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Register a new userThe Swift/UVOT catalogue of NGC4321 star forming sources: A case against density wave theory
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Ignacio Ferreras
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We study the star forming regions in the spiral galaxy NGC4321, taking advantage of the spatial resolution (2.5 arcsec FWHM) of the Swift/UVOT camera and the availability of three UV passbands in the region 1600-3000 A, in combination with optical and IR imaging from SDSS, KPNO/Ha and Spitzer/IRAC, to obtain a catalogue of 787 star forming regions out to three disc scale lengths. We determine the properties of the young stellar component and its relationship with the spiral arms. The Ha luminosities of the sources have a strong decreasing radial trend, suggesting more massive star forming regions in the central part of the galaxy. When segregated with respect to NUV-optical colour, blue sources have a significant excess of flux in the IR at 8 micron, revealing the contribution from PAHs, although the overall reddening of these sources stays below E(B-V)=0.2 mag. The distribution of distances to the spiral arms is compared for subsamples selected according to Ha luminosity, NUV-optical colour, or ages derived from a population synthesis model. An offset is expected between these subsamples as a function of radius if the pattern speed of the spiral arm were constant - as predicted by classic density wave theory. No significant offsets are found, favouring instead a mechanism where the pattern speed has a radial dependence.
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We present the first Swift Ultraviolet/Optical Telescope Serendipitous Source Catalogue (UVOTSSC). The catalogue was compiled from 23,059 Swift datasets taken within the first five years of observations with the Swift UVOT. A purpose-built processing pipeline, based around the standard Swift processing tools, was employed. The catalogue contains positions, photometry in three UV and three optical bands, morphological information and data quality flags. In total, the catalogue contains 6,200,016 unique sources of which more than 2 million have multiple observations in the catalogue.
H{alpha} observations centred on galaxies selected from the HI Parkes All Sky Survey (HIPASS, Barnes et al. 2001) typically show one and sometimes two star-forming galaxies within the approximately 15-arcminute beam of the Parkes 64-m HI detections. In our Survey of Ionization in Neutral Gas Galaxies (SINGG, Meurer et al. 2006) we found fifteen cases of HIPASS sources containing four or more emission line galaxies (ELGs). We name these fields Choir groups. In the most extreme case we found a field with at least nine ELGs. In this paper we present a catalogue of Choir group members in the context of the wider SINGG sample. The dwarf galaxies in the Choir groups would not be individually detectable in HIPASS at the observed distances if they were isolated, but are detected in SINGG narrow-band imaging due to their membership of groups with sufficiently large total HI mass. The ELGs in these groups are similar to the wider SINGG sample in terms of size, H{alpha} equivalent width, and surface brightness. Eight of these groups have two large spiral galaxies with several dwarf galaxies and may be thought of as morphological analogues of the Local Group. However, on average our groups are not significantly HI-deficient, suggesting that they are at an early stage of assembly, and more like the M81 group. The Choir groups are very compact at typically only 190 kpc in projected distance between the two brightest members. They are very similar to SINGG fields in terms of star formation efficiency (the ratio of star formation rate to HI mass; SFE), showing an increasing trend in SFE with stellar mass.
We study the star formation rates (SFRs) of galaxies as a function of local galaxy density at 0.6<z<0.9. We used a low-dispersion prism in IMACS on the 6.5-m Baade (Magellan I) telescope to obtain spectra and measured redshifts to a precision of sigma_z/(1+z)=1% for galaxies with z<23.3 AB mag. We utilized a stellar mass-limited sample of 977 galaxies above M>1.8x10^{10} Msun to conduct our main analysis. With three different SFR indicators, (1) Spitzer MIPS 24-micron imaging, (2) SED fitting, and (3) [OII]3727 emission, we find the median specific SFR (SSFR) and SFR to decline from the low-density field to the cores of groups and a rich cluster. For the SED and [OII] based SFRs, the decline in SSFR is roughly an order of magnitude while for the MIPS based SFRs, the decline is a factor of ~4. We find approximately the same magnitude of decline in SSFR even after removing the sample of galaxies near the cluster. Galaxies in groups and a cluster at these redshifts therefore have lower star formation (SF) activity than galaxies in the field, as is the case at z~0. We investigated whether the decline in SFR with increasing density is caused by a change in the proportion of quiescent and star forming galaxies (SFGs) or by a decline in the SFRs of SFGs. Using the rest-frame U-V and V-J colors to distinguish quiescent galaxies from SFGs we find the fraction of quiescent galaxies increases from ~32% to 79% from low to high density. In addition, we find the SSFRs of SFGs, selected based on U-V and V-J colors, to decline with increasing density by factors of ~5-6 for the SED and [OII] based SFRs. The MIPS based SSFRs for SFGs decline with a shallower slope. The order of magnitude decline in the SSFR-density relation at 0.6<z<0.9 is therefore driven by both a combination of declining SFRs of SFGs as well as a changing mix of SFGs and quiescent galaxies [ABRIDGED].
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Observations in the near- and mid-ultraviolet (NUV: 2000--3500$AA$) performed with the NASA Swift UVOT instrument have revealed that optically-normal SNe Ia feature NUV-optical color evolution that can be divided into NUV-blue and NUV-red groups, with roughly one-third of the observed events exhibiting NUV-blue color curves. Combined with an apparent correlation between NUV-blue events and the detection of unburned carbon in the optical spectra, the grouping might point to a fundamental difference within the normal SN Ia classfication. Recognizing the dramatic temporal evolution of the NUV-optical colors for all SNe Ia, as well as the existence of this sub-division, is important for studies that compare nearby SNe Ia with intermediate or high-$z$ events, for the purpose of the cosmological utilization of SNe Ia. SN 2011fe is shown to be of the NUV-blue groups, which will be useful towards interpretation of the gamma-ray line results from the INTEGRAL SPI campaign on SN 2011fe.
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