The Star Formation History and Extended Structure of the Hercules Milky Way Satellite

Sand, David J.; Olszewski, Edward W.; Willman, Beth; Zaritsky, Dennis; Seth, Anil; Harris, Jason; Piatek, Slawomir; Saha, A.

doi:10.1088/0004-637x/704/2/898

Cited by 92 publications

(181 citation statements)

References 55 publications

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“…Sand et al 2009;Musella et al 2012), as recently confirmed by Brown et al (2012) with Hubble Space Telescope (HST) data. Martin et al (2008) measured the galaxy half-light radius (r h = 8.6 ), and Sand et al (2009) estimated the tidal radius (r t = 35.25 ). Hercules appears highly elongated with an ellipticity e 0.65 (Coleman et al 2007;Martin et al 2008) and shows indications of a possible tidal disruption (e.g.…”

Section: Introductionsupporting

confidence: 53%

“…Since the earliest groundbased observations, it was recognized that the galaxy CMD resembles that of an old and metal-poor GGC (t 10 ± 2 Gyr and Z ∼ 10 −4 , see e.g. Sand et al 2009;Musella et al 2012), as recently confirmed by Brown et al (2012) with Hubble Space Telescope (HST) data. Martin et al (2008) measured the galaxy half-light radius (r h = 8.6 ), and Sand et al (2009) estimated the tidal radius (r t = 35.25 ).…”

Section: Introductionmentioning

confidence: 95%

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A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members

Fabrizio

Raimondo

Brocato

et al. 2014

A&A

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Context. Discovered in the last decade as overdensities of resolved stars, the ultra-faint dwarfs (UFDs) are among the least luminous, most dark-matter dominated, and most metal-poor galaxies known today. They appear as sparse, loose objects with high mass-to-light ratios. Hercules is the prototype of the UFD galaxies. To date, there are still no firm constraints on its total luminosity due to the difficulty of disentangling Hercules bona-fide stars from the severe Galactic field contamination. Aims. To better constrain Hercules properties, we aim at removing foreground and background contaminants in the galaxy field using the proper motions of the Milky Way stars and the colour-colour diagram. Methods. We have obtained images of Hercules in the r Sloan , B Bessel and U spec bands with the Large Binocular Telescope (LBT) and LBC-BIN mode capabilities. The r Sloan new dataset combined with data from the LBT archive span a time baseline of about 5 yr, allowing us to measure proper motions of stars in the Hercules direction for the first time. The U spec data along with existing LBT photometry allowed us to use colour-colour diagram to further remove the field contamination.Results. Thanks to a highly-accurate procedure to derive the r Sloan -filter geometric distortion solution for the LBC-red, we were able to measure stellar relative proper motions to a precision of better than 5 mas yr −1 down to r Sloan 22 mag and disentangle a significant fraction (>90%) of Milky Way contaminants. We ended up with a sample of 528 sources distributed over a large portion of the galaxy body (∼0.12 deg 2 ). Of these sources, 171 turned out to be background galaxies and additional foreground stars from the analysis of the U spec − B Bessel vs. B Bessel − r Sloan colour-colour diagram. This leaves us with a sample of 357 likely members of the Hercules UFD. We compared the cleaned colour-magnitude diagram (CMD) with evolutionary models and synthetic CMDs, confirming the presence in Hercules of an old population (t = 12 ± 2 Gyr) with wide spread metallicity (−3.3 < [Fe/H] < −1.8). Conclusions. Our procedure to estimate star proper motions proved to be a very effective way to identify likely members for stellar systems as far as 130 kpc. The present selection, based on both proper motions and colour-colour diagram, provides a robust identification of Hercules members and a new target list for more spectroscopic investigations.

