skip to main content


Search for: All records

Creators/Authors contains: "Hirschauer, Alec S."

Note: When clicking on a Digital Object Identifier (DOI) number, you will be taken to an external site maintained by the publisher. Some full text articles may not yet be available without a charge during the embargo (administrative interval).
What is a DOI Number?

Some links on this page may take you to non-federal websites. Their policies may differ from this site.

  1. Abstract In this work, we present 299 candidate young stellar objects (YSOs) in 30 Doradus discovered using Spitzer and Herschel point-source catalogs, 276 of which are new. We study the parental giant molecular clouds in which these YSO candidates form using recently published Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 7 observations of 12 CO and 13 CO. The threshold for star formation in 30 Doradus inferred by the LTE-based mass surface density is 178 M ⊙ pc −2 , 40% higher than the threshold for star formation in the Milky Way. This increase in star formation threshold in comparison to the Milky Way and increase in line width seen in clumps 11 pc away in comparison to clumps 45 pc away from the R136 super star cluster could be due to injected turbulent energy, increase in interstellar medium pressure, and/or local magnetic field strength. Of the 299 YSO candidates in this work, 62% are not associated with 12 CO molecular gas. This large fraction can be explained by the fact that 75%–97% of the H 2 gas is not traced by CO. We fit a Kroupa initial mass function to the YSO candidates and find that the total integrated stellar mass is 18,000 M ⊙ and that the region has a star formation rate (SFR) of 0.18 M ⊙ yr −1 . The initial mass function determined here applies to the four 150″ × 150″ (37.5 pc × 37.5 pc) subfields and one 150″ × 75″ (37.5 pc × 18.8 pc) subfield observed with ALMA. The SFR in 30 Doradus has increased in the past few million years. 
    more » « less
  2. ABSTRACT

    We re-examine the extremely metal-poor dwarf galaxy AGC 198691 using a high quality spectrum obtained by the LBT’s MODS instrument. Previous spectral observations obtained from KOSMOS on the Mayall 4-m and the Blue channel spectrograph on the MMT 6.5-m telescope did not allow for the determination of sulfur, argon, or helium abundances. We report an updated and full chemical abundance analysis for AGC 198691, including confirmation of the extremely low “direct” oxygen abundance with a value of 12 + log (O/H) = 7.06 ± 0.03. AGC 198691’s low metallicity potentially makes it a high value target for helping determine the primordial helium abundance (Yp). Though complicated by a Na i night sky line partially overlaying the He i λ5876 emission line, the LBT/MODS spectrum proved adequate for determining AGC 198691’s helium abundance. We employ the recently expanded and improved model of Aver et al., incorporating higher Balmer and Paschen lines, augmented by the observation of the infrared helium emission line He i λ10830 obtained by Hsyu et al. Applying our full model produced a reliable helium abundance determination, consistent with the expectation for its metallicity. Although this is the lowest metallicity object with a detailed helium abundance, unfortunately, due to its faintness [EW(Hβ) < 100 Å] and the compromised He i λ5876, the resultant uncertainty on the helium abundance is too large to allow a significant improvement on the measurement of Yp.

     
    more » « less
  3. 12CO and 13CO emission maps of the 30 Doradus molecular cloud in the Large Magellanic Cloud, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) during Cycle 7. See the associated article in the Astrophysical Journal, and README file, for details. Please cite the article if you use these data. 
    more » « less
  4. Abstract

    We present results of a wide-field (approximately 60 × 90 pc) Atacama Large Millimeter/submillimeter Array mosaic of CO(2–1) and13CO(2–1) emission from the molecular cloud associated with the 30 Doradus star-forming region in the Large Magellanic Cloud (LMC). Three main emission complexes, including two forming a bow-tie-shaped structure extending northeast and southwest from the central R136 cluster, are resolved into complex filamentary networks. Consistent with previous studies, we find that the central region of the cloud has higher line widths at a fixed size relative to the rest of the molecular cloud and to other LMC clouds, indicating an enhanced level of turbulent motions. However, there is no clear trend in gravitational boundedness (as measured by the virial parameter) with distance from R136. Structures observed in13CO are spatially coincident with filaments and are close to a state of virial equilibrium. In contrast,12CO structures vary greatly in virialization, with low CO surface brightness structures outside of the main filamentary network being predominantly unbound. The low surface brightness structures constitute ∼10% of the measured CO luminosity; they may be shredded remnants of previously star-forming gas clumps, or alternatively the CO-emitting parts of more massive, CO-dark structures.

     
    more » « less
  5. We present new multi-configuration VLA HI spectral line imaging of the dwarf galaxies AGC 198691 (hereafter, Leoncino) and UGC 5186. Leoncino is one of 82 galaxies in the "Survey of HI in Extremely Low-mass Dwarfs" ("SHIELD"), a multi-wavelength investigation of low-mass galaxies that were cataloged by the Arecibo Legacy Fast ALFA (ALFALFA) survey. Leoncino is one of the lowest metallicity galaxies known in the local universe (Hirschauer et al. 2016). A single pointing with the VLA allows us to study the HI morphology and dynamics of both galaxies simultaneously. We present HI images at a variety of angular resolutions in order to explore both the global and the resolved properties of the neutral ISM. 
    more » « less