454 research outputs found
Completely dark galaxies: their existence, properties, and strategies for finding them
There are a number of theoretical and observational hints that large numbers
of low-mass galaxies composed entirely of dark matter exist in the field. The
theoretical considerations follow from the prediction of cold dark matter
theory that there exist many low-mass galaxies for every massive one. The
observational considerations follow from the observed paucity of these low-mass
galaxies in the field but not in dense clusters of galaxies; this suggests that
the lack of small galaxies in the field is due to the inhibition of star
formation in the galaxies as opposed to the fact that their small dark matter
halos do not exist. In this work we outline the likely properties of low-mass
dark galaxies, and describe observational strategies for finding them, and
where in the sky to search. The results are presented as a function of the
global properties of dark matter, in particular the presence or absence of a
substantial baryonic dark matter component. If the dark matter is purely cold
and has a Navarro, Frenk and White density profile, directly detecting dark
galaxies will only be feasible with present technology if the galaxy has a
maximum velocity dispersion in excess of 70 km/s, in which case the dark
galaxies could strongly lens background objects. This is much higher than the
maximum velocity dispersions in most dwarf galaxies. If the dark matter in
galaxy halos has a baryonic component close to the cosmic ratio, the
possibility of directly detecting dark galaxies is much more realistic; the
optimal method of detection will depend on the nature of the dark matter. A
number of more indirect methods are also discussed.Comment: 12 pages, 4 figures, MNRAS in pres
The Luminosity Function of Nearby Galaxy Clusters II: Redshifts and Luminosity Function for Galaxies in the Region of the Centaurus Cluster
We acquired spectra for a random sample of galaxies within a 0.83 square
degree region centered on the core of the Centaurus cluster. Radial velocities
were obtained for 225 galaxies to limiting magnitudes of V < 19.5. Of the
galaxies for which velocities were obtained, we find 35% to be member galaxies.
Of the 78 member galaxies, magnitudes range from 11.8 < V < 18.5 (-21.6 < M_{V}
< -14.9 for H_o = 70 km s^-1 Mpc^-1) with a limiting central surface brightness
of \mu_o < 22.5 mag arcsec^-2. We constructed the cluster galaxy luminosity
function by using these spectroscopic results to calculate the expected
fraction of cluster members in each magnitude bin. The faint-end slope of the
luminosity function using this method is shallower than the one obtained using
a statistical method to correct for background galaxy contamination. We also
use the spectroscopy results to define surface brightness criteria to establish
membership for the full sample. Using these criteria, we find a luminosity
function very similar to the one constructed with the statistical background
correction. For both, we find a faint-end slope alpha ~ -1.4. Adjusting the
surface brightness membership criteria we find that the data are consistent
with a faint-end slope as shallow as -1.22 or as steep as -1.50. We describe in
this paper some of the limitations of using these methods for constructing the
galaxy luminosity function.Comment: 16 pages, 12 figures, accepted by A
Weak lensing observations of the "dark" cluster MG2016+112
We investigate the possible existence of a high-redshift (z=1) cluster of
galaxies associated with the QSO lens system MG2016+112. From an ultra-deep R-
and less deep V- and I-band Keck images and a K-band mosaic from UKIRT, we
detect ten galaxies with colors consistent with the lensing galaxy within
225h^{-1} kpc of the z=1.01 lensing galaxy. This represents an overdensity of
more than ten times the number density of galaxies with similar colors in the
rest of the image. We also find a group of seven much fainter objects closely
packed in a group only 27h^{-1} kpc north-west of the lensing galaxy. We
perform a weak lensing analysis on faint galaxies in the R-band image and
detect a mass peak of a size similar to the mass inferred from X-ray
observations of the field, but located 64" northwest of the lensing galaxy.