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Section: Introductionsupporting

confidence: 53%

Section: Introductionmentioning

confidence: 95%

A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members

Fabrizio

Raimondo

Brocato

et al. 2014

A&A

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“…To compute the late-time SFR we follow (Orban et al 2008) assuming that about 1% of the total stellar mass content of dSph is produced in the latest Gyr. We use SFR results reported in (Dolphin et al 2005) (Carina and Fornax), (deBoer et al 2012) (Sculptor), and (Sand et al 2009) (Hercules). For a very recent comprehensive study of SFR of dwarf galaxies in the Local Group, see (Weisz et al 2014).…”

Section: Cosmic-ray Sourcesmentioning

confidence: 99%

“…In the second, we instead consider the magnetic field to be induced only by the SF in the latest Gyr and assume (following (Orban et al 2008)) that 1% of the total stellar mass content of the dSph is produced in such range of time. We consider the same SFR estimates reported in Section 4.2 (i.e., (Dolphin et al 2005) for Carina and Fornax, (deBoer et al 2012) for Sculptor, and (Sand et al 2009) for BootesII, Hercules, and Segue2).…”

Section: Sf Rmentioning

confidence: 99%

Local Group dSph radio survey with ATCA – II. Non-thermal diffuse emission

Regis

Richter

Colafrancesco

et al. 2015

Monthly Notices of the Royal Astronomical Society

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Our closest neighbours, the Local Group dwarf spheroidal (dSph) galaxies, are extremely quiescent and dim objects, where thermal and non-thermal diffuse emissions lack, so far, of detection. In order to possibly study the dSph interstellar medium, deep observations are required. They could reveal non-thermal emissions associated with the very-low level of star formation, or to particle dark matter annihilating or decaying in the dSph halo. In this work, we employ radio observations of six dSphs, conducted with the Australia Telescope Compact Array in the frequency band 1.1-3.1 GHz, to test the presence of a diffuse component over typical scales of few arcmin and at an rms sensitivity below 0.05 mJy/beam. We observed the dSph fields with both a compact array and long baselines. Short spacings led to a synthesized beam of about 1 arcmin and were used for the extended emission search. The high-resolution data mapped background sources, which in turn were subtracted in the short-baseline maps, to reduce their confusion limit. We found no significant detection of a diffuse radio continuum component. After a detailed discussion on the modelling of the cosmic-ray (CR) electron distribution and on the dSph magnetic properties, we present bounds on several physical quantities related to the dSphs, such that the total radio flux, the angular shape of the radio emissivity, the equipartition magnetic field, and the injection and equilibrium distributions of CR electrons. Finally, we discuss the connection to far-infrared and X-ray observations.

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“…At M V = −6.6 mag, Hercules is one of the brighter UFDs. Hercules is a typical, metal-poor, old stellar population (>12 Gyr) that has experienced little to no star formation in the past 12 Gyr (Sand et al 2009;Musella et al 2012; but cf. Brown et al 2012).…”

Section: Introductionmentioning

confidence: 99%

Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

Koch

Feltzing

Adén

et al. 2013

A&A

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We present an assessment of the barium abundance ratios for red giant member stars in the faint Hercules dwarf spheroidal (dSph) galaxy. Our results are drawn from intermediate-resolution FLAMES/GIRAFFE spectra around the Ba II 6141.71 Å absorption line at low signal-to-noise ratios. For three brighter stars we were able to gain estimates from direct equivalent-width measurements, while for the remaining eight stars only upper limits could be obtained. These results are investigated in a statistical manner and indicate very low Ba abundances of log ε(Ba) < ∼ 0.7 dex (3σ). We discuss various possible systematic biasses, first and foremost, a blend with the Fe I 6141.73 Å-line, but most of those would only lead to even lower abundances. A better match with metal-poor halo and dSph stars can only be reached by including a large uncertainty in the continuum placement. This contrasts with the high dispersions in iron and calcium (in excess of 1 dex) in this galaxy. While the latter spreads are typical of the very low luminosity, dark-matter dominated dSphs, a high level of depletion in heavy elements suggests that chemical enrichment in Hercules was governed by very massive stars, coupled with a very low star formation efficiency. While very low abundances of some heavy elements are also found in individual stars of other dwarf galaxies, this is the first time that a very low Ba abundance is found within an entire dSph over a broad metallicity range.

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The Star Formation History and Extended Structure of the Hercules Milky Way Satellite

Cited by 92 publications

References 55 publications

A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members

A bag of tricks: Using proper motions of Galactic stars to identify the Hercules ultra-faint dwarf galaxy members

Local Group dSph radio survey with ATCA – II. Non-thermal diffuse emission

Neutron-capture element deficiency of the Hercules dwarf spheroidal galaxy

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