559 research outputs found
The fragmentation of expanding shells II: Thickness matters
We study analytically the development of gravitational instability in an
expanding shell having finite thickness. We consider three models for the
radial density profile of the shell: (i) an analytic uniform-density model,
(ii) a semi-analytic model obtained by numerical solution of the hydrostatic
equilibrium equation, and (iii) a 3D hydrodynamic simulation. We show that all
three profiles are in close agreement, and this allows us to use the first
model to describe fragments in the radial direction of the shell. We then use
non-linear equations describing the time-evolution of a uniform oblate spheroid
to derive the growth rates of shell fragments having different sizes. This
yields a dispersion relation which depends on the shell thickness, and hence on
the pressure confining the shell. We compare this dispersion relation with the
dispersion relation obtained using the standard thin-shell analysis, and show
that, if the confining pressure is low, only large fragments are unstable. On
the other hand, if the confining pressure is high, fragments smaller than
predicted by the thin-shell analysis become unstable. Finally, we compare the
new dispersion relation with the results of 3D hydrodynamic simulations, and
show that the two are in good agreement.Comment: 9 pages, 9 figures, accepted by MNRA
Triggered massive-star formation on the borders of Galactic HII regions. IV- Star formation at the periphery of Sh2-212
Aims: We wish to establish whether sequential star formation is taking place
at the periphery of the Galactic HII region Sh2-212.
Methods: We present CO millimetre observations of this region obtained at the
IRAM 30-m telescope to investigate the distribution of associated molecular
material. We also use deep JHK observations obtained at the CFHT to study the
stellar content of the region, and radio observations obtained at the VLA to
look for the presence of an ultra-compact (UC) HII region and for maser
emission.
Results: In the optical, Sh2-212 is spherically symmetric around its central
exciting cluster. This HII region is located along a molecular filament. A
thin, well-defined half ring of molecular material surrounds the brightest part
of the HII region at the rear and is fragmented. The most massive fragment
(~200 solar masses) contains a massive young stellar object displaying a
near-IR excess; its spectral energy distribution indicates a high-mass
(~14solar masses), high-temperature (~30000K), and high-luminosity (~17000
solar luminosities) source. This object ionizes a UC HII region.
Conclusions: Sh2-212 is a good example of massive-star formation triggered
via the collect and collapse process. The massive YSO observed at its periphery
is a good candidate for a massive star formed in isolation.Comment: 12 pages, 14 figures. To be published in A&
Triggered massive-star formation on the borders of Galactic HII regions. III. Star formation at the periphery of Sh2-219
Context. Massive-star formation triggered by the expansion of HII regions.
Aims. To understand if sequential star formation is taking place at the
periphery of the HII region Sh2-219. Methods. We present 12CO(2-1) line
observations of this region, obtained at the IRAM 30-m telescope (Pico Veleta,
Spain). Results. In the optical, Sh2-219 is spherically symmetric around its
exciting star; furthermore it is surrounded along three quarters of its
periphery by a ring of atomic hydrogen. This spherical symmetry breaks down at
infrared and millimetre wavelengths. A molecular cloud of about 2000\msol lies
at the southwestern border of Sh2-219, in the HI gap. Two molecular
condensations, elongated along the ionization front, probably result from the
interaction between the expanding HII region and the molecular cloud. In this
region of interaction there lies a cluster containing many highly reddened
stars, as well as a massive star exciting an ultracompact HII region. More
surprisingly, the brightest parts of the molecular cloud form a `chimney',
perpendicular to the ionization front. This chimney is closed at its south-west
extremity by H-alpha walls, thus forming a cavity. The whole structure is 7.5
pc long. A luminous H-alpha emission-line star, lying at one end of the chimney
near the ionization front, may be responsible for this structure. Confrontation
of the observations with models of HII region evolution shows that Sh2-219 is
probably 10^5 yr old. The age and origin of the near-IR cluster observed on the
border of Sh2-219 remain unknown.Comment: 11 pages, 10 figures. To be published in A&
Triggered massive-star formation on the borders of Galactic HII regions. II. Evidence for the collect and collapse process around RCW 79
We present SEST-SIMBA 1.2-mm continuum maps and ESO-NTT SOFI JHK images of
the Galactic HII region RCW 79. The millimetre continuum data reveal the
presence of massive fragments located in a dust emission ring surrounding the
ionized gas. The two most massive fragments are diametrically opposite each
other in the ring. The near-IR data, centred on the compact HII region located
at the south-eastern border of RCW 79, show the presence of an IR-bright
cluster containing massive stars along with young stellar objects with near-IR
excesses. A bright near- and mid-IR source is detected towards maser emissions,
1.2 pc north-east of the compact HII region centre. Additional information,
extracted from the Spitzer GLIMPSE survey, are used to discuss the nature of
the bright IR sources observed towards RCW 79. Twelve luminous Class I sources
are identified towards the most massive millimetre fragments. All these facts
strongly indicate that the massive-star formation observed at the border of the
HII region RCW 79 has been triggered by its expansion, most probably by the
collect and collapse process.Comment: 14 pages, 15 figures. Accepted for publication in A&A. The images
have been highly compressed for astro-ph. A version of this paper with
higher-resolution figures is available at
http://www.oamp.fr/matiere/rcw79.pd
The escape fraction of ionizing photons from high redshift galaxies
The fraction of ionizing photons which escape their host galaxy and so are
able to ionize hydrogen in the inter-galactic medium (IGM) is a critical
parameter in studies of the reionization era and early galaxy formation. In
this paper we combine observations of Lyman-alpha absorption towards high
redshift quasars with the measured UV luminosity function of high redshift
galaxies to constrain the escape fraction (f_esc) of ionizing photons from
galaxies at z ~ 5.5-6. The observed Lyman-alpha transmission constrains the
escape fraction to lie in the range f_esc ~ 10-25 % (at z ~ 5.5-6). Excluding
halos with M< 10^10 M_sun (as might be expected if galaxy formation is
suppressed due to the reionization of the IGM) implies a larger escape fraction
of f_esc ~ 20-45 %. Using the numerical results to calibrate an analytic
relation between the escape fraction and minimum galaxy halo mass we also
extrapolate our results to a mass (M~10^8 M_sun) corresponding to the hydrogen
cooling threshold. In this case we find f_esc ~ 5-10 %, consistent with
observed estimates at lower redshift. We find that the escape fraction of high
redshift galaxies must be greater than 5 % irrespepctive of galaxy mass. Based
on these results we use a semi-analytic description to model the reionization
history of the IGM, assuming ionizing sources with escape fractions suggested
by our numerical simulations. We find that the IBG observed at z ~ 5.5-6
implies a sufficient number of ionizing photons to have reionized the Universe
by z ~ 6. However, if the minimum mass for star-formation were greater than
10^9 M_sun, the IBG would be over-produced at redshifts less than z ~ 5. In
summary, our results support a scenario in which the IGM was reionized by low
mass galaxies.Comment: 14 pages, 9 figure
The far-ultraviolet emission of early-type galaxies
We have assembled a UV-flux selected sample of 82 early-type galaxies and
collected additional information at other wavelengths. These data confirm a
large spread of the UV-V color in the range 2 to 5. The spread in UV-V is
accompanied by a spread in B-V that is mainly attributed to the range of
morphological types and luminosities. A large fraction of the objects have red
colors, UV-V = 4 +-0.4, corresponding to a weak UV-upturn as observed with IUE.
If the current interpretation for the UV emission from early-type galaxies is
applicable to our sample, the PAGB (Post-Asymptotic Giant Branch) tracks are
the most common evolution path for the low-mass stars responsible for the UV
emission. A small number of very blue (UV-V < 1.4) objects have been found that
can be reasonably interpreted as harbouring some low level of star formation.
