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Register a new userHidden Galaxies in the Fornax Cluster
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Authors
M.J. Drinkwater
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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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The Advanced Camera for Surveys (ACS) Fornax Cluster Survey is a Hubble Space Telescope program to image 43 early-type galaxies in the Fornax cluster, using the F475W and F850LP bandpasses of the ACS. We employ both 1D and 2D techniques to characterize the properties of the stellar nuclei in these galaxies, defined as the central luminosity excesses relative to a Sersic model fitted to the underlying host. We find 72+/-13% of our sample (31 galaxies) to be nucleated, with only three of the nuclei offset by more than 0.5 from their galaxy photocenter, and with the majority of nuclei having colors bluer than their hosts. The nuclei are observed to be larger, and brighter, than typical Fornax globular clusters, and to follow different structural scaling relations. A comparison of our results to those from the ACS Virgo Cluster Survey reveals striking similarities in the properties of the nuclei belonging to these different environments. We briefly review a variety of proposed formation models and conclude that, for the low-mass galaxies in our sample, the most important mechanism for nucleus growth is probably infall of star clusters through dynamical friction, while for higher mass galaxies, gas accretion triggered by mergers, accretions and tidal torques is likely to dominate, with the relative importance of these two processes varying smoothly as a function of galaxy mass. Some intermediate-mass galaxies in our sample show a complexity in their inner structure that may be the signature of hybrid nuclei that arose through parallel formation channels.
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.
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.
Using the photometric data from the Next Generation Fornax Survey, we find a significant radial alignment signal among the Fornax dwarf galaxies. For the first time, we report that the radial alignment signal of nucleated dwarfs is stronger than that of non-nucleated ones at 2.4$sigma$ confidence level, and the dwarfs located in the outer region ($R>R_{rm{vir}}/3$; $R_{rm{vir}}$ is the Fornax virial radius) show slightly stronger radial alignment signal than those in the inner region ($R<R_{rm{vir}}/3$) at $1.5sigma$ level. We also find that the significance of radial alignment signal is independent of the luminosities or sizes of the dwarfs.
Based on FORS2-VLT long-slit spectroscopy, the analysis of the central absorption line indices of 9 S0 galaxies in the Fornax Cluster is presented. Central indices correlate with central velocity dispersions as observed in ellipticals. However, the stellar population properties of these S0s indicates that the observed trends are produced by relative differences in age and alpha-element abundances and not in metallicity ([Fe/H]) as previous studies have found in elliptical galaxies. The observed scatter in the line indices vs. velocity dispersion relations can be partially explained by the rotationally-supported nature of many of these systems. The presence of tighter line indices vs. maximum (circular) rotational velocity relations confirms this statement. It was also confirmed that the dynamical mass is the driving physical property of all these correlations and in our Fornax S0s it has to be estimated assuming rotational support.
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