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Far-Ultraviolet and X-Ray Observations of VV 114: Feedback in a Local Analog to Lyman Break Galaxies
We have analyzed Far Ultraviolet Spectroscopic Explorer (FUSE),XMM-Newton, and Chandra observations of VV 114, a local galaxy mergerwith strong similarities to typical high-redshift Lyman break galaxies(LBGs). Diffuse thermal X-ray emission encompassing VV 114 has beenobserved by Chandra and XMM-Newton. This region of hot (kT~0.59 keV) gashas an enhanced α/Fe element ratio relative to solar abundancesand follows the same relations as typical starbursts between itsproperties (luminosity, size, and temperature) and those of thestarburst galaxy (star formation rate, dust temperature, and galaxymass). These results are consistent with the X-ray gas having beenproduced by shocks driven by a galactic superwind. The FUSE observationsof VV 114 show strong, broad interstellar absorption lines with apronounced blueshifted component (similar to what is seen in LBGs). Thisimplies an outflow of material moving at ~300-400 km s-1relative to VV 114. The properties of the strong O VI absorption lineare consistent with radiative cooling at the interface between the hotoutrushing gas seen in X-rays and the cooler material seen in the otheroutflowing ions in the FUSE data. We show that the wind in VV 114 has not created a ``tunnel'' that enables more than a small fraction(
| Optical Star Formation Rate Indicators
Using integrated optical spectrophotometry for 412 star-forming galaxiesat z~0, and fiber-aperture spectrophotometry for 120,846 SDSS galaxiesat z~0.1, we investigate the Hα λ6563, Hβλ4861, [O II] λ3727, and [O III] λ5007 nebularemission lines and the U-band luminosity as quantitative star formationrate (SFR) indicators. We demonstrate that the extinction-correctedHα λ6563 luminosity is a reliable SFR tracer even in highlyobscured star-forming galaxies. We find that variations in dustreddening dominate the systematic uncertainty in SFRs derived from theobserved Hβ, [O II], and U-band luminosities, producing a factor of~1.7, ~2.5, and ~2.1 scatter in the mean transformations, respectively.We show that [O II] depends weakly on variations in oxygen abundanceover a wide range in metallicity, 12+log(O/H)=8.15-8.7(Z/Zsolar=0.28-1.0), and that in this metallicity intervalgalaxies occupy a narrow range in ionization parameter(-3.8<~logU<~-2.9). We show that the scatter in [O III]λ5007 as a SFR indicator is a factor of 3-4 due to itssensitivity to metal abundance and ionization. We develop empirical SFRcalibrations for Hβ and [O II] parameterized in terms of the B-bandluminosity, which remove the systematic effects of reddening andmetallicity and reduce the SFR scatter to +/-40% and +/-90%,respectively, although individual galaxies may deviate substantiallyfrom the median relations. Finally, we compare the z~0 relations betweenblue luminosity and reddening, ionization, and [O II]/Hα ratioagainst measurements at z~1 and find broad agreement. We emphasize,however, that optical emission-line measurements including Hα forlarger samples of intermediate- and high-redshift galaxies are needed totest the applicability of our locally derived SFR calibrations todistant galaxies.
| High-Density Molecular Gas in the Infrared-bright Galaxy System VV 114
The new high-resolution CO (3-2) interferometric map of the IR-brightinteracting galaxy system VV 114 observed with the Submillimeter Arrayreveals a substantial amount of warm and dense gas in the IR-bright butoptically obscured galaxy, VV 114E, and the overlap region connectingthe two nuclei. A 1.8×1.4 kpc concentration of CO (3-2) emittinggas with a total mass of 4×109 Msolarcoincides with the peaks of near-IR, mid-IR, and radio continuumemission found previously by others, identifying the dense fuel for theactive galactic nucleus and/or the starburst activity there. ExtensiveCO (2-1) emission is also detected, revealing detailed distribution andkinematics that are consistent with the earlier CO (1-0) results. Thewidely distributed molecular gas traced in CO (2-1) and the distributeddiscrete peaks of CO (3-2) emission suggest that a spatially extendedintense starbursts may contribute significantly to its large IRluminosity. These new observations further support the notion that VV114 is approaching its final stage of merger, when a violent centralinflow of gas triggers intense starburst activity possibly boosting theIR luminosity above the ultraluminous threshold.
| A Bias in Optical Observations of High-Redshift Luminous Infrared Galaxies
We present evidence for the dramatically different morphology betweenthe rest-frame UV and 7 μm mid-IR emission of VV 114 and Arp 299, twonearby (z~0) violently interacting luminous infrared galaxies (LIRGs).Nearly all LIRGs are interacting systems, and it is currently acceptedthat they dominate the IR emission at z>1. LIRGs located at z=1-2could easily be detected as unresolved sources in deep optical/near-IRground-based surveys as well as in upcoming 24 μm surveys with theSpace Infrared Telescope Facility. We demonstrate that the spatialresolution of these surveys will result in blending of the emission fromunresolved interacting components. An increased scatter will thus beintroduced in the observed optical-to-mid-IR colors of these galaxies,leading to a systematic underestimation of their dust content.
