A spatially resolved study of ionized regions in galaxies at different ...
A spatially resolved study of ionized regions in galaxies at different ... A spatially resolved study of ionized regions in galaxies at different ...
148 4 • Long-slit spectrophotometry of multiple knots of Hii galaxies 1400 1350 B C Hα velocity [SII] 6717 Å velocity 1,2e-15 1e-15 velocity (km/s) 1300 1250 A D 8e-16 6e-16 4e-16 Flux Hα (erg s -1 cm -2 Å) 1200 2e-16 1150 -20 -10 0 10 0 20 spatial distribution (arcsecs) Figure 4.21: Rotation curve of IIZw71. Filled circles represent heliocentric velocity derived from Hα and open squares from [Sii] 6717 Å. Solid line represents the spatial distribution of flux of Hα to be compared with the rotation curve. gas surrounding the knots of star formation which reaches 60 km/s in the case of knot C. Assuming that we see the polar ring of the galaxy edge-on, with a radial velocity of 85 km/s, and considering an optical radius of 20 arcsec, which is where we can measure the emission lines with a good enough signal-to-noise ratio, we calculated a dynamical mass inside this radius of (2.8 ± 0.2) ×10 9 M ⊙ . We then calculated the L B luminosity inside the radius of 20 arcsec, internal to the polar ring, from the B brightness distribution given in Cairós et al. (2001), obtaining L B = 7.2×10 8 L ⊙ . Therefore, we obtain a value of 3.9 for the M/L B ratio inside the star forming-ring. This is close to the value of 2.8 found by Reshetnikov and Combes (1994) from optical observations within a distance of 30 arcsec from the centre. Considering that their blue light probably encompasses the light coming from the emission knots in the ring, that luminosity probably constitutes an upper limit and provides a lower limit to the M/L B .
Chapter 5 Conclusions and future work In the present thesis we have studied star formation processes in galaxies at different scales: looking inside giant Hii regions of the Local Universe and individual starburst knots in Hii galaxies. For NGC 5471, a GEHR in the outskirts of the spiral galaxy M101, we have performed a cluster and resolved stellar photometry study to derive the star formation history of this complex. Integrated photometry of the whole region, using data from GALEX (ultraviolet), HST/WFPC2 (optical) and TNG (near infrared), yields two possible solutions for a single stellar population: one that corresponds to an age of ∼ 8 Myr with moderate extinction and one with an age around 50 Myr and very low extinction. This degeneration is typical of this kind of approach when spatially integrated data are used and reflects the complexity of GEHR which include clusters with different ages and extinction values. From the photometric analysis of the eleven clusters defined on the IR H image a clear correlation emerges in the sense that redder knots have less Hα flux, showing a clear aging trend. The ages range from 3 Myr for the youngest cluster, up to 10 Myr for the oldest one. The complex history of star formation of NGC 5471 revealed by the cluster analysis is confirmed by the resolved stellar analysis. From the CMD it is clear that star formation has been proceeding more or less continuously for the last 100 Myr. The well-defined mainsequence gives a youngest age of about 4 Myr, while red stars, which span a range of 3-4 magnitudes, indicate that star formation has occurred during the last 15-70 Myr. We have found that the masses of the ionized gas and of the stellar cluster in the current star formation event are both of the same order of magnitude, close to 10 6 M ⊙ , which may be understood in terms of a very high efficiency in star formation. This implies that there 149
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- Page 210 and 211: 190 REFERENCES Bertelli, G., Bressa
- Page 212 and 213: 192 REFERENCES Girardi, L. & Bertel
- Page 214 and 215: 194 REFERENCES Kunth, D. & Sargent,
- Page 216 and 217: 196 REFERENCES Pérez-Montero, E.,
Chapter<br />
5<br />
Conclusions and future work<br />
In the present thesis we have studied star form<strong>at</strong>ion processes <strong>in</strong> <strong>galaxies</strong> <strong>at</strong> <strong>different</strong><br />
scales: look<strong>in</strong>g <strong>in</strong>side giant Hii <strong>regions</strong> <strong>of</strong> the Local Universe and <strong>in</strong>dividual starburst<br />
knots <strong>in</strong> Hii <strong>galaxies</strong>.<br />
For NGC 5471, a GEHR <strong>in</strong> the outskirts <strong>of</strong> the spiral galaxy M101, we have performed<br />
a cluster and <strong>resolved</strong> stellar photometry <strong>study</strong> to derive the star form<strong>at</strong>ion history <strong>of</strong> this<br />
complex. Integr<strong>at</strong>ed photometry <strong>of</strong> the whole region, us<strong>in</strong>g d<strong>at</strong>a from GALEX (ultraviolet),<br />
HST/WFPC2 (optical) and TNG (near <strong>in</strong>frared), yields two possible solutions for a s<strong>in</strong>gle<br />
stellar popul<strong>at</strong>ion: one th<strong>at</strong> corresponds to an age <strong>of</strong> ∼ 8 Myr with moder<strong>at</strong>e ext<strong>in</strong>ction<br />
and one with an age around 50 Myr and very low ext<strong>in</strong>ction. This degener<strong>at</strong>ion is typical <strong>of</strong><br />
this k<strong>in</strong>d <strong>of</strong> approach when <strong>sp<strong>at</strong>ially</strong> <strong>in</strong>tegr<strong>at</strong>ed d<strong>at</strong>a are used and reflects the complexity <strong>of</strong><br />
GEHR which <strong>in</strong>clude clusters with <strong>different</strong> ages and ext<strong>in</strong>ction values.<br />
From the photometric analysis <strong>of</strong> the eleven clusters def<strong>in</strong>ed on the IR H image a clear<br />
correl<strong>at</strong>ion emerges <strong>in</strong> the sense th<strong>at</strong> redder knots have less Hα flux, show<strong>in</strong>g a clear ag<strong>in</strong>g<br />
trend. The ages range from 3 Myr for the youngest cluster, up to 10 Myr for the oldest one.<br />
The complex history <strong>of</strong> star form<strong>at</strong>ion <strong>of</strong> NGC 5471 revealed by the cluster analysis is<br />
confirmed by the <strong>resolved</strong> stellar analysis. From the CMD it is clear th<strong>at</strong> star form<strong>at</strong>ion<br />
has been proceed<strong>in</strong>g more or less cont<strong>in</strong>uously for the last 100 Myr. The well-def<strong>in</strong>ed ma<strong>in</strong>sequence<br />
gives a youngest age <strong>of</strong> about 4 Myr, while red stars, which span a range <strong>of</strong> 3-4<br />
magnitudes, <strong>in</strong>dic<strong>at</strong>e th<strong>at</strong> star form<strong>at</strong>ion has occurred dur<strong>in</strong>g the last 15-70 Myr.<br />
We have found th<strong>at</strong> the masses <strong>of</strong> the <strong>ionized</strong> gas and <strong>of</strong> the stellar cluster <strong>in</strong> the current<br />
star form<strong>at</strong>ion event are both <strong>of</strong> the same order <strong>of</strong> magnitude, close to 10 6 M ⊙ , which may<br />
be understood <strong>in</strong> terms <strong>of</strong> a very high efficiency <strong>in</strong> star form<strong>at</strong>ion. This implies th<strong>at</strong> there<br />
149
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