163 research outputs found
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
Quantifying the faint structure of galaxies: the late-type spiral NGC 2403
Ground-based surveys have mapped the stellar outskirts of Local Group disc galaxies in unprecedented detail, but extending this work to other galaxies is necessary in order to overcome stochastic variations in evolutionary history and provide more stringent constraints on cosmological galaxy formation models. As part of our continuing programme of ultra-deep imagery of galaxies beyond the Local Group, we present a wide-field analysis of the isolated late-type spiral NGC 2403 using data obtained with Suprime-Cam on the Subaru telescope. The surveyed area reaches a maximum projected radius of 30 kpc or a deprojected radius of Rdp∼ 60 kpc. The colour-magnitude diagram reaches 1.5 mag below the tip of the metal-poor red giant branch (RGB) at a completeness rate >50 per cent for Rdp > rsim 12 kpc. Using the combination of diffuse light photometry and resolved star counts, we are able to trace the radial surface brightness (SB) profile over a much larger range of radii and SB than is possible with either technique alone. The exponential disc as traced by RGB stars dominates the SB profile out to ≳8 disc scalelengths, or Rdp∼ 18 kpc, and reaches a V-band SB of μV∼ 29 mag arcsec−2. Beyond this radius, we find evidence for an extended structural component with a significantly flatter SB profile than the inner disc and which we trace to Rdp∼ 40 kpc and μV∼ 32 mag arcsec−2. This component can be fit with a power-law index of γ∼ 3, has an axial ratio consistent with that of the inner disc and has a V-band luminosity integrated over all radii of 1-7 per cent that of the whole galaxy. At Rdp∼ 20 − 30 kpc, we estimate a peak metallicity [M/H] =−1.0 ± 0.3 assuming an age of 10 Gyr and zero α-element enhancement. Although the extant data are unable to discriminate between stellar halo or thick disc interpretations of this component, our results support the notion that faint, extended stellar structures are a common feature of all disc galaxies, even isolated, low-mass system
A Minor Axis Surface Brightness Profile for M31
We use data from the Isaac Newton Telescope Wide Field Camera survey of M31
to determine the surface brightness profile of M31 along the south-east minor
axis. We combine surface photometry and faint red giant branch star counts to
trace the profile from the innermost regions out to a projected radius of 4
degrees (~55 kpc) where the V-band surface brightness is 32 mag per square
arcsec; this is the first time the M31 minor axis profile has been mapped over
such a large radial distance using a single dataset. We confirm the finding by
Pritchet & van den Bergh (1994) that the minor axis profile can be described by
a single de Vaucouleurs law out to a projected radius of 1.4 degrees or ~20
kpc. Beyond this, the surface brightness profile flattens considerably and is
consistent with either a power-law of index -2.3 or an exponential of
scalelength 14 kpc. The fraction of the total M31 luminosity contained in this
component is ~2.5%. While it is tempting to associate this outer component with
a true Population II halo in M31, we find that the mean colour of the stellar
population remains approximately constant at V-i~1.6 from 0.5-3.5 degrees along
the minor axis. This result suggests that the same metal-rich stellar
population dominates both structural components.Comment: 11 pages, 3 figures, ApJ Letters in press, extremely minor
modification
The Progenitor of the Peculiar Galaxy NGC3077
We present a study of the structural properties and metallicity distribution
of the nearby peculiar galaxy NGC3077. Using data from our survey of the M81
Group with the Hyper Suprime-Cam on the Subaru Telescope, we construct deep
color-magnitude diagrams that are used to probe the old red giant branch
population of NGC3077. We map these stars out to and beyond the nominal tidal
radius, which allows us to derive the structural properties and stellar content
of the peripheral regions. We show that NGC3077 has an extended stellar halo
and pronounced ``S-shaped" tidal tails that diverge from the radial profile of
the inner region. The average metallicity of the old population in NGC3077 is
estimated from individual RGBs to be , which
decreases with the distance from the galaxy center as dex
. The metallicity of the S-shaped structure is similar to that
of the regions lying at ~kpc), indicating that
the stellar constituents of the tidal tails have come from the outer envelope
of NGC3077. These results suggest that this peculiar galaxy was probably a
rather normal dwarf elliptical galaxy before the tidal interaction with M81 and
M82. We also examine the evidence in our dataset for the six recently-reported
ultra-faint dwarf candidates around NGC3077. We recover a spatial overdensity
of sources coinciding with only one of these.Comment: 13 pages, 10 figures, Accepted for publication in Ap
The PAndAS view of the Andromeda satellite system - I. A Bayesian search for dwarf galaxies using spatial and color-magnitude information
We present a generic algorithm to search for dwarf galaxies in photometric
catalogs and apply it to the Pan-Andromeda Archaeological Survey (PAndAS). The
algorithm is developed in a Bayesian framework and, contrary to most
dwarf-galaxy-search codes, makes use of both the spatial and color-magnitude
information of sources in a probabilistic approach. Accounting for the
significant contamination from the Milky Way foreground and from the structured
stellar halo of the Andromeda galaxy, we recover all known dwarf galaxies in
the PAndAS footprint with high significance, even for the least luminous ones.
Some Andromeda globular clusters are also recovered and, in one case,
discovered. We publish a list of the 143 most significant detections yielded by
the algorithm. The combined properties of the 39 most significant isolated
detections show hints that at least some of these trace genuine dwarf galaxies,
too faint to be individually detected. Follow-up observations by the community
are mandatory to establish which are real members of the Andromeda satellite
system. The search technique presented here will be used in an upcoming
contribution to determine the PAndAS completeness limits for dwarf galaxies.
Although here tuned to the search of dwarf galaxies in the PAndAS data, the
algorithm can easily be adapted to the search for any localised overdensity
whose properties can be modeled reliably in the parameter space of any catalog.Comment: 20 pages, 16 figures, 1 table; accepted for publication in ApJ. High
res pdf available at
https://www.dropbox.com/s/7zk7pme2wunwkjv/PAndAS_dwarf_galaxies.pd
Discovery of Recent Star Formation in the Extreme Outer Regions of Disk Galaxies
We present deep Halpha images of three nearby late-type spiral galaxies
(NGC628, NGC1058 and NGC6946), which reveal the presence of HII regions out to,
and beyond, two optical radii (defined by the 25th B-band isophote). The
outermost HII regions appear small, faint and isolated, compared to their inner
disk counterparts, and are distributed in organized spiral arm structures,
likely associated with underlying HI arms and faint stellar arms. The
relationship between the azimuthally--averaged Halpha surface brightness
(proportional to star formation rate per unit area) and the total gas surface
density is observed to steepen considerably at low gas surface densities. We
find that this effect is largely driven by a sharp decrease in the covering
factor of star formation at large radii, and not by changes in the rate at
which stars form locally. An azimuthally--averaged analysis of the
gravitational stability of the disk of NGC6946 reveals that while the existence
of star formation in the extreme outer disk is consistent with the Toomre-Q
instability model, the low rates observed are only compatible with the model
when a constant gaseous velocity dispersion is assumed. We suggest that
observed behaviour could also be explained by a model in which the star
formation rate has an intrinsic dependence on the azimuthally-averaged gas
volume density, which decreases rapidly in the outer disk due to the vertical
flaring of the gas layer.Comment: 10 pages, 2 embedded postscript files, 3 jpeg images; accepted for
publication in ApJ Letter
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