Seeking large-scale magnetic fields in a pure-disk dwarf galaxy NGC 2976

Drzazga, Robert T.; Chyży, K. T.; Heald, G.; Elstner, D.; Gallagher, John S.

doi:10.1051/0004-6361/201527236

Cited by 11 publications

(10 citation statements)

References 67 publications

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“…We particularly note that we focus here on the disordered component of the magnetic field, which yields the dominant contribution to the radio flux. For comparison, studies by Drzazga et al (2016) have shown that the ordered component in the outer parts of the galaxy is also affected by tidal interactions. The density of the warm neutral gas can be estimated as Σ/2H.…”

Section: Basic Assumptionsmentioning

confidence: 99%

Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

Schleicher

Beck

2016

A&A

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The far-infrared -radio correlation connects star formation and magnetic fields in galaxies, and has been confirmed over a large range of far-infrared / radio luminosities, both in the local Universe and even at redshifts of z ∼ 2. Recent investigations indicate that it may even hold in the regime of local dwarf galaxies, and we therefore explore here the expected behavior in the regime of star formation surface densities below 0.1 M⊙ kpc −2 yr −1 . We derive two conditions that can be particularly relevant for inducing a change in the expected correlation: a critical star formation surface density to maintain the correlation between star formation rate and the magnetic field, and a critical star formation surface density below which cosmic ray diffusion losses dominate over their injection via supernova explosions. For rotation periods shorter than 1.5 × 10 7 (H/kpc) 2 yrs, with H the scale height of the disk, the first correlation will break down before diffusion losses are relevant, as higher star formation rates are required to maintain the correlation between star formation rate and magnetic field strength. For high star formation surface densities ΣSFR, we derive a characteristic scaling of the non-thermal radio to the far-infrared / infrared emission with Σ 1/3 SFR , corresponding to a scaling of the non-thermal radio luminosity Ls with the infrared luminosity L th as L 4/3 th . The latter is expected to change when the above processes are no longer steadily maintained. In the regime of long rotation periods, we expect a transition towards a steeper scaling with Σ 2/3 SFR , implying Ls ∝ L 5/3th , while the regime of fast rotation is expected to show a considerably enhanced scatter, as a well-defined relation between star formation and magnetic field strength is not maintained. The scaling relations above explain the increasing thermal fraction of the radio emission observed within local dwarfs, and can be tested with future observations by LOFAR as well as the SKA and its precursor radio telescopes.

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Section: Basic Assumptionsmentioning

confidence: 99%

Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

Schleicher

Beck

2016

A&A

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“…We believe that this scenario could be relevant to producing large-scale magnetic fields in dwarf galaxies, and potentially explain certain observations of these objects . Note that Drzazga et al (2016), when discussing observations of a dwarf galaxy NGC 2976 performed in order to isolate a large-scale magnetic pattern, discuss the necessity of a mechanism similar to that one suggested here.…”

Section: Discussionmentioning

confidence: 82%

Galactic winds and the origin of large-scale magnetic fields

Moss

Sokoloff

2017

A&A

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Context. Observations of dwarf galaxies suggest the presence of large-scale magnetic fields. However the size and slow rotation of these galaxies appear insufficient to support a mean-field dynamo action to excite such fields. Aims. Here we suggest a new mechanism to explain large-scale magnetic fields in galaxies that are too small to support mean-field dynamo action. The key idea is that we do not identify large-scale and mean magnetic fields. In our scenario the the magnetic structures originate from a small-scale dynamo which produces small-scale magnetic field in the galactic disc and a galactic wind that transports this field into the galactic halo where the large turbulent diffusion increases the scale and order of the field. As a result, the magnetic field becomes large-scale; however its mean value remains vanishing in a strict sense. Methods. We verify the idea by numerical modelling of two distinct simplified configurations, a thin disc model using the no-z approximation, and an axisymmetric model using cylindrical r, z coordinates. Results. Each of these allows reduction of the problem to two spatial dimensions. Taken together, the models support the proposition that the general trends will persist in a fully 3D model. We demonstrate that a pronounced large-scale pattern can develop in the galactic halo for a wide choice of the dynamo governing parameters. Conclusions. We believe that our mechanism can be relevant to explaining the presence of the fields observed in the halos of dwarf galaxies. We emphasize that detailed modelling of the proposed scenario needs 3D simulations, and adjustment to the specific dynamo governing parameters of dwarf galaxies.

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“…This relatively low x-ray luminosity can give tighter constraints for annihilating dark matter. Furthermore, the magnetic field strength of NGC 2976 is B = 6.6 ± 1.8 µG, which is relatively higher than the local group dwarf galaxies (B = 4.2 ± 1.8 µG) (Drzazga et al 2016) so that the cooling rate of high-energy electrons is higher than the diffusion rate. The cooling timescale for a 1 GeV electron in NGC 2976 is t c = 1/b = 7 × 10 15 s while the diffusion timescale is…”

Section: The X-ray Constraintsmentioning

confidence: 90%

“…−15 erg cm −2 s −1 and S ≤ 1.76×10 −15 erg cm −2 s −1 respectively (Drzazga et al 2016). By using Eq.…”

Section: The Radio Constraintsmentioning

confidence: 94%

A New Target Object for Constraining Annihilating Dark Matter

Chan

2017

ApJ

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In the past decade, gamma-ray observations and radio observations of our Milky Way and the Milky Way dwarf spheroidal satellite galaxies put very strong constraints on annihilation cross sections of dark matter. In this article, we suggest a new target object (NGC 2976) that can be used for constraining annihilating dark matter. The radio and x-ray data of NGC 2976 can put very tight constraints on the leptophilic channels of dark matter annihilation. The lower limits of dark matter mass annihilating via e + e − , µ + µ − and τ + τ − channels are 200 GeV, 130 GeV and 110 GeV respectively with the canonical thermal relic cross section. We suggest that this kind of large nearby dwarf galaxies with relatively high magnetic field can be good candidates for constraining annihilating dark matter in future analysis.

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Seeking large-scale magnetic fields in a pure-disk dwarf galaxy NGC 2976

Cited by 11 publications

References 67 publications

Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

Star-forming dwarf galaxies: the correlation between far-infrared and radio fluxes

Galactic winds and the origin of large-scale magnetic fields

A New Target Object for Constraining Annihilating Dark Matter

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