263 research outputs found
The dark matter halos of the bluest low surface brightness galaxies
We present BVI photometry and long-slit Halpha rotation curve data obtained
with ESO VLT/FORS2 for six low surface brightness galaxies with extremely blue
colours and very faint central regions. We find no evidence for a steep central
density cusp of the type predicted by many N-body simulations of cold dark
matter (CDM) halos. Our observations are instead consistent with dark matter
halos characterized by cores of roughly constant density, in agreement with
previous investigations. While unremarkable in terms of the central density
slope, these galaxies appear very challenging for existing CDM halo models in
terms of average central halo density, as measured by the Delta_(V/2)
parameter. Since most of our target galaxies are bulgeless disks, our
observations also disfavour a recently suggested mechanism for lowering the
central mass concentration of the halo by means of a fast collapse phase, as
this scenario predicts that the original CDM profile should still be detectable
in bulgeless galaxies. Other potential ways of reconciling the CDM predictions
with these observations are discussed.Comment: 14 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
Graphene Nanogap for Gate Tunable Quantum Coherent Single Molecule Electronics
We present atomistic calculations of quantum coherent electron transport
through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We
predict that three difficult problems in molecular electronics with single
molecules may be solved by utilizing graphene contacts: (1) a back gate
modulating the Fermi level in the graphene leads facilitate control of the
device conductance in a transistor effect with high on/off current ratio; (2)
the size mismatch between leads and molecule is avoided, in contrast to the
traditional metal contacts; (3) as a consequence, distinct features in charge
flow patterns throughout the device are directly detectable by scanning
techniques. We show that moderate graphene edge disorder is unimportant for the
transistor function.Comment: 8 pages, 6 figure
Feedback in the local LBG Analog Haro 11 as probed by far-UV and X-ray observations
We have re-analyzed FUSE data and obtained new Chandra observations of Haro
11, a local (D_L=88 Mpc) UV luminous galaxy. Haro 11 has a similar far-UV
luminosity (10^10.3 L_\odot), UV surface brightness (10^9.4 L_\odot kpc^-2),
SFR, and metallicity to that observed in Lyman Break Galaxies (LBGs). We show
that Haro 11 has extended, soft thermal (kT~0.68 keV) X-ray emission with a
luminosity and size which scales with the physical properties (e.g. SFR,
stellar mass) of the host galaxy. An enhanced alpha/Fe, ratio of ~4 relative to
solar abundance suggests significant supernovae enrichment. These results are
consistent with the X-ray emission being produced in a shock between a
supernovae driven outflow and the ambient material. The FUV spectra show strong
absorption lines similar to those observed in LBG spectra. A blueshifted
absorption component is identified as a wind outflowing at ~200-280 km/s.
OVI\lambda\lambda1032,1038 emission, the dominant cooling mechanism for coronal
gas at T~10^5.5 K is also observed. If associated with the outflow, the
luminosity of the OVI emission suggests that <20% of the total mechanical
energy from the supernovae and solar winds is being radiated away. This implies
that radiative cooling through OVI is not significantly inhibiting the growth
of the outflowing gas. In contradiction to the findings of Bergvall et al 2006,
we find no convincing evidence of Lyman continuum leakage in Haro 11. We
conclude that the wind has not created a `tunnel' allowing the escape of a
significant fraction of Lyman continuum photons and place a limit on the escape
fraction of f_{esc}<2%. Overall, both Haro 11 and a previously observed LBG
analogue VV 114, provide an invaluable insight into the X-ray and FUV
properties of high redshift LBGs.Comment: Accepted for publication in ApJ, 40 pages, 17 figure
The Red Halo Phenomenon
Optical and near-IR observations of the halos of disk galaxies and blue
compact galaxies have revealed a very red spectral energy distribution, which
cannot easily be reconciled with a normal, metal-poor stellar population like
that in the stellar halo of the Milky Way. Here, spectral evolutionary models
are used to explore the consequences of these observations. We demonstrate that
a stellar population of low to intermediate metallicity, but with an extremely
bottom-heavy initial mass function, can explain the red halos around both types
of objects. Other previously suggested explanations, like nebular emission or
very metal-rich stars, are shown to fail in this respect. This indicates that,
if the reported halo colours are correct, halo populations dominated by
low-mass stars may be a phenomenon common to galaxies of very different Hubble
types. Potential tests of this hypothesis are discussed, along with its
implications for the baryonic dark matter content of galaxies.Comment: 6 pages, 3 figures, Accepted for publication in Ap
Deep multiband surface photometry on star forming galaxies: II. A volume limited sample of 21 emission lines galaxies
We present deep surface photometry of a volume--limited sample of 21 UM
emission line galaxies in broadband optical UBVRI and near infra-red (NIR) HKs
filters. The sample comprises 19 blue compact galaxies (BCGs) and two spirals.
