HI Observations of Nearby Dwarf Galaxies

Monthly Notices of The Royal Astronomical Society, 2005

We present a deep Giant Metrewave Radio Telescope (GMRT) search for HI 21-cm emission from three dwarf galaxies, viz. POX 186, SC 24 and KKR 25. Based, in part, on previous single-dish HI observations, these galaxies have been classified as a blue compact dwarf (BCD), a dwarf irregular and a transition galaxy, respectively. However, in conflict with previous single-dish detections,

1989

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Publications of the Astronomical Society of Australia, 1999

After reviewing the HIcontent and distribution in extreme dwarf irregular (dIrr) and dwarf elliptical (dE) galaxies, previous searches for HIin dwarf spheroidal (dSph) galaxies will be discussed. I will report on the recent detections of HIprobably associated with the Local Group (LG) dSph Sculptor and dIrr/dSph Phoenix, obtained with the ATCA, along with a similar detection in the Centaurus Group dSph CEN_41. Data obtained for Sculptor, using the Parkes Multibeam system, will also be presented and the advantage of the wide field for such nearby objects will be emphasised. Finally, the possible origin of the gas and the general problem of the missing ISM in dSph galaxies will be discussed.

2005

An \hi survey of 10 dE/dS0 galaxies in the nearby Sculptor and Centaurus A groups was made using the Australia Telescope Compact Array (ATCA). The observed galaxies have accurate distances derived by Jerjen et al (1998; 2000b) using the surface brightness fluctuation technique. Their absolute magnitudes are in the range $-9.5 > M_B > -15.3$. Only two of the ten galaxies were detected at our detection limit ($\sim 1.0 \times 10^6$ \msol for the Centaurus group and $\sim 5.3 \times 10^5$ \msol for the Sculptor group), the two dS0 galaxies ESO384-016 in the Centaurus A Group and NGC 59 in the Sculptor Group, with \hi masses of $6.0 \pm 0.5 \times 10^6$ \msol and $1.4 \pm 0.1 \times 10^7$ \msol respectively. Those two detections were confirmed using the Green Bank Telescope. These small \hi reservoirs could fuel future generations of low level star formation and could explain the bluer colors seen at the center of the detected galaxies. Similarly to what is seen with the Virgo dEs, the two objects with \hi appear to be on the outskirt of the groups.

Bulletin of the American Astronomical Society, 1993

We present the results of a survey of the HI content of a large, complete sample of actively star-forming dwarf galaxies. All observations were obtained with the 305-m telescope of the Arecibo Observatory. The sample consists of all galaxies cataloged in the Markarian, Case, Michigan, Wasilewski, and Haro surveys which were known to have M_B > -17.0 and which lie within the Arecibo declination range. In all, 139 galaxies were observed, of which 122 were detected. The data consist of high S/N HI profiles, obtained with high velocity resolution (1 -- 2 km/s channel spacing) to ensure adequate sampling of the profiles, which are often quite narrow. The neutral gas properties of the sample are analyzed and combined with optical data when the latter are available. These objects possess a surprisingly wide range in the levels of their neutral gas content. Some of these star-forming galaxies appear to be very gas-poor even when compared with samples of less active dwarfs, but many others have large HI mass to blue light ratios. It is suggested that many of the currently starbursting dwarfs evolved from progenitor galaxies which initially had extremely high MHI/L_B and were likely to be ultra-low surface brightness galaxies. These results hint at the presence of a large population of optically faint gas-rich dwarf galaxies, such as envisioned by Impey, Bothun & Malin (1988) and others.

1996

The Blue Compact Dwarf (BCD) galaxy SBS 0335--052 is one of the most metal--deficient galaxies known and one of the best candidates for a young dwarf galaxy in the process of formation. A VLA HI map reveals an unusual structure: the neutral gas is distributed in a very extended disk, about 15 times larger than the Holmberg optical diameter of the BCG. There are two peaks of high density. The eastern peak is close to the position of SBS 0335--052 whereas the western HI peak is associated with a faint compact optical galaxy. A 6--m telescope spectrum of this object shows Halpha, Hbeta, and [OIII] 5007 emission with a redshift close to that of SBS 0335--052. We suggest that it may be a young and chemically unevolved dwarf galaxy.