| Dust-to-gas ratios in the starburst regions of luminous infrared galaxies
We investigate the properties of dust and dust-to-gas ratios in variousstarburst regions of luminous infrared galaxies (LIGs). We refer to thesample of seven LIGs recently observed in the mid-infrared by Soifer etal. using the Keck Telescope with spatial resolution approaching thediffraction limit. These seven objects are among the closest LIGs andhave been classified as starburst galaxies from optical spectroscopy.Our goal consists in modelling the continuum spectral energydistribution of each galaxy, particularly in the infrared range. Modelsare further constrained by observed emission-line ratios in the opticalrange. Multicloud models consistently account for the coupled effects ofshock, photoionization by hot stars and diffuse secondary radiation fromthe shock-heated gas. Emission from clouds in the neighbourhood ofevolved starbursts and with high shock velocities (~500 kms-1) explains both the bremsstrahlung and reradiation fromdust in the mid-infrared. Clouds with lower velocity (100 kms-1), which correspond to younger starbursts, also contributeto both line and continuum spectra. Both low- and high-velocity cloudsare thus present in nearly all the sample galaxies. For all thegalaxies, an old stellar population is revealed by blackbody emission inthe optical/near-infrared range. Dust-to-gas ratios vary in differentregions of individual galaxies.
| Extended mid-infrared emission from VV 114: Probing the birth of a ULIRG
We present our 5-16 mu m spectro-imaging observations of VV114, an infrared luminous early-stage merger of two galaxiesVV 114E and VV 114W, taken with the ISOCAM camera on-board the InfraredSpace Observatory. We find that only 40% of the mid-infrared (MIR) fluxis associated with a compact nuclear region of VV 114E, while the restof the emission originates from a rather diffuse component extended overseveral kpc in the regions between VV 114E and VV 114W. This is in starkcontrast with the very compact MIR starbursts usually seen in luminousand ultraluminous infrared galaxies. A secondary peak of MIR emission isassociated with an extra-nuclear star forming region of VV 114W whichdisplays the largest Hα equivalent width in the whole system.Comparing our data with the distribution of the molecular gas and colddust, as well as with radio observations, it becomes evident that theconversion of molecular gas into stars can be triggered over large areasat the very first stages of an interaction. This extended star formationalong with the extreme nuclear starburst observed in VV 114E can easilylead to the heating of dust grains found in the tidally disturbed disksof the progenitor galaxies and subsequently traced via their MIRemission. The presence of a very strong continuum at the 5-6.5 mu mrange in the spectrum of VV 114E indicates that an enshrouded activegalactic nucleus (AGN) may contribute to ~ 40% of its MIR flux. Wefinally note that the relative variations in the UV to radio spectralproperties between VV 114E and VV 114W provide evidence that theextinction-corrected star formation rate of similar objects at high z,such as those detected in optical deep surveys, cannot be accuratelyderived from their rest-frame UV properties. Based on observations withthe ISO satellite, an ESA project with instruments funded by ESA MemberStates (especially the PI countries: France, Germany, the Netherlandsand the United Kingdom) and with the participation of ISAS and NASA.
| H I line observations of luminous infrared galaxy mergers
A total of 19 luminous infrared galaxy mergers, with LIR>~ 2 1011 Lsun, for H0=75 kms-1 Mpc-1, have been observed in the H I line atNançay and four of them were observed at Arecibo as well. Ofthese 19, ten had not been observed before. Six were clearly detected,one of which for the first time. The objective was to statisticallysample the H I gas mass in luminous infrared mergers along a starburstmerger sequence where the molecular CO gas content is already known. Wealso searched the literature for H I data and compared these with ourobservations.
| VV 114, a high infrared luminosity interacting galaxy system
VV 114 is a nearby example of a far-infrared (FIR) bright, highluminosity (LFIR greater than 1011 solarluminosity) interacting galaxy pair. At a redshift of z = 0.02 itprovides an opportunity to study such interacting galaxies at afavorable spatial scale (390 pc/arcsec). This paper presents new highresolution near-infrared (1.25 to 3.7 micrometer) and visible images,and visible spectra of VV 114. A picture emerges of a system withwidespread massive star formation throughout both interacting galaxies.The brighter visible galaxy (VV 114W) shows H II region-like emission inboth visual spectra and near-infrared colors, with no more than twomagnitudes of visual extinction. The brightest peak of infrared andradio emission (VV 114E) has extreme near-infrared colors and is locatedat a minimum of visible emission. This indicates a large concentrationof dust in the nucleus of VV 114E that is nearly entirely obscuring amajor luminosity source in this system.
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