Mark Hancock

The Spitzer Spirals, Bridges, and Tails Interacting Galaxy Survey: Interaction-Induced Star Formation in the Mid-Infrared

We present Spitzer mid-infrared images from a survey of three dozen pre-merger strongly interacting galaxy pairs selected from the Arp Atlas. The global mid-infrared colors of these galaxies and their tidal tails and bridges are similar to those of normal spiral galaxies, thus this optically selected sample of interacting galaxies does not have strongly enhanced normalized star formation rates in their disks or tidal features. Despite distortion and disturbance these systems continue to form stars at a normal rate on average. The morphology of these galaxies is generally smoother in the shorter wavelength IRAC bands than at 8

CANDIDATE TIDAL DWARF GALAXIES IN Arp 305: LESSONS ON DWARF DETACHMENT AND GLOBULAR CLUSTER FORMATION

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Large-Scale Star Formation Triggering in the Low-Mass Arp 82 System: A Nearby Example of Galaxy Downsizing Based on UV/Optical/Mid-IR Imaging

m, where dozens of clumps of star formation are detected.

THE ORIGIN OF THE 4.5 μm EXCESS FROM DWARF GALAXIES

To search for Tidal Dwarf Galaxies (TDGs) and to study star formation (SF) in tidal features, we are conducting a large UV imaging survey of interacting galaxies selected from the Arp (1996) Atlas using the Galaxy Evolution Explorer (GALEX) telescope. As part of that study, we present a GALEX UV and Sloan Digital Sky Survey and SARA optical study of the gas-rich interacting galaxy pair Arp 305 (NGC 4016/7). The GALEX UV data reveal much extended diffuse UV emission and SF outside the disks. This includes a luminous star-forming region between the two galaxies, and a number of such regions in tidal tails. We have identified 45 young star-forming clumps in Arp 305, including several TDG candidates. By comparing the UV and optical colors to population synthesis models, we determined that the clumps are very young, with several having ages ~6 Myr. We do not find many intermediate age clumps in spite of the fact that the last closest encounter was about 300 Myr ago. We have used a smooth particle hydrodynamics code to model the interaction and determine the fate of the star clusters and candidate TDGs.

An assessment of broad-band optical colours as age indicators for star clusters

As part of our Spitzer Spirals, Bridges, and Tails project to help understand the effects of galaxy interactions on star formation, we analyze Galaxy Evolution Explorer UV, Southeastern Association for Research in Astronomy optical, and Spitzer IR images of the interacting galaxy pair Arp 82 (NGC 2535/6) and compare to a numerical simulation of the interaction. We investigate the multiwavelength properties of several individual star-forming complexes (clumps). Using optical and UV colors, EW(Hα), and population synthesis models we constrain the ages of the clumps and find that the median clump age is ~9 Myr. The clumps have masses ranging from a few; x 10^6 to 10^9 M☉. In general, the clumps in the tidal features have ages similar to those in the spiral region, but are less massive. The clumps provide 33%, 36%, and 70% of the far-UV, 8.0 µm, and 24 µm emission, respectively. The 8 and 24 µm luminosities are used to estimate the far-IR luminosities and the star formation rates of the clumps. The total clump star formation rate is ~2.0 ± 0.8M☉yr^-1, while the entire Arp 82 system is forming stars at a rate of ~4.9 ± 2.0 M☉ yr^-1. We find, for the first time, stars in the H I arc to the southeast of the NGC 2535 disk. Population synthesis models indicate that all of the observed populations have young to intermediate ages. We conclude that, although the gas disks and some old stars may have formed early on, the progenitors may have been of late-type or low surface brightness, and the evolution of these galaxies seems to have halted until the recent encounter.

