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Register a new userExtended UV Disks and UV-Bright Disks in Low-Mass E/S0 Galaxies
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Amanda J. Moffett
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We have identified 15 XUV disks in a largely field sample of 38 E/S0 galaxies with stellar masses primarily below ~4 x 10^10 M_sun and comparable numbers on the red and blue sequences. We use a new purely quantitative XUV disk definition requiring UV extension relative to a UV-defined star formation threshold radius. The 39(+-9)% XUV-disk frequency for these E/S0s is roughly twice the ~20% reported for late types, possibly indicating that XUV disks are associated with galaxies experiencing weak or inefficient star formation. Consistent with this interpretation, the XUV disks in our sample do not correlate with enhanced outer-disk star formation as traced by blue optical outer-disk colors. However, UV-Bright (UV-B) disk galaxies with blue UV colors outside their optical 50% light radii do display enhanced optical outer-disk star formation as well as enhanced atomic gas content. UV-B disks occur with a 42(+9/-8)% frequency, and the combined XUV/UV-B frequency is 61(+-9)%. For both types, UV colors typically imply <1 Gyr ages. XUV disks occur over the full sample mass range and on both sequences, suggesting an association with galaxy interactions or another general evolutionary process. In contrast, UV-B disks favor the blue sequence and may also prefer low masses, perhaps reflecting the onset of cold-mode accretion or another mass-dependent evolutionary process. Virtually all blue E/S0s in the gas-rich regime below stellar mass M_t ~ 5 x 10^9 M_sun (the gas-richness threshold mass) display UV-B disks. [abridged]
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We describe HST imaging of recent star formation complexes located in the extended UV disk (XUV-disk) component of NGC 5236 (M 83), NGC 5055 (M 63), and NGC 2090. Photometry in four FUV--visible bands permits us to constrain the type of resolved stars and effective age of clusters, in addition to extinction. The preliminary results given herein focus on CMD analysis and clustering properties in this unique star-forming environment.
We summarize the main properties of the extended UV (XUV) emission found in roughly 30% of the nearby spiral galaxies observed by the GALEX satellite. Two different classes of XUV disks are identified, the Type 1 XUV disks where significant, structured UV-bright features are found beyond the classical azimuthally-averaged star-formation threshold, and the Type 2 XUV disks, which are characterized by very extended (seven times the area where most of the stellar mass is found), blue [(FUV-K)<5mag] outer disks. These latter disks are extreme examples of galaxies growing inside-out. The few XUV disks studied in detail to date are rich in HI but relatively poor in molecular gas, have stellar populations with luminosity-weighted ages of ~1 Gyr, and ionized-gas metal abundances of ~Zsun/10. As part of the CAHA-XUV project we are in the process of obtaining deep multi-wavelength imaging and spectroscopy of 65 XUV-disk galaxies so to determine whether or not these properties are common among XUV disks.
Most of the massive star-forming galaxies are found to have `inside-out stellar mass growth modes, which means the inner parts of the galaxies mainly consist of the older stellar population, while the star forming in the outskirt of the galaxy is still ongoing. The high-resolution HST images from Hubble Deep UV Legacy Survey (HDUV) and Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) projects with the unprecedented depth in both F275W and F160W bands are the perfect data sets to study the forming and formed stellar distribution directly. We selected the low redshift ($0.05 < z_{rm spec} < 0.3$) galaxy sample from the GOODS-North field where the HST F275W and F160W images are available. Then we measured the half light radius in F275W and F160W bands, which are the indicators of the star formation and stellar mass. By comparing the F275W and F160W half light radius, we find the massive galaxies are mainly follow the `inside-out growth which is consistent with the previous results. Moreover, the HST F275W and F160W images reveal that some of the low-mass galaxies ($<10^8M_odot$) have the `outside-in growth mode: their images show a compact UV morphology, implying an ongoing star formation in the galaxy centre, the stars in the outskirts of the galaxies are already formed. The two modes transit smoothly at stellar mass range about $10^{8-9}M_odot$ with a large scatter. We also try to identify the possible neighbour massive galaxies from the SDSS data, which represent the massive galaxy sample. We find that all of the spec-z selected galaxies have no massive galaxy nearby. Thus the `outside-in mode we find in the low-mass galaxies are not likely originated from the environment.
The recent discovery of extended ultraviolet (XUV) disks around a large fraction of late-type galaxies provides evidence for unexpectedly large-scale disk building at recent epochs. Combining GALEX UV observations with deep optical and Spitzer IR imaging, we search for XUV disks in a sample of nearby low-to-intermediate mass E/S0 galaxies to explore evidence for disk rebuilding after mergers. Preliminary visual classification yields ten XUV-disk candidates from the full sample of 30, intriguingly similar to the ~30% frequency for late-type galaxies. These XUV candidates occur at a wide range of masses and on both the red and blue sequences in color vs. stellar mass, indicating a possible association with processes like gas accretion and/or galaxy interactions that would affect the galaxy population broadly. We go on to apply the quantitative Type 1 and Type 2 XUV-disk definitions to a nine-galaxy subsample analyzed in detail. For this subsample, six of the nine are Type 1 XUVs, i.e., galaxies with UV structure beyond the expected star formation threshold. The other three come close to satisfying the Type 2 definition, but that definition proves problematic to apply to this sample: the NUV-derived star formation threshold radii for our E/S0s often lie inside the 80% Ks-band light (K80) radii, violating an implicit assumption of the Type 2 definition, or lie outside but not as far as the definition requires. Nonetheless, the three otherwise Type 2-like galaxies (modified Type 2 XUVs) have higher star formation rates and bluer FUV - NUV colors than the Type 1 XUVs in the sample. We propose that Type 1 XUVs may reflect early or inefficient stages of star formation, while modified Type 2 XUVs perhaps reflect inside-out disk regrowth.
We identify a high frequency of Type 1 XUV disks, reflecting recent outer disk star formation, in a sample of 31 E/S0s with stellar masses primarily below M_* ~ 4 x 10^10 M_sun. Our ~40% identification rate is roughly twice the 20% fraction reported for late-type galaxies. Intriguingly, in the dwarf mass regime (below M_* ~ 5 x 10^9 M_sun) where gas fractions clearly rise, Type 1 XUV disks occur in ~70% of red-sequence E/S0s but only ~20% of blue-sequence E/S0s, a population recently linked to active disk rebuilding, especially in the dwarf regime. Our statistics are preliminary, but could indicate that for dwarf E/S0s Type 1 XUV disks are primarily related to weak or inefficient outer-disk star formation rather than to star formation capable of driving substantial disk growth. Substantial growth may instead be associated with populations that have low XUV-disk frequency, possibly explaining the similar ~20% frequencies for normal late types and low-mass blue-sequence E/S0s.
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