High-velocity Clouds Research Articles

Discovery of Two Small High-velocity Compact Clouds in the Central 10 pc of Our Galaxy

Abstract We discovered two small high-velocity compact clouds (HVCCs) in the HCN J = 4–3 and J = 3–2 maps of the central 20 pc of our Galaxy. Both HVCCs have broad velocity widths ( km s−1) and compact sizes ( ), and originate from the dense molecular clouds in the position–velocity space. One of them has a faint counterpart in a Paschen-α image. Their spatial structure, kinematics, and absence of luminous stellar object are compatible with the notion that each of the small HVCCs is driven by the plunge of an invisible compact object into a molecular cloud. Such objects are most likely inactive, isolated black holes.

Condensation of Halo, Circumgalactic, and Intergalactic Gas onto Massive High-velocity Clouds

Abstract High-velocity clouds (HVCs) hydrodynamically interact with their surroundings. In scenarios with small clouds, the net result is erosion of the HVC, but in scenarios with large, massive clouds, it is capture and cooling of environmental gas by the cloud. In order to examine these effects over long periods of time, we made detailed three-dimensional hydrodynamic FLASH simulations of massive HVCs (1.35 × 105 M ☉ to 1.35 × 108 M ☉) traveling through hot, low-density media like that in the extended Galactic halo, circumgalactic media, and intergalactic space for at least 200 Myr. By setting the metallicity of the clouds to differ from that of the ambient gas, we were able to track the transfer of material between the two media. We found that massive clouds condense substantial amounts of ambient gas, up to 100% of their initial mass in certain cases. This gas cools to temperatures below 1.0 × 104 K, but retains some high ions. The amount of condensed gas depends on the cloud mass, the ambient density, and the cloud density and temperature, but apparently not on the velocity. We discuss the ramifications for cloud survivability and for the transport of halo, circumgalactic, and intergalactic gas to the disk of the Galaxy aboard massive HVCs.

The Discovery and Origin of a Very High-velocity Cloud Toward M33

Abstract We report the detection of a largely ionized very high-velocity cloud (VHVC; km ) toward M33 with the Hubble Space Telescope/Cosmic Origin Spectrograph. The VHVC is detected in O i, C ii, Si ii, and Si iii absorption along five sightlines separated by . On sub-degree scales, the velocities and ionic column densities of the VHVC remain relatively smooth with standard deviations of ±14 km and ±0.15 dex between the sightlines, respectively. The VHVC has a metallicity of [O i/H i] = −0.56 ± 0.17 dex (Z = 0.28 ± 0.11 ). Despite the position–velocity proximity of the VHVC to the ionized Magellanic Stream, the VHVC’s higher metallicity makes it unlikely to be associated with the Stream, highlighting the complex velocity structure of this region of sky. We investigate the VHVC’s possible origin by revisiting its surrounding H i environment. We find that the VHVC may be (1) a MW CGM cloud, (2) related to a nearby H i VHVC—Wright’s Cloud, or (3) connected to M33's northern warp. Furthermore, the VHVC could be a bridge connecting Wright’s Cloud and M33's northern warp, which would make it a Magellanic-like structure in the halo of M33.

Distance to the Low-velocity Cloud in the Direction of the High-velocity Compact Cloud CO–0.40–0.22

Abstract CO–0.40–0.22 is a peculiar molecular cloud that is compact and has an extraordinary broad velocity width. It is found in the central molecular zone (CMZ) of our Galaxy. In this direction, there is another cloud with an H2O maser spot at a lower velocity. A collision with this low-velocity cloud could be responsible for the broad velocity width of CO–0.40–0.22. We performed phase-referencing very long baseline interferometry (VLBI) astrometry with VERA and detected the annual parallax of the H2O maser spot in the low-velocity cloud to be 0.33 ± 0.14 mas, which corresponds to a distance of from the Sun. This implies that the low-velocity cloud is located in the Galactic disk on the near side of the CMZ.

