767 research outputs found
Galaxy Evolution and Star Formation Efficiency in the Last Half of the Universe
We present the results of a CO(1-0) emission survey with the IRAM 30m of 30
galaxies at moderate redshift (z ~ 0.2-0.6) to explore galaxy evolution and in
particular the star formation efficiency, in the redshift range filling the gap
between local and very high-z objects. Our detection rate is about 50%. One of
the bright objects was mapped at high resolution with the IRAM interferometer,
and about 50% of the total emission found in the 27 arcsec (97 kpc) single dish
beam is recovered by the interferometer, suggesting the presence of extended
emission. The FIR-to-CO luminosity ratio is enhanced, following the increasing
trend observed between local and high-z ultra-luminous starbursts.Comment: 6 pages, 5 figures, To appear in the proceedings of "SF2A-2007:
Semaine de l'Astrophysique Francaise", (J. Bouvier, A. Chalabaev, and C.
Charbonnel eds
Magnetic fields in the absence of spiral density waves - NGC 4414
We present three-frequency VLA observations of the flocculent spiral galaxy
NGC 4414 made in order to study the magnetic field structure in absence of
strong density wave flows. NGC 4414 shows a regular spiral pattern of observed
polarization B-vectors with a radial component comparable in strength to the
azimuthal one. The average pitch angle of the magnetic field is about
20\degr, similar to galaxies with a well-defined spiral pattern. This
provides support for field generation by a turbulent dynamo without significant
``contamination'' from streaming motions in spiral arms. While the stellar
light is very axisymmetric, the magnetic field structure shows a clear
asymmetry with a stronger regular field and a smaller magnetic pitch angle in
the northern disk. Extremely strong Faraday rotation is measured in the
southern part of the disk, becoming Faraday thick at 6cm. The distribution of
Faraday rotation suggests a mixture of axisymmetric and higher-mode magnetic
fields. The strong Faraday effects in the southern region suggest a much
thicker magnetoionic disk and a higher content of diffuse ionized gas than in
the northern disk portion. An elongation of the 20cm total power emission is
also seen towards the South. Although NGC 4414 is currently an isolated spiral,
the asymmetries in the polarized radio emission may be sensitive tracers of
previous encounters, including weak interactions which would chiefly affect the
diffuse gas component without generating obvious long-term perturbations in the
optical structure.Comment: 12 pages, 14 figures, A&A accepte
Star Formation in Collision Debris: Insights from the modeling of their Spectral Energy Distribution
During galaxy-galaxy interactions, massive gas clouds can be injected into
the intergalactic medium which in turn become gravitationally bound, collapse
and form stars, star clusters or even dwarf galaxies. The objects resulting
from this process are both "pristine", as they are forming their first
generation of stars, and chemically evolved because the metallicity inherited
from their parent galaxies is high. Such characteristics make them particularly
interesting laboratories to study star formation. After having investigated
their star-forming properties, we use photospheric, nebular and dust modeling
to analyze here their spectral energy distribution (SED) from the
far-ultraviolet to the mid-infrared regime for a sample of 7 star-forming
regions. Our analysis confirms that the intergalactic star forming regions in
Stephan's Quintet, around Arp 105, and NGC 5291, appear devoid of stellar
populations older than 10^9 years. We also find an excess of light in the
near-infrared regime (from 2 to 4.5 microns) which cannot be attributed to
stellar photospheric or nebular contributions. This excess is correlated with
the star formation rate intensity suggesting that it is probably due to
emission by very small grains fluctuating in temperature as well as the
polycyclic aromatic hydrocarbons (PAH) line at 3.3 micron. Comparing the
attenuation via the Balmer decrement to the mid-infrared emission allows us to
check the reliability of the attenuation estimate. It suggests the presence of
embedded star forming regions in NGC 5291 and NGC 7252. Overall the SED of
star-forming regions in collision debris (and Tidal Dwarf Galaxies) resemble
more that of dusty star-forming regions in galactic disks than to that of
typical star-forming dwarf galaxies.Comment: 22 pages, 24 figures, accepted for publication in A
Uncovering Spiral Structure in Flocculent Galaxies
We present K'(2.1 micron) observations of four nearby flocculent spirals,
which clearly show low-level spiral structure and suggest that kiloparsec-scale
spiral structure is more prevalent in flocculent spirals than previously
supposed. In particular, the prototypical flocculent spiral NGC 5055 is shown
to have regular, two-arm spiral structure to a radius of 4 kpc in the near
infrared, with an arm-interarm contrast of 1.3. The spiral structure in all
four galaxies is weaker than that in grand design galaxies. Taken in unbarred
galaxies with no large, nearby companions, these data are consistent with the
modal theory of spiral density waves, which maintains that density waves are
intrinsic to the disk. As an alternative, mechanisms for driving spiral
structure with non-axisymmetric perturbers are also discussed. These
observations highlight the importance of near infrared imaging for exploring
the range of physical environments in which large-scale dynamical processes,
such as density waves, are important.Comment: 12 pages AASTeX; 3 compressed PS figures can be retrieved from
ftp://ftp.astro.umd.edu/pub/michele as file thornley.tar (1.6Mbytes).
