Stellar population

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Study of the stellar population properties in the discs of ten spiral galaxies

Study of the stellar population properties in the discs of ten spiral galaxies

2008, 2012; Seidel et al. 2015) for which the assumption of SSP is a good approximation. To date, only a few papers focused on the stellar populations of galactic discs have been published because of their low surface brightness and the presence of intense nebular emission lines which make the spectroscopic analysis quite diffi- cult. In addition, the discs of galaxies are a reservoir of molecular gas (Davis et al. 2012) feeding more than one single episode of star formation. Therefore, the SSP approach cannot be used in discs and multiple stellar populations are required to correctly recover the star formation history and the stellar population properties in galaxy discs (Morelli et al. 2013; Gonz´alez Delgado et al. 2014; McDermid et al. 2015).
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Stellar population effects on the inferred photon density at reionization

Stellar population effects on the inferred photon density at reionization

One reason for such uncertainty is the unfortunate necessity of extrapolation below the limits of observational data. Only a few, rare z > 5 galaxies are bright enough for detailed spectroscopic analysis. Observations of high-redshift galaxies are typically limited to their broad-band colours in the rest-frame ultraviolet (and sometimes op- tical) and perhaps a measurement of particularly strong emission lines (such as Lyman α at 1216 Å; e.g. Labb´e et al. 2013; Caruana et al. 2014; Oesch et al. 2015; Smit et al. 2015). Given a measured rest-frame 1500 Å flux continuum density, usually derived from photometry in a broad bandpass, the number of ionizing photons shortwards of 912 Å (the ionization limit of hydrogen or ‘Lyman break’) must be inferred (e.g. Madau, Haardt & Rees 1999; Bunker et al. 2004; Robertson et al. 2015). This ‘Lyman-continuum’ flux is estimated through the use of stellar population synthesis models, fitted to the available data. For a stellar population of known age, metallicity and ultraviolet flux, the ionizing photon flux can be reli- ably estimated. However, difficulties arise when any of these proper- ties are unknown. A young starburst will contain a larger proportion of hot, massive stars than an older one and so emit more ionizing photons for a given 1500 Å continuum measurement. By contrast, a stellar population that has formed stars continuously over its life- time will show an ultraviolet spectral energy distribution (SED) to which both young stars and older sources contribute, resulting in a modified but far more stable ionizing photon-to-continuum ratio. At low metallicities, different stellar evolution pathways, including those which result from binary star interactions or rotation, become increasingly important — again resulting in a modified ionizing photon output (Eldridge, Izzard & Tout 2008; Stanway et al. 2014; Zhang et al. 2013; Topping & Schull 2015). While some of these variations can be inferred from stellar population modelling, this has traditionally been tuned to match the properties of the local galaxy distribution – largely comprising mature galaxies and stellar populations with a near-solar average metallicity.
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SDSS IV MaNGA: stellar population gradients as a function of galaxy environment

SDSS IV MaNGA: stellar population gradients as a function of galaxy environment

One downside of these large spectroscopic surveys is that only a small subregion of the galaxy is sampled, defined by the location of the light collecting fiber. Therefore neglecting the complex and rich internal structure of galaxies where important environmental effects, on proper- ties such stellar population gradients, might be seen (La Barbera et al. 2011a). In order to decipher the internal components of the galaxy and understand in detail the dependence of stellar population gradients on environment, it is necessary to use integral field spectroscopy (IFS). A number of spatially resolved measurements on local galaxies have already been made (SAURON (de Zeeuw et al. 2002), DiskMass (Bershady et al. 2010), ATLAS 3D (Cappellari et al. 2011), CALIFA (Sánchez et al. 2012), SAMI survey (Allen et al. 2015)), which have provided evidence for inside-out mass assembly of galaxies (Pérez et al. 2013) and ‘sub maximality’ of disks (Bershady et al. 2011), and probed the internal chemical composition of galaxies. There has also been efforts to map stellar content of galaxies in very high spatial resolution IFS data (e.g. Bacon et al. 1995; McDermid et al. 2006; Davies et al. 2007; Riffel et al. 2010, 2011; Storchi-Bergmann et al. 2012; Kamann et al. 2016).
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Magellan/M2FS Spectroscopy of Galaxy Clusters: Stellar Population Model and Application to Abell 267

Magellan/M2FS Spectroscopy of Galaxy Clusters: Stellar Population Model and Application to Abell 267

In addition to cosmological constraints from cluster masses, the galaxy spectra themselves convey a multitude of informa- tion about their stellar populations. In recent years, with the development of more robust statistical techniques, there has been great progress in the fi tting of galaxy spectra to extract stellar population information. These efforts have focused on building a more robust statistical framework around the early methods of stellar population synthesis ( Tinsley 1972; Searle et al. 1973; Larson & Tinsley 1978 ) used for modeling the spectral energy distributions ( SEDs ) of galaxies. These early stellar population synthesis methods have been improved over the years to incorporate a more complete understanding of galactic processes ( see Walcher et al. 2011 for a review ) . In the past few years, new efforts have been made to apply Bayesian techniques to fi t these stellar population models. BayeSED ( Han & Han 2014 ) and BEAGLE ( Chevallard & Charlot 2016 ) are two recently developed Bayesian models aimed at fi tting SEDs of galaxies over a large wavelength coverage. However, these models are geared towards SEDs, which sample only a few band passes over a large wavelength range ( from γ -rays to IR ) . And most recently, Meneses-Goytia et al. ( 2015 ) developed a single stellar population model with Bayesian statistical techniques to fi t spectra in the near-infrