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

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 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

The Stellar Halo and Outer Disk of M33

We present first results from a Keck DEIMOS spectroscopic survey of red giant branch (RGB) stars in M33. The radial velocity distributions of the stars in our fields are well described by three Gaussian components, corresponding to a candidate halo component with an uncorrected radial velocity dispersion of σ ≃ 50 km s^(-1), a candidate disk component with a dispersion σ ≃ 16 km s^(-1), and a third component offset from the disk by ~50 km s^(-1), but for which the dispersion is not well constrained. By comparing our data to a model of M33 based on its H I rotation curve, we find that the stellar disk is offset in velocity by ~25 km s^(-1) from the H I disk, consistent with the warping that exists between these components. The spectroscopic metallicity of the halo component is [Fe/H] ≃ -1.5, significantly more metal-poor than the implied metallicity of the disk population ([Fe/H] ≃ -0.9), which also has a broader color dispersion than the halo population. These data represent the first detections of individual stars in the halo of M33 and, despite being ~10 times less massive than M31 or the Milky Way, all three of these disk galaxies have stellar halo components with a similar metallicity. The color distribution of the third component is different from the disk and the halo but is similar to that expected for a single, coeval, stellar population, and could represent a stellar stream. More observations are required to determine the true nature of this intriguing third kinematic component in M33

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 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

Inferring the Andromeda Galaxy's mass from its giant southern stream with Bayesian simulation sampling

M31 has a giant stream of stars extending far to the south and a great deal of other tidal debris in its halo, much of which is thought to be directly associated with the southern stream. We model this structure by means of Bayesian sampling of parameter space, where each sample uses an N-body simulation of a satellite disrupting in M31's potential. We combine constraints on stellar surface densities from the Isaac Newton Telescope survey of M31 with kinematic data and photometric distances. This combination of data tightly constrains the model, indicating a stellar mass at last pericentric passage of log(M_s / Msun) = 9.5+-0.1, comparable to the LMC. Any existing remnant of the satellite is expected to lie in the NE Shelf region beside M31's disk, at velocities more negative than M31's disk in this region. This rules out the prominent satellites M32 or NGC 205 as the progenitor, but an overdensity recently discovered in M31's NE disk sits at the edge of the progenitor locations found in the model. M31's virial mass is constrained in this model to be log(M200) = 12.3+-0.1, alleviating the previous tension between observational virial mass estimates and expectations from the general galactic population and the timing argument. The techniques used in this paper, which should be more generally applicable, are a powerful method of extracting physical inferences from observational data on tidal debris structures.Comment: 27 pages, 10 figures. Accepted by MNRA

A Deep Photometric Look at Two of Andromeda's Dwarf Spheroidals: X and XVII

We use deep wide-field photometry from the Large Binocular Camera to study the stellar and structural properties of the recently discovered Andromeda X and Andromeda XVII (And X and And XVII) dwarf galaxies. Using the mean apparent magnitude of the horizontal branch (HB), we derive distances of 621 +- 20 kpc to And X and 734+- 23 kpc to And XVII, closer by >60 kpc than the previous estimates which were based on red giant branch (RGB) observations. Thus our results warrant against the use of the RGB tip method for determining distances to systems with sparsely populated RGBs, and show how crucial HB observations are in obtaining accurate distances in systems such as these. We find that And X is a relatively faint (MV = -7.36), highly elongated (e = 0.48) system at a distance of 174 +- 62 kpc from Andromeda. And XVII is brighter (MV = -8.61) with an M31-centric distance of 73 kpc which makes it one of the closest satellites to Andromeda. Both galaxies are metal-poor: we derive =-2.2 for And X, while And XVII shows = -2.0, consistent with the relation of higher luminosity dwarfs being more metal- rich. Additionally, both galaxies show considerable intrinsic spreads in metallicity (0.2 and 0.3 dex for And X and And XVII respectively), consistent with multiple stellar populations.Comment: 8 pages, 6 figure