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 ...
12 2 • The star formation history of NGC 5471 800 600 400 NGC 5471 ¢DEC (arcsec) 200 0 −200 −400 −600 −1000 −800 −600 −400 −200 0 200 400 600 800 ¡RA (arcsec) Figure 2.1: Large scale image of M101. The grayscale is the Digital Sky Survey image and the contour image is the H i from the VLA (Braun, 1995). NGC 5471 is located in the outskirts of the galaxy, in an arm concentration of H i. the study of the interrelation between stars, gas and dust components is highly relevant to understand the nature of these regions. 30 Doradus in the Large Magellanic Cloud (LMC) and NGC 604 in M33 are the two largest GEHRs in the Local Group and they have been the subject of very detailed analyses. Both are dominated by very young (2-4 Myr) ionizing stellar populations, but they present two rather different stellar distributions. While NGC 604 is an extended Scaled OB Association (SOBA) with ∼ 200 O+WR massive stars surrounded by nebular filamentary structures, 30 Doradus is powered by a Super Star Cluster (SSC) – a much more compact core (Maíz-Apellániz, 2001) – and a halo, with different stellar populations ranging from 2 to 20 Myr (Walborn and Blades, 1997). Both GEHRs include an ongoing star formation with young stars still embedded in their parent molecular cloud. The structural similarities and differences between these well studied and spatially well resolved Local Group GEHRs have made them obvious ideal laboratories in which to study massive star formation, but in
2.2. Observations and Data Reduction 13 order to extrapolate our knowledge to distant luminous unresolved starbursts it is critically necessary to go a step further and to analyze in detail the somewhat more distant GEHRs just outside the neighbourhood of the Local Group. M101 (NGC 5457) is a giant spiral galaxy located at a distance of 7.2 Mpc (Stetson et al., 1998), yielding a linear scale of 34.9 pc/ ′′ .This galaxy contains a large number of very luminous GEHRs, of which NGC 5471 (α 2000 = 14 h 04 m 29 s , δ 2000 = +54 ◦ 23 ′ 48 ′′ ; l = 101. ◦ 78, b = +59. ◦ 63) is one of the outermost, at a galactocentric radius of about 25 kpc (see Figure 2.1). The Hα morphology of NGC 5471 shows multiple cores with surrounding nebular filaments, extending over a diameter of ∼ 17 ′′ , or ∼ 600 pc. Ground-based photometry shows five bright knots, designated by Skillman (1985) as components A, B, C, D and E. Component A is as luminous as 30 Doradus, while components B, C and E are comparable to NGC 604, the two prototypical spatially resolved mini-starbursts in the Local Group. Thus NGC 5471 is truly a giant star forming region. All these features make of NGC 5471 an excellent candidate for the study of complex star formation, being close to the limit distance between the spatially well resolved Local Group GEHRs and the more distant unresolved starbursts, thus providing an intermediate step in our understanding of very massive star forming regions. This chapter is organized as follows. Section 2.2 describes the observations and data reduction. Section 2.3 presents the integrated analysis of both the individual large clusters and the whole of NGC 5471, and the photometry of the resolved stars. The results found in section 2.3 are elaborated and discussed in section 2.4. 2.2 Observations and Data Reduction For NGC 5471 we have obtained near infrared JHK images and have retrieved the broad and narrow band optical images available in the HST archive. Ultraviolet NUV and FUV images of M101 have also been retrieved from the GALEX archive. 2.2.1 GALEX data The Galaxy Evolution Explorer (GALEX) far-ultraviolet (FUV; λ ref = 1530 Å, ∆λ = 400 Å) and near-ultraviolet (NUV; λ ref = 2310 Å, ∆λ = 1000 Å) images of M101 were retrieved from the Nearby Galaxies Survey (NGS). The GALEX instrument is described by Martin et al. (2005) and its on-orbit performance by Morrissey et al. (2005). GALEX FUV and NUV imaging was obtained with total exposure times of 1041 s in each band. The 1σ NUV (FUV) sensitivity limit of the GALEX images is 27.6 (27.5) AB mag arcsec −2 for these data (Bianchi et al., 2005). At the distance of M101, the linear scale of 34.9 pc/ ′′ implies that the GALEX point spread function (PSF) of 4.6 arcsec translates to 160 pc; this is barely sufficient to resolve some of the structure in NGC
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12 2 • The star form<strong>at</strong>ion history <strong>of</strong> NGC 5471<br />
800<br />
600<br />
400<br />
NGC 5471<br />
¢DEC (arcsec)<br />
200<br />
0<br />
−200<br />
−400<br />
−600<br />
−1000 −800 −600 −400 −200 0 200 400 600 800<br />
¡RA (arcsec)<br />
Figure 2.1: Large scale image <strong>of</strong> M101. The grayscale is the Digital Sky Survey image and the<br />
contour image is the H i from the VLA (Braun, 1995). NGC 5471 is loc<strong>at</strong>ed <strong>in</strong> the outskirts <strong>of</strong> the<br />
galaxy, <strong>in</strong> an arm concentr<strong>at</strong>ion <strong>of</strong> H i.<br />
the <strong>study</strong> <strong>of</strong> the <strong>in</strong>terrel<strong>at</strong>ion between stars, gas and dust components is highly relevant to<br />
understand the n<strong>at</strong>ure <strong>of</strong> these <strong>regions</strong>.<br />
30 Doradus <strong>in</strong> the Large Magellanic Cloud (LMC) and NGC 604 <strong>in</strong> M33 are the two<br />
largest GEHRs <strong>in</strong> the Local Group and they have been the subject <strong>of</strong> very detailed analyses.<br />
Both are dom<strong>in</strong><strong>at</strong>ed by very young (2-4 Myr) ioniz<strong>in</strong>g stellar popul<strong>at</strong>ions, but they<br />
present two r<strong>at</strong>her <strong>different</strong> stellar distributions. While NGC 604 is an extended Scaled OB<br />
Associ<strong>at</strong>ion (SOBA) with ∼ 200 O+WR massive stars surrounded by nebular filamentary<br />
structures, 30 Doradus is powered by a Super Star Cluster (SSC) – a much more compact<br />
core (Maíz-Apellániz, 2001) – and a halo, with <strong>different</strong> stellar popul<strong>at</strong>ions rang<strong>in</strong>g from<br />
2 to 20 Myr (Walborn and Blades, 1997). Both GEHRs <strong>in</strong>clude an ongo<strong>in</strong>g star form<strong>at</strong>ion<br />
with young stars still embedded <strong>in</strong> their parent molecular cloud. The structural similarities<br />
and differences between these well studied and <strong>sp<strong>at</strong>ially</strong> well <strong>resolved</strong> Local Group GEHRs<br />
have made them obvious ideal labor<strong>at</strong>ories <strong>in</strong> which to <strong>study</strong> massive star form<strong>at</strong>ion, but <strong>in</strong>
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