12,000 research outputs found
Light on Dark Matter
Galaxies are lighthouses that sit atop peaks in the density field. There is
good observational evidence that these lighthouses do not provide a uniform
description of the distribution of dark matter.Comment: 4 pages, 5 figures. Paper presented at workshop `Structure and
Dynamics in the Local Universe' 24-26 November, 2003, Sydney, Australi
The Local Void is Really Empty
Are voids in the distribution of galaxies only places with reduced matter
density and low star formation efficiency or are they empty of matter? There is
now compelling evidence of expansion away from the Local Void at very high
velocities. The motion is most reasonably interpreted as an evacuation of the
void, which requires that the void be very large and very empty.Comment: Proceedings IAU Symp. 244: Dark Galaxies and Lost Baryons. Cardiff,
25-29 June, 2007. 6 pages, 5 figure
The faint end of the galaxy luminosity function
We present and discuss optical measurements of the faint end of the galaxy
luminosity function down to M_R = -10 in five different local environments of
varying galaxy density and morphological content. The environments we studied,
in order of decreasing galaxy density, are the Virgo Cluster, the NGC 1407
Group, the Coma I Group, the Leo Group and the NGC 1023 Group. Our results come
from a deep wide-angle survey with the NAOJ Subaru 8 m Telescope on Mauna Kea
and are sensitive down to very faint surface-brightness levels. Galaxies were
identified as group or cluster members on the basis of their surface brightness
and morphology. The faintest galaxies in our sample have R ~ 22.5. There were
thousands of fainter galaxies but we cannot distinguish cluster members from
background galaxies at these faint limits so do not attempt to determine a
luminosity function fainter than M_R = -10.
In all cases, there are far fewer dwarfs than the numbers of low mass halos
anticipated by cold dark matter theory. The mean logarithmic slope of the
luminosity function between M_R = -18 and M_R = -10 is alpha ~ -1.2, far
shallower than the cold dark matter mass function slope of alpha ~ -1.8. We
would therefore need to be missing about 90 per cent of the dwarfs at the faint
end of our sample in all the environments we study to achieve consistency with
CDM theory.Comment: 23 pages, 26 figs, MNRAS in pres
Environmental Dependencies in the Luminosity Function of Galaxies
The evidence is becoming strong that the luminosity function of galaxies
varies with environment. Higher density, more dynamically evolved regions
appear to have more dwarfs per giant. The situation is becoming clearer as a
result of wide field imaging surveys with the Canada-France-Hawaii and Subaru
telescopes and spectroscopy of faint dwarfs with the Keck telescope. We report
here on extensive observations of the small but dense NGC 5846 Group. The faint
end of the luminosity function rises relatively steeply in this case.Comment: Proceedings IAU Colloq. 198 "Near-Field Cosmology with Dwarf
Elliptical Galaxies" 6 pages, 4 figure
The Luminosity Function in Groups of Galaxis
With targeted imaging of groups in the local volume, the regions of collapse
around bright galaxies can be clearly identified by the distribution of dwarfs
and luminosity functions can be established to very faint levels. In the case
of the M81 Group there is completion to M_R ~ -9. In all well studied cases,
the faint end slopes are in the range -1.35 < alpha < -1.2, much flatter than
the slope for the bottom end of the halo mass spectrum anticipated by LambdaCDM
hierarchical clustering theory. Small but significant variations are found with
environment. Interestingly, the populations of dwarf galaxies are roughly
constant per unit halo mass. With the numbers of dwarfs as an anchor point,
evolved environments (dominated by early morphological types) have relatively
fewer intermediate luminosity systems and at least one relatively more
important galaxy at the core. The variations with environment are consistent
with a scenario of galaxy merging. However it is questionable if the universal
dearth of visible dwarf systems is a consequence of an astrophysical process
like reionization.Comment: 4 pages, 4 figures. `A Universe of Dwarf Galaxies', 14-18 June, Lyon,
Franc
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