EDP Sciences
Free Access
Issue
A&A
Volume 584, December 2015
Article Number A101
Number of page(s) 3
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201527332
Published online 27 November 2015

© ESO, 2015

1. Introduction

Hydra II is a dwarf galaxy in the Local Group, newly-discovered during the Survey of the Magellanic Stellar History (SMASH) using the Dark Energy Survey Camera DECam (Nidever & Smash Team 2015; Martin et al. 2015). It lies at a distance of 134 kpc from the Sun with Vhel = 303 km s-1, and has physical properties, such as a half light radius of 66 pc, a high dynamical mass-to-luminosity ratio, and a metallicity ⟨ | Fe / H | ⟩ = −2.02 ± 0.08, indicating that it is a dwarf galaxy and not a cluster in the Milky Way’s halo (Martin et al. 2015; Kirby et al. 2015). It is located in the same region of the sky as the leading arm of the Magellanic Stream and has a similar velocity (Nidever et al. 2008, 2010). The distance to the leading arm is not known and estimates range from 21 kpc to >100 kpc (McClure-Griffiths et al. 2008; Besla et al. 2012; Martin et al. 2015). If Hydra II originated in the Magellanic Clouds it may have a considerably different history and properties than other Milky Way dwarfs (Martin et al. 2015). For this reason, we made very sensitive observations of the 21 cm H I  emission in Hydra II using the Green Bank Telescope, which has been used to produce some of the most strict limits on the H I  content of other Local Group dwarf galaxies (Spekkens et al. 2014).

2. Observations and analysis

Measurements of 21 cm emission from Hydra II were made with the 100-m diameter Robert C. Byrd Green Bank Telescope (GBT) of the National Radio Astronomy Observatory1 on 2015, July 7–8 and July 17, centered at the position J2000 = 12h21m42.1s −31°59′07″ given by Martin et al. (2015). The GBT 1.4 GHz receiver had a total system temperature toward Hydra II of about 21 K. The GBT VEGAS spectrometer was used to measure spectra with both in-band frequency switching for 23 min and position-switching to a reference location displaced ±5m30s in right ascension from the source for 110 min. The FWHM beam of the GBT at 21 cm completely encompasses Hydra II, which is estimated to have an angular size of about on the sky (Kirby et al. 2015).

The 21 cm spectra covered ±2000 km s-1 centered on zero velocity in the local standard of rest (LSR). In the position-switched observations, emission at Milky Way velocities between 70 and +180 km s-1 is partially cancelled, but as the stars in Hydra II have a mean VLSR = + 301 km s-1, this does not affect our results. The data were reduced using the standard GBTIDL routines and calibrated correcting for atmospheric attenuation to produce as a function of VLSR. Frequency-switched data were calibrated and corrected for stray radiation using the method described in Boothroyd et al. (2011). The final spectra, smoothed to a velocity resolution of 0.30 km s-1 from the intrinsic resolution of 0.15 km s-1, are shown in Fig. 1. We combine the noise limits from position- and frequency-switched spectra for a final estimated rms noise in of 9.1 mK (4.55 mJy) in an 0.30 km s-1 channel.

thumbnail Fig. 1

GBT 21 cm H I  spectra toward Hydra II over the velocity range appropriate to that galaxy. The grey and black are the frequency- and position-switched spectra, respectively. Their differing noise level is a consequence of the difference in exposure times: 23 min for frequency switching and 110 min for position switching. The combined rms noise is 9.1 mK in an 0.30 km s-1 channel. The dip in the position-switched spectrum near 150 km s-1 results from partial cancellation of Milky Way emission.

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thumbnail Fig. 2

The integrated 21 cm H I  emission in the field of Hydra II from the GASS survey (McClure-Griffiths et al. 2009; Kalberla et al. 2010) over 20 km s-1 around the stellar velocity of Hydra II, whose location at is marked with the circle. No significant emission is detected. The brightest H I  within a degree of the galaxy is consistent with a 2.3σ noise feature.

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thumbnail Fig. 3

5σ H I  mass limits of local dwarf galaxies from Spekkens et al. (2014; S14 open symbols) as a function of heliocentric distance, with the Hydra II measurement as the filled symbol. The limit on MHI of Hydra II is consistent with that of other dwarfs near the Milky Way.

