Search for: All records

Abstract Understanding the interplay of stellar feedback and turbulence in the interstellar medium (ISM) is essential to modeling the evolution of galaxies. To determine the timescales over which stellar feedback drives turbulence in the ISM, we performed a spatially resolved, multiwavelength study of the nearby star-forming dwarf galaxy UGC 4305. As indicators of turbulence on local scales (400 pc), we utilized ionized gas velocity dispersion derived from IFU Hαobservations and atomic gas velocity dispersion and energy surface densities derived from Hisynthesis observations with the Very Large Array. These indicators of turbulence were tested against star formation histories over the past 560 Myr derived from color–magnitude diagrams using Spearman’s rank correlation coefficient. The strongest correlation identified at the 400 pc scale is between measures of Hiturbulence and star formation 70–140 Myr ago. We repeated our analysis of UGC 4305's current turbulence and past star formation activity on multiple physical scales (∼560 and 800 pc) to determine whether there are indications of changes in the correlation timescale with changes to the physical scale. No notable correlations were found at larger physical scales, emphasizing the importance of analyzing star formation-driven turbulence as a local phenomenon. 

Abstract The ratio of baryonic-to-dark matter in present-day galaxies constrains galaxy formation theories and can be determined empirically via the baryonic Tully–Fisher relation (BTFR), which compares a galaxy’s baryonic mass ( M bary ) to its maximum rotation velocity ( V max ). The BTFR is well determined at M bary > 10 8 M ⊙ , but poorly constrained at lower masses due to small samples and the challenges of measuring rotation velocities in this regime. For 25 galaxies with high-quality data and M bary ≲ 10 8 M ⊙ , we estimate M bary from infrared and H i observations and V max from the H i gas rotation. Many of the V max values are lower limits because the velocities are still rising at the edge of the detected H i disks ( R max ); consequently, most of our sample has lower velocities than expected from extrapolations of the BTFR at higher masses. To estimate V max , we map each galaxy to a dark matter halo assuming density profiles with and without cores. In contrast to noncored profiles, we find the cored profile rotation curves are still rising at R max values, similar to the data. When we compare the V max values derived from the cored density profiles to our M bary measurements, we find a turndown of the BTFR at low masses that is consistent with Λ cold dark matter predictions and implies baryon fractions of 1%–10% of the cosmic value. Although we are limited by the sample size and assumptions inherent in mapping measured rotational velocities to theoretical rotation curves, our results suggest that galaxy formation efficiency drops at masses below M bary ∼ 10 8 M ⊙ , corresponding to M 200 ∼ 10 10 M ⊙ . 

Abstract Stellar feedback is fundamental to the modeling of galaxy evolution, as it drives turbulence and outflows in galaxies. Understanding the timescales involved are critical for constraining the impact of stellar feedback on the interstellar medium. We analyzed the resolved star formation histories along with the spatial distribution and kinematics of the atomic and ionized gas of four nearby star-forming dwarf galaxies (NGC 4068, NGC 4163, NGC 6789, and UGC 9128) to determine the timescales over which stellar feedback drives turbulence. The four galaxies are within 5 Mpc and have a range of properties including current star formation rates of 0.0005–0.01M_⊙yr⁻¹, log(M_*/M_⊙) between 7.2 and 8.2, and log(M_Hi/M_⊙) between 7.2 and 8.3. Their color–magnitude diagram derived star formation histories over the past 500 Myr were compared to their atomic and ionized gas velocity dispersion and Hienergy surface densities as indicators of turbulence. The Spearman’s rank correlation coefficient was used to identify any correlations between their current turbulence and their past star formation activity on local scales (∼400 pc). The strongest correlation found was between the Hiturbulence measures and the star formation rate 100–200 Myr ago. This suggests a coupling between the star formation activity and atomic gas on this timescale. No strong correlation between the ionized gas velocity dispersion and the star formation activity between 5 and 500 Myr ago was found. The sample and analysis are the foundation of a larger program aimed at understanding the timescales over which stellar feedback drives turbulence. 

Abstract The LIGO HET Response (LIGHETR) project is an enterprise to follow up optical transients (OTs) discovered as gravitational-wave merger sources by the LIGO/Virgo collaboration (LVC). Early spectroscopy has the potential to constrain crucial parameters such as the aspect angle. The LIGHETR collaboration also includes the capacity to model the spectroscopic evolution of mergers to facilitate a real-time direct comparison of models with our data. The principal facility is the Hobby–Eberly Telescope. LIGHETR uses the massively replicated VIRUS array of spectrographs to search for associated OTs and obtain early blue spectra, and in a complementary role, the low-resolution LRS2 spectrograph is used to obtain spectra of viable candidates as well as a densely sampled series of spectra of true counterparts. Once an OT is identified, the anticipated cadence of spectra would match or considerably exceed anything achieved for GW170817 = AT2017gfo for which there were no spectra in the first 12 hr and thereafter only roughly once daily. We describe special HET-specific software written to facilitate the program and attempts to determine the flux limits to undetected sources. We also describe our campaign to follow up OT candidates during the third observational campaign of the LIGO and Virgo Scientific Collaborations. We obtained VIRUS spectroscopy of candidate galaxy hosts for five LVC gravitational-wave events and LRS2 spectra of one candidate for the OT associated with S190901ap. We identified that candidate, ZTF19abvionh = AT2019pip, as a possible Wolf–Rayet star in an otherwise unrecognized nearby dwarf galaxy. 

ABSTRACT Deciphering the distribution of metals throughout galaxies is fundamental in our understanding of galaxy evolution. Nearby, low-metallicity, star-forming dwarf galaxies, in particular, can offer detailed insight into the metal-dependent processes that may have occurred within galaxies in the early Universe. Here, we present VLT/MUSE observations of one such system, JKB 18, a blue diffuse dwarf galaxy with a metallicity of only 12 + log(O/H)=7.6 ± 0.2 (∼0.08 Z⊙). Using high spatial resolution integral-field spectroscopy of the entire system, we calculate chemical abundances for individual H ii regions using the direct method and derive oxygen abundance maps using strong-line metallicity diagnostics. With large-scale dispersions in O/H, N/H, and N/O of ∼0.5–0.6 dex and regions harbouring chemical abundances outside this 1σ distribution, we deem JKB 18 to be chemically inhomogeneous. We explore this finding in the context of other chemically inhomogeneous dwarf galaxies and conclude that neither the accretion of metal-poor gas, short mixing time-scales or self-enrichment from Wolf–Rayet stars are accountable. Using a galaxy-scale, multiphase, hydrodynamical simulation of a low-mass dwarf galaxy, we find that chemical inhomogeneities of this level may be attributable to the removal of gas via supernovae and the specific timing of the observations with respect to star formation activity. This study not only draws attention to the fact that dwarf galaxies can be chemically inhomogeneous, but also that the methods used in the assessment of this characteristic can be subject to bias. 

The ALFALFA blind HI survey has populated the low-mass end of the HI mass function for the first time, allowing an unprecedented opportunity to explore the physical properties of the galaxies that inhabit this extreme portion of parameter space. Using the now-complete ALFALFA dataset, we have constructed the "Survey of HI in Extremely Low-mass Dwarfs" ("SHIELD"), a complete sample of 82 galaxies that provides a unique opportunity to further our understanding of these cosmologically important systems. Here we present new VLA imaging of the final 3 of the 82 sources to be observed at low angular resolution in the HI spectral line: AGC215284, AGC731448, and AGC732041 (program VLA/18A-177). We compare the HI images to ground-based H-alpha imaging in order to examine the star formation properties of each system.