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Astron. Astrophys. 333, 803-808 (1998)

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4. Results and discussion

4.1. Extinction curve

The calculated values of the slope of the regression lines alongwith the values reported by Savage & Mathis (1979) for our Galaxy are given in Table 2. The value of [FORMULA], i.e. the ratio of total extinction [FORMULA] in the V band to the selective extinction E [FORMULA] between B and V bands, is found to be 2.70 [FORMULA], whereas its Galactic value is reported as 3.1 (Savage & Mathis 1979, Rieke & Lebofsky 1985). The smaller value of RV as compared to that of our Galaxy shows that the average dust particle size of this galaxy is smaller than in our Galaxy. Goudfrooij et al. (1994b) have shown that the dust particles appear larger because of foreground light and/or forward scattering, implying that the dust particle size in this galaxy is indeed smaller than in our Galaxy. The extinction curve for this galaxy is plotted alongwith the Galactic extinction curve in Fig. 3. The extiction curve of NGC 2076 runs almost parallel to the Galactic extinction curve which shows that the properties of dust in it are similar to those in our Galaxy.


[TABLE]

Table 2. Linear fits of extinction


[FIGURE] Fig. 3. Extinction curve for NGC 2076 alongwith the Galactic extinction curve.

Using the extinction curve and extinction efficiency [FORMULA] of spherical dielectric grains (Greenberg 1968), we have tried to estimate the relative dust grain size responsible for extinction. From Fig. 3, it is obvious that the extinction curve varis linearly with the inverse wavelength in the optical part of the spectrum, which is also consistent with the prediction that for small grain size [FORMULA], [FORMULA], where [FORMULA] is the grain size in dimensionless parameters and `a' is the grain radius. The shift of extinction curve along the increasing value of [FORMULA] for a fixed value of [FORMULA] essentially indicates the decrease in characteristic particle size `a'. The relative grain size ([FORMULA] / [FORMULA] ; [FORMULA] is the characteristic grain size of dust in our Galaxy) is obtained by shifting the extinction curve along 1/ [FORMULA] axis untill it best matches with the Galactic curve. The relative grain size thus obtained is [FORMULA] / [FORMULA] =0.89 [FORMULA] 0.07.

4.2. Dust mass estimation

[FORMULA] Dust mass from total extinction:
We have made an attempt to derive the dust content of this galaxy using the extinction values, obtained in section 3.1. For a given grain size distribution n(a), the specific grain mass density [FORMULA], and length of the dust column [FORMULA], the dust column density can be calculated using the relation

[EQUATION]

We have used the dust size distribution of Mathis et al. (1977) i.e.

[EQUATION]

The upper and lower dust particle sizes are taken to be

[EQUATION]

respectively. The value of [FORMULA] is 0.89 [FORMULA] 0.07 for NGC 2076 as calculated in section 4.1. We have assumed that the dust is composed of silicate and graphite grains with equal abundance ratio and typical grain size of [FORMULA] = 0.1µm, [FORMULA] = 0.05µm. Extinction efficiencies are taken from literature and parametrized for the V-band extinction (Goudfrooij et al. 1994b). The dust mass estimated in this manner turns out to be 3.2 [FORMULA] 106 [FORMULA].

We have also estimated the neutral hydrogen mass and dust mass from the optical color-excess using the relation given by Bohlin et al.(1978) and assuming that gas-to-dust ratio of 100 is valid for NGC 2076 also. It gives mass of total neutral hydrogen as 1.8 [FORMULA] 108 [FORMULA] and dust mass as 1.8 [FORMULA] 106 [FORMULA]. The dust mass calculated using optical color-excess involves the assumption of gas-to-dust ratio similar to the Galactic value, which may not be a good approximation for early-type galaxies.

[FORMULA] Dust mass from FIR data:
We have made use of FIR flux from Knapp et al. (1989) to derive dust temperature, dust mass, total FIR luminosity and mass of the molecular hydrogen. Dust mass has been estimated from the relation in Thronson et al. (1986). The derived temperature of dust is 36 K, Infra-red luminosity is 2.98 [FORMULA] [FORMULA], dust mass is 3.67 [FORMULA] 106 [FORMULA].

