Magnetic support of the optical emission line filaments in NGC 1275

Abstract

The giant elliptical galaxy NGC 1275, at the centre of the Perseus cluster, is surrounded by a well-known giant nebulosity of emission-line filaments 1,2 , which are plausibly about >10 8 yr old 3 . The filaments are dragged out from the centre of the galaxy by the radio bubbles rising buoyantly in the hot intracluster gas 4 before later falling back. They act as dramatic markers of the feedback process by which energy is transferred from the central massive black hole to the surrounding gas. The mechanism by which the filaments are stabilized against tidal shear and dissipation into the surrounding 4×10 7 K gas has been unclear. Here we report new observations that resolve thread-like structures in the filaments. Some threads extend over 6 kpc, yet are only 70 pc wide. We conclude that magnetic fields in the threads, in pressure balance with the surrounding gas, stabilize the filaments, so allowing a large mass of cold gas to accumulate and delay star formation.The images presented here (Figs. 1-4) were taken with the Advanced Camera for Surveys (ACS) on the NASA Hubble Space Telescope (HST) using three filters; F625W in the red contains the Hα line, F550M is mostly continuum and F435W in the blue which highlights young stars. In Fig. 2 we show part of the Northern filament ~27 kpc from the nucleus (we adopt H 0 =71 km s -1 Mpc -1 which at a redshift 0.0176 for NGC 1275 gives 352 pc arcsec -1 ). The filaments seen in the WIYN groundbased image (right) are just resolved into narrow threads with the HST ACS (see Supplementary Information). This also occurs in many other filaments including the north-west "horseshoe" filament ( Fig. 3) which lies immediately interior to the outer ghost bubble in X-ray images 5 . A fine thread of emission is seen in the Northern filament system extending about 16 arcsec or 5.8 kpc. Averaged over kpc strips it is about 4 pixels (0.2 arcsec) or about 70 pc wide. (This is an upper limit as the point spread function of the ACS is about one half this value.) The aspect ratio (length / thickness) therefore approaches 100. The top of the horseshoe which is about 6 kpc across is similar, as are many other relatively isolated filaments.In order to estimate the required magnetic field we need to know the properties of a filament and its surroundings. We shall concentrate on a thread of radius 35 pc and length 6 kpc at a distance of 25 kpc from the nucleus of NGC 1275 (Fig. 2) as a basic structural unit typical of what is now resolved in the filaments. To estimate the mass for such a thread we scale from the total gas mass of 10 8 M⊙ inferred from CO emission 6 observed in a 22 arcsec IRAM beam on the same Northern filament complex. Assuming that the mass scales with Hα emission, which is the case for the H 2 emission measured with Spitzer 7 , then our fiducial thread has a mass of about 10 6 M⊙. Its mean density is then ~2 cm -3 and perpendicular column density N ~ 4×10 20 cm -2 or Σ ┴ ~ 7×10 -4 g cm -2 . The lengthwise column density, Σ∥, is l/2r times larger.The variation in projected rad...

A very extended molecular web around NGC 1275

“…This will help lower the model predictions of the cold gas velocities. These single dish observations do not resolve the very thin filaments as observed with the HST (Fabian et al 2008), so we certainly also mixed several filaments and averaged their individual velocities. The CO observed line widths are large.…”

“…These large line widths are difficult to explain by motions within a single filament. It is more likely that in our beam, we observe an ensemble of very thin sub-filaments (as observed in the optical with the HST by Fabian et al 2008) that consist of clumps of molecular clouds. Furthermore, these individual sub-filaments are very likely to have different projection angles, which would also increase the observed line widths, when observed with a larger beam.…”

“…In the simulation however, at temperature below 10 6 K, the cooling gas decouples from the ambient gas and falls towards the centre. This discrepancy would be removed if another physical process, like viscosity or magnetic fields (Fabian et al 2008) increase the coupling between the different gas phases.…”

ALMA observations of cold molecular gas filaments trailing rising radio bubbles in PKS 0745−191

“…The SE filament is ∼ 3 kpc in length with a radius of 0.3 kpc and a molecular gas mass of 1.9 × 10 9 M . Following Fabian et al (2008), for a radial filament the lengthwise column density N ∼ 7 × 10 23 cm −2 and the surface density Σ ∼ 1.2 g cm −2 . The gravitational acceleration at the centre of PKS 0745-191 g ∼ 10 −8 cm s −2 (Sanders et al 2014).…”

Far-ultraviolet morphology of star-forming filaments in cool core brightest cluster galaxies

“…Moreover, some blue star-forming filaments apparently lack cospatial Hα emission (e.g. the 'blue loop' in Perseus; Fabian et al 2008;Canning et al 2010Canning et al , 2014. Hα traces the contemporary SFR via the instantaneous flux of ionizing photons from the most massive (M 15 M ) O and early B-type stars, while the more heavily extincted UV excess associated with the photospheres of less massive (M 5 M ) young stars can shine long after those most massive stars powering the Hα flux are gone.…”