H. Alyson Ford

National Radio Astronomy Observatory, Charlottesville, Virginia, United States

Are you H. Alyson Ford?

Claim your profile

Publications (14)71.76 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We present HI spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only 3% Z$_{\odot}$, making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the mass-metallicity and luminosity-metallicity relationships. The origin of such a low oxygen abundance in DDO 68 presents a challenge for models of the chemical evolution of galaxies. One possible solution to this problem is the infall of pristine neutral gas, potentially initiated during a gravitational interaction. Using archival HI spectral-line imaging obtained with the Karl G. Jansky Very Large Array, we have discovered a previously unknown companion of DDO 68. This low-mass (M$_{\rm HI}$ $=$ 2.8$\times$10$^{7}$ M$_{\odot}$), recently star-forming (SFR$_{\rm FUV}$ $=$ 1.4$\times$10$^{-3}$ M$_{\odot}$ yr$^{-1}$, SFR$_{\rm H\alpha}$ $<$ 7$\times$10$^{-5}$ M$_{\odot}$ yr$^{-1}$) companion has the same systemic velocity as DDO 68 (V$_{\rm sys}$ $=$ 506 km s$^{-1}$; D $=$ 12.74$\pm$0.27 Mpc) and is located at a projected distance of 42 kpc. New HI maps obtained with the 100m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time. Low surface brightness HI gas forms a bridge between these objects.
    The Astrophysical Journal Letters 04/2014; 787(1). · 6.35 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We report the discovery of a new faint dwarf galaxy, GHOSTS I, using HST/ACS data from one of our GHOSTS (Galaxy Halos, Outer disks, Substructure, Thick disk, and Star clusters) fields. Its detected individual stars populate an approximately one magnitude range of its luminosity function (LF). Using synthetic color-magnitude diagrams (CMDs) to compare with the galaxy's CMD, we find that the colors and magnitudes of GHOSTS I's individual stars are most consistent with being young helium-burning and asymptotic giant branch stars at a distance of 12 +/- 2 Mpc. Morphologically, GHOSTS I appears to be actively forming stars, so we tentatively classify it as a dwarf irregular (dIrr) galaxy, although future HST observations deep enough to resolve a larger magnitude range in its LF are required to make a more secure classification. GHOSTS I's absolute magnitude is $M_V = -9.85^{+ 0.40}_{- 0.33}$, making it one of the least luminous dIrr galaxies known, and its metallicity is lower than [Fe/H] =-1.5 dex. The half-light radius of GHOSTS I is 226 +/- 38 pc and its ellipticity is 0.47 +/- 0.07, similar to Milky Way and M31 dwarf satellites at comparable luminosity. There are no luminous massive galaxies or galaxy clusters within ~ 4 Mpc from GHOSTS I that could be considered as its host, making it a very isolated dwarf galaxy in the Local Universe.
    The Astrophysical Journal 12/2013; 780(2). · 6.73 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We have detected hundreds of HI clouds in the lower Galactic halo within the Galactic All-Sky Survey pilot region, a region spanning 325∘≤ l ≤ 343∘ and |b|≤ 20∘. The motions of these clouds are governed by Galactic rotation, they are seen within the disk and up to heights of 2.5 kpc, and they span a wide range of Galactocentric radii. However, these clouds are not uniformly distributed and appear to be associated with Galactic structures such as spiral arms or superbubbles.
    EAS Publications Series 09/2012;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A survey of the Milky Way disk and the Magellanic System at the wavelengths of the 21-cm atomic hydrogen (HI) line and three 18-cm lines of the OH molecule will be carried out with the Australian Square Kilometre Array Pathfinder telescope. The survey will study the distribution of HI emission and absorption with unprecedented angular and velocity resolution, as well as molecular line thermal emission, absorption, and maser lines. The area to be covered includes the Galactic plane (|b|< 10deg) at all declinations south of delta = +40deg, spanning longitudes 167deg through 360deg to 79deg at b=0deg, plus the entire area of the Magellanic Stream and Clouds, a total of 13,020 square degrees. The brightness temperature sensitivity will be very good, typically sigma_T ~ 1 K at resolution 30arcsec and 1 km/s. The survey has a wide spectrum of scientific goals, from studies of galaxy evolution to star formation, with particular contributions to understanding stellar wind kinematics, the thermal phases of the interstellar medium, the interaction between gas in the disk and halo, and the dynamical and thermal states of gas at various positions along the Magellanic Stream.
    Publications of the Astronomical Society of the Pacific 07/2012; · 3.69 Impact Factor
  • Source
    H. Alyson Ford, Joel N. Bregman
    [Show abstract] [Hide abstract]
