40 research outputs found
Improved photometry of SDSS crowded field images: Structure and dark matter content in the dwarf spheroidal galaxy Leo I
We explore how well crowded field point-source photometry can be accomplished
with SDSS data: We present a photometric pipeline based on DoPhot, and tuned
for analyzing crowded-field images from the SDSS. Using Monte Carlo simulations
we show that the completeness of source extraction is above 80% to i < 21 (AB)
and a stellar surface density of about 200 sq.amin. Hence, a specialized data
pipeline can efficiently be used for e.g. nearby resolved galaxies in SDSS
images, where the standard SDSS photometric package Photo, when applied in
normal survey mode, gives poor results. We apply our pipeline to an area of
about 3.55sq.deg. around the dwarf spheroidal galaxy (dSph) Leo I, and
construct a high S/N star-count map of Leo I via an optimized filter in
color-magnitude space (g,r,i). Although the radial surface-density profile of
the dwarf deviates from the best fit empirical King model towards outer radii,
we find no evidence for tidal debris out to a stellar surface-density of
4*10^(-3) of the central value. We determine the total luminosity of Leo I, and
model its mass using the spherical and isotropic Jeans equation. Assuming that
'mass follows light' we constrain a lower limit of the total mass of the dSph
to be (1.7+/-0.2)*10^7 Msol. Contrary, if the mass in Leo I is dominated by a
constant density dark-matter (DM) halo, then the mass within the central 12' is
(2+/-0.6)*10^8 Msol. This leads to a mass-to-light ratio of >>6 (Ic_sol), and
possibly >75 if the DM halo dominates the mass and extends further out than
12'. In summary, our results show that Leo I is a symmetric, relaxed and bound
system; this supports the idea that Leo I is a dark-matter dominated system.Comment: 13 pages, 11 figures; accepted for publication in A
Dynamical modelling of the elliptical galaxy NGC 2974
In this paper we analyse the relations between a previously described oblate
Jaffe model for an ellipsoidal galaxy and the observed quantities for NGC 2974,
and obtain the length and velocity scales for a relevant elliptical galaxy
model. We then derive the finite total mass of the model from these scales, and
finally find a good fit of an isotropic oblate Jaffe model by using the
Gauss-Hermite fit parameters and the observed ellipticity of the galaxy NGC
2974. The model is also used to predict the total luminous mass of NGC 2974,
assuming that the influence of dark matter in this galaxy on the image,
ellipticity and Gauss-Hermite fit parameters of this galaxy is negligible
within the central region, of radius Comment: 7 figure
A Spitzer/IRAC Census of the Asymptotic Giant Branch Populations in Local Group Dwarfs. I. WLM
We present Spitzer/IRAC observations at 3.6 and 4.5 microns along with
optical data from the Local Group Galaxies Survey to investigate the evolved
stellar population of the Local Group dwarf irregular galaxy WLM. These
observations provide a nearly complete census of the asymptotic giant branch
(AGB) stars. We find 39% of the infrared-detected AGB stars are not detected in
the optical data, even though our 50% completeness limit is three magnitudes
fainter than the red giant branch tip. An additional 4% of the
infrared-detected AGBs are misidentified in the optical, presumably due to
reddening by circumstellar dust. We also compare our results with those of a
narrow-band optical carbon star survey of WLM, and find the latter study
sensitive to only 18% of the total AGB population. We detect objects with
infrared fluxes consistent with them being mass-losing AGB stars, and derive a
present day total mass-loss rate from the AGB stars of 0.7-2.4 x 10^(-3) solar
masses per year. The distribution of mass-loss rates and bolometric
luminosities of AGBs and red supergiants are very similar to those in the LMC
and SMC and the empirical maximum mass-loss rate observed in the LMC and SMC is
in excellent agreement with our WLM data.Comment: Accepted by ApJ, 34 pages, 13 figures, version with high-resolution
figures available at: http://webusers.astro.umn.edu/~djackson
Evidence for alternative electron sinks to photosynthetic carbon assimilation in the high mountain plant species Ranunculus glacialis
