skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Extreme CO Isotopic Abundances in the ULIRG IRAS 13120-5453: An Extremely Young Starburst or Top-heavy Initial Mass Function

Abstract

We present ALMA {sup 12}CO (J = 1-0, 3-2 and 6-5), {sup 13}CO (J = 1-0), and C{sup 18}O (J = 1-0) observations of the local ultraluminous infrared galaxy (ULIRG) IRAS 13120-5453. The morphologies of the three isotopic species differ, as {sup 13}CO shows a hole in emission toward the center. We measure integrated brightness temperature line ratios of {sup 12}CO/{sup 13}CO ≥ 60 (exceeding 200) and {sup 13}CO/C{sup 18}O ≤ 1 in the central region. Assuming optical thin emission, C{sup 18}O is more abundant than {sup 13}CO in several regions. The abundances within the central 500 pc are consistent with the enrichment of the interstellar medium via a young starburst (<7 Myr), a top-heavy initial mass function, or a combination of both.

Authors:
 [1];  [2];  [3];  [4]
  1. MPI for Astronomy, Königstuhl 17, D-69117 Heidelberg (Germany)
  2. Department of Physics and Astronomy, McMaster University, Hamilton, ON L8S 4M1 (Canada)
  3. Department of Earth and Space Sciences, Chalmers University of Technology, Onsala Space Observatory, SE-439 94 Onsala (Sweden)
  4. Instituto de Astrofśica, Facultad de Física, Pontificia Universidad Católica de Chile, Casilla 306, Santiago 22 (Chile)
Publication Date:
OSTI Identifier:
22654483
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 840; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABUNDANCE; CARBON; CARBON 12; CARBON 13; CARBON 18; CARBON MONOXIDE; EMISSION; GALAXIES; INTERACTIONS; ISOTOPES; MASS; RADIANT HEAT TRANSFER