    ABSTRACT: Small amounts of star formation in elliptical galaxies are suggested by several results: surprisingly young ages from optical line indices, cooling X-ray gas, and mid-IR dust emission. Such star formation has previously been difficult to directly detect, but using UV Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) imaging, we have identified individual young stars and star clusters in four nearby ellipticals. This technique is orders of magnitude more sensitive than other methods, allowing detections of star formation to 10^(-5) Msun/yr. Ongoing star formation is detected in all galaxies, including three ellipticals that have previously exhibited potential signposts of star forming conditions (NGC 4636, NGC 4697, and NGC 4374), as well as the typical "red and dead" NGC 3379. The current star formation in our closest targets, where we are most complete, is between 1-8x10^(-5) Msun/yr. The star formation history was roughly constant from 0.5-1.5 Gyr (at 3-5x10^(-4) Msun/yr), but decreased by a factor of several in the past 0.3 Gyr. Most star clusters have a mass between 10^2 - 10^4 Msun. The specific star formation rates of ~10^(-16) yr^(-1) (at the present day) or ~10^(-14) yr^(-1) (when averaging over the past Gyr) imply that a fraction 10^(-8) of the stellar mass is younger than 100 Myr and 10^(-5) is younger than 1 Gyr, quantifying the level of frosting of recent star formation over the otherwise passive stellar population. There is no obvious correlation between either the presence or spatial distribution of postulated star formation indicators and the star formation we detect.
    The Astrophysical Journal 05/2012; 770(2). · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Using data from the Galactic All-Sky Survey, we have compared the properties and distribution of HI clouds in the disk-halo transition at the tangent points in mirror-symmetric regions of the first quadrant (QI) and fourth quadrant (QIV) of the Milky Way. Individual clouds are found to have identical properties in the two quadrants. However, there are 3 times as many clouds in QI as in QIV, their scale height is twice as large, and their radial distribution is more uniform. We attribute these major asymmetries to the formation of the clouds in the spiral arms of the Galaxy, and suggest that the clouds are related to star formation in the form of gas that has been lifted from the disk by superbubbles and stellar feedback, and fragments of shells that are falling back to the plane.
    01/2011;
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Parkes Galactic All-Sky Survey (GASS) is a survey of Galactic atomic hydrogen (HI) emission in the southern sky observed with the Parkes 64-m Radio Telescope. The first data release was published by McClure-Griffiths et al. (2009). We remove instrumental effects that affect the GASS and present the second data release. We calculate the stray-radiation by convolving the all-sky response of the Parkes antenna with the brightness temperature distribution from the Leiden/Argentine/Bonn (LAB) all sky 21-cm line survey, with major contributions from the 30-m dish of the Instituto Argentino de Radioastronomia (IAR) in the southern sky. Remaining instrumental baselines are corrected using the LAB data for a first guess of emission-free baseline regions. Radio frequency interference is removed by median filtering. After applying these corrections to the GASS we find an excellent agreement with the Leiden/Argentine/Bonn (LAB) survey. The GASS is the highest spatial resolution, most sensitive, and is currently the most accurate HI survey of the Galactic HI emission in the southern sky. We provide a web interface for generation and download of FITS cubes. Comment: minor changes, 12 pages, 9 figures, A&A in press
    Astronomy and Astrophysics 07/2010; · 5.08 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The description of Column 6 of Table 1 in Section 3.2 of our paper was based on a previous version of the catalog and does not apply to the published version of Table 1. The correct description of the H I column density, , that is presented in Table 1 is: the H I column density, , at the cloud center is 1.94 × 1018T pkΔv cm–2 in the optically thin limit, an assumption that is reasonable because the emission is faint. An integrated intensity map was not used to subtract a background when determining , and was only used when determining θmaj, θmin, and . The background subtraction from the line profile in the determination of T pk was inherently included in the calculation of .
    The Astrophysical Journal 02/2010; 710(2):1868. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Using 21 cm H I observations from the Parkes Radio Telescope's Galactic All-Sky Survey, we measure 255 H I clouds in the lower Galactic halo that are located near the tangent points at 16fdg9
    The Astrophysical Journal 01/2010; 722:367-379. · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Parkes Galactic All-Sky Survey (GASS) is a survey of Galactic atomic hydrogen (HI) emission in the Southern sky covering declinations $\delta \leq 1^{\circ}$ using the Parkes Radio Telescope. The survey covers $2\pi$ steradians with an effective angular resolution of ~16', at a velocity resolution of 1.0 km/s, and with an rms brightness temperature noise of 57 mK. GASS is the most sensitive, highest angular resolution survey of Galactic HI emission ever made in the Southern sky. In this paper we outline the survey goals, describe the observations and data analysis, and present the first-stage data release. The data product is a single cube at full resolution, not corrected for stray radiation. Spectra from the survey and other data products are publicly available online. Comment: 35 pages, 16 figures, accepted for publication in ApJS. Full resolution version available at ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/GASS.1.ps.gz