From the weak lensing data we rule out a similar sized mass peak centered on
the lensing galaxy at the 2 sigma level.Comment: 9 pages, 10 figures, submitted to A&A version with figure 4 at higher
resolution can be downloaded from
http://www.mpa-garching.mpg.de/~clowe/mg2016aa.ps.g
HST ultraviolet spectral energy distributions for three ultraluminous infrared galaxies
We present HST Faint Object Camera ultraviolet (230 nm and 140 nm) images of
three ultraluminous infrared galaxies (ULIG: L_ir > 10^12 L_sun) selected from
the IRAS Revised Bright Galaxy Sample. The purpose is to estimate spectral
energy distributions (SEDs) to facilitate the identification of similar objects
at high redshift in deep optical, infrared, and submm surveys.
All three galaxies (VII Zw031 = IRAS F12112+0305, and IRAS F22491-1808) were
well detected at 230 nm. Two of the three were marginally detected at 140 nm.
The fluxes, together with ground-based optical and infrared photometry, are
used to compute SEDs over a wide wavelength range. The measured SEDs drop from
the optical to the ultraviolet, but the magnitude of the drop ranges from a
factor of ~3 in IRAS F22491-1808 to a factor of ~100 in VIIZw031. This is most
likely due to different internal extinctions. Such an interpretation is also
suggested by extrapolating to ultraviolet wavelengths the optical internal
extinction measured in VIIZw031. K-corrections are calculated to determine the
colors of the sample galaxies as seen at high redshifts. Galaxies like VIIZw031
have very low observed rest-frame UV fluxes which means that such galaxies at
high redshift will be extremely red or even missing in optical surveys. On the
other hand, galaxies like IRAS F12112+0305 and IRAS F22491-1808, if seen at
high redshift, would be sufficiently blue that they would not easily be
distinguished from normal field galaxies, and therefore, identified as ULIGs.
The implication is then that submillimeter surveys may be the only means of
properly identifying the majority of ULIGs at high redshift.Comment: AJ in press, TeX, 23 pages, 7 tab, 17 figs available also (at higher
resolution) from http://www.ast.cam.ac.uk~trentham/ufigs.htm
An upper mass limit for the progenitor of the TypeII-P supernova SN1999gi
Masses and progenitor evolutionary states of TypeII supernovae remain almost
unconstrained by direct observations. Only one robust observation of a
progenitor (SN1987A) and one plausible observation (SN1993J) are available.
Neither matched theoretical predictions and in this Letter we report limits on
a third progenitor (SN1999gi). The Hubble Space Telescope has imaged the site
of the TypeII-P supernova SN1999gi with the WFPC2 in two filters (F606W and
F300W) prior to explosion. The distance to the host galaxy (NGC3184) of 7.9Mpc
means that the most luminous, massive stars are resolved as single objects in
the archive images. The supernova occurred in a resolved, young OB association
2.3kpc from the centre of NGC3184 with an association age of about 4Myrs.
Follow-up images of SN1999gi with WFPC2 taken 14 months after discovery
determine the precise position of the SN on the pre-explosion frames. An upper
limit of the absolute magnitude of the progenitor is estimated (M_v >= -5.1).
By comparison with stellar evolutionary tracks this can be interpreted as a
stellar mass, and we determine an upper mass limit of 9(+3/-2)M_solar. We
discuss the possibility of determining the masses or mass limits for numerous
nearby core-collapse supernovae using the HST archive enhanced by our current
SNAP programme.Comment: To appear in ApJ Letters, 16 pages, 3 figure
The importance of radio sources in accounting for the highest mass black holes
The most massive black holes lie in the most massive elliptical galaxies, and
at low-z all radio-loud AGNs lie in giant ellipticals. This strongly suggests a
link between radio-loudness and black hole mass. We argue that the increase in
the radio-loud fraction with AGN luminosity in optically-selected quasar
samples is consistent with this picture. We also use the ratio of black holes
today to quasars at z~2 to conclude that the most bolometrically-luminous AGN,
either radio-loud or radio quiet, are constrained to have lifetimes <~10^8 yr.
If radio sources are associated with black holes of >~10^9 M_sun at all
redshifts, then the same lifetime constraint applies to all radio sources with
luminosities above L_5GHz ~ 10^24 W/Hz/sr.Comment: 6 pages, 2 figures. To appear in "Lifecycles of Radio Galaxies", ed
J. Biretta et al., New Astronomy Review
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