The Galaxy’s veil of excited hydrogen

Many of the baryons in our Galaxy probably lie outside the well-known disk and bulge components. Despite a wealth of evidence for the presence of some gas in galactic halos—including absorption line systems in the spectra of quasars, high-velocity neutral hydrogen clouds in our Galaxy halo, line-emitting ionized hydrogen originating from galactic winds in nearby starburst galaxies and the X-ray coronas surrounding the most massive galaxies—accounting for the gas in the halo of any galaxy has been observationally challenging, primarily because of the low density in these expansive regions. The most sensitive measurements come from detecting absorption due to the intervening gas in the spectra of distant objects, such as quasars or distant halo stars, but these have typically been limited to a few lines of sight to sufficiently bright objects. Extensive spectroscopic surveys of millions of objects provide an alternative approach to the problem. Here, we present evidence for a newly discovered, widely distributed, neutral, excited hydrogen component of the Galaxy’s halo. It is observed as the slight (0.779 ± 0.006%) absorption of flux near the rest wavelength of Hα in the combined spectra of hundreds of thousands of galaxy spectra and is ubiquitous in high-latitude lines of sight. This observation provides an avenue to tracing, both spatially and kinematically, the majority of the gas in the halo of our Galaxy. The stacking of nearly three-quarters of a million spectra has unearthed a previously unknown component of the Galactic halo: a widely distributed, neutral, excited hydrogen layer that could harbour a sizeable proportion of the Milky Way’s baryons.

Detection of an Optical Counterpart to the ALFALFA Ultra-compact High-velocity Cloud AGC 249525

Abstract We report on the detection at >98% confidence of an optical counterpart to AGC 249525, an ultra-compact high-velocity cloud (UCHVC) discovered by the Arecibo Legacy Fast ALFA survey blind neutral hydrogen survey. UCHVCs are compact, isolated H i clouds with properties consistent with their being nearby low-mass galaxies, but without identified counterparts in extant optical surveys. Analysis of the resolved stellar sources in deep g- and i-band imaging from the WIYN pODI camera reveals a clustering of possible red giant branch stars associated with AGC 249525 at a distance of 1.64 ± 0.45 Mpc. Matching our optical detection with the H i synthesis map of AGC 249525 from Adams et al. shows that the stellar overdensity is exactly coincident with the highest-density H i contour from that study. Combining our optical photometry and the H i properties of this object yields an absolute magnitude of , a stellar mass between and , and an H i to stellar mass ratio between 9 and 144. This object has stellar properties within the observed range of gas-poor ultra-faint dwarfs in the Local Group, but is gas-dominated.

The Effect of Mixing on the Observed Metallicity of the Smith Cloud

Abstract Measurements of high-velocity clouds’ metallicities provide important clues about their origins, and hence on whether they play a role in fueling ongoing star formation in the Galaxy. However, accurate interpretation of these measurements requires compensating for the galactic material that has been mixed into the clouds. In order to determine how much the metallicity changes as a result of this mixing, we have carried out three-dimensional wind-tunnel-like hydrodynamical simulations of an example cloud. Our model cloud is patterned after the Smith Cloud, a particularly well studied cloud of mass . We calculated the fraction of the high-velocity material that had originated in the galactic halo, , for various sight lines passing through our model cloud. We find that generally increases with distance from the head of the cloud, reaching ∼0.5 in the tail of the cloud. Models in which the metallicities (relative to solar) of the original cloud, , and of the halo, , are in the approximate ranges and , respectively, are in rough agreement with the observations. Models with and are also in rough agreement with the observations, but such a low halo metallicity is inconsistent with recent independent measurements. We conclude that the Smith Cloud’s observed metallicity may not be a true reflection of its original metallicity and that the cloud’s ultimate origin remains uncertain.

Redshift, metallicity and size of two extended dwarf Irregular galaxies. A link between dwarf Irregulars and ultra diffuse galaxies?

We present the results of the spectroscopic and photometric follow-up of two field galaxies that were selected as possible stellar counterparts of local high velocity clouds. Our analysis shows that the two systems are distant (D>20 Mpc) dwarf irregular galaxies unrelated to the local HI clouds. However, the newly derived distance and structural parameters reveal that the two galaxies have luminosities and effective radii very similar to the recently identified Ultra Diffuse Galaxies (UDGs). At odds with classical UDGs, they are remarkably isolated, having no known giant galaxy within ~2.0 Mpc. Moreover, one of them has a very high gas content compared to galaxies of similar stellar mass, with a HI to stellar mass ratio M_HI/M_* ~90, typical of almost-dark dwarfs. Expanding on this finding, we show that extended dwarf irregulars overlap the distribution of UDGs in the M_V vs. log(r_e) plane and that the sequence including dwarf spheroidals, dwarf irregulars and UDGs appears as continuously populated in this plane.

TRACING DENSE AND DIFFUSE NEUTRAL HYDROGEN IN THE HALO OF THE MILKY WAY

ABSTRACT We have combined observations of Galactic high-velocity H i from two surveys: a very sensitive survey from the Green Bank 140 ft Telescope with limited sky coverage, and the less sensitive but complete Galactic All Sky Survey from the 64 m Parkes Radio Telescope. The two surveys preferentially detect different forms of neutral gas due to their sensitivity. We adopt a machine learning approach to divide our data into two populations that separate across a range in column density: (1) a narrow line-width population typical of the majority of bright high velocity cloud components, and (2) a fainter, broad line-width population that aligns well with that of the population found in the Green Bank survey. We refer to these populations as dense and diffuse gas, respectively, and find that diffuse gas is typically located at the edges and in the tails of high velocity clouds, surrounding dense components in the core. A fit to the average spectrum of each type of gas in the Galactic All Sky Survey data reveals the dense population to have a typical line width of ∼20 km s−1 and brightness temperature of ∼0.3 K, while the diffuse population has a typical line width of ∼30 km s−1 and a brightness temperature of ∼0.2 K. Our results confirm that most surveys of high velocity gas in the Milky Way halo are missing the majority of the ubiquitous diffuse gas, and that this gas is likely to contribute at least as much mass as the dense gas.

G181.1+9.5, a new high-latitude low-surface brightness supernova remnant

More than 90% of the known Milky Way supernova remnants are within 5 degrees of the Galactic Plane. We present the discovery of the supernova remnant G181.1+9.5, a new high-latitude SNR, serendipitously discovered in an ongoing survey of the Galactic Anti-centre High-Velocity Cloud complex, observed with the DRAO Synthesis Telescope in the 21~cm radio continuum and HI spectral line. We use radio continuum observations (including the linearly polarized component) at 1420~MHz (observed with the DRAO ST) and 4850~MHz (observed with the Effelsberg 100-m radio telescope) to map G181.1+9.5 and determine its nature as a SNR. High-resolution 21~cm HI line observations and HI emission and absorption spectra reveal the physical characteristics of its local interstellar environment. Finally, we estimate the basic physical parameters of G181.1+9.5 using models for highly-evolved SNRs. G181.1+9.5 has a circular shell-like morphology with a radius of about 16~pc at a distance of 1.5 kpc some 250 pc above the mid-plane. The radio observations reveal highly linearly polarized emission with a non-thermal spectrum. Archival ROSAT X-ray data reveal high-energy emission from the interior of G181.1+9.5 indicative of the presence of shock-heated ejecta. The SNR is in the advanced radiative phase of SNR evolution, expanding into the HVC inter-cloud medium with a density of 1$~cm$^{-3}$. Basic physical attributes of G181.1+9.5 calculated with radiative SNR models show an upper-limit age of 16,000 years, a swept-up mass of more than 300 solar masses, and an ambient density in agreement with that estimated from HI observations. G181.1+9.5 shows all characteristics of a typical mature shell-type SNR, but its observed faintness is unusual and requires further study.

Identifying galaxy candidates in WSRT H i imaging of ultra-compact high velocity clouds

Ultra-compact high velocity clouds (UCHVCs) were identified in the ALFALFA HI survey as potential gas-bearing dark matter halos. Here we present higher resolution neutral hydrogen (HI) observations of twelve UCHVCS with the Westerbork Synthesis Radio Telescope (WSRT). The UCHVCs were selected based on a combination of size, isolation, large recessional velocity and high column density as the best candidate dark matter halos. The WSRT data were tapered to image the UCHVCs at 210" (comparable to Arecibo) and 105" angular resolution. In a comparison of the single-dish to interferometer data, we find that the line flux recovered in the WSRT observations is comparable to that from the single-dish ALFALFA data. In addition, any structure seen in the ALFALFA data is reproduced in the WSRT maps at the same angular resolution. At 210'" resolution all the sources are generally compact with a smooth HI morphology, as expected from their identification as UCHVCs. At the higher angular resolution, a majority of the sources break into small clumps contained in a diffuse envelope. These UCHVCs also have no ordered velocity motion and are most likely Galactic halo clouds. We identify two UCHVCs, AGC 198606 and AGC 249525, as excellent galaxy candidates based on maintaining a smooth HI morphology at higher angular resolution and showing ordered velocity motion consistent with rotation. A third source, AGC 249565, lies between these two populations in properties and is a possible galaxy candidate. If interpreted as gas-bearing dark matter halos, the three candidate galaxies have rotation velocities of 8-15 km/s, HI masses of 0.6-50 x 10^5 Msun, HI radii of 0.3-2 kpc, and dynamical masses of 2-20 x 10^7 Msun for a range of plausible distances. These are the UCHVCs with the highest column density values in the ALFALFA HI data and we suggest this is the best way to identify further candidates.

The distribution and kinematics of interstellar O vi in the Milky Way

We present the results of a survey of interstellar O~{\small VI} absorption in the Milky Way (MW) towards {69} stars in the Large Magellanic Cloud (LMC) obtained with the \textit{Far Ultraviolet Spectroscopic Explorer (FUSE)}. The integrated MW O~{\small VI} column densities log N(O~{\small VI}) are in the range from 13.68 to 14.73 with a mean of {14.26$_{-0.09}^{+0.07}$} atoms cm$^{-2}$. The O~{\small VI} exponential scale height is found to be {2.28$\pm$1.06} kpc. The O~{\small VI} column density correlates with the Doppler parameter $\it b$. The O~{\small VI} velocity dispersion ranges from {14.0 to 91.6 with an average value of 62.7 km s$^{-1}$}. These high values of velocity dispersion reveal the effect of turbulence, multiple velocity components and collision on broad O~{\small VI} profiles. There is a significant variation of O~{\small VI} column density on all scales studied {(0.0025$\degr$ - 6.35$\degr$)}. The smallest scale for which O~{\small VI} column density variations has been found is {$\bigtriangleup \theta$ $\sim$~9$\arcsec$}. Comparison of the O~{\small VI} velocity profiles with Fe~{\small II} indicates the presence of intermediate velocity cloud (IVC) and/or high velocity cloud (HVC) components in the O~{\small VI} absorption.

The properties of ‘dark’ ΛCDM haloes in the Local Group

We examine the baryon content of low-mass {\Lambda}CDM halos $(10^8<M_{200}/{\rm M_\odot}<5\times 10^{9})$ using the APOSTLE cosmological hydrodynamical simulations. Most of these systems are free of stars and have a gaseous content set by the combined effects of cosmic reionization, which imposes a mass-dependent upper limit, and of ram pressure stripping, which reduces it further in high-density regions. Halos mainly affected by reionization (RELHICs; REionization-Limited HI Clouds) inhabit preferentially low-density regions and make up a population where the gas is in hydrostatic equilibrium with the dark matter potential and in thermal equilibrium with the ionizing UV background. Their thermodynamic properties are well specified, and their gas density and temperature profiles may be predicted in detail. Gas in RELHICs is nearly fully ionized but with neutral cores that span a large range of HI masses and column densities and have negligible non-thermal broadening. We present predictions for their characteristic sizes and central column densities: the massive tail of the distribution should be within reach of future blind HI surveys. Local Group RELHICs (LGRs) have some properties consistent with observed Ultra Compact High Velocity Clouds (UCHVCs) but the sheer number of the latter suggests that most UCHVCs are not RELHICs. Our results suggest that LGRs (i) should typically be beyond 500 kpc from the Milky Way or M31; (ii) have positive Galactocentric radial velocities; (iii) HI sizes not exceeding 1 kpc, and (iv) should be nearly round. The detection and characterization of RELHICs would offer a unique probe of the small-scale clustering of cold dark matter.

