45,707 research outputs found
Discovery of a very extended X-ray halo around a quiescent spiral galaxy - the "missing link" of galaxy formation
Hot gaseous haloes surrounding galaxies and extending well beyond the
distribution of stars are a ubiquitous prediction of galaxy formation
scenarios. The haloes are believed to consist of gravitationally trapped gas
with a temperature of millions of Kelvin. The existence of such hot haloes
around massive elliptical galaxies has been established through their X-ray
emission. While gas out-flowing from starburst spiral galaxies has been
detected, searches for hot haloes around normal, quiescent spiral galaxies have
so far failed, casting doubts on the fundamental physics in galaxy formation
models. Here we present the first detection of a hot, large-scale gaseous halo
surrounding a normal, quiescent spiral galaxy, NGC 5746, alleviating a
long-standing problem for galaxy formation models. In contrast to starburst
galaxies, where the X-ray halo can be powered by the supernova energy, there is
no such power source in NGC 5746. The only compelling explanation is that we
are here witnessing a galaxy forming from gradually in-flowing hot and dilute
halo gas.Comment: New Astronomy, in pres
The most ancient spiral galaxy: a 2.6-Gyr-old disk with a tranquil velocity field
We report an integral-field spectroscopic (IFS) observation of a
gravitationally lensed spiral galaxy A1689B11 at redshift . It is the
most ancient spiral galaxy discovered to date and the second kinematically
confirmed spiral at . Thanks to gravitational lensing, this is also
by far the deepest IFS observation with the highest spatial resolution (
400 pc) on a spiral galaxy at a cosmic time when the Hubble sequence is about
to emerge. After correcting for a lensing magnification of 7.2 0.8, this
primitive spiral disk has an intrinsic star formation rate of 22 2
yr, a stellar mass of 10 and a
half-light radius of kpc, typical of a main-sequence
star-forming (SF) galaxy at . However, the H\alpha\ kinematics show a
surprisingly tranquil velocity field with an ordered rotation ( =
200 12 km/s) and uniformly small velocity dispersions ( = 23 4 km/s and = 15 2 km/s).
The low gas velocity dispersion is similar to local spiral galaxies and is
consistent with the classic density wave theory where spiral arms form in
dynamically cold and thin disks. We speculate that A1689B11 belongs to a
population of rare spiral galaxies at that mark the formation epoch
of thin disks. Future observations with JWST will greatly increase the sample
of these rare galaxies and unveil the earliest onset of spiral arms.Comment: 18 pages, 13 figures, 1 table; accepted for publication in Ap
Star formation and ISM morphology in tidally induced spiral structures
Tidal encounters are believed to be one of the key drivers of galactic spiral
structure in the Universe. Such spirals are expected to produce different
morphological and kinematic features compared to density wave and dynamic
spiral arms. In this work we present high resolution simulations of a tidal
encounter of a small mass companion with a disc galaxy. Included are the
effects of gas cooling and heating, star formation and stellar feedback. The
structure of the perturbed disc differs greatly from the isolated galaxy,
showing clear spiral features that act as sites of new star formation, and
displaying interarm spurs. The two arms of the galaxy, the bridge and tail,
appear to behave differently; with different star formation histories and
structure. Specific attention is focused on offsets between gas and stellar
spiral features which can be directly compared to observations. We find some
offsets do exist between different media, with gaseous arms appearing mostly on
the convex side of the stellar arms, though the exact locations appear highly
time dependent. These results further highlight the differences between tidal
spirals and other theories of arm structure.Comment: 17 pages, 19 colour figures, accepted for publication in MNRA
Spatial Distributions of Cold and Warm Interstellar Dust in M101 Resolved with AKARI/Far-Infrared Surveyor (FIS)
The nearby face-on spiral galaxy M101 has been observed with the Far-Infrared
Surveyor (FIS) onboard AKARI. The far-infrared four-band images reveal fine
spatial structures of M101, which include global spiral patterns, giant HII
regions embedded in outer spiral arms, and a bar-like feature crossing the
center. The spectral energy distribution of the whole galaxy shows the presence
of the cold dust component (18 K) in addition to the warm dust component (55
