of galaxies. Among the unresolved or marginally resolved targets we have found five objects which are actually at the redshift of the Fornax Cluster, i.e. they are extremely compact dwarf galaxies or extremely large star clusters. All five have absorption line spectra. With intrinsic sizes less than 1.1 arc second HWHM (corresponding to approximately 100 pc at the distance of the cluster), they are more compact and significantly less luminous than other known compact dwarf galaxies, yet much brighter than any globular cluster. In this letter we present new ground based optical observations of these enigmatic objects. In addition to having extremely high central surface brightnesses, these objects show no evidence of any surrounding low surface brightness envelopes down to much fainter limits than is the case for, e.g., nucleated dwarf ellipticals. Thus, if they are not merely the stripped remains of some other type of galaxy, then they appear to have properties unlike any previously known type of stellar system.

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arXiv2001

Public Release of 2dF data from the Fornax Cluster Spectroscopic Survey

Thanks to the 2dF spectrograph on the Anglo-Australian Telescope, we have recently completed the first stage of a complete spectroscopic survey more than one order of magnitude larger than any previous study, measuring 7000 spectra in a 6 sq.deg. area as part of our study of the Fornax Cluster. In this article we describe the public release of 3600 spectra from our first field. We hope that this public release will encourage colleagues making surveys for rare objects to choose these fields, as much of the follow-up spectroscopy that might be required is available from our data.

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arXiv2001

Ultra-compact dwarf galaxies: a new class of compact stellar system discovered in the Fornax Cluster

We have used the 2dF spectrograph on the Anglo-Australian Telescope to obtain a complete spectroscopic sample of all objects in the magnitude range, Bj= 16.5 to 19.8, regardless of morphology, in an area centred on the Fornax Cluster of galaxies. Among the unresolved targets are five objects which are members of the Fornax Cluster. They are extremely compact stellar systems with scale lengths less than 40 parsecs. These ultra-compact dwarfs are unlike any known type of stellar system, being more compact and significantly less luminous than other compact dwarf galaxies, yet much brighter than any globular cluster.

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arXiv2000

Hidden Galaxies in the Fornax Cluster

We are using the Multibeam 21cm receiver on the Parkes Telescope combined with the optical Two degree Field spectrograph (2dF) of the Anglo-Australian Telescope to obtain the first complete spectroscopic sample of the Fornax cluster. In the optical the survey is unique in that all objects (both ``stars'' and ``galaxies'') within our magnitude limits (Bj=16.5 to 19.7) are measured, producing the most complete survey of cluster members irrespective of surface brightness. We have detected two new classes of high surface brightness dwarf galaxy in the cluster. With 2dF we have discovered a population of very low luminosity (Mb approx -12) objects which are unresolved from the ground and may be the stripped nuclei of dwarf galaxies; they are unlike any known galaxies. In a survey of the brighter (Bj=16.5 to 18) galaxies with the FLAIR-II spectrograph we have found a number of new high surface brightness dwarf galaxies and show that the fraction of star-forming dwarf galaxies in the cluster is about 30 per cent, about twice that implied by earlier morphological classifications. Our radio observations have greatly improved upon the sensitivity of the standard Multibeam survey by using a new ``basket weave'' scanning pattern. Our initial analysis shows that we are detecting new cluster members with HI masses of order 10-to-the-8 Msun and HI mass-to-light ratios of 1-2 Msun/Lsun.

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arXiv2000

Compact Stellar Systems in the Fornax Cluster: Super-massive Star Clusters or Extremely Compact Dwarf Galaxies?

We describe a population of compact objects in the centre of the Fornax Cluster which were discovered as part of our 2dF Fornax Spectroscopic Survey. These objects have spectra typical of old stellar systems, but are unresolved on photographic sky survey plates. They have absolute magnitudes -13<Mb<-11, so they are 10 times more luminous than any Galactic globular clusters, but fainter than any known compact dwarf galaxies. These objects are all within 30 arcminutes of the central galaxy of the cluster, NGC 1399, but are distributed over larger radii than the globular cluster system of that galaxy. We suggest that these objects are either super-massive star clusters (intra-cluster globular clusters or tidally stripped nuclei of dwarf galaxies) or a new type of low-luminosity compact elliptical dwarf (M32-type) galaxy. The best way to test these hypotheses will be to obtain high resolution imaging and high-dispersion spectroscopy to determine their structures and mass-to-light ratios. This will allow us to compare them to known compact objects and establish if they represent a new class of hitherto unknown stellar system.

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arXiv2000

The Fornax Spectroscopic Survey I. Survey Strategy and Preliminary Results on the Redshift Distribution of a Complete Sample of Stars and Galaxies

The Fornax Spectroscopic Survey will use the Two degree Field spectrograph (2dF) of the Anglo-Australian Telescope to obtain spectra for a complete sample of all 14000 objects with 16.5<=Bj<=19.7 in a 12 square degree area centred on the Fornax Cluster. By selecting all objects---both stars and galaxies---independent of morphology, we cover a much larger range of surface brightness and scale size than previous surveys. In this paper we present results from the first 2dF field. Redshift distributions and velocity structures are shown for all observed objects in the direction of Fornax, including Galactic stars, galaxies in and around the Fornax Cluster, and for the background galaxy population. The velocity data for the stars show the contributions from the different Galactic components, plus a small tail to high velocities. We find no galaxies in the foreground to the cluster in our 2dF field. The Fornax Cluster is clearly defined kinematically. The mean velocity from the 26 cluster members having reliable redshifts is 1560+/-80 km/s. They show a velocity dispersion of 380+/-50 km/s. Large-scale structure can be traced behind the cluster to a redshift beyond z=0.3. Background compact galaxies and low surface brightness galaxies are found to follow the general galaxy distribution.

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arXiv1998

Low Surface Brightness Dwarf Galaxies in the Bristol - Anglo-Australian Observatory Virgo Cluster Survey

We describe a new, deep photographic survey of the Virgo Cluster which uses multiple exposures on Tech Pan film with the United Kingdom Schmidt Telescope to probe the dwarf population to fainter surface brightness limits than previous surveys. We have identified galaxies having sizes (>= 3 arcsec scale length) and surface brightnesses (<= 24.5 R mag arcsec^-2) characteristic of those expected for dwarf spheroidal galaxies in the cluster. The survey is providing substantial samples of extremely low luminosity galaxies outside the environment of the Local Group and nearby groups for the first time. An initial study of two small areas has found dwarf spheroidal candidates in large numbers (500 deg^-2) which indicate a steep, continuously rising luminosity function at low luminosities.

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