Gravitational lensing is a powerful astrophysical and cosmological probe and is particularly valuable at submillimeter wavelengths for the study of the statistical and individual properties of dusty star-forming galaxies. However, the identification of gravitational lenses is often time-intensive, involving the sifting of large volumes of imaging or spectroscopic data to find few candidates. We used early data from the Herschel Astrophysical Terahertz Large Area Survey to demonstrate that wide-area submillimeter surveys can simply and easily detect strong gravitational lensing events, with close to 100% efficiency.

We analyze optical spectra of a sample of 502 galaxies in close pairs and n-tuples, separated by ≤ 50h −1 kpc. We extracted the sample objectively from the CfA2 redshift survey, without regard to the surroundings of the tight systems; we re-measure the spectra with longer exposures, to explore the spectral characteristics of the galaxies. We use the new spectra to probe the relationship between star formation and the dynamics of the systems of galaxies.The equivalent widths of Hα (EW(Hα)) and other emission lines anti-correlate strongly with pair spatial separation (∆D) and velocity separation; the anti-correlations do not result from any large-scale environmental effects that we detect. We use the measured EW(Hα) and the starburst models of Leitherer et al. to estimate the time since the most recent burst of star formation began for galaxies in our sample. In the absence of a large contribution from an old stellar population to the continuum around Hα that correlates with the orbit parameters, the observed ∆D -EW(Hα) correlation signifies that starbursts with larger separations on the sky are, on average, older. We also find a population of galaxies with small to moderate amounts of Balmer absorption. These galaxies suport our conclusion that the sample includes many aging bursts of star formation; they have a narrower distribution of velocity separations, consistent with a population of orbiting galaxies near apogalacticon.By matching the dynamical timescale to the burst timescale, we show that the data support a simple picture in which a close pass initiates a starburst; EW(Hα) decreases with time as the pair separation increases, accounting for the anti-correlation. Recent n-body/SPH simulations of interacting pairs suggest a physical basis for the correlation -for galaxies with shallow central potentials, they predict gas infall before the final merger. This picture leads to a method for measuring the duration and the initial mass function of interaction-induced starbursts: our data are compatible with the starburst models and orbit models in many respects, as long as the starburst lasts longer than ∼10 8 years and the delay between the close pass and the initiation of the starburst is less than a few×10 7 years. If there is no large contribution from an old stellar population to the continuum around Hα, the Miller-Scalo and cutoff (M≤ 30 M ⊙ ) Salpeter initial mass functions fit the data much better than a standard Salpeter IMF.

We present multiband photometry of 185 type-Ia supernovae (SNe Ia), with over 11,500 observations. These were acquired between 2001 and 2008 at the F. L. Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics (CfA). This sample contains the largest number of homogeneously observed and reduced nearby SNe Ia (z 0.08) published to date. It more than doubles the nearby sample, bringing SN Ia cosmology to the point where systematic uncertainties dominate. Our natural system photometry has a precision of 0.02 mag in BV RI r i and 0.04 mag in U for points brighter than 17.5 mag. We also estimate a systematic uncertainty of 0.03 mag in our SN Ia standard system BV RI r i photometry and 0.07 mag for U. Comparisons of our standard system photometry with published SN Ia light curves and comparison stars, where available for the same SN, reveal agreement at the level of a few hundredths mag in most cases. We find that 1991bg-like SNe Ia are sufficiently distinct from other SNe Ia in their color and light-curve-shape/ luminosity relation that they should be treated separately in light-curve/distance fitter training samples. The CfA3 sample will contribute to the development of better light-curve/distance fitters, particularly in the few dozen cases where near-infrared photometry has been obtained and, together, can help disentangle host-galaxy reddening from intrinsic supernova color, reducing the systematic uncertainty in SN Ia distances due to dust.

We derive the first luminosity-metallicity relation for a large objectively selected sample of local galaxy pairs and we compare the pairs LZ relation with the relation for the Nearby Field Galaxy Survey (NFGS). Galaxy pair members with small projected separations (s < 20 kpc/h) have systematically lower metallicities (∼ 0.2 dex on average) than the field galaxies, or than more widely separated pairs at the same luminosity. There is a strong correlation between metallicity and central burst strength in the galaxy pairs. All five galaxies in the pairs sample with strong central bursts have close companions and metallicities lower than the comparable field galaxies. Our results provide strong observational evidence for a merger scenario where galaxy interactions cause gas flows towards the central regions, carrying less enriched gas from the outskirts of the galaxy into the central regions. The less enriched gas dilutes the pre-existing nuclear gas to produce a lower metallicity than would be obtained prior to the interaction. These gas flows trigger central bursts of star formation, causing strong central burst strengths, and possibly aiding the formation of blue bulges. We show that the timescale and central gas dilution required by this scenario are consistent with predictions from hydrodynamic merger models.

We present the first systematic investigation into the metallicity gradients in galaxy close pairs. We determine the metallicity gradients for 8 galaxies in close pairs using H II region metallicities obtained with high signal-to-noise multi-slit observations with the Keck LRIS Spectrograph. We show that the metallicity gradients in close pairs are significantly shallower than gradients in isolated spiral galaxies such as the Milky Way, M83, and M101. These observations provide the first solid evidence that metallicity gradients in interacting galaxies are systematically different from metallicity gradients in isolated spiral galaxies. Our results suggest that there is a strong relationship between metallicity gradients and the gas dynamics in galaxy interactions and mergers.

