Star Formation Density and Halpha Luminosity Function of an Emission Line Selected Galaxy Sample at z ~ 0.24

We use narrowband imaging (FWHM = 70 A) to select a sample of emission line galaxies between 0.20 <~ z <~ 1.22 in two fields covering 0.5 sq. deg. We use spectroscopic follow-up to select a sub-sample of Halpha emitting galaxies at z ~ 0.24 and determine the Halpha luminosity function and star formation density at z ~ 0.24 for both of our fields. Corrections are made for imaging and spectroscopic incompleteness, extinction and interloper contamination on the basis of the spectroscopic data. When compared to each other, we find the field samples differ by \Delta \alpha = 0.2 in faint end slope and \Delta \log [ L* (ergs/s) ] = 0.2 in luminosity. In the context of other recent surveys, our sample has comparable faint end slope, but a fainter L* turn-over. We conclude that systematic uncertainties and differences in selection criteria remain the dominant sources of uncertainty between Halpha luminosity functions at this redshift. We also investigate average star formation rates as a function of local environment and find typical values consistent with the field densities that we probe, in agreement with previous results. However, we find tentative evidence for an increase in star formation rate with respect to the local density of star forming galaxies, consistent with the scenario that galaxy-galaxy interactions are triggers for bursts of star formation.Comment: Accepted for publication in MNRAS. The paper contains 14 figures and 7 table

Triggered Star Formation in Galaxy Pairs at z=0.08-0.38

We measure the strength, frequency, and timescale of tidally triggered star formation at redshift z=0.08-0.38 in a spectroscopically complete sample of galaxy pairs drawn from the magnitude-limited redshift survey of 9,825 Smithsonian Hectospec Lensing Survey (SHELS) galaxies with R<20.3. To examine the evidence for tidal triggering, we identify a volume-limited sample of major (|\Delta M_R|1/5) pair galaxies with $M_R < -20.8 in the redshift range z=0.08-0.31. The size and completeness of the spectroscopic survey allows us to focus on regions of low local density. The spectrophotometric calibration enables the use of the 4000 Ang break (D_n4000), the H\alpha specific star formation rate (SSFR_{H\alpha}), and population models to characterize the galaxies. We show that D_n4000 is a useful population classification tool; it closely tracks the identification of emission line galaxies. The sample of major pair galaxies in regions of low local density with low D_n4000 demonstrates the expected anti-correlation between pair-wise projected separation and a set of star formation indicators explored in previous studies. We measure the frequency of triggered star formation by comparing the SSFR_{H\alpha} in the volume-limited sample in regions of low local density: 32 +/-7% of the major pair galaxies have SSFR_{H\alpha} at least double the median rate of the unpaired field galaxies. Comparison of stellar population models for pair and for unpaired field galaxies implies a timescale for triggered star formation of ~300-400 Myr.Comment: 25 pages, 15 figures. Accepted to A

Star Formation Indicators and Line Equivalent Width in Lyman Alpha Galaxies

The equivalent width (EW) of the Lyman Alpha (Lya) line is directly related to the ratio of star formation rates determined from Lya flux and UV flux density [SFR(Lya)/SFR(UV)]. We use published data --in the literature EW and SFR(Lya)/SFR(UV) are treated as independent quantities-- to show that the predicted relation holds for the vast majority of observed Lya emitting galaxies (LAEs). We show that the relation between EW and SFR(Lya)/SFR(UV) applies irrespective of a galaxy's `true' underlying star formation rate, and that its only source of scatter is the variation in the spectral slope of the UV continuum between individual galaxies. The derived relation, when combined with the observed EW distribution, implies that the ratio SFR(UV)/SFR(Lya) is described well by a log-normal distribution with a standard deviation of ~0.3-0.35. This result is useful when modelling the statistical properties of LAEs. We further discuss why the relation between EW and SFR(Lya)/SFR(UV) may help identifying galaxies with unusual stellar populations.Comment: 6 pages, 4 figures, accepted to MNRA

Evolution of the Halpha luminosity function

The Smithsonian Hectospec Lensing Survey (SHELS) is a window on the star formation history over the last 4 Gyr. SHELS is a spectroscopically complete survey for Rtot < 20.3 over 4 square degrees. We use the 10k spectra to select a sample of pure star forming galaxies based on their Halpha emission line. We use the spectroscopy to determine extinction corrections for individual galaxies and to remove active galaxies in order to reduce systematic uncertainties. We use the large volume of SHELS with the depth of a narrowband survey for Halpha galaxies at z ~ 0.24 to make a combined determination of the Halpha luminosity function at z ~ 0.24. The large area covered by SHELS yields a survey volume big enough to determine the bright end of the Halpha luminosity function from redshift 0.100 to 0.377 for an assumed fixed faint-end slope alpha = -1.20. The bright end evolves: the characteristic luminosity L* increases by 0.84 dex over this redshift range. Similarly, the star formation density increases by 0.11 dex. The fraction of galaxies with a close neighbor increases by a factor of 2-5 for L(Halpha) >~ L* in each of the redshift bins. We conclude that triggered star formation is an important influence for star forming galaxies with Halpha emission.Comment: 26 pages, 23 figures, submitted to ApJ; version with high resolution figures available at http://www.cfa.harvard.edu/~ewestra/publications