Citation Formats

Sliwa, Kazimierz, Wilson, Christine D., Aalto, Susanne, and Privon, George C., E-mail: sliwa@mpia-hd.mpg.de. Extreme CO Isotopic Abundances in the ULIRG IRAS 13120-5453: An Extremely Young Starburst or Top-heavy Initial Mass Function. United States: N. p., 2017. Web. doi:10.3847/2041-8213/AA6EA4.
Sliwa, Kazimierz, Wilson, Christine D., Aalto, Susanne, & Privon, George C., E-mail: sliwa@mpia-hd.mpg.de. Extreme CO Isotopic Abundances in the ULIRG IRAS 13120-5453: An Extremely Young Starburst or Top-heavy Initial Mass Function. United States. doi:10.3847/2041-8213/AA6EA4.
Sliwa, Kazimierz, Wilson, Christine D., Aalto, Susanne, and Privon, George C., E-mail: sliwa@mpia-hd.mpg.de. 2017. "Extreme CO Isotopic Abundances in the ULIRG IRAS 13120-5453: An Extremely Young Starburst or Top-heavy Initial Mass Function". United States. doi:10.3847/2041-8213/AA6EA4.
@article{osti_22654483,
title = {Extreme CO Isotopic Abundances in the ULIRG IRAS 13120-5453: An Extremely Young Starburst or Top-heavy Initial Mass Function},
author = {Sliwa, Kazimierz and Wilson, Christine D. and Aalto, Susanne and Privon, George C., E-mail: sliwa@mpia-hd.mpg.de},
abstractNote = {We present ALMA {sup 12}CO (J = 1-0, 3-2 and 6-5), {sup 13}CO (J = 1-0), and C{sup 18}O (J = 1-0) observations of the local ultraluminous infrared galaxy (ULIRG) IRAS 13120-5453. The morphologies of the three isotopic species differ, as {sup 13}CO shows a hole in emission toward the center. We measure integrated brightness temperature line ratios of {sup 12}CO/{sup 13}CO ≥ 60 (exceeding 200) and {sup 13}CO/C{sup 18}O ≤ 1 in the central region. Assuming optical thin emission, C{sup 18}O is more abundant than {sup 13}CO in several regions. The abundances within the central 500 pc are consistent with the enrichment of the interstellar medium via a young starburst (<7 Myr), a top-heavy initial mass function, or a combination of both.},
doi = {10.3847/2041-8213/AA6EA4},
journal = {Astrophysical Journal Letters},
number = 2,
volume = 840,
place = {United States},
year = 2017,
month = 5
}
  • We examine variations of the stellar initial mass function (IMF) in extreme environments within the formalism derived by Hennebelle and Chabrier. We focus on conditions encountered in progenitors of massive early-type galaxies and starburst regions. We show that, when applying the concept of turbulent Jeans mass as the characteristic mass for fragmentation in a turbulent medium, the peak of the IMF in such environments is shifted toward smaller masses, leading to a bottom-heavy IMF, as suggested by various observations. In very dense and turbulent environments, we predict that the high-mass tail of the IMF can become even steeper than themore » standard Salpeter IMF, with a limit for the power-law exponent α ≅ –2.7, in agreement with recent observational determinations. This steepening is a direct consequence of the high densities and Mach values in such regions but also of the time dependence of the fragmentation process, as incorporated in the Hennebelle-Chabrier theory. We provide analytical parameterizations of these IMFs in such environments to be used in galaxy evolution calculations. We also calculate the star-formation rates and the mass-to-light ratios expected under such extreme conditions and show that they agree well with the values inferred in starburst environments and massive high-redshift galaxies. This reinforces the paradigm of star formation as being a universal process, i.e., the direct outcome of gravitationally unstable fluctuations in a density field initially generated by large-scale, shock-dominated turbulence. This globally enables us to infer the variations of the stellar IMF and related properties for atypical galactic conditions.« less
  • We present new observations of the nuclear star cluster in the central parsec of the Galaxy with the adaptive optics assisted, integral field spectrograph SINFONI on the ESO/VLT. Our work allows the spectroscopic detection of early- and late-type stars to m{sub K} >= 16, more than 2 mag deeper than our previous data sets. Our observations result in a total sample of 177 bona fide early-type stars. We find that most of these Wolf Rayet (WR), O-, and B-stars reside in two strongly warped disks between 0.''8 and 12'' from Sgr A*, as well as a central compact concentration (themore » S-star cluster) centered on Sgr A*. The later type B-stars (m{sub K} >15) in the radial interval between 0.''8 and 12'' seem to be in a more isotropic distribution outside the disks. The observed dearth of late-type stars in the central few arcseconds is puzzling, even when allowing for stellar collisions. The stellar mass function of the disk stars is extremely top heavy with a best-fit power law of dN/dm propor to m {sup -0.45+}-{sup 0.3}. WR/O-stars were formed in situ in a single star formation event approx6 Myr ago, this mass function probably reflects the initial mass function (IMF). The mass functions of the S-stars inside 0.''8 and of the early-type stars at distances beyond 12'' are compatible with a standard Salpeter/Kroupa IMF (best-fit power law of dN/dm propor to m {sup -2.15+}-{sup 0.3}).« less
