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tokens: arcmin away, in a low density environment (Cernicharo et al. 1998 ; Rosado et al. 1999 ; Yusef-Zadeh, Biretta Wardle 2005b ). Evidence of shocked gas associated with core TC1 has also been provided by Yusef-Zadeh, Biretta Geball ( 2005a ). Finally, Rho et al. ( 2004 ) reported the detection of 304 X-ray sources with the Chandra satellite, 70 per cent of which having near-infrared (near-IR) counterparts. X-ray emission was detected from within condensation TC1 and possibly TC4, two star-forming cores with bipolar wings and associated Class 0 candidates (Rho et al. 2004 ). A recent comprehensive review of the Trifid star-forming region has been presented by Rho et al. ( 2008 ). In order to study in detail the principal star-forming cores within M 20, we present here new far-IR images from the Herschel Infrared Galactic Plane Survey (HI-GAL; Molinari et al. 2010 ), combined with a new set of deep near-IR images obtained through broad- and narrow-band filters from the Calar Alto 3.5-m telescope. In addition, archival Spitzer Infrared Array Camera (IRAC) images (Fazio et al. 2004 ; Werner et al. 2004 ), part of its Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) survey (Benjamin et al. 2003 ), are used to supplement these observations. The observations are described in Section 2 , whereas in Section 3, we present the results and discussion. Finally, Section 4 lists our conclusions. 2 OBSERVATIONS AND DATA ANALYSIS 2.1 HI-GAL images and photometry The Trifid nebula has been observed in the far-IR as part of the Herschel satellite (Pilbratt et al. 2010 ) HI-GAL survey at 70,160, 250, 350, and 500 μm. The images were reduced using the HI-GAL standard pipeline (Traficante et al. 2011 ). From these, source extraction and photometry were performed using the CuTEx package (Curvature Threshold Extractor), independently at each band (Molinari et al. 2016 ). Fig. 1 (lower right-hand panel) illustrates the Herschel three-colour image of an area of 12 × 12 arcmin 2 around the Trifid nebula, composed from the 70 μm (blue), 160 μm (green), and 250 μm (red) single images. Figure 1. 12 × 12 arcmin 2 (7.0 × 7.0 pc) colour-coded images of the Trifid nebula. Lower right-hand panel: Herschel at 70 μm (blue), 160 μm (green), and 250 μm (red); lower left-hand panel: Spitzer /IRAC at 3.6, 4.5 and 5.8 μm; upper left-hand panel: CAHA 3.5m in the JHK s bands; upper right-hand panel: a visible public image from the European Southern Observatory. All images have the same scale and are centred at the same position. The white contours are from the Herschel 160 μm frame. The white labels indicate the TCs that were studied in this work. The yellow arrows indicate the location of the early-type trapezium system HD 164492, which ionizes the H ii region, and the Herbig–Haro jet HD 399 associated with TC2. The brightest dense cores have been associated with those found at millimetre wavelengths (Lefloch et al. 2008 ) as labelled in Fig. 1 . In addition, filamentary