    The Astrophysical Journal Supplement Series 01/2009; · 16.24 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have detected over 400 HI clouds in the lower halo of the Galaxy within the pilot region of the Galactic All-Sky Survey (GASS), a region of the fourth quadrant that spans 18 degrees in longitude, 40 degrees in latitude and is centered on the Galactic equator. These clouds have a median peak brightness temperature of 0.6 K, a median velocity width of 12.8 km/s, and angular sizes <1 degree. The motion of these clouds is dominated by Galactic rotation with a random cloud-to-cloud velocity dispersion of 18 km/s. A sample of clouds likely to be near tangent points was analyzed in detail. These clouds have radii on the order of 30 pc and a median HI mass of 630 Msun. The population has a vertical scale height of 400 pc and is concentrated in Galactocentric radius, peaking at R=3.8 kpc. This confined structure suggests that the clouds are linked to spiral features, while morphological evidence that many clouds are aligned with loops and filaments is suggestive of a relationship with star formation. The clouds might result from supernovae and stellar winds in the form of fragmenting shells and gas that has been pushed into the halo rather than from a galactic fountain. Comment: 16 pages. Accepted for publication in ApJ
    The Astrophysical Journal 07/2008; · 6.73 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The Leading Arm of the Magellanic System is a tidally formed HI feature extending $\sim 60\arcdeg$ from the Magellanic Clouds ahead of their direction of motion. Using atomic hydrogen (HI) data from the Galactic All Sky-Survey (GASS), supplemented with data from the Australia Telescope Compact Array, we have found evidence for an interaction between a cloud in the Leading Arm and the Galactic disk where the Leading Arm crosses the Galactic plane. The interaction occurs at velocities permitted by Galactic rotation, which allows us to derive a kinematic distance to the cloud of 21 kpc, suggesting that the Leading Arm crosses the Galactic Plane at a Galactic radius of $R\approx 17$ kpc. Comment: 14 pages, 5 figures, accepted to Astrophysical Journal Letters. Full resolution version available at ftp://ftp.atnf.csiro.au/pub/people/nmcclure/papers/LeadingArm_apjl.pdf
    The Astrophysical Journal 12/2007; · 6.73 Impact Factor
  • Source
    H Alyson Ford, Joel N Bregman
    [Show abstract] [Hide abstract]
    ABSTRACT: Small amounts of star formation in elliptical galaxies are suggested by several results: surprisingly young ages from optical line indices, cooling X-ray gas, and mid-IR dust emission. Such star for-mation has previously been difficult to detect, but using UV Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) imaging, we have identified individual young stars and star clusters in four nearby ellipticals. This technique is orders of magnitude more sensitive than other methods, allowing detections of star formation to 10 −5 M ⊙ yr −1 . Ongoing star formation is detected in all galaxies, including three ellipticals that have previously exhibited potential signposts of star forming condi-tions (NGC 4636, NGC 4697, and NGC 4374), and our control galaxy, the typical red and dead NGC 3379. The current star formation in our closest targets, where we are most complete, is between 2 − 8 × 10 −5 M ⊙ yr −1 . The star formation history was roughly constant from 0.5 − 1.5 Gyr (at 3 − 5 × 10 −4 M ⊙ yr −1), but decreased by a factor of several in the past 0.3 Gyr. Most star clusters have a mass between 10 2 − 10 4 M ⊙ . The specific star formation rates of ∼ 10 −16 yr −1 (at the present day) or ∼ 10 −14 yr −1 (when averaging over the past Gyr) would require timescales 4-6 orders of magnitude longer than the age of the Universe to build up the stellar mass of the galaxies, quantifying for the first time the level of quenching they have experienced relative to their average value. There is no obvious correlation between either the presence or spatial distribution of postulated star formation indicators and the star formation we detect.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The Parkes Galactic All-Sky Survey (GASS) is a survey of Galactic atomic hydrogen (HI) emission in the Southern sky covering declinations < 1° using the Parkes Radio Telescope with an effective angular resolution of 16'. The survey covers 2pi sterradians at a velocity resolution of 1.0 km/s and an rms brightness temperature noise of 57 mK. GASS is the most sensitive, highest angular resolution survey of Galactic HI emission ever made in the Southern sky. We present the first-stage data release in the form of a single cube at full resolution not corrected for stray radiation. Spectra from the survey and other data products are publicly available online. We also present some initial results on small HI clouds in the Galactic halo and the interaction of the Galactic disk and halo highlighting some of the strengths of GASS for studies of the Milky Way.

Publication Stats

75 Citations
71.76 Total Impact Points

Institutions

  • 2014
    • National Radio Astronomy Observatory
      Charlottesville, Virginia, United States
  • 2012
    • University of Michigan
      • Department of Astronomy
      Ann Arbor, Michigan, United States
  • 2008–2011
    • Swinburne University of Technology
      • Centre for Astrophysics and Supercomputing
      Melbourne, Victoria, Australia