skip to main content


Search for: All records

Creators/Authors contains: "Cannon, John M."

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

    Around 400 Myr after the big bang, the ultraviolet emission from star-forming galaxies reionized the Universe. Ionizing radiation (Lyman continuum, LyC) is absorbed by cold neutral hydrogen gas (H i) within galaxies, hindering the escape of LyC photons. Since the H i reservoir of LyC emitters has never been mapped, major uncertainties remain on how LyC photons escape galaxies and ionize the intergalactic medium. We have directly imaged the neutral gas in the nearby reionization-era analogue galaxy Haro 11 with the 21 cm line to identify the mechanism enabling ionizing radiation escape. We find that merger-driven interactions have caused a bulk offset of the neutral gas by about $6\,$ kpc from the centre of the galaxy, where LyC emission production sites are located. This could facilitate the escape of ionizing radiation into our line of sight. Galaxy interactions can cause both elevated LyC production and large-scale displacement of H i from the regions where these photons are produced. They could contribute to the anisotropic escape of LyC radiation from galaxies and the reionization of the Universe. We argue for a systematic assessment of the effect of environment on LyC production and escape.

     
    more » « less
  2. Abstract

    We present results from an optical search for Local Group dwarf galaxy candidates associated with the Ultra-Compact High Velocity Clouds (UCHVCs) discovered by the ALFALFA neutral hydrogen survey. The ALFALFA UCHVCs are isolated, compact Hiclouds with projected sizes, velocities, and estimated Himasses that suggest they may be nearby dwarf galaxies, but that have no clear counterpart in existing optical survey data. We observed 26 UCHVCs with the WIYN 3.5 m telescope and One Degree Imager (ODI) in two broadband filters and searched the images for resolved stars with properties that match those of stars in typical dwarf galaxies at distances <2.5 Mpc. We identify one promising dwarf galaxy candidate at a distance of ∼570 kpc associated with the UCHVC AGC 268071, and five other candidates that may deserve additional follow-up. We carry out a detailed analysis of ODI imaging of a UCHVC that is close in both projected distance and radial velocity to the outer-halo Milky Way globular cluster Pal 3. We also use our improved detection methods to reanalyze images of five UCHVCs that were found to have possible optical counterparts during the first phase of the project, and confirm the detection of a possible stellar counterpart to the UCHVC AGC 249525 at an estimated distance of ∼2 Mpc. We compare the optical and Hiproperties of the dwarf galaxy candidates to the results from recent theoretical simulations that model satellite galaxy populations in group environments, as well as to the observed properties of galaxies in and around the Local Group.

     
    more » « less
  3. 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 ⊙ . 
    more » « less
  4. Abstract

    We present deep optical imaging and photometry of four objects classified as “Almost-Dark” galaxies in the Arecibo Legacy Fast Arecibo L-band Feed Array (ALFALFA) survey because of their gas-rich nature and extremely faint or missing optical emission in existing catalogs. They have Himasses of 107–109Mand distances of ∼9–100 Mpc. Observations with the WIYN 3.5 m telescope and One Degree Imager reveal faint stellar components with central surface brightnesses of ∼24–25magarcsec2in thegband. We also present the results of Hisynthesis observations with the Westerbork Synthesis Radio Telescope. These Almost-Dark galaxies have been identified as possible tidal dwarf galaxies (TDGs) based on their proximity to one or more massive galaxies. We demonstrate that AGC 229398 and AGC 333576 likely have the low dark matter content and large effective radii representative of TDGs. They are located much farther from their progenitors than previously studied TDGs, suggesting they are older and more evolved. AGC 219369 is likely dark matter dominated, while AGC 123216 has a dark matter content that is unusually high for a TDG, but low for a normal dwarf galaxy. We consider possible mechanisms for the formation of the TDG candidates such as a traditional major merger scenario and gas ejection from a high-velocity flyby. Blind Hisurveys like ALFALFA enable the detection of gas-rich, optically faint TDGs that can be overlooked in other surveys, thereby providing a more complete census of the low-mass galaxy population and an opportunity to study TDGs at a more advanced stage of their life cycle.