STAR FORMATION AND THE INTERSTELLAR MEDIUM IN NEARBY TIDAL STREAMS (SAINTS): SPITZER MID-INFRARED SPECTROSCOPY AND IMAGING OF INTERGALACTIC STAR-FORMING OBJECTS

Dwarf galaxies tend to have redder [3.6 μm] – [4.5 μm] Spitzer broadband colors than spirals. To investigate this effect, for a large sample of dwarf galaxies we combine Spitzer fluxes with data at other wavelengths and compare to population synthesis models. Lower metallicity systems are found to have redder [3.6] – [4.5] colors on average, but with considerable scatter. The observed range in [3.6] – [4.5] color is too large to be accounted for solely by variations in stellar colors due to age or metallicity differences; interstellar effects must contribute as well. For the reddest systems, the 4.5 μm luminosity may not be a good tracer of stellar mass. We identify three factors that redden this color in dwarfs. First, in some systems, strong Brα emission contributes significantly to the 4.5 μm emission. Second, in some cases high optical depths lead to strong reddening of the starlight in the Spitzer bands. Third, in some galaxies, the nebular continuum dominates the 4.5 μm flux, and in extreme cases, the 3.6 μm flux as well. The harder UV radiation fields in lower metallicity systems produce both more gaseous continuum in the infrared and more Brα per star formation rate. The combination of these three factors can account for the 4.5 μm excess in our sample galaxies, thus it is not necessary to invoke a major contribution from hot dust to the 4.5 μm band. However, given the uncertainties, we are not able to completely rule out hot dust emission at 4.5 μm. More spectroscopic observations in the 3-5 μm range are needed to disentangle these effects.

ERRATUM: “SPIRALS, BRIDGES, AND TAILS: A GALAXY EVOLUTION EXPLORER UV ATLAS OF INTERACTING GALAXIES” (2010, AJ, 139, 1212)

We present an empirical assessment of the use of broad-band optical colours as age indicators for unresolved extragalactic clusters and investigate stochastic sampling effects on integrated colours. We use the integrated properties of Galactic open clusters (OCs) as models for unresolved extragalactic clusters. The population synthesis code STARBURST99 (SB99) and four optical colours were used to estimate how well we can recover the ages of 62 well-studied Galactic OCs with published ages. We provide a method for estimating the ages of unresolved clusters and for reliably determining the uncertainties in the age estimates. Our results support earlier conclusions based on comparisons to synthetic clusters, namely the (U - B) colour is critical to the estimation of the ages of star-forming regions. We compare the observed optical colours with those obtained from SB99 using the published ages and get good agreement. The scatter in the (B - V) observed - (B - V) model is larger for lower luminosity clusters, perhaps due to stochastic effects.

SPIRALS, BRIDGES, AND TAILS: A GALAXY EVOLUTION EXPLORER ULTRAVIOLET ATLAS OF INTERACTING GALAXIES

A spectroscopic analysis of 10 intergalactic star-forming objects (ISFOs) and a photometric analysis of 67 ISFOs in a sample of 14 interacting systems is presented. The majority of the ISFOs have relative polycyclic aromatic hydrocarbon (PAH) band strengths similar to those of nearby spiral and starburst galaxies. In contrast to what is observed in blue compact dwarfs (BCDs) and local giant H II regions in the Milky Way (NGC 3603) and the Magellanic Clouds (30 Doradus and N 66), the relative PAH band strengths in ISFOs correspond to models with a significant PAH ion fraction ( 3.7, i.e., enhanced non-stellar emission, most likely due to PAHs, relative to normal spirals, dwarf irregulars, and BCD galaxies. The relative strength of the 8 μm emission compared to that at 3.6 μm or 24 μm separates ISFOs from dwarf galaxies in Spitzer two-color diagrams. The infrared power in two-thirds of the ISFOs is dominated by emission from grains in a diffuse interstellar medium. One in six ISFOs have significant emission from PDRs, contributing ~30%-60% of the total power. ISFOs are young knots of intense star formation.

STOCHASTIC “BEADS ON A STRING” IN THE ACCRETION TAIL OF ARP 285

Due to an error at the publisher, the name of the last author was accidentally omitted from the published article. The complete list of authors appears above. IOP Publishing sincerely regrets this error.