747 research outputs found
A survey for PAH emission in H II regions, planetary and proto-planetary nebulae
The results of a systematic investigation of polycyclic aromatic hydrocarbon (PAH) emission in H II regions, planetary nebulae (PN), and proto-planetary nebulae (PNN), are reported. Data is obtained from the low resolution spectra (LRS) of IRAS. The results show that: PAHs are formed in carbon rich objects; and PAH emission is ubiquitous in general interstellar medium and requires the presence of ultraviolet photons, in planetary and proto-planetary nebulae, PAH emission is seen only where an ionizing flux is present and in carbon rich objects
Physical properties of 6dF dwarf galaxies
Spectral synthesis is basically the decomposition of an observed spectrum in
terms of the superposition of a base of simple stellar populations of various
ages and metallicities, producing as output the star formation and chemical
histories of a galaxy, its extinction and velocity dispersion. The STARLIGHT
code provides one of the most powerful spectral synthesis tools presently
available. We have applied this code to the entire Six-Degree-Field Survey
(6dF) sample of nearby star-forming galaxies, selecting dwarf galaxy candidates
with the goal of: (1) deriving the age and metallicity of their stellar
populations and (2) creating a database with the physical properties of our
sample galaxies together with the FITS files of pure emission line spectra
(i.e. the observed spectra after subtraction of the best-fitting synthetic
stellar spectrum). Our results yield a good qualitative and quantitative
agreement with previous studies based on the Sloan Digital Sky Survey (SDSS).
However, an advantage of 6dF spectra is that they are taken within a twice as
large fiber aperture, much reducing aperture effects in studies of nearby dwarf
galaxies.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy
Formation and Evolution", P. Papaderos, S. Recchi, G. Hensler (eds.). Lisbon,
September 2010, Springer Verlag, in pres
Clues to the Origin of the Mass-Metallicity Relation: Dependence on Star Formation Rate and Galaxy Size
We use a sample of 43,690 galaxies selected from the Sloan Digital Sky Survey
Data Release 4 to study the systematic effects of specific star formation rate
(SSFR) and galaxy size (as measured by the half light radius, r_h) on the
mass-metallicity relation. We find that galaxies with high SSFR or large r_h
for their stellar mass have systematically lower gas phase-metallicities (by up
to 0.2 dex) than galaxies with low SSFR or small r_h. We discuss possible
origins for these dependencies, including galactic winds/outflows, abundance
gradients, environment and star formation rate efficiencies.Comment: Accepted by ApJ Letter
Rejuvenation and overaging in a colloidal glass under shear
We report the modifications of the microscopic dynamics of a colloidal glass
submitted to shear. We use multispeckle diffusing wave spectroscopy to monitor
the evolution of the spontaneous slow relaxation processes after the sample
have been submitted to various straining. We show that high shear rejuvenates
the system and accelerates its dynamics whereas moderate shear overage the
system. We analyze this phenomena within the frame of the Bouchaud's trap
model.Comment: 4 pages, 4 figures, to be published in PR
A Search for Near-Infrared Emission From the Halo of NGC 5907 at Radii of 10 kpc to 30 kpc
We present a search for near-infrared (3.5-5 micron) emission from baryonic
dark matter in the form of low-mass stars and/or brown dwarfs in the halo of
the nearby edge-on spiral galaxy NGC 5907. The observations were made using a
256 by 256 InSb array with a pixel scale of 17" at the focus of a
liquid-helium-cooled telescope carried above the Earth's atmosphere by a
sounding rocket. In contrast to previous experiments which have detected a halo
around NGC 5907 in the V, R, I, J and K bands at galactic radii 6kpc < r <
10kpc, our search finds no evidence for emission from a halo at 10kpc < r <
30kpc. Assuming a halo mass density scaling as r^(-2), which is consistent with
the flat rotation curves that are observed out to radii of 32kpc, the lower
limit of the mass-to-light ratio at 3.5-5 microns for the halo of NGC 5907 is
250 (2 sigma) in solar units. This is comparable to the lower limit we have
found previously for NGC 4565 (Uemizu et al. 1998). Based on recent models, our
non-detection implies that hydrogen- burning stars contribute < 15% of the mass
of the dark halo of NGC 5907. Our results are consistent with the previous
detection of extended emission at r < 10kpc if the latter is caused by a
stellar population that has been ejected from the disk because of tidal
interactions. We conclude that the dark halo of NGC 5907, which is evident from
rotation curves that extend far beyond 10kpc, is not comprised of hydrogen
burning stars.Comment: 12 pages, LateX, plus 6 ps figures. Accepted by ApJ. minor changes,
added references, corrected typo
The Star Formation History of IZw18
The star formation history in IZw18 has been inferred from HST/WFPC2 archival
data. This is done by comparing the derived V, B-V and V, V-I color-magnitude
diagrams and luminosity functions with synthetic ones, based on various sets of
stellar evolutionary tracks. At a distance of 10 Mpc, the stars resolved in the
field of IZw18 allow for a lookback time up to 1 Gyr. We find that the main
body is not experiencing its first episode of star formation. Instead, it has
been forming stars over the last 0.5-1 Gyr, at a rate of ~ 1-2 * 10**(-2) Msol
per year per kpc**2. A more intense activity of 6-16 * 10**(-2) Msol per year
per kpc**2 has taken place between 15 and 20 Myr ago. For the secondary body,
the lookback time is 0.2 Gyr at most and the uncertainty is much higher, due to
the shallower diagrams and the small number of resolved stars. The derived
range of star formation rate is 3-10 * 10**(-3) Msol per year per kpc**2. The
IMF providing the best fit to the observed stellar populations in the main body
has a slope 1.5, much flatter than in any similar galaxy analyzed with the same
method. In the secondary body, it is peaked at 2.2, closer to Salpeter's slope
(2.35).Comment: 70 pages including 18 figures, to be published in The Astronomical
Journa
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