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Register a new userA Very Large Array Survey of Luminous Extranuclear Star-forming Regions in Luminous Infrared Galaxies in GOALS
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We present the first results of a high-resolution Karl G. Jansky Very Large Array (VLA) imaging survey of luminous and ultra-luminous infrared galaxies (U/LIRGs) in the Great Observatories All-Sky LIRG Survey (GOALS). From the full sample of 68 galaxies, we have selected 25 LIRGs that show resolved extended emission at sufficient sensitivity to image individual regions of star-formation activity beyond the nucleus.~With wideband radio continuum observations, which sample the frequency range from $3-33$ GHz, we have made extinction-free measurements of the luminosities and spectral indicies for a total of 48 individual star-forming regions identified as having de-projected galactocentric radii ($r_{G}$) that lie outside the 13.2$mu$m core of the galaxy.~The median $3-33$ GHz spectral index and 33 GHz thermal fraction measured for these extranuclear regions is $-0.51 pm 0.13$ and $65 pm 11%$ respectively.~These values are consistent with measurements made on matched spatial scales in normal star-forming galaxies, and suggests that these regions are more heavily-dominated by thermal free-free emission relative to the centers of local ULIRGs.~Further, we find that the median star-formation rate derived for these regions is $sim 1 M_{odot}$ yr$^{-1}$, and when we place them on the sub-galactic star-forming main sequence of galaxies (SFMS), we find they are offset from their host galaxies globally-averaged specific star-formation rates (sSFRs).~We conclude that while nuclear starburst activity drives LIRGs above the SFMS, extranuclear star-formation still proceeds in a more extreme fashion relative to what is seen in local spiral galaxies.
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We present 33 GHz imaging for 112 pointings towards galaxy nuclei and extranuclear star-forming regions at $approx$2 resolution using the Karl G. Jansky Very Large Array (VLA) as part of the Star Formation in Radio Survey. A comparison with 33 GHz Robert C. Byrd Green Bank Telescope single-dish observations indicates that the interferometric VLA observations recover $78pm4 %$ of the total flux density over 25 regions ($approx$ kpc-scales) among all fields. On these scales, the emission being resolved out is most likely diffuse non-thermal synchrotron emission. Consequently, on the $approx30-300$ pc scales sampled by our VLA observations, the bulk of the 33 GHz emission is recovered and primarily powered by free-free emission from discrete HII regions, making it an excellent tracer of massive star formation. Of the 225 discrete regions used for aperture photometry, 162 are extranuclear (i.e., having galactocentric radii $r_{rm G} geq 250$ pc) and detected at $>3sigma$ significance at 33 GHz and in H$alpha$. Assuming a typical 33 GHz thermal fraction of 90 %, the ratio of optically-thin 33 GHz-to-uncorrected H$alpha$ star formation rates indicate a median extinction value on $approx30-300$ pc scales of $A_{rm Halpha} approx 1.26pm0.09$ mag with an associated median absolute deviation of 0.87 mag. We find that 10 % of these sources are highly embedded (i.e., $A_{rm Halpha}gtrsim3.3$ mag), suggesting that on average HII regions remain embedded for $lesssim1$ Myr. Finally, we find the median 33 GHz continuum-to-H$alpha$ line flux ratio to be statistically larger within $r_{rm G}<250$ pc relative the outer-disk regions by a factor of $1.82pm0.39$, while the ratio of 33 GHz-to-24 $mu$m flux densities are lower by a factor of $0.45pm0.08$, which may suggest increased extinction in the central regions.
We present $HST$ narrow-band near-infrared imaging of Pa$alpha$ and Pa$beta$ emission of 48 local Luminous Infrared Galaxies (LIRGs) from the Great Observatories All-Sky LIRG Survey (GOALS). These data allow us to measure the properties of 810 spatially resolved star-forming regions (59 nuclei and 751 extra-nuclear clumps), and directly compare their properties to those found in both local and high-redshift star-forming galaxies. We find that in LIRGs, the star-forming clumps have radii ranging from $sim90-900$ pc and star formation rates (SFRs) of $sim1times10^{-3}$ to 10 M$_odot$yr$^{-1}$, with median values for extra-nuclear clumps of 170 pc and 0.03 M$_odot$yr$^{-1}$. The detected star-forming clumps are young, with a median stellar age of $8.7$ Myrs, and a median stellar mass of $5times10^{5}$ M$_odot$. The SFRs span the range of those found in normal local star-forming galaxies to those found in high-redshift star-forming galaxies at $rm{z}=1-3$. The luminosity function of the LIRG clumps has a flatter slope than found in lower-luminosity, star-forming galaxies, indicating a relative excess of luminous star-forming clumps. In order to predict the possible range of star-forming histories and gas fractions, we compare the star-forming clumps to those measured in the MassiveFIRE high-resolution cosmological simulation. The star-forming clumps in MassiveFIRE cover the same range of SFRs and sizes found in the local LIRGs and have total gas fractions that extend from 10 to 90%. If local LIRGs are similar to these simulated galaxies, we expect future observations with ALMA will find a large range of gas fractions, and corresponding star formation efficiencies, among the star-forming clumps in LIRGs.
