arXiv subjects
Publications and source records attributed to Debra Elmegreen.
At the XXXth General Assembly in Vienna in 2018, Resolution A1 called for a mid-term review of the IAU Strategic Plan 2020-2030 in time for the General Assembly in 2024. This report constitutes the 2024 mid-term review of the Strategic Plan, which was presented at the 110th Executive Committee meeting in April 2024.
We present results of a Hubble Space Telescope (HST) UBVI-band study of star clusters in tidal tails, using new WFC3 and ACS imaging to complement existing WFPC2 data. We survey 12 tidal tails across seven merging systems, deriving ages and masses for 425 star cluster candidates (SCCs). The stacked mass distribution across all systems follows a power law of the form $dN/dM \propto M^β$, with $β= -2.02 \pm 0.15$, consistent with what is seen in other star forming environments. GALEX and Swift UV imaging provide star formation rates (SFRs) for our tidal tails, which when compared with ages and masses of our SCCs, allows for a determination of the cluster formation efficiency (CFE). We find the CFE increases with increasing SFR surface density, matching the theoretical model. We confirm this fit down at SFR densities lower than previously measured (log $Σ_\text{SFR} \: (\text{M}_\odot \: \text{yr}^{-1} \: \text{kpc}^{-2}) \approx -4.2$), as related to the CFE. We determine the half-light radii for a refined sample of 57 SCCs with our HST WFC3 and ACS imaging, and calculate their dynamical age, finding the majority of them to be gravitationally bound. We also provide evidence of only low-mass ($< 10^4 \: \text{M}_\odot$) cluster formation in our nearest galaxy, NGC 1487, consistent with the theory that this system is a dwarf merger.
One of the outstanding problems of current observational cosmology is to understand the nature of sources that produced the bulk of the ionizing radiation after the Cosmic Dark Age. Direct detection of these reionization sources is practically infeasible at high redshift due to the steep decline of intergalactic medium transmission. However, a number of low-redshift analogs emitting Lyman continuum at~900 Angstrom ~restframe are now detected at $z< 0.4$ and there are detections in the range $ 2.5< z< 3.5$ also. Here, we report the detection of Lyman continuum emission with a high escape fraction (>20%) from a low-mass clumpy galaxy at z=1.42, in the middle of the redshift range where no detection has been made before and near the peak of the Cosmic Star-formation history. The observation was made in the Hubble Extreme Deep field by the wide-field Ultra-Violet-Imaging Telescope onboard AstroSat. This is the first detection of Extreme Ultraviolet radiation from a distant galaxy at a rest-frame wavelength of 600 Angstrom, and it opens up a new window to constrain the shape of the ionization spectrum. Further observations with AstroSat should significantly increase the sample of Lyman continuum leaking galaxies at Cosmic Noon.
International efforts play a key role in driving all areas of astrophysics. Strategic planning is essential to explore possible partnerships and joint projects that otherwise could not be afforded, and to maximize their scientific return. For these reasons, the International Astronomical Union (IAU) established in 2016 the Working Group on Global Coordination of Ground and Space Astrophysics. The Working Group held a Kavli IAU Workshop on "Global Coordination: Future Space-Based Ultraviolet-Optical-Infrared Telescopes" in July 2017, bringing together scientific leaders from 17 countries to discuss compelling science drivers, technical requirements, political constraints, and opportunities for a future large-scale UV/optical/Infrared space mission. This report summarizes the workshop and recommends that astronomers worldwide intensify their activities to explore the possibilities for science with such a mission, including consideration of instrumentation and technology definition and development, construction, launch, data analysis, and complementary science. The workshop presentations are available at https://www.strw.leidenuniv.nl/KavliIAU2017.
We study the m=1 distortions (lopsidedness) in the stellar components of 167 nearby galaxies that span a wide range of morphologies and luminosities. We confirm the previous findings of 1) a high incidence of lopsidedness in the stellar distributions, 2) increasing lopsidedness as a function of radius out to at least 3.5 exponential scale lengths, and 3) greater lopsidedness, over these radii, for galaxies of later type and lower surface brightness. Additionally, the magnitude of the lopsidedness 1) correlates with the character of the spiral arms (stronger arm patterns occur in galaxies with less lopsidedness), 2) is not correlated with the presence or absence of a bar, or the strength of the bar when one is present, 3) is inversely correlated to the stellar mass fraction, f_*, within one radial scale length, and 4) correlates directly with f_* measured within the radial range over which we measure lopsidedness. We interpret these findings to mean that lopsidedness is a generic feature of galaxies and does not, generally, depend on a rare event, such as a direct accretion of a satellite galaxy onto the disk of the parent galaxy. While lopsidedness may be caused by several phenomena, moderate lopsidedness ( _i + _o)/2 < 0.3) is likely to reflect halo asymmetries to which the disk responds or a gravitationally self-generated mode . We hypothesize that the magnitude of the stellar response depends both on how centrally concentrated the stars are with respect to the dark matter and whether there are enough stars in the region of the lopsidedness that self-gravity is dynamically important.
With the aim of constructing accurate 2D maps of the stellar mass distribution in nearby galaxies from S4G 3.6 and 4.5 micron images, we report on the separation of the light from old stars from the emission contributed by contaminants (e.g. hot dust and the 3.3 micron PAH feature). Results for a small sample of six disk galaxies (NGC 1566, NGC 2976, NGC 3031, NGC 3184, NGC 4321, and NGC 5194) with a range of morphological properties, dust contents and star formation histories are presented to demonstrate our approach. We use an Independent Component Analysis (ICA) technique designed to separate statistically independent source distributions, maximizing the distinction in the [3.6]-[4.5] colors of the sources. The technique also removes emission from intermediate-age evolved red objects with a low mass-to-light ratio, such as asymptotic giant branch (AGB) and red supergiant (RSG) stars, revealing maps of the underlying old distribution of light with [3.6]-[4.5] colors consistent with the colors of K and M giants. Contaminants are identified via comparison to the non-stellar emission imaged at 8 microns, which is dominated by the broad PAH feature. Using the measured 3.6/8 micron ratio to select the individual contaminants, we find that hot dust and PAH together contribute between ~5-15% to the integrated light at 3.6 microns, while light from regions dominated by intermediate-age stars accounts for only 1-5%. Locally, however, the contribution from either contaminant can reach much higher levels; dust contributes on average 22% to the emission in star-forming regions throughout the sample, while intermediate age-stars contribute upwards of 50% in localized knots. The removal of these contaminants with ICA leaves maps of the old stellar disk that retain a high degree of structural information and are ideally suited for tracing the stellar mass, as will be the focus in a companion paper.