The Absence of Stars in Compact High‐Velocity Clouds

Abstract. We are investigating the hypothesis that Compact High-Velocity Clouds (CHVCs) are the left-over building blocks of Local Group galaxies. To this end, we are searching for their embedded stellar populations using FORS at the VLT. The search is done with single-star photometry in V and I bands, which is sensitive to both, young and old, stellar populations. Five CHVCs of our sample have been observed so far down to I=24. We pointed the VLT towards the highest HI column density regions, as determined in Effelsberg radio data. In an alternate approach, we searched 2MASS public data towards those 5 CHVCs down to K=16. While the VLT data probe the central regions out to distance moduli of about 27, the 2MASS data are sensitive to a population of red giant stars to distance moduli of about 20. The 2MASS data, on the other hand, cover a much wider field of view than the VLT data (radius of 1 degree versus FORS field of 6.8 arcmin). We did not find a stellar population intrinsic to the CHVCs in either data. In this paper, we illustrate our search methods.

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“…Our conclusions agree with and extend those reported by Simon & Blitz (2002), who did not detect stars in CHVCs on processed POSS scans.…”

Section: Discussionsupporting

confidence: 93%

Searching for an Intrinsic Stellar Population in Compact High-Velocity Clouds

Hopp

Schulte-Ladbeck

Kerp

2003

The Evolution of Galaxies

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“…Simon & Blitz 2002;Willman et al 2002;Hopp et al 2003Hopp et al , 2007Siegel et al 2005, and references therein). The novelty is that, on the other hand, some more distant counterpart begins to emerge (Paper I, Bellazzini et al 2015b), especially among the GALFA-HI candidates (Sand et al 2015).…”

Section: Discussionmentioning

confidence: 99%

The StEllar Counterparts of COmpact high velocity clouds (SECCO) survey

Beccari

Bellazzini

Battaglia

et al. 2016

A&A

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The searching for StEllar Counterparts of COmpact high velocity clouds (SECCO) survey is devoted to the search for stellar counterparts within ultra compact high velocity clouds that are candidate low-mass, low-luminosity galaxies. We present the results of a set of simulations aimed at the quantitative estimate of the sensitivity of the survey as a function of the total luminosity, size, and distance of the stellar systems we are looking for. For all of our synthetic galaxies we assumed an exponential surface brightness profile and an old and metal-poor population. The synthetic galaxies are simulated both on the images and on the photometric catalogues, taking all the observational effects into account. In the fields where the available observational material is of top quality ( 36% of the SECCO fields), we detect synthetic galaxies as ≥5σ over-densities of resolved stars down to µ V,h 30.0 mag/arcsec 2 , for D ≤ 1.5 Mpc, and down to µ V,h 29.5 mag/arcsec 2 , for D ≤ 2.5 Mpc. In the field with the worst observational material of the whole survey, we detect synthetic galaxies with µ V,h ≤ 28.8 mag/arcsec 2 out to D ≤ 1.0 Mpc, and those with µ V,h ≤ 27.5 mag/arcsec 2 out to D ≤ 2.5 Mpc. Dwarf galaxies with M V = −10.0, with sizes in the range spanned by known dwarfs, are detected by visual inspection of the images up to D = 5 Mpc independent of the image quality. In the best quality images, dwarfs are partially resolved into stars up to D = 3.0 Mpc and completely unresolved at D = 5 Mpc. As an independent test of the sensitivity of our images to low surface brightness galaxies, we report on the detection of several dwarf spheroidal galaxies probably located in the Virgo cluster with M V −8.0 and µ V,h 26.8 mag/arcsec 2 . The nature of the previously discovered SECCO 1 stellar system, also likely located in the Virgo cluster, is rediscussed in comparison with these dwarfs. While specific for the SECCO survey, our study may also provide general guidelines for the detection of faint stellar systems with 8 m class telescopes.

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“…An optical study of 264 high velocity clouds down to a surface brightness limit of μ V ∼ 26 mag arcsec −2 resulted in no detections (Simon & Blitz 2002). Even for one of the most massive high velocity clouds in the Milky Way, complex H (Hulsbosch 1971;Dieter 1971), infrared, and millimeter wavelength observations did not reveal any evidence of either current star formation or an evolved population (Simon et al 2006).…”

Section: Hi Clouds and Stellar Counterpartsmentioning

confidence: 97%

Stellar structures in the outer regions of M 33

Grossi

Hwang

Corbelli

et al. 2011

A&A

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Aims. We present Subaru/Suprime-Cam deep V and I imaging of seven fields in the outer regions of M 33. Our aim is to search for stellar structures corresponding to extended Hi clouds found in a recent 21-cm survey of the galaxy. Three fields probe a large Hi complex to the southeastern (SE) side of the galaxy. An additional three fields cover the northwestern (NW) side of the galaxy along the Hi warp. A final target field was chosen further north, at a projected distance of approximately 25 kpc, to study part of the large stellar plume recently discovered around M 33. Methods. We analyse the stellar population at R > 10 kpc by means of V, I colour magnitude diagrams reaching the red clump. We constrain the age and metallicity of the different stellar populations, search for density enhancements that correspond to the Hi features, and investigate the radial surface distribution of the stars. Results. We find evolved stellar populations in all fields out to 120 (∼30 kpc), while a diffuse population of young stars (∼200 Myr) is detected out to a galactocentric radius of 15 kpc. The mean metallicity in the southern fields remains approximately constant at [M/H] = -0.7 beyond the edge of the optical disc, from 40 out to 80 . Along the northern fields probing the outer Hi disc, we also find a metallicity of [M/H] = -0.7 between 35 and 70 from the centre, which decreases to [M/H] = -1.0 at larger angular radii out to 120 . In the northernmost field, outside the disc extent, the stellar population of the large stellar plume possibly related to a M 33-M 31 interaction is on average more metal-poor ([M/H] = -1.3) and older ( ∼ >6 Gyr).Conclusions. An exponential disc with a large scale-length (∼7 kpc) fits well the average distribution of stars detected in both the SE and NW regions from a galactocentric distance of 11 kpc out to 30 kpc. The stellar disc extends beyond the Hi disc. The stellar distribution at large radii is disturbed and, although there is no clear correlation between the stellar substructures and the location of the Hi clouds, this gives evidence of tidal interaction or accretion events.

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The Absence of Stars in Compact High‐Velocity Clouds

Cited by 47 publications

References 58 publications

Searching for an Intrinsic Stellar Population in Compact High-Velocity Clouds

Searching for an Intrinsic Stellar Population in Compact High-Velocity Clouds

The StEllar Counterparts of COmpact high velocity clouds (SECCO) survey

Stellar structures in the outer regions of M 33

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