In contrast with a previous sample based on IUE observations, no correlation is
found between the UV-V color and the Mg_2 spectral line index; possible
explanations are reviewed. The potential of a more extended UV survey like
GALEX is briefly presented.Comment: accepted for publication on Astronomy and Astrophysic
Far-Ultraviolet Radiation from Elliptical Galaxies
Far-ultraviolet radiation is a ubiquitous, if unanticipated, phenomenon in
elliptical galaxies and early-type spiral bulges. It is the most variable
photometric feature associated with old stellar populations. Recent
observational and theoretical evidence shows that it is produced mainly by
low-mass, small-envelope, helium-burning stars in extreme horizontal branch and
subsequent phases of evolution. These are probably descendents of the dominant,
metal rich population of the galaxies. Their lifetime UV outputs are remarkably
sensitive to their physical properties and hence to the age and the helium and
metal abundances of their parents. UV spectra are therefore exceptionally
promising diagnostics of old stellar populations, although their calibration
requires a much improved understanding of giant branch mass loss, helium
enrichment, and atmospheric diffusion.Comment: 46 pages; includes LaTeX text file, 9 PS figures, 1 JPG figure, 2
style files. Full resolution figures and PS version available at
http://www.astro.virginia.edu/~rwo/araa99/. Article to appear in Annual
Reviews of Astronomy & Astrophysics, 199
Constraints on the Lyman continuum radiation from galaxies: first results with FUSE on Mrk 54
We present Far Ultraviolet Spectroscopic Explorer observations of the
star-forming galaxy Mrk 54 at z = 0.0448. The Lyman continuum radiation is not
detected above the HI absorption edge in our Galaxy. An upper limit is
evaluated by comparison with the background measured in regions of the detector
adjacent to the observed spectrum. A spectral window of 16 A, reasonably free
of additional HI Lyman series line absorption is used. No correction is needed
for molecular hydrogen absorption in our Galaxy but a foreground extinction of
0.29 mag is accounted for. An upper limit of 6.15 10^{-16} erg/cm^2/s/A is
obtained for the flux at ~ 900 A in the rest frame of Mrk 54. By comparison
with the number of ionizing photons derived from the H-alpha flux, this limit
translates into an upper limit of f_esc < 0.062 for the fraction of Lyman
continuum photons that escape the galaxy without being absorbed by interstellar
material. This limit compares with the limits obtained in three other nearby
galaxies and is compatible with the escape fractions predicted by models.
The upper limits obtained in nearby galaxies contrasts with the detection of
Lyman continuum flux in the composite spectrum of Lyman-break galaxies at z ~
3.4. The difficulties and implications of a comparison are discussed.Comment: 9 pages, 3 figures, accepted for publication in A&A include aa.cls
v5.0
What Controls the Star Formation in Luminous Starburst Mergers ?
In order to understand what controls the star formation process in luminous
starburst mergers (e.g., NGC 6240, Arp 220, and so on), we investigate
observational properties of two samples of high-luminosity starburst galaxies
mapped in CO(=1--0) independently using both the Owens Valley Radio
Observatory (Scoville et al. 1991) and the IRAM interferometer (Downes &
Solomon 1998). We find that the surface density of far-infrared luminosity,
(FIR), is proportional linearly to the H surface mass density,
(H), for the two samples; (FIR) (H) with a correlation coefficient of 0.96. It is
often considered that (FIR) provides a good measure of the star
formation rate per unit area, (SFR). It is also known that molecular
gas is dominated in circumnuclear regions in the luminous starburst mergers;
i.e., (gas) (H). Therefore, the above relationship
suggests a star formation law; (SFR) (gas). We suggest
that this star formation law favors the gravitational instability scenario
rather than the cloud-cloud collision one.Comment: 14 pages, 2 figures. The Astrophysical Journal (Letters), in pres
The ultraviolet extragalactic background light: dust extinction and the evolution of the cosmic star formation rate from z=0 to 0.6
We show that the accumulated light of galaxies in the ultraviolet can be
evaluated from their luminosity density as a function of the evolution of the
cosmic star formation rate and dust extinction properties. Constraints on the
evolution rate are expected in future. Data available at the moment are
consistent with an evolution rate at low z steeper than (1+z)^(3.5). A
shallower rate remains possible if the luminosity-weighted dust extinction at
2000 A, as suggested by some data, is lower than about 1.2.Comment: 4 pages, 1 figure, accepted for publication in MNRA
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