Gas properties

The excitation, physical conditions, dust and metal content of the interstellar gas in He 2-10 can be explored using selected ratios between the measured emission lines.

Dust extinction, electron density and gas excitation

The left panel of Fig.3.2illustrates the dust extinction map, derived through the Balmer decrement Hα/Hβ, assuming aCalzetti et al. (2000) attenuation law and a fixed tem- perature of Te = 104 K. The dust attenuation shows the highest values in the two star

forming clumps detached ∼ 18” to the SW of the main galaxy and in the Eastern region where the line and continuum emission is less prominent. These locations correspond to the position of the CO gas (Kobulnicky et al.,1995b). Moreover, the extinction is high on one of the central star forming regions (AV = 2.3, knot 1+2). It is interesting to

compare Fig.3.2with the extinction map obtained by near-IR IFU data for the central region of the galaxy by Cresci et al. (2010) from the Br12/Brγ line ratio, in which the the extinction towards the two brightest star forming region at the center of the galaxy was AV = 7 − 8. This difference is probably due to the fact that IR observations are

capable to probe deeper in the highly embedded star forming clusters. The right panel of Fig.3.2shows the electron density nemap, estimated using the [S ii]λ6717/[S ii]λ6731

ratio (Osterbrock,1989). The highest ne values (ne∼ 1500 cm−3) come from the West-

ern central star forming region (knot 4), while the density distribution appears to be flat in the rest of the galaxy (ne∼ 100 cm−3).

The dominant ionisation source for the line emitting gas in each MUSE spaxel can be explored using the [N ii], [S ii] and [O i] BPT diagrams (e.g. Baldwin et al. 1981;Kewley et al. 2006), in which spatially resolved regions of galaxies dominated by SF, AGN (Seyfert-type), low ionisation emission line Regions (LI(N)ERs,Belfiore et al. 2016), or shocks populate different regions. The three BPT diagrams for the spaxels with S/N > 3 for all the lines involved with the corresponding maps are shown in Fig.3.3, in which the dominant source of ionisation is marked with different colors. Clearly, all the line emitting gas in the galaxy, even at the location of the compact radio source, is dominated by ionisation from young stars, as different sources are limited to few noisier

Figure 3.2: Extinction map as derived from the Hα/Hβ line ratio (left panel) and elec- tron density ne from [S ii]λ6716/[S ii]λ6731 (right panel) with Hα contours overplotted.

Both maps are displaying the spaxels with S/N > 3 in all the emission lines involved. This figure is taken from Cresci et al.(2017).

spaxels at the edges of the galaxy. Assuming that all the line emitting gas is ionised by young stars, the total SFR in the galaxy can be estimated from the integrated stellar continuum subtracted spectrum extracted in an aperture of 120 spaxels in radius (24” or 960 pc). An intrinsic Hα luminosity of L(Hα) ∼ 1.48 × 1041 erg/s, after correcting for the dust extinction derived from the Balmer decrement (AH = 1.32. This value of

L(Hα) converts into a SF R = 0.76 M/yr, using the calibration byKennicutt & Evans

(2012).

These pieces of evidence confirm that the central regions of He 2-10 host dense, dust embedded, young and highly star forming star clusters, a common feature in starburst galaxies (see e.g. Vanzi & Sauvage 2006) and possibly in galaxies in general (e.g. F¨orster Schreiber et al. 2011).

Metallicity and ionisation

Since the gas ionisation is dominated by young stars across all the galaxy, it is possible to use selected line ratios of the most intense lines to derive the chemical enrichment of the interstellar gas (i.e. SEL). The integrated metal abundance of He 2-10 was already derived byKobulnicky et al.(1999), yielding a super solar abundance of 12 + log(O/H) = 8.93. Such a high value is not unexpected given the relative higher mass of He 2-10 and

Figure 3.3: Resolved BPT diagrams for He 2-10. The [N ii], [S ii] and [O i] BPT diagrams for each spaxel with S/N > 3 in each line are shown on the left, while the corresponding maps, marking each spaxel according to the dominant excitation, are shown on the right (SF in blue, AGN in green, and LINER/shock in red and composite regions in magenta). The contours of Hα line emission are overplotted in black, and the location of the nuclear radio source classified as accreting BH by Reines et al. (2011) is marked with a yellow cross. This figure is taken fromCresci et al.(2017).