A very dark stellar system lost in Virgo: kinematics and metallicity of SECCO 1 with MUSE

We present the results of VLT-MUSE integral field spectroscopy of SECCO1, a faint, star-forming stellar system recently discovered as the stellar counterpart of an Ultra Compact High Velocity Cloud (HVC274.68+74.0), very likely residing within a substructure of the Virgo cluster of galaxies. We have obtained the radial velocity of a total of 38 individual compact sources identified as HII regions in the main and secondary body of the system, and derived the metallicity for 18 of them. We provide the first direct demonstration that the two stellar bodies of SECCO1 are physically associated and that their velocities match the HI velocities. The metallicity is quite uniform over the whole system, with a dispersion sigma_12+log(O/H/)=0.08, lower than the uncertainty on individual metallicity estimates. The mean abundance, 12+log(O/H)=8.44, is much higher than the typical values for local dwarf galaxies of similar stellar mass. This strongly suggests that the SECCO~1 stars were born from a pre-enriched gas cloud, possibly stripped from a larger galaxy. Using archival HST images we derive a total stellar mass of ~1.6 X 10^5 M_sun for SECCO1, confirming that it has a very high HI to stellar mass ratio for a dwarf galaxy, M_HI/M_*~ 100. The star formation rate, derived from the H_alpha flux is a factor of more than 10 higher than in typical dwarf galaxies of similar luminosity.

Synchrotron emission from dark matter in galactic subhalos. A look into the Smith cloud

One of the key predictions of the ``WIMP" paradigm for Dark Matter (DM) is that DM particles can annihilate into charged particles. These annihilations will proceed in e.g. Galactic subhalos such as dwarf Galaxies or, as recently pointed out, high velocity clouds such as the ``Smith Cloud''. In this note, we focus on the radio emission associated with DM annihilations into electrons and positrons occurring in the Smith Cloud. The phenomenology of this emission is discussed in quite some detail. We argue that the uncertainties in the propagation can be captured by the typical diffusion-loss length parameter (Syrovatskii variable) but that the angle-integrated radio fluxes are independent of the propagation. We conclude that if the Smith Cloud is indeed dominated by DM, radio signals from DM annihilation stand out amongst other messengers. Furthermore, low frequencies such as the ones observed by e.g. the Low Frequency Array (LOFAR) and the next-generation Square Kilometre Array (SKA) are optimal for searches for DM in the Smith Cloud. As a practical application, we set conservative constraints on dark matter annihilation cross section using data of continuum radio emission from the Galaxy at 22 MHz and at 1.4 GHz. Stronger constraints could be reached by background subtraction, exploiting the profile and frequency dependence of the putative DM signal. We set stronger but tentative limits using the median noise in brightness temperature from the Green Bank Telescope and the LOFAR sensitivities.

THE FIRST DISTANCE CONSTRAINT ON THE RENEGADE HIGH-VELOCITY CLOUD COMPLEX WD

ABSTRACT We present medium-resolution, near-ultraviolet Very Large Telescope/FLAMES observations of the star USNO-A0600-15865535. We adapt a standard method of stellar typing to our measurement of the shape of the Balmer ϵ absorption line to demonstrate that USNO-A0600-15865535 is a blue horizontal branch star, residing in the lower stellar halo at a distance of 4.4 kpc from the Sun. We measure the H &amp; K lines of singly ionized calcium and find two isolated velocity components, one originating in the disk, and one associated with the high-velocity cloud complex WD. This detection demonstrated that complex WD is closer than ∼4.4 kpc and is the first distance constraint on the +100 km s−1 Galactic complex of clouds. We find that complex WD is not in corotation with the Galactic disk, which has been assumed for decades. We examine a number of scenarios and find that the most likely scenario is that complex WD was ejected from the solar neighborhood and is only a few kiloparsecs from the Sun.