     
    more » « less
  5. Abstract Using Hubble Space Telescope imaging of the resolved stellar population of KK 242 = NGC 6503-d1 =PGC 4689184, we measure the distance to the galaxy to be 6.46 ± 0.32 Mpc and find that KK 242 is a satellite of the low-mass spiral galaxy NGC 6503 located on the edge of the Local Void. Observations with the Karl G. Jansky Very Large Array show signs of a very faint H i signal at the position of KK 242 within a velocity range of V hel = −80 ± 10 km s −1 . This velocity range is severely contaminated by H i emission from the Milky Way and from NGC 6503. The dwarf galaxy is classified as the transition type, dIrr/dSph, with a total H i mass of < 10 6 M ⊙ and a star formation rate SFR(H α ) = −4.82 dex ( M ⊙ yr −1 ). Being at a projected separation of 31 kpc with a radial velocity difference of—105 km s −1 relative to NGC 6503, KK 242 gives an estimate of the halo mass of the spiral galaxy to be log ( M / M ⊙ ) = 11.6. Besides NGC 6503, there are eight more detached low-luminosity spiral galaxies in the Local Volume: M33, NGC 2403, NGC 7793, NGC 1313, NGC 4236, NGC 5068, NGC 4656, and NGC 7640, from whose small satellites we have estimated the average total mass of the host galaxies and their average total mass-to- K -band-luminosity 〈 M T / M ⊙ 〉 = (3.46 ± 0.84) × 10 11 and (58 ± 19) M ⊙ / L ⊙ , respectively. 
    more » « less
  6. Abstract We use panoramic optical spectroscopy obtained with the Very Large Telescope/MUSE to investigate the nature of five candidate extremely isolated low-mass star-forming regions (Blue Candidates; hereafter, BCs) toward the Virgo cluster of galaxies. Four of the five (BC1, BC3, BC4, and BC5) are found to host several H ii regions and to have radial velocities fully compatible with being part of the Virgo cluster. All the confirmed candidates have mean metallicity significantly in excess of that expected from their stellar mass, indicating that they originated from gas stripped from larger galaxies. In summary, these four candidates share the properties of the prototype system SECCO 1, suggesting the possible emergence of a new class of stellar systems, intimately linked to the complex duty cycle of gas within clusters of galaxies. A thorough discussion of the nature and evolution of these objects is presented in a companion paper, where the results obtained here from the MUSE data are complemented with Hubble Space Telescope (optical) and Very Large Array (H i ) observations. 
    more » « less
  7. Abstract We present results from deep H i and optical imaging of AGC 229101, an unusual H i source detected at v helio =7116 km s −1 in the Arecibo Legacy Fast ALFA (ALFALFA) blind H i survey. Initially classified as a candidate “dark” source because it lacks a clear optical counterpart in Sloan Digital Sky Survey (SDSS) or Digitized Sky Survey 2 (DSS2) imaging, AGC 229101 has 10 9.31±0.05 M ⊙ of H i , but an H i line width of only 43 ± 9 km s −1 . Low-resolution Westerbork Synthesis Radio Telescope (WSRT) imaging and higher-resolution Very Large Array (VLA) B-array imaging show that the source is significantly elongated, stretching over a projected length of ∼80 kpc. The H i imaging resolves the source into two parts of roughly equal mass. WIYN partially populated One Degree Imager (pODI) optical imaging reveals a faint, blue optical counterpart coincident with the northern portion of the H i . The peak surface brightness of the optical source is only μ g ∼ 26.6 mag arcsec −2 , well below the typical cutoff that defines the isophotal edge of a galaxy, and its estimated stellar mass is only 10 7.32±0.33 M ⊙ , yielding an overall neutral gas-to-stellar mass ratio of M / M * = 98 − 52 + 111 . We demonstrate the extreme nature of this object by comparing its properties with those of other H i -rich sources in ALFALFA and the literature. We also explore potential scenarios that might explain the existence of AGC 229101, including a tidal encounter with neighboring objects and a merger of two dark H i clouds. 
    more » « less
  8. Abstract

    We discuss five blue stellar systems in the direction of the Virgo cluster, analogous to the enigmatic object SECCO 1 (AGC 226067). These objects were identified based on their optical and UV morphology and followed up with Hiobservations with the Very Large Array (and Green Bank Telescope), Multi Unit Spectroscopic Explorer (on the Very Large Telescope) optical spectroscopy, and Hubble Space Telescope imaging. These new data indicate that one system is a distant group of galaxies. The remaining four are extremely low mass (M*∼ 105M), are dominated by young blue stars, have highly irregular and clumpy morphologies, are only a few kiloparsecs across, yet host an abundance of metal-rich,12+log(O/H)>8.2, Hiiregions. These high metallicities indicate that these stellar systems formed from gas stripped from much more massive galaxies. Despite the young age of their stellar populations, only one system is detected in Hi, while the remaining three have minimal (if any) gas reservoirs. Furthermore, two systems are surprisingly isolated and have no plausible parent galaxy within ∼30′ (∼140 kpc). Although tidal stripping cannot be conclusively excluded as the formation mechanism of these objects, ram pressure stripping more naturally explains their properties, in particular their isolation, owing to the higher velocities, relative to the parent system, that can be achieved. Therefore, we posit that most of these systems formed from ram-pressure-stripped gas removed from new infalling cluster members and survived in the intracluster medium long enough to become separated from their parent galaxies by hundreds of kiloparsecs and that they thus represent a new type of stellar system.

     
    more » « less
  9. null (Ed.)