Star-Formation in Close Pairs Selected from the Sloan Digital Sky Survey Overview The effect of galaxy interactions on star formation has been investigated using Data Release 1 of the Sloan Digital Sky Survey (SDSS). The spectroscopic and imaging data products from SDSS were used to construct a catalogue of nearest companions for a volume-limited sample of galaxies (0.03 < z < 0.1) drawn from the Main Galaxy Sample of SDSS galaxies were identified as having companions with separations less than 300 kpc. Star formation rates (SFRs) for the volume-limited sample have been calculated from aperture corrected Hα luminosities and, where available, IRAS data. Specific star formation rates (SSFRs) were calculated by estimating galaxy masses from z-band luminosities. The r-band inverse concentration index was used to distinguish between morphological class. The mean specific star formation rate is found to be significantly enhanced for projected separations less than 25 kpc. For late-type galaxies, the correlation extends out to projected separations of 300 kpc. A tight relationship exists between galaxy concentration index and projected pair separation; the mean concentration index peaks at pair separation of  75 kpc, falling rapidly for smaller separations. We find no dependence of the star formation enhancement on morphological type or mass of the companion galaxy. Selection of Primary Catalogue Using the Main Galaxy Sample from SDSS Data Release 1 (Strauss et al. 2002) covering 1360 deg 2, we select a complete, volume and luminosity sub-sample of galaxies with spectroscopic data – the Primary Catalogue. The Primary Catalogue of galaxies was selected according to the following criteria: –Removal of sources which had been de-blended into multiple sources and removal of duplicate observations. –Volume limited 0.03 < z helio < the lower redshift limit avoids large aperture corrections whilst the upper redshift limit allows selection of a complete sample of galaxies by applying a luminosity constraint. –Luminosity limited - M r < only galaxies bright enough to be observed as part of the main galaxy sample even if they fall at the edge of the sample volume are retained. –Redshift confidence > selects galaxies with secure redshifts without introducing significant bias towards active emission line systems. –These constraints yielded a sample of  galaxies. Galaxies believed to contain AGN were removed using the standard technique of examining the position of the galaxy on the [N II]/H  vs [O III]/H  plot (Villieux & Osterbrock 1987). After AGN removal  galaxies. Harriet Cullen, Bojan Nikolic, Paul Alexander Cavendish Astrophysics University of Cambridge, Department of Physics Sloan: An Introduction The SDSS is a new, digital, photometric & spectroscopic sky survey. Imaging is done in five broad photometric bands (ugriz) with a photometric camera, pixel size 0.4 arcsec. Based on the imaging data several samples of systems are identified for spectroscopic follow up, including the Main Galaxy Sample - galaxies with r-band Pertrosian magnitudes brighter than Spectroscopy is performed using a medium resolution spectrograph (R  ), wavelength range Angstroms. Morphological Classification: Concentration Index The SDSS imaging pipeline measures the galaxy light distribution as a function of radius: it measures the radii containing 50% and 90% of the Petrosian flux of the galaxy. The ratio of these radii is the (inverse) concentration index: C = 50% light radius / 90% light radius and correlates with morphological type (e.g. Morgan 1958) Low values of concentration index = systems with centrally condensed light profiles – de Vaucouleurs profiles – galaxies of early morphological type. Possible to distinguish between early and late-type galaxies. Using the concentration index measured from the SDSS r-band imaging data we select three morphological sub-samples of galaxies: early-type C  0.33, mixed systems. Construction of the Pair Catalogue To investigate the effects of galaxy interactions on star formation we identify the nearest companion galaxy to each member of our primary, volume-limited catalogue. Spectroscopic fibres could not target two galaxies with separation less than 55 arcsec. This introduces a significant bias against the detection of close pairs within the Main Galaxy Sample. We therefore based our search for close companions on the imaging data using measured redshifts when available. The algorithm used to establish the closest pair out to a maximum radius of 300 kpc is illustrated in Figure 2. In the absence of redshift information galaxy pairs are selected based on their z-band magnitude difference - approximates a selection by mass. Examination of  z spectral-spectral pairs indicates a completeness of 80% for  z = 2 We expect no significant contamination by interlopers out to separations  150 kpc. Star Formation Rates Spectroscopy is available for the entire primary catalogue => Hα fluxes used to measure the SFR using calibration of Kennicutt (1998). Hα line need not be prominent to be measured. A number of corrections are applied to the Hα fluxes before computation of the SFR. –Hα and H  lines corrected for stellar absorption. –Correction for dust obscuration estimated from Hα to H  line ratio assuming an intrinsic ratio of 2.86 and the extinction curve of Cardelli et al. (1989) –Correction made for 3 arcsec diameter of SDSS spectroscopic fibres - smaller than most galaxies in the sample - Hα flux rescaled based on the ratio of the r-band flux measured using the spectroscopic fibre and the total r-band flux (Hopkins et al 2003). Major uncertainties arise from stellar absorption corrections, the dust extinction and aperture corrections. Normalization of Star Formation Rates Star formation rates scale with galaxy mass – more suitable measure of tidal triggering is the specific star formation rate – SFR divided by stellar mass. z-band luminosity used to estimate galaxy mass and normalise SFRs. z-band best tracer of galaxy mass available for entire primary catalogue - only marginally affected by extinction For galaxies in the Two Micron All Sky Survey Extended Source Catalogue – we observe a good correlation between SDSS z-band magnitude and 2MASS K-band magnitude. Figure 1. The Sloan telescope. Figure 2. The Pair Finding Algorithm Figure 3. Some examples of close galaxy pairs.