the well known relation between mass and metallicity. As an example, assuming the mass metallicity relation ofTremonti et al.(2004) and taking into account a stellar mass of 1 × 1010 M, the oxygen abundance would be 12 + log(O/H) = 8.95, while if the

SFR is assumed as third parameter in the relation, following Mannucci et al. (2010), 12 + log(O/H) = 8.70, somehow lower than the measured value. However, Esteban et al. (2014) obtained a new measure of the oxygen metallicity of 12 + log(O/H) = 8.55 ± 0.02 integrated on the central 8”×3”, using faint pure recombination lines. The new metallicity value byEsteban et al.(2014) is again comparable with what is expected by a computation of the mass metallicity relation obtained with such new diagnostics, as Andrews & Martini(2013) would predict 12+log(O/H) ∼ 8.65 using only the stellar mass and 12 + log(O/H) ∼ 8.50 taking into account the SFR as well.

The first row of Fig. 3.4 shows the spatial variation of the diagnostic ratio O3N2 and the corresponding metallicity map, using the calibrations by Curti et al. (2017), on the left and on the right, respectively. The global average metallicity in the disk is compatible with the value derived by Esteban et al. (2014), while the eastern region of the galaxy and the central highly star forming regions show lower metallicity values (12 + log(O/H) = 8.2−8.3). Highly star forming regions with lower gas metallicity than the surrounding galaxy have been interpreted as signatures of pristine, low metallicity gas accretion, that dilutes the metal content of the ISM and boosts star formation, both locally (e.g. S´anchez Almeida et al. 2014) and at high-z (Cresci et al., 2010). However, in the case of He 2-10 the abundance gradient amplitudes is of the same order of magnitude of the intrinsic scatter in the calibrations (e.g. 0.2 dex for O3N2, probably due to variation in the ionisation in different sources).

The map of the ionisation parameter can be derived trough the calibration ofKew- ley & Dopita (2002_{) using the line ratio [S iii]λλ9069,9532/[S ii]λλ6717,31}2 _{(Fig. 3.4,}

third panel). It can be seen from Fig. 3.4 (third panel) that this parameter is one order of magnitude higher in the two nuclear star forming region, further suggesting that the line ratio variation is due to ionisation effects in extreme environments such as these embedded, highly star forming regions and not to a metallicity variation.

To further explore this, the fourth panel of Fig. 3.4 shows the metallicity map obtained from the new diagnostic diagram by Dopita et al. (2016) (shown in Fig. 3.5) which makes use of [O iii]λ5007, Hβ, [N ii]λ6584, [S ii]λλ6717,31 and Hα lines to con- strain both metallicity and the ionisation parameter using photoionisation models. In Fig.3.5, the different grids shows the variation of metallicity and log(U) for different val- ues of ISM pressure, while the spaxels corresponding to the central star forming regions

2

Since [S iii]λ9532 is not covered by the wavelength range observed by MUSE, a theoretical ratio of [S iii]λλ9532/[S iii]λλ9069 = 2.47 (Vilchez & Esteban,1996), fixed by atomic physics, can be adopted.

define two clear sequences towards high metallicity and high U, reported in magenta and blue. The location of these spaxels are plotted in the upper right panel superimposed to the Hα map of He 2-10. The spaxels with metallicity 12 + log(O/H) < 8.55 are marked in green, both in the left and in the right panel. This confirms that the Eastern part of the galaxy has lower metallicity than the rest by 0.2 dex, possibily in agreement with the merger interpretation of the origin of He 2-10, while the different line ratios in the central star forming regions are due to higher ionisation parameter.

In the Dopita et al. (2016) diagram the two star forming regions actually show very high supersolar metallicities, probably due to efficient metal enrichment in those extreme environments. As shown in Fig. 3.5, lower right panel, these regions form a definite structure in the BPT diagram as well. This result suggests caution in the interpretation of a single line ratio as a variation in metallicity of the ISM, and confirms the importance of a large wavelength range to exploit multiple physical diagnostics.