The Astrophysical Journal, 2015

We report five Local Volume dwarf galaxies (two of which are presented here for the first time) uncovered during a comprehensive archival search for optical counterparts to ultra-compact high velocity clouds (UCHVCs). The UCHVC population of HI clouds are thought to be candidate gas-rich, low mass halos at the edge of the Local Group and beyond, but no comprehensive search for stellar counterparts to these systems has been presented. Careful visual inspection of all publicly available optical and ultraviolet imaging at the position of the UCHVCs revealed six blue, diffuse counterparts with a morphology consistent with a faint dwarf galaxy beyond the Local Group. Optical spectroscopy of all six candidate dwarf counterparts show that five have an Hα-derived velocity consistent with the coincident HI cloud, confirming their association; the sixth diffuse counterpart is likely a background object. The size and luminosity of the UCHVC dwarfs is consistent with other known Local Volume dwarf irregular galaxies. The gas fraction (M HI /M star) of the five dwarfs are generally consistent with that of dwarf irregular galaxies in the Local Volume, although ALFALFA-Dw1 (associated with ALFALFA UCHVC HVC274.68+74.70−123) has a very high M HI /M star ∼40. Despite the heterogenous nature of our search, we demonstrate that the current dwarf companions to UCHVCs are at the edge of detectability due to their low surface brightness, and that deeper searches are likely to find more stellar systems. If more sensitive searches do not reveal further stellar counterparts to UCHVCs, then the dearth of such systems around the Local Group may be in conflict with ΛCDM simulations.

2013

H i line widths are typically interpreted as a measure of ISM turbulence, which is potentially driven by star formation. In an effort to better understand the possible connections between line widths and star formation, we have characterized H i kinematics in a sample of nearby dwarf galaxies by co-adding line-of-sight spectra after removing the rotational velocity to produce an average, global H i line profile. These "superprofiles" are composed of a central narrow peak (∼ 6 − 10 km s −1 ) with higher-velocity wings to either side that contain ∼ 10 − 15% of the total flux. The superprofiles are all very similar, indicating a universal global H i profile for dwarf galaxies. We compare characteristics of the superprofile to various galaxy properties, such as mass and measures of star formation (SF), with the assumption that the superprofile represents a turbulent peak with energetic wings to either side. We use these quantities to derive average scale heights for the sample galaxies. When comparing to physical properties, we find that the velocity dispersion of the central peak is correlated with Σ HI . The fraction of mass and characteristic velocity of the high velocity wings are correlated with measures of SF, consistent with the picture that SF drives surrounding H i to higher velocities. While gravitational instabilities provide too little energy, the SF in the sample galaxies does provide enough energy through supernova, with realistic estimates of the coupling efficiency, to produce the observed superprofiles.

The Astronomical Journal, 2006

Parkes 1 neutral hydrogen 21 cm line (Hi) observations of the surroundings of 9 early-type Local Group dwarfs are presented. We detected numerous Hi clouds in the general direction of those dwarfs and these clouds are often offset from the optical center of the galaxies. Although all the observed dwarfs, except Antlia, occupy phase-space regions where the High-Velocity Cloud (HVC) density is well above average, the measured offsets are smaller than one would expect from a fully random cloud distribution. Possible association is detected for 11 of the 16 investigated clouds, while, for two galaxies, Sextans and Leo I, no Hi was detected. The galaxies where Hi clouds were found not to coincide with the optical, yet have a significant probability of being associated are: Sculptor

Proceedings of the International Astronomical Union

We know that the observed H i (and H2) content cannot explain the SFR observed in galaxies. The only way galaxies can sustain that SFR is by accreting HI-rich dwarf galaxies or Inter-Galactic HI clouds. However, no observation to detect those accretion events has been conclusive so far. Instruments having the necessary sensitivity (e.g. GBT) lack the necessary spatial resolution and those with the proper resolution (e.g. VLA) lack the sensitivity. I will show that both are necessary to detect those illusive Hi clouds. The SKA precursor MeerKAT is starting its operation as we speak and will start the Large Survey Programs at the end of 2018. FAST has started its observations in drift scan mode with CRAFTS (Commensal Radio Astronomy Fast Survey). In the near future (2019-20), the best combination to study low column density H i will be to combine the sensitivity of FAST with the spatial resolution of MeerKAT.