A HIGH-VELOCITY CLOUD IMPACT FORMING A SUPERSHELL IN THE MILKY WAY

ABSTRACT Neutral atomic hydrogen (H i) gas in interstellar space is largely organized into filaments, loops, and shells, the most prominent of which are “supershells.” These gigantic structures, which require erg to form, are generally thought to be produced by either the explosion of multiple supernovae (SNe) in OB associations or, alternatively, by the impact of high-velocity clouds (HVCs) falling into the Galactic disk. Here, we report the detection of a kiloparsec (kpc)-size supershell in the outskirts of the Milky Way with the compact HVC 040 + 01−282 (hereafter, CHVC040) at its geometrical center using the “Inner-Galaxy Arecibo L-band Feed Array” H i 21 cm survey data. The morphological and physical properties of both objects suggest that CHVC040, which is either a fragment of a nearby disrupted galaxy or a cloud that originated from an intergalactic accreting flow, collided with the disk ∼5 Myr ago to form the supershell. Our results show that some compact HVCs can survive their trip through the Galactic halo and inject energy and momentum into the Milky Way disk.

Statistical Study of High-Velocity Compact Clouds Based on the Complete CO Imagings of the Central Molecular Zone

High-velocity compact clouds (HVCCs) is one of the populations of peculiar clouds detected in the Central Molecular Zone (CMZ) of our Galaxy. They have compact appearances (&lt; 5 pc) and large velocity widths (&gt; 50 km s−1). Several explanations for the origin of HVCC were proposed; e.g., a series of supernovae (SN) explosions (Oka et al. 1999) or a gravitational kick by a point-like gravitational source (Oka et al. 2016). To investigate the statistical property of HVCCs, a complete list of them is acutely necessary. However, the previous list is not complete since the identification procedure included automated processes and manual selection (Nagai 2008). Here we developed an automated procedure to identify HVCCs in a spectral line data.

Signature of an Intermediate-Mass Black Hole in the Central Molecular Zone of Our Galaxy

AbstractThe high-velocity compact cloud CO–0.40–0.22 was mapped in 22 molecular lines with the NRO 45 m radio telescope and the ASTE 10 m telescope. The map of each detected line shows that this cloud has a compact appearance (d≃3 pc) and extremely broad velocity width (Δ V≃100 km s−1). The representative position–velocity map along the major axis shows that CO–0.40–0.22 consists of an intense region with a shallow velocity gradient and a less intense high-velocity wing. This kinematical structure can be attributed to a gravitational kick to the molecular cloud caused by an invisible compact object with a mass of ~105M⊙. Its compactness and the absence of a counterpart at other wavelengths suggest that this massive object is an intermediate-mass black hole.

Kinematics of the Ultra-High-Velocity Gas in the Expanding Molecular Shell Adjacent to the W44 Supernova Remnant

High-velocity compact cloud (HVCC) is a peculiar category of molecular clouds detected in the central molecular zone of our Galaxy (Oka et al. 1998, 2007, and 2012). They are characterized by compact appearances (d &lt; 5 pc) and very large velocity widths (Δ V &gt; 50 km s−1). Some of them show high CO J=3–2/J=1–0 intensity ratios (≥ 1.5), indicating that they consist of dense and warm molecular gas. Dispite a number of efforts, we have not reached a comprehensive interpretation of HVCCs. Recently, we detected an extraordinaly broad velocity width feature, the ‘Bullet’, in the molecular cloud interacting with the W44 supernova remnant. The Bullet shares essential properties with HVCCs. Because of its proximity, a close inspection of the Bullet must contribute to the understanding of HVCCs.