International audienceThe high mountain plant species Ranunculus glacialis has a low antioxidative scavenging capacity and a low activity of thermal dissipation of excess light energy despite its growth under conditions of frequent light and cold stress. In order to examine whether this species is protected from over-reduction by matching photosystem II (PSII) electron transport (ETR) and carbon assimilation, both were analysed simultaneously at various temperatures and light intensities using infrared gas absorption coupled with chlorophyll fluorescence. ETR exceeded electron consumption by carbon assimilation at higher light intensities and at all temperatures tested, necessitating alternative electron sinks. As photorespiration might consume the majority of excess electrons, photorespiration was inhibited by either high internal leaf CO2 molar ratio (C-i), low oxygen partial pressure (0.5% oxygen), or both. At 0.5% oxygen ETR was significantly lower than at 21% oxygen. At 21% oxygen, however, ETR still exceeded carbon assimilation at high C-i, suggesting that excess electrons are transferred to another oxygen consuming reaction when photorespiration is blocked. Nevertheless, photorespiration does contribute to electron consumption. While the activity of the water -water cycle to electron consumption is not known in leaves of R. glacialis, indirect evidence such as the high sensitivity to oxidative stress and the low initial NADP-malate dehydrogenase (NADP-MDH) activity suggests only a minor contribution as an alternative electron sink. Alternatively, the plastid terminal oxidase (PTOX) may transfer excess electrons to oxygen. This enzyme is highly abundant in R. glacialis leaves and exceeds the PTOX content of every other plant species so far examined, including those of transgenic tomato leaves overexpressing the PTOX protein. Finally, PTOX contents strongly declined during deacclimation of R. glacialis plants, suggesting their important role in photoprotection. Ranunculus glacialis is the first reported plant species with such a high PTOX protein content
SN 1998A: Explosion of a Blue Supergiant
We present spectroscopic and photometric observations of the peculiar Type II
supernova (SN) 1998A. The light curves and spectra closely resemble those of SN
1987A, suggesting that the SN 1998A progenitor exploded when it was a compact
blue supergiant. However, the comparison with SN 1987A also highlights some
important differences: SN 1998A is more luminous and the spectra show bluer
continua and larger expansion velocities at all epochs. These observational
properties indicate that the explosion of SN 1998A is more energetic than SN
1987A and more typical of SNe II. Comparing the observational data to
simulations, we deduce that the progenitor of SN 1998A was a massive star (~ 25
Mo) with a small pre-supernova radius (< 6 x 10^{12} cm). The Ba II lines,
unusually strong in SN 1987A and some faint II--P events, are almost normal in
the case of SN 1998A, indicating that the temperature plays a key role in
determining their strength.Comment: 15 pages, 16 figures, accepted for publication in MNRA
The Local Group Census: searching for PNe in IC 1613, WLM and GR8
In the framework of the Local Group Census (LGC), a survey of the Local Group
(LG) galaxies above Dec=, which is aimed at surveying their
populations with strong emission lines, we have searched for planetary nebulae
(PNe) in the low-metallicity dwarf irregular galaxies IC1613, WLM, GR8. Two new
candidate PNe have been found in IC1613, one in WLM and none in GR8. The
observations presented in this paper, together with the previous results from
the LGC, represent the first step in the study of the PN population in
low-metallicity, dwarf irregular galaxies of the Local Group. They will be
followed by deep spectroscopy to confirm their nature and to study their
physical-chemical properties. We used the observed number of PNe in each LG
galaxy to estimate a lower limit to the mass of the intermediate-age population
which was compared to the Star Formation Rate (SFR) of LG dwarf galaxies. These
results are in agreement with those from accurate star formation history (SFH)
analysis for these small galaxy systems.Comment: 10 pages, 6 figures, MNRAS accepte
Problems involving -Laplacian type equations and measures
summary:In this paper I discuss two questions on -Laplacian type operators: I characterize sets that are removable for Hölder continuous solutions and then discuss the problem of existence and uniqueness of solutions to with zero boundary values; here is a Radon measure. The joining link between the problems is the use of equations involving measures