In absence of neutral as well as molecular hydrogen observations it is very difficult to have a reliable estimate of total ISM content of this galaxy. However, estimation of dust content from FIR as well as optical data does indicate that it contains significant amount of ISM. A rough estimate of molecular hydrogen mass may be obtained from the dust mass derived from FIR data and making some assumptions about the ratio of molecular hydrogen to dust mass. Taking a value of [FORMULA] 700 for this ratio as given by Wiklind et al. (1995) for early-type galaxies, the molecular hydrogen mass turns out to be [FORMULA] 4.6 [FORMULA] 109 [FORMULA]. Thus, this galaxy appears to be extremely ISM rich.

4.3. Star formation rate (SFR) and efficiency

The dust temperature of 36 K is similar to the findings of Brosch et al. (1991), which shows that this galaxy contains warm dust (Sanders & Mirabel 1996). By virtue of its log(f(60 µm)/f(100µm)) = - 0.33 and log(f(12µm)/f(25µm)) = - 0.07 ratio it occupies an intermediate position in the phenomenological diagram of Helou (1986), where the cool component as well as the newly formed massive stars are responsible for heating the dust, and more than 50% of the IR emission can be attributed to young stars. This indicates an ongoing star formation in this galaxy. Therefore, we have tried to estimate the SFR using the FIR luminosity and total blue luminosity of NGC 2076.

The present day star formation rate, averaged over past 2 [FORMULA] 106 yrs. under the assumption that all the FIR emission comes from dust heated by young stars is estimated using the relation
[FORMULA] = 6.5 [FORMULA] 10-10 [FORMULA]
given by Thronson & Telesco (1986). It gives SFR of 110.5 [FORMULA] yr-1. However, considering the phenomenological model of Helou (1986) the expected SFR will be [FORMULA] 55 - 60 [FORMULA] yr-1.

The SFR for massive O, B, A stars, obtained from the relation
[FORMULA] = 7.7 [FORMULA] 10-11 [FORMULA],
given by Young et al. (1986), and assuming that the star formation process lasts for [FORMULA] 109 yrs., is 13.63 [FORMULA] yr-1. This estimate would be true provided that the observed luminosity is produced primarily by massive O, B, A type stars and therefore it can be taken as an upper limit of SFR for NGC 2076.

Gallagher et al. (1984) have used the total blue luminosity of a galaxy to estimate the recent SFR averaged over the past (0.4 - 6)109 yrs.:
[FORMULA] yr-1 = 6.5 [FORMULA] 10-9 [FORMULA].
Applying this relation for the case of NGC 2076 gives a value of 20.9 [FORMULA] yr-1 for the recent SFR averaged over the life time of stars, that dominate the blue light.
The comparison of SFR calculated for different scenarios reveals that the present day SFR averaged over 2 [FORMULA] 106 yrs. is much higher than the SFR for massive O, B, A stars averaged over [FORMULA] 109 yrs.. The SFR for low as well as high mass stars averaged over (0.4 - 6)109 yrs. is 20.9 [FORMULA] yr-1, which is greater than but consistent with its value for massive O, B, A type stars. This indicates that the star formation rate for this galaxy is rather high in the present epoch.

4.4. Origin of dust

A consensus is growing for an external origin of dust in early-type galaxies. The prominent dust lane observed in NGC 2076 may be attributed to either merger or interaction with a gas-rich companion. We do not find any suitable companion galaxy from which this galaxy could have accreted the observed dust. However, the observed strong dust obscuration and FIR emission in this galaxy suggest that it may have accreted an entire gas-rich galaxy. Further, very high star formation rate at the present epoch indicates that the cannibal process might have occured in not-so-distant past. Detailed invesigations, specially on its kinematics, are needed to arrive at any firm conclusion as regards the origin of dust in NGC 2076.

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© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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