We use cosmological SPH simulations to study the kinematic signatures of cool gas accretion onto a pair of wellresolved galaxy halos. Cold-flow streams and gas-rich mergers produce a circum-galactic component of cool gas that generally orbits with high angular momentum about the galaxy halo before falling in to build the disk. This signature of cosmological accretion should be observable using background-object absorption line studies as features that are offset from the galaxys systemic velocity by ∼ 100 km/s. Accreted gas typically co-rotates with the central disk in the form of a warped, extended cold flow disk, such that the observed velocity offset is in the same direction as galaxy rotation, appearing in sight lines that avoid the galactic poles. This prediction provides a means to observationally distinguish accreted gas from outflow gas: the accreted gas will show large one-sided velocity offsets in absorption line studies while radial/bi-conical outflows will not (except possibly in special polar projections). This rotation signature has already been seen in studies of intermediate redshift galaxy-absorber pairs; we suggest that these observations may be among the first to provide indirect observational evidence for cold accretion onto galactic halos. Cold mode halo gas typically has ∼ 3 − 5 times more specific angular momentum than the dark matter. The associated cold mode disk configurations are likely related to extended HI/XUV disks seen around galaxies in the local universe. The fraction of galaxies with extended cold flow disks and associated offset absorption-line gas should decrease around bright galaxies at low redshift, as cold mode accretion dies out.

We present UBVRI photometry of 44 type-Ia supernovae (SN Ia) observed from 1997 to 2001 as part of a continuing monitoring campaign at the Fred Lawrence Whipple Observatory of the Harvard-Smithsonian Center for Astrophysics. The data set comprises 2190 observations and is the largest homogeneously observed and reduced sample of SN Ia to date, nearly doubling the number of well-observed, nearby SN Ia with published multicolor CCD light curves. The large sample of U-band photometry is a unique addition, with important connections to SN Ia observed at high redshift. The decline rate of SN Ia U-band light curves correlates well with the decline rate in other bands, as does the U −B color at maximum light. However, the U-band peak magnitudes show an increased dispersion relative to other bands even after accounting for extinction and decline rate, amounting to an additional ∼40% intrinsic scatter compared to B-band.Subject headings: supernovae: general -techniques: photometric Data and Reduction DiscoveryOur program of supernova photometry consists solely of follow-up; we search only our email, not the sky, to find new supernovae. A number of observers, both amateur and professional, are engaged in searching for supernovae. We rely on these searches, as well as prompt notification of candidates, coordinated by Dan Green and Brian Marsden of the IAU's Central Bureau for Astronomical Telegrams (CBAT), with confirmed SN reported in the IAU Circulars. In some cases the SN discoverers provide spectroscopic classification of the new objects, but generally spectroscopy is obtained by others, and reported separately in the IAU Circulars. With our spectroscopic SN follow-up program at the F. L. Whipple Observatory 1.5m telescope and FAST spectrograph (Fabricant et al. 1998), we have classified a large fraction of the new, nearby supernovae reported over the last several years and compiled a large spectroscopic database (Matheson et al. 2005, in preparation).Given a newly discovered and classified supernova, several factors help determine whether or not we include it in our monitoring program. Because of their importance, SN Ia are often given higher priority over other types, but factors such as ease of observability (southern targets and those discovered far to the west are less appealing), supernova phase (objects whose spectra indicate they are after maximum light are given lower priority), redshift (more nearby objects are favored), as well as the number of objects we are already monitoring are significant. Our final sample of well-observed SN Ia is not obtained from a single well-defined set of criteria, and selection effects in both the searches and follow-up may make this sample unsuitable for some applications (such as determining the intrinsic luminosity function of SN Ia, for example). A thorough discussion of the selection biases in the Calán/Tololo supernova search and follow-up campaign can be found in Hamuy & Pinto (1999).The discovery data for the sample of SN Ia presented here are given in Table 1. All of the ...

A rare class of 'superluminous' supernovae that are about ten or more times more luminous at their peaks than other types of luminous supernova has recently been found at low to intermediate redshifts. A small subset of these events have luminosities that evolve slowly and result in radiated energies of up to about 10(51) ergs. Therefore, they are probably examples of 'pair-instability' or 'pulsational pair-instability' supernovae with estimated progenitor masses of 100 to 250 times that of the Sun. These events are exceedingly rare at low redshift, but are expected to be more common at high redshift because the mass distribution of the earliest stars was probably skewed to high values. Here we report the detection of two superluminous supernovae, at redshifts of 2.05 and 3.90, that have slowly evolving light curves. We estimate the rate of events at redshifts of 2 and 4 to be approximately ten times higher than the rate at low redshift. The extreme luminosities of superluminous supernovae extend the redshift limit for supernova detection using present technology, previously 2.36 (ref. 8), and provide a way of investigating the deaths of the first generation of stars to form after the Big Bang.