  • We have studied the star formation history and the initial mass function (IMF) using the age and mass derived from spectral energy distribution (SED) fitting and from color-magnitude diagrams (CMDs). We also examined the physical and structural parameters of more than 1000 pre-main-sequence stars in NGC 2264 using the online SED fitting tool (SED fitter) of Robitaille et al. We have compared the physical parameters of central stars from SED fitter and other methods. The temperature of the central star is, in many cases, much higher than that expected from its spectral type. The mass and age from SED fittermore » are not well matched with those from CMDs. We have made some suggestions to improve the accuracy of temperature estimates in the SED fitter. In most cases, these parameters of individual stars from the SED fitter in a star-forming region (SFR) or in the whole cluster showed nearly no systematic variation with age or with any other relevant parameter. On the other hand, the median properties of stars in NGC 2264 SFRs showed an evident evolutionary effect and were interrelated to each other. Such differences are caused by a larger age spread within an SFR than between them. The cumulative distribution of stellar ages showed a distinct difference among SFRs. A Kolmogorov-Smirnov test gave a very low probability of them being from the same population. The results indicate that star formation in NGC 2264 started at the surface region (Halo and Field regions) about 6-7 Myr ago, propagated into the molecular cloud and finally triggered the recent star formation in the Spokes cluster. The kind of sequential star formation that started in the low-density surface region (Halo and Field regions) implies that star formation in NGC 2264 was triggered by an external source. The IMF of NGC 2264 was determined in two different ways. One method used the stellar mass from the SED fitting tool, the other used the stellar mass from CMDs. The first IMF showed a distinct peak at m {approx} 2 M{sub sun}, but the second did not. We attribute the peak as an artifact of the SED fitting tool because there is no such IMF with a peak at m {approx} 2 M{sub sun}. The slope of the IMF of NGC 2264 for massive stars (log m {>=} 0.5) is -1.7 {+-} 0.1, which is somewhat steeper than the so-called standard Salpeter-Kroupa IMF. We also present data for 79 young brown dwarf candidates.« less
  • We present deep Hubble Space Telescope NICMOS 2 F160W band observations of the central 56'' x 57'' (14 pc x 14.25 pc) region around R136 in the starburst cluster 30 Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to derive the stellar initial mass function (IMF) down to approx1 M{sub sun} in order to test whether the IMF in a massive metal-poor cluster is similar to that observed in nearby young clusters and the field in our Galaxy. We estimate the mean age of the cluster to be 3 Myr by combining our F160W photometry withmore » previously obtained HST WFPC2 optical F555W and F814W band photometry and comparing the stellar locus in the color-magnitude diagram with main sequence and pre-main sequence isochrones. The color-magnitude diagrams show the presence of differential extinction and possibly an age spread of a few megayear. We convert the magnitudes into masses adopting both a single mean age of 3 Myr isochrone and a constant star formation history from 2 to 4 Myr. We derive the IMF after correcting for incompleteness due to crowding. The faintest stars detected have a mass of 0.5 M{sub sun} and the data are more than 50% complete outside a radius of 5 pc down to a mass limit of 1.1 M{sub sun} for 3 Myr old objects. We find an IMF of dN/d log M propor to M{sup -1.20+}-{sup 0.2} over the mass range 1.1-20 M{sub sun} only slightly shallower than a Salpeter IMF. In particular, we find no strong evidence for a flattening of the IMF down to 1.1 M{sub sun} at a distance of 5 pc from the center, in contrast to a flattening at 2 M{sub sun} at a radius of 2 pc, reported in a previous optical HST study. We examine several possible reasons for the different results including the possible presence of mass segregation and the effects of differential extinction, particularly for the pre-main sequence sources. If the IMF determined here applies to the whole cluster, the cluster would be massive enough to remain bound and evolve into a relatively low-mass globular cluster.« less
  • Westerlund 1 is the most important starburst cluster in the Galaxy due to its massive star content. We have performed BVI{sub C} and JK{sub S} photometry to investigate the initial mass function (IMF). By comparing the observed color with the spectral-type-intrinsic-color relation, we obtain the mean interstellar reddening of (E(B - V)) = 4.19 {+-} 0.23 and (E(J - K{sub S} )) = 1.70 {+-} 0.21. Due to the heavy extinction toward the cluster, the zero-age main sequence fitting method based on optical photometry proved to be inappropriate for the distance determination, while the near-infrared photometry gave a reliable distancemore » to the cluster-3.8 kpc from the empirical relation. Using the recent theoretical stellar evolution models with rotation, the age of the cluster is estimated to be 5.0 {+-} 1.0 Myr. We derived the IMF in the massive part and obtained a fairly shallow slope of {Gamma} = -0.8 {+-} 0.1. The integration of the IMF gave a total mass for the cluster in excess of 5.0 Multiplication-Sign 10{sup 4} M{sub Sun }. The IMF shows a clear radial variation indicating the presence of mass segregation. We also discuss the possible star formation history of Westerlund 1 from the presence of red supergiants and relatively low luminosity yellow hypergiants.« less