Accepted to Ap.J. Letters.(Figures now also available here, and from
ftp://ftp.astro.umd.edu/pub/michele , in GIF format.
Infrared dust emission in the outer disk of M51
We examine faint infrared emission features detected in Spitzer Space
Telescope images of M51, which are associated with atomic hydrogen in the outer
disk and tidal tail at R greater than R_25 (4.9', ~14 kpc at d=9.6 Mpc). The
infrared colors of these features are consistent with the colors of dust
associated with star formation in the bright disk. However, the star formation
efficiency (as a ratio of star formation rate to neutral gas mass) implied in
the outer disk is lower than that in the bright disk of M51 by an order of
magnitude, assuming a similar relationship between infrared emission and star
formation rate in the inner and outer disks.Comment: 13 pages in manuscript form, 2 figures; download PDF of manuscript
with original-resolution Figure 1 at
http://www.eg.bucknell.edu/physics/thornley/thornleym51.pd
Constraints on UV Absorption in the Intracluster Medium of Abell 1030
We present results from an extensive HST spectroscopic search for UV
absorption lines in the spectrum of the quasar B2~1028+313, which is associated
with the central dominant galaxy in the cluster Abell~1030 (). This is
one of the brightest known UV continuum sources located in a cluster, and
therefore provides an ideal opportunity to obtain stringent constraints on the
column densities of any cool absorbing gas that may be associated with the
intracluster medium (ICM). Our HST spectra were obtained with the FOS and GHRS,
and provide continuous coverage at rest-frame wavelengths from to
4060~\AA, thereby allowing the investigation of many different elements and
ionization levels. We utilize a new technique that involves simultaneous
fitting of large numbers of different transitions for each species, thereby
yielding more robust constraints on column densities than can be obtained from
a single transition. This method yields upper limits of cm on the column densities of a wide range of molecular, atomic
and ionized species that may be associated with the ICM. We also discuss a
possible \Lya and C IV absorption system associated with the quasar. We discuss
the implications of the upper limits on cool intracluster gas in the context of
the physical properties of the ICM and its relationship to the quasar.Comment: Astrophysical Journal, in press, 19 pages, includes 5 PostScript
figures. Latex format, uses aas2pp4.sty and epsfig.sty file
Star formation in M33 (HerM33es)
Within the key project "Herschel M33 extended survey" (HerM33es), we are
studying the physical and chemical processes driving star formation and
galactic evolution in the nearby galaxy M33, combining the study of local
conditions affecting individual star formation with properties only becoming
apparent on global scales. Here, we present recent results obtained by the
HerM33es team. Combining Spitzer and Herschel data ranging from 3.6um to 500um,
along with HI, Halpha, and GALEX UV data, we have studied the dust at high
spatial resolutions of 150pc, providing estimators of the total infrared (TIR)
brightness and of the star formation rate. While the temperature of the warm
dust at high brightness is driven by young massive stars, evolved stellar
populations appear to drive the temperature of the cold dust. Plane-parallel
models of photon dominated regions (PDRs) fail to reproduce fully the [CII],
[OI], and CO maps obtained in a first spectroscopic study of one 2'x2'
subregion of M33, located on the inner, northern spiral arm and encompassing
the HII region BCLMP302.Comment: 6 pages, to appear in the proceedings of the 5th Zermatt ISM
Symposium "Conditions and impact of star formation: New results with Herschel
and beyond
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