For some targets the exposure times are the deepest to date. For the BCG UM462
we observe a previously undetected second disk component beyond a surface
brightness level of mu_B=26 mag arcsec^{-2}. This is a true low surface
brightness component with central surface brightness mu_0=24.1 mag arcsec^{-2}
and scale length h_r=1.5 kpc. All BCGs are dwarfs, with M_B>=-18, and very
compact, with an average scale length of h_r~1 kpc. We separate the burst and
host populations for each galaxy and compare them to stellar evolutionary
models with and without nebular emission contribution. We also measure the
A_{180} asymmetry in all filters and detect a shift from optical to NIR in the
average asymmetry of the sample. This shift seems to be correlated with the
morphological class of the BCGs. Using the color-asymmetry relation, we
identify five BCGs in the sample as mergers, which is confirmed by their
morphological class. Though clearly separated from normal galaxies in the
concentration-asymmetry parameter space, we find that it is not possible to
distinguish luminous starbursting BCGs from the merely star forming low
luminosity BCGs.Comment: 48 pages, 39 figures, submitte
Penetrating the Deep Cover of Compton Thick Active Galactic Nuclei
We analyze observations obtained with the Chandra X-ray Observatory of bright
Compton thick active galactic nuclei (AGNs), those with column densities in
excess of 1.5 x 10^{24} cm^{-2} along the lines of sight. We therefore view the
powerful central engines only indirectly, even at X-ray energies. Using high
spatial resolution and considering only galaxies that do not contain
circumnuclear starbursts, we reveal the variety of emission AGNs alone may
produce. Approximately 1% of the continuum's intrinsic flux is detected in
reflection in each case. The only hard X-ray feature is the prominent Fe K
alpha fluorescence line, with equivalent width greater than 1 keV in all
sources. The Fe line luminosity provides the best X-ray indicator of the unseen
intrinsic AGN luminosity. In detail, the morphologies of the extended soft
X-ray emission and optical line emission are similar, and line emission
dominates the soft X-ray spectra. Thus, we attribute the soft X-ray emission to
material that the central engines photoionize. Because the resulting spectra
are complex and do not reveal the AGNs directly, crude analysis techniques such
as hardness ratios would mis-classify these galaxies as hosts of intrinsically
weak, unabsorbed AGNs and would fail to identify the luminous, absorbed nuclei
that are present. We demonstrate that a three-band X-ray diagnostic can
correctly classify Compton thick AGNs, even when significant soft X-ray line
emission is present. The active nuclei produce most of the galaxies' total
observed emission over a broad spectral range, and much of their light emerges
at far-infrared wavelengths. Stellar contamination of the infrared emission can
be severe, however, making long-wavelength data alone unreliable indicators of
the buried AGN luminosity.Comment: To appear in ApJ, September 1, 200
A search for Low Surface Brightness galaxies in the near-infrared I. Selection of the sample
A sample of about 3,800 Low Surface Brightness (LSB) galaxies was selected
using the all-sky near-infrared (J, H and K_s-band) 2MASS survey. The selected
objects have a mean central surface brightness within a 5 arcsec radius around
their centre fainter than 18 mag/sq.arcsec in the K_s band, making them the
lowest surface brightness galaxies detected by 2MASS. A description is given of
the relevant properties of the 2MASS survey and the LSB galaxy selection
procedure, as well as of basic photometric properties of the selected objects.
The latter properties are compared to those of other samples of galaxies, of
both LSBs and `classical' high surface brightness (HSB) objects, which were
selected in the optical. The 2MASS LSBs have a (B_T_c)-(K_T) colour which is on
average 0.9 mag bluer than that of HSBs from the NGC. The 2MASS sample does not
appear to contain a significant population of red objects.Comment: accepted for publication in Astronomy and Astrophysics on 24/2/2003;
62 page
The merging/AGN connection II. Ionization of the circumnuclear regions
We report the first results of a study of a sample of 20 galaxy
mergers/interacting systems, using the VIMOS and PMAS integral field
spectrographs. For each object, we extracted the integrated spectrum of the
central regions and analyzed the ionization state using classical diagnostic
diagrams (Veilleux & Osterbrock 1987). There is evidence of AGN ionization in 4
of the objects, i.e. 20% of the sample, a considerably higher fraction than
found in previous studies ~4%Comment: 4 pages, 2 figures, accepted for publishing in A&A Letter
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