K). The distribution of the cold dust is mostly concentrated near the center,
and exhibits smoothly distributed over the entire extent of the galaxy, whereas
the distribution of the warm dust indicates some correlation with the spiral
arms, and has spotty structures such as four distinctive bright spots in the
outer disk in addition to a bar-like feature near the center tracing the CO
intensity map. The star-formation activity of the giant HII regions that
spatially correspond to the former bright spots is found to be significantly
higher than that of the rest of the galaxy. The latter warm dust distribution
implies that there are significant star-formation activities in the entire bar
filled with molecular clouds. Unlike our Galaxy, M101 is a peculiar normal
galaxy with extraordinary active star-forming regions.Comment: 18 pages, 9 figures, accepted for publication in PASJ AKARI special
issu
Zoom-in cosmological hydrodynamical simulation of a star-forming barred, spiral galaxy at redshift z=2
Accepted for publication in MNRASWe present gas and stellar kinematics of a high-resolution zoom-in cosmological chemodynamical simulation, which fortuitously captures the formation and evolution of a star-forming barred spiral galaxy, from redshift to at the peak of the cosmic star formation rate. The galaxy disc grows by accreting gas and substructures from the environment. The spiral pattern becomes fully organised when the gas settles from a thick (with vertical dispersion 50 km/s) to a thin ( km/s) disc component in less than 1 Gyr. Our simulated disc galaxy also has a central X-shaped bar, the seed of which formed by the assembly of dense gas-rich clumps by . The star formation activity in the galaxy mainly happens in the bulge and in several clumps along the spiral arms at all redshifts, with the clumps increasing in number and size as the simulation approaches . We find that stellar populations with decreasing age are concentrated towards lower galactic latitudes, being more supported by rotation, and having also lower velocity dispersion; furthermore, the stellar populations on the thin disc are the youngest and have the highest average metallicities. The pattern of the spiral arms rotates like a solid body with a constant angular velocity as a function of radius, which is much lower than the angular velocity of the stars and gas on the thin disc; moreover, the angular velocity of the spiral arms steadily increases as function of time, always keeping its radial profile constant. The origin of our spiral arms is also discussed.Peer reviewe
Tracing PAHs and Warm Dust Emission in the Seyfert Galaxy NGC 1068
We present a study of the nearby Seyfert galaxy NGC 1068 using mid- and far-
infrared data acquired with the IRAC, IRS, and MIPS instruments aboard the
Spitzer Space Telescope. The images show extensive 8 um and 24 um emission
coinciding with star formation in the inner spiral approximately 15" (1 kpc)
from the nucleus, and a bright complex of star formation 47" (3 kpc) SW of the
nucleus. The brightest 8 um PAH emission regions coincide remarkably well with
knots observed in an Halpha image. Strong PAH features at 6.2, 7.7, 8.6, and
11.3 um are detected in IRS spectra measured at numerous locations inside,
within, and outside the inner spiral. The IRAC colors and IRS spectra of these
regions rule out dust heated by the AGN as the primary emission source; the
SEDs are dominated by starlight and PAH emission. The equivalent widths and
flux ratios of the PAH features in the inner spiral are generally consistent
with conditions in a typical spiral galaxy ISM. Interior to the inner spiral,
the influence of the AGN on the ISM is evident via PAH flux ratios indicative
of a higher ionization parameter and a significantly smaller mean equivalent
width than observed in the inner spiral. The brightest 8 and 24 um emission
peaks in the disk of the galaxy, even at distances beyond the inner spiral, are
located within the ionization cones traced by [O III]/Hbeta, and they are also
remarkably well aligned with the axis of the radio jets. Although it is
possible that radiation from the AGN may directly enhance PAH excitation or
trigger the formation of OB stars that subsequently excite PAH emission at
these locations in the inner spiral, the orientation of collimated radiation
from the AGN and star formation knots in the inner spiral could be
coincidental. (abridged)Comment: 20 pages, 11 figures; AJ, accepted; full resolution version available
at http://spider.ipac.caltech.edu/staff/jhhowell/astro/howelln1068.pd
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