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Register a new userAre Newly Discovered HI High Velocity Clouds Minihalos in the Local Group?
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Added by
Riccardo Giovanelli
Publication date
2009
fields
Physics
and research's language is
English
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A set of HI sources extracted from the north Galactic polar region by the ongoing ALFALFA survey has properties that are consistent with the interpretation that they are associated with isolated minihalos in the outskirts of the Local Group (LG). Unlike objects detected by previous surveys, such as the Compact High Velocity Clouds of Braun & Burton (1999), the HI clouds found by ALFALFA do not violate any structural requirements or halo scaling laws of the LambdaCDM structure paradigm, nor would they have been detected by extant HI surveys of nearby galaxy groups other than the LG. At a distance of d Mpc, their HI masses range between $5 x 10^4 d^2 and 10^6 d^2 solar and their HI radii between <0.4d and 1.6 d kpc. If they are parts of gravitationally bound halos, the total masses would be on order of 10^8--10^9 solar, their baryonic content would be signifcantly smaller than the cosmic fraction of 0.16 and present in a ionized gas phase of mass well exceeding that of the neutral phase. This study does not however prove that the minihalo interpretation is unique. Among possible alternatives would be that the clouds are shreds of the Leading Arm of the Magellanic Stream.
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High-velocity clouds (HVCs) are clouds of HI seen around the Milky Way with velocities inconsistent with Galactic rotation, have unknown distances and masses and controversial origins. One possibility is that HVCs are associated with the small dark matter halos seen in models of galaxy formation and distributed at distances of 150 kpc - 1 Mpc. We report on our attempts to detect the analogs to such putative extragalactic clouds in three groups of galaxies similar to our own Local Group using the ATNF Parkes telescope and Compact Array. Eleven dwarf galaxies were found, but no HI clouds lacking stars were detected. Using the population of compact HVCs around the Milky Way as a template, we find that our non-detection of analogs implies that they must be clustered within 160 kpc of the Milky Way (and other galaxies) with an average HI mass <4x10^5 M(sun) at the 95% confidence level. This is in accordance with recent limits derived by other authors. If our groups are true analogs to the Local Group, then this makes the original Blitz et al. and Braun & Burton picture of HVCs residing out to 1 Mpc from the Milky Way extremely unlikely. The total HI mass in HVCs, < 10^8 M(sun), implies that there is not a large reservoir of neutral hydrogen waiting to be accreted onto the Milky Way. Any substantial reservoir of baryonic matter must be mostly ionized or condensed enough as to be undetectable.
Motivated by the apparent order-of-magnitude discrepancy between the observed number of Local Group satellite galaxies, and that predicted by Lambda-CDM hierarchical clustering cosmologies, we explore an alternate suggestion - perhaps the missing satellites are not actually ``missing, but are instead ``in disguise. The disguise we consider here is that of the classical HI High-Velocity Clouds. Is it possible that what have been thought of traditionally as a ``Galactic phenomenon, are actually the building blocks of the Local Group? We discuss the strengths and weaknesses of this hypothesis, and highlight avenues of future research which may provide an unequivocal resolution to this contentious issue.
Context: We present a newly discovered class of low-luminosity, dusty, evolved objects in the Magellanic Clouds. These objects have dust excesses, stellar parameters, and spectral energy distributions similar to those of dusty post-asymptotic giant branch (post-AGB) stars. However, they have lower luminosities and hence lower masses. We suggest that they have evolved off the red giant branch (RGB) instead of the AGB as a result of binary interaction. Aims: In this study we aim to place these objects in an evolutionary context and establish an evolutionary connection between RGB binaries (such as the sequence E variables) and our new sample of objects. Methods: We compared the theoretically predicted birthrates of the progeny of RGB binaries to the observational birthrates of the new sample of objects. Results: We find that there is order-of-magnitude agreement between the observed and predicted birthrates of post-RGB stars. The sources of uncertainty in the birthrates are discussed; the most important sources are probably the observational incompleteness factor and the post-RGB evolution rates. We also note that mergers are relatively common low on the RGB and that stars low on the RGB with mid-IR excesses may recently have undergone a merger. Conclusions: Our sample of dusty post-RGB stars most likely provides the first observational evidence for a newly discovered phase in binary evolution: post-RGB binaries with circumstellar dust.
We have conducted an HI 21 cm emission-line survey using the Parkes 20cm multibeam instrument and the Australia Telescope Compact Array (ATCA) of six loose groups of galaxies chosen to be analogs to the Local Group. The goal of this survey is to make a census of the HI-rich galaxies and high-velocity clouds (HVCs) within these groups and compare these populations with those in the Local Group. The Parkes observations covered the entire volume of each group with a rms M(HI) sensitivity of 4-10x10^5 M(sun) per 3.3 km/s channel. All potential sources detected in the Parkes data were confirmed with ATCA observations at ~2 resolution and the same M(sun) sensitivity. All the confirmed sources have associated stellar counterparts; no starless HI clouds--HVC analogs--were found in the six groups. In this paper, we present a description of the survey parameters, its sensitivity and completeness. Using the population of compact HVCs (CHVCs) around the Milky Way as a template coupled with the detailed knowledge of our survey parameters, we infer that our non-detection of CHVC analogs implies that, if similar populations exist in the six groups studied, the CHVCs must be clustered within 90 kpc of group galaxies, with average M(HI) < 4x10^5 M(sun) at the 95% confidence level. The corollary is that the same must apply to Milky Way CHVCs. This is consistent with our previous results from a smaller sample of groups, and in accordance with recent observational and theoretical constraints from other authors. These results confirm that there is very little neutral matter around galaxies, and that any substantial reservoir of baryons must be in other phases.
The SKA and its pathfinders will enable studies of HI emission at higher redshifts than ever before. In moving beyond the local Universe, this will require the use of cosmologically appropriate formulae that have traditionally been simplified to their low-redshift approximations. In this paper, we summarise some of the most important relations for tracing HI emission in the SKA era, and present an online calculator to assist in the planning and analysis of observations (hifi.icrar.org).
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