325 research outputs found
The Evolution of Stellar Exponential Discs
Models of disc galaxies which invoke viscosity-driven radial flows have long
been known to provide a natural explanation for the origin of stellar
exponential discs, under the assumption that the star formation and viscous
timescales are comparable. We present models which invoke simultaneous star
formation, viscous redistribution of gas and cosmologically-motivated gaseous
infall and explore the predictions such models make for the scale length
evolution and radial star formation history of galactic stellar discs. While
the inclusion of viscous flows is essential for ensuring that the stellar disc
is always exponential over a significant range in radius, we find that such
flows play essentially no role in determining the evolution of the disc scale
length. In models in which the main infall phase precedes the onset of star
formation and viscous evolution, we find the exponential scale length to be
rather invariant with time. On the other hand, models in which star
formation/viscous evolution and infall occur concurrently result in a smoothly
increasing scale length with time, reflecting the mean angular momentum of
material which has fallen in at any given epoch. The disc stellar populations
in these models are predominantly young (ie. ages < 5 Gyr) beyond a few scale
lengths. In both cases, viscous flows are entirely responsible for transporting
material to very large radii. We discuss existing observational constraints on
these models from studies of both local and moderate redshift disc galaxies. In
particular, a good agreement is found between the solar neighbourhood star
formation history predicted by our infall model and the recent observational
determination of this quantity by Rocha-Pinto et al (2000).Comment: 12 pages inclding 9 figures, MNRAS accepte
Constraints on Galaxy Formation from Stars in the Far Outer Disk of M31
Numerical simulations of galaxy formation within the cold dark matter (CDM)
hierarchical clustering framework are unable to produce large disk galaxies
without invoking some form of feedback to suppress gas cooling and collapse to
a redshift of unity or below. An important observational consequence of
delaying the epoch of disk formation until relatively recent times is that the
stellar populations in the extended disk should of be predominantly
young-to-intermediate age. We use a deep HST/WFPC2 archival pointing to
investigate the mean age and metallicity of the stellar population in a
disk-dominated field at 30 kpc along the major axis of M31. Our analysis of the
color-magnitude-diagram reveals the dominant population to have significant
mean age (>~8 Gyr) and a moderately-high mean metallicity ([Fe/H]~-0.7);
tentative evidence is also presented for a trace population of ancient (>10
Gyr) metal-poor stars. These characteristics are unexpected in CDM models and
we discuss the possible implications of this result, as well as alternative
interpretations.Comment: Accepted to ApJ Letters, 4 pages, uses emulateapj5.sty. Minor
revisions/additions to previously posted versio
The Spectacular Ionized Interstellar Medium of NGC55
We present deep Halpha+[NII], [SII] (6716,6731A) and [OII] (3726,3729A)
images of the highly inclined, actively star--forming SBm galaxy NGC 55,
located in the nearby Sculptor Group. Due to its proximity, NGC 55 provides a
unique opportunity to study the disk--halo interface in a late--type galaxy
with unprecedented spatial resolution. Our images reveal a spectacular variety
of ionized gas features, ranging from giant HII region complexes, to supergiant
filamentary and shell features, to patches of very faint diffuse emission. Many
of these features protrude well above the plane of the galaxy, including a very
faint fragmented shell of emission which is visible at 2.6 kpc above the disk.
We identify candidate `chimneys' extending out of the disk, which could be the
conduits into the halo for hot gas around disk star-forming regions, and could
also provide low-density paths for the passage of UV photons from the disk to
the halo. Several of the identified chimneys are `capped' with clumps of
ionized gas, one of which, located at 1.5 kpc above the disk plane, appears to
be the site of recent star formation. Emission--line ratios
([OII]/Halpha+[NII], [SII]/Halpha+[NII]) constrain the ionization mechanism of
the gas, and our images allow the first measurement of [OII]/Halpha+[NII] in
extra-planar diffuse ionized gas. The diffuse gas is characterized by
emission--line ratios which are enhanced on average by a factor of two compared
to those of bright HII regions. Each line ratio increases in value smoothly
from the cores of HII regions, through the haloes of HII regions, into the
diffuse ionized gas. Such a continuous trend is predicted by models in which
the diffuse gas is ionized by photons produced by massive stars in HII regions.Comment: 19 pages, including 4 figures, plus 2 external tables. Accepted for
publication in the Astronomical Journal. Compressed, postscript versions of
the plates are available at ftp://skysrv.pha.jhu.edu/ferguson
A faint red stellar halo around an edge-on disc galaxy in the Hubble Ultra Deep Field
We analyse the detailed structure of a highly-inclined (i>~80 degrees) disc
galaxy which lies within the Hubble Ultra Deep Field (UDF). The unprecedented
depth of the UDF data allow disc and extraplanar emission to be reliably traced
to surface brightness levels of mu_{V,i,z}~29-30 mag/arcsec^2 (corresponding to
rest-frame equivalents of mu_{g,r,i}~28-29 mag/arcsec^2) in this redshift
z=0.32 system. We detect excess emission above the disc which is characterised
by a moderately-flattened (b/a~0.6) power-law (I proportional to R^(-2.6)). The
structure and colour of this component are very similar to the stellar halo
detected in an SDSS stacking analysis of local disc galaxies (Zibetti, White
and Brinkmann 2004) and lend support to the idea that we have detected a
stellar halo in this distant system. Although the peculiar colours of the halo
are difficult to understand in terms of normal stellar populations, the
consistency found between the UDF and SDSS analyses suggests that they cannot
be easily discounted.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letters. Figure
1 substantially degraded, full resolution version available at
http://www.mpa-garching.mpg.de/~zibetti/UDFhalo.pd
Near-Infrared Integral Field Spectroscopy of Damped Lyman-alpha Systems
We assess the feasibility of detecting star formation in damped Lyman-alpha
systems (DLAs) at z>1 through near-infrared spectroscopy using the forthcoming
integral field units on 8m-class telescopes. Although their relation to
galaxies is not well established, high-z DLAs contain most of the neutral gas
in the Universe, and this reservoir is depleted with time - presumably through
star formation. Line emission should be an indicator of star formation
activity, but searches based on Lyman-alpha are unreliable because of the
selective extinction of this resonant UV line. Using more robust lines such as
H-alpha forces a move to the near-infrared at z>1. For line emission searches,
spectroscopy is more sensitive than imaging, but previous long-slit
spectroscopic searches have been hampered by the likelihood that any star
forming region in the DLA galaxy disk would fall outside the narrow slit. The
new integral field units such as CIRPASS on Gemini will cover sufficient solid
angles to intercept these, even in the extreme case of large galactic disks at
high redshift. On an 8m-class telescope, star formation rates of <1M_sun/yr
will be reached at z~1.4 with H-alpha in the H-band. Such star formation rates
are well below L* for the high-z Lyman-break population, and are comparable
locally to the luminous giant HII complexes in M101. It appears that
instruments such as CIRPASS on Gemini will have both the sensitivity and the
survey area to measure star formation rates in z>1 DLAs. These observations
will probe the nature of damped Lyman-alpha systems and address their relation
to galaxies.Comment: To appear in the proceedings of the ESO/ECF workshop on "Deep
Fields", 9-12 October 2000, Garching. 4 page
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