We present the results of a {it Hubble Space Telescope} ACS/HRC FUV, ACS/WFC optical study into the cluster populations of a sample of 22 Luminous Infrared Galaxies in the Great Observatories All-Sky LIRG Survey. Through integrated broadband photometry we have derived ages and masses for a total of 484 star clusters contained within these systems. This allows us to examine the properties of star clusters found in the extreme environments of LIRGs relative to lower luminosity star-forming galaxies in the local Universe. We find that by adopting a Bruzual & Charlot simple stellar population (SSP) model and Salpeter initial mass function, the age distribution of clusters declines as $dN/dtau = tau^{-0.9 +/- 0.3}$, consistent with the age distribution derived for the Antennae Galaxies, and interpreted as evidence for rapid cluster disruption occuring in the strong tidal fields of merging galaxies. The large number of $10^{6} M_{odot}$ young clusters identified in the sample also suggests that LIRGs are capable of producing more high-mass clusters than what is observed to date in any lower luminosity star-forming galaxy in the local Universe. The observed cluster mass distribution of $dN/dM = M^{-1.95 +/- 0.11}$ is consistent with the canonical -2 power law used to describe the underlying initial cluster mass function (ICMF) for a wide range of galactic environments. We interpret this as evidence against mass-dependent cluster disruption, which would flatten the observed CMF relative to the underlying ICMF distribution.
We present recent results from an adaptive optics imaging survey of 40 Luminous IR Galaxies (LIRGs) searching for obscured core collapse supernovae and studying the galaxies themselves. Here, in particular, we discuss the Super Star Clusters (SSC) populations in the LIRGs. We have constructed the first statistically significant samples of Luminosity Functions (LF) of SSCs in the near-IR, and find evidence that the LF slopes in LIRGs are shallower than in more quiescent spiral galaxies. Distance and blending effects were investigated in detail paving the way for SSC studies further out than done previously. We have also correlated the luminosities of the brightest clusters with the star formation rates (SFR) of the hosts. The relation is similar, though somewhat steeper than that found in the optical and at lower SFR levels, suggesting systematic extinction and/or age effects. We find that the characteristics of the relation suggest an underlying physical driver rather than solely a size-of-sample effect. In particular, a truncated luminosity/mass function would naturally explain the small scatter we find. Finally, we are modelling the ages and masses of our near-IR detected clusters in conjunction with HST optical data and present early results of using SSC properties to trace the histories of the target LIRG systems.
We present a high spatial (diffraction-limited) resolution (~0.3) mid-infrared (MIR) spectroscopic study of the nuclei and star-forming regions of 4 local luminous infrared galaxies (LIRGs) using T-ReCS on the Gemini South telescope. We investigate the spatial variations of the features seen in the N-band spectra of LIRGs on scales of ~100 pc, which allow us to separate the AGN emission from that of the star formation (SF). We compare our Gemini T-ReCS nuclear and integrated spectra of LIRGs with those obtained with Spitzer IRS. The 9.7um silicate absorption feature is weaker in the nuclei of the LIRGs than in the surrounding regions. This is probably due to the either clumpy or compact environment of the central AGN or young, nuclear starburst. We find that the [NeII] luminosity surface density is tightly and directly correlated with that of Pa-alpha for the LIRG star-forming regions (slope of 1.00+-0.02). Although the 11.3um PAH feature shows also a trend with Pa-alpha, this is not common for all the regions. We also find that the [NeII]Pa-alpha ratio does not depend on the Pa-alpha equivalent width (EW), i.e., on the age of the ionizing stellar populations, suggesting that, on the scales probed here, the [NeII] emission line is a good tracer of the SF activity in LIRGs. On the other hand, the 11.3um PAHPa-alpha ratio increases for smaller values of the Pa-alpha EW (increasing ages), indicating that the 11.3um PAH feature can also be excited by older stars than those responsible for the Pa-alpha emission. Additional high spatial resolution observations are essential to investigate, in a statistical way, the star formation in local LIRGs at the smallest scales and to probe ultimately whether they share the same physical properties as high-z LIRGs, ULIRGs and submillimiter galaxies.
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