16 research outputs found
Gas flows, star formation and galaxy evolution
In the first part of this article we show how observations of the chemical
evolution of the Galaxy: G- and K-dwarf numbers as functions of metallicity,
and abundances of the light elements, D, Li, Be and B, in both stars and the
interstellar medium (ISM), lead to the conclusion that metal poor HI gas has
been accreting to the Galactic disc during the whole of its lifetime, and is
accreting today at a measurable rate, ~2 Msun per year across the full disc.
Estimates of the local star formation rate (SFR) using methods based on stellar
activity, support this picture. The best fits to all these data are for models
where the accretion rate is constant, or slowly rising with epoch. We explain
here how this conclusion, for a galaxy in a small bound group, is not in
conflict with graphs such as the Madau plot, which show that the universal SFR
has declined steadily from z=1 to the present day. We also show that a model in
which disc galaxies in general evolve by accreting major clouds of low
metallicity gas from their surroundings can explain many observations, notably
that the SFR for whole galaxies tends to show obvious variability, and
fractionally more for early than for late types, and yields lower dark to
baryonic matter ratios for large disc galaxies than for dwarfs. In the second
part of the article we use NGC 1530 as a template object, showing from
Fabry-Perot observations of its Halpha emission how strong shear in this
strongly barred galaxy acts to inhibit star formation, while compression acts
to stimulate it.Comment: 20 pages, 10 figures, to be presented at the "Penetrating Bars
through Masks of Cosmic Dust" conference in South Africa, proceedings
published by Kluwer, Eds. D.L. Block, K.C. Freeman, I. Puerari, & R. Groes
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FUSE and STIS observations of the warm-hot intergalactic medium toward PG 1259+593
Highly ionized gas in the galactic halo and the high-velocity clouds toward PG 1116+215
We have obtained high resolution FUSE and HST/STIS echelle observations of the quasar PG 1116+215. The semi-continuous coverage of the ultraviolet spectrum over the wavelength range 916-2800 provides detections of Galactic and high velocity cloud (HVC) absorption over a wide range of ionization species over the local standard of rest velocity range -100 - +200 km/s. The high dispersion of these spectra (6.5-20 km/s) reveals that low ionization species consist of five discrete components, three at low-to-intermediate velocities, and two at high velocities (v = +100, +184 km/s). Over the same velocity range, the higher ionization species show continuous absorption with column density peaks at v = +10 km/s +184 km/s. The absorption kinematics of the v=+184 km/s HVC suggest a scenario in which a low-ionization cloud of gas is streaming through a hot external medium that is stripping gas from this cloud. Using the OI and HI column densities, we estimate [O/H]=-0.66, with a substantial uncertainty due to saturation of the HI Lyman series. If the ionization of the cloud core is photonionization by the extragalactic UV background, we estimate the cloud has a density of 10^-2.7 cm^-3. If photons escaping the Galactic disk are also included, the density could be higher by nearly 2 dex. In either case, the relative abundances of O, Si, and Fe in the cloud core are readily explained by a solar pattern. Magellanic Stream gas is a possible origin for this gas and is consistent with the location of the HVC on the sky, as well as its high positive velocity, the ionization, and metallicity. [Abridged
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Probing O VI Emission in the Halos of Edge-on Spiral Galaxies
We have used the Far Ultraviolet Spectroscopic Explorer to search for O VI λλ1031.926, 1037.617 emission in the halos of the edge-on spiral galaxies NGC 4631 and NGC 891. In NGC 4631, we detected O VI in emission toward a soft X-ray bubble above a region containing numerous Hα arcs and filaments. The line-of-sight component of the motion of the O VI gas appears to match the underlying disk rotation. The observed O VI luminosities can account for 0.2%-2% of the total energy input from supernovae (assuming a full O VI- emitting halo) and yield mass flux cooling rates between 0.48 and 2.8 M yr-1 depending on the model used in the derivations. On the basis of these findings, we believe it is likely that we are seeing cooling, galactic fountain gas. No emission was detected from the halo of NGC 891, a galaxy in a direction with considerably high foreground Galactic extinction
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Physical properties, baryon content, and evolution of the Ly alpha forest: New insights from high-resolution observations at z less than or similar to 0.4
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A high-resolution survey of low-redshift QSO absorption lines: Statistics and physical conditions of OVI absorbers
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Pre-heating by pre-virialization and its impact on galaxy formation
We use recent observations of the H i mass function to constrain galaxy formation. The data conflict with the standard model where most of the gas in a low-mass dark matter halo is assumed to settle into a disc of cold gas that is depleted by star formation and supernova-driven outflows until the disc becomes gravitationally stable. Assuming a star formation threshold density supported by both theory and observations, this model predicts H i masses that are much too large. The reason is simple: supernova feedback requires star formation, which in turn requires a high surface density for the gas. Heating by the ultraviolet background can reduce the amount of cold gas in haloes with masses \u3c109.5 h−1 M⊙, but is insufficient to explain the observed H i mass function. A consistent model can be found if low-mass haloes are embedded in a pre-heated medium, with a specific gas entropy ∼10 keV cm2. In addition, such a model simultaneously matches the faint-end slope of the galaxy luminosity function without the need for any supernova-driven outflows. We propose a pre-heating model where the medium around low-mass haloes is pre-heated by gravitational pancaking. Because gravitational tidal fields suppress the formation of low-mass haloes while promoting that of pancakes, the formation of massive pancakes precedes that of the low-mass haloes within them. We demonstrate that the progenitors of present-day dark matter haloes with M≲ 1012 h−1 M⊙ were embedded in pancakes of masses ∼5 × 1012 h−1 M⊙ at z∼ 2. The formation of such pancakes heats the gas to a temperature of 5 × 105 K and compresses it to an overdensity of ∼10. Such gas has a cooling time that exceeds the age of the Universe at z≲ 2, and has a specific entropy of ∼15 keV cm2, almost exactly the amount required to explain the stellar and H i mass functions
The low-redshift Ly alpha forest towards 3C 273
We present an analysis of the Ly-a forest toward 3C 273 from the Space
Telescope Imaging Spectrograph at ~7 km/s resolution, along with re-processed
data from the Far Ultraviolet Spectroscopic Explorer. The high UV flux of 3C
273 allows us to probe the weak, low z absorbers. The main sample consists of
21 HI absorbers that we could discriminate to a sensitivity of log NHI~ 12.5.
The redshift density for absorbers with 13.1<log NHI<14.0 is ~1.5 sigma below
the mean for other lines of sight; for log NHI >= 12.5, it is consistent with
numerical model predictions. The Doppler parameter distribution is consistent
with other low z samples. We find no evidence for a break in the column density
power-law distribution to log NHI=12.3. A broad Ly-a absorber (BLA) is within
Delta v =< 50 km/s and 1.3 local frame Mpc of two ~0.5L* galaxies, with an OVI
absorber ~700 km/s away, similarly close to three galaxies and indicating
overdense environments. We detect clustering on the Delta v<1000 km/s scale at
3.4 sigma significance for log NHI >= 12.6, consistent with the level predicted
from hydrodynamical simulations, and indication for a Ly-a forest void at
0.09<z<0.12. We find at least two components for the z=0.0053 Virgo absorber,
but the total NHI column is not significantly changed.Comment: 30 pages, 10 figures, MNRAS, in pres