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thumbnail Fig. 4

5σ H I  mass to LV ratios of local dwarf galaxies from Spekkens et al. (2014; S14) as a function of heliocentric distance with open symbols, and the Hydra II measurement as a filled symbol using LV from Martin et al. (2015). The limit for Hydra II is consistent with that of other dwarfs near the Milky Way.

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No signal from H I  associated with Hydra II is detected. The 5σ limit on the velocity-integrated flux density over 15 km s-1 is mJy km s-1. The H I  mass within the beam is given by (1)(Roberts 1975), where D is the source distance in Mpc, and Sdv is the velocity integrated flux in Jy km s-1. We assume an H I  linewidth of 15 km s-1, consistent with that used in the analysis of other local dwarfs (Spekkens et al. 2014). At the 0.134 Mpc distance of Hydra II, our measurements give a 5σ limit MH I ≤ 210 M.

To explore whether there might be some H I  associated with Hydra II but displaced somewhat in position or velocity from the stars, we integrated the GASS H I  survey, made with the Parkes radio telescope at an angular resolution of 14.́4 (McClure-Griffiths et al. 2009; Kalberla et al. 2010) over 20 km s-1 around the velocity of the galaxy. Figure 2 shows the resulting noise map. No H I  is detected. The brightest GASS pixel within one degree of Hydra II has an integrated intensity of 0.7 K-km s-1, or 2.3σ of the measured noise. Over the area of the Parkes telescope beam anywhere in this field, the 5σ limit on MH I is 9350 M.

3. Discussion

We do not detect any H I  directly toward Hydra II, nor is there any evidence of significant H I  nearby that galaxy, in position or velocity. Our 5σ limit MHI ≤ 210 M shown in Fig. 3, is consistent with values derived for other Local Group dwarf galaxies that lie within the 300 kpc virial radius of the Milky Way (Blitz & Robishaw 2000; Grcevich & Putman 2009; Spekkens et al. 2014; Westmeier et al. 2015). Likewise, the derived H I  mass-luminosity limit (using LV from Martin et al. 2015) of MHI/ is also similar to that found for dwarfs near the Milky Way, as shown in Fig. 4. Dwarf galaxies at larger distances from the Milky Way typically have MHI/LV ≈ 1 (McConnachie 2012).

The lack of detectable H I  in nearby dwarf galaxies is usually ascribed to tidal or ram pressure stripping in the Milky Way’s hot halo (Blitz & Robishaw 2000; Mayer et al. 2006; Grcevich & Putman 2009; Gatto et al. 2013). Presumably, similar processes have removed the gas from Hydra II. In this regard, Hydra II’s location near the leading arm of the Magellanic Stream has apparently not affected its neutral Hydrogen properties as compared with other dwarfs at similar distances.


1

The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.

Acknowledgments

We thank K. Spekkens for useful discussions and Richard Prestage for comments on the manuscript. These measurements were performed as part of the 2015 NRAO/NAIC Single Dish Summer School program.

References

All Figures

thumbnail Fig. 1

GBT 21 cm H I  spectra toward Hydra II over the velocity range appropriate to that galaxy. The grey and black are the frequency- and position-switched spectra, respectively. Their differing noise level is a consequence of the difference in exposure times: 23 min for frequency switching and 110 min for position switching. The combined rms noise is 9.1 mK in an 0.30 km s-1 channel. The dip in the position-switched spectrum near 150 km s-1 results from partial cancellation of Milky Way emission.

Open with DEXTER
In the text
thumbnail Fig. 2

The integrated 21 cm H I  emission in the field of Hydra II from the GASS survey (McClure-Griffiths et al. 2009; Kalberla et al. 2010) over 20 km s-1 around the stellar velocity of Hydra II, whose location at is marked with the circle. No significant emission is detected. The brightest H I  within a degree of the galaxy is consistent with a 2.3σ noise feature.

Open with DEXTER
In the text
thumbnail Fig. 3

5σ H I  mass limits of local dwarf galaxies from Spekkens et al. (2014; S14 open symbols) as a function of heliocentric distance, with the Hydra II measurement as the filled symbol. The limit on MHI of Hydra II is consistent with that of other dwarfs near the Milky Way.

Open with DEXTER
In the text
thumbnail Fig. 4

5σ H I  mass to LV ratios of local dwarf galaxies from Spekkens et al. (2014; S14) as a function of heliocentric distance with open symbols, and the Hydra II measurement as a filled symbol using LV from Martin et al. (2015). The limit for Hydra II is consistent with that of other dwarfs near the Milky Way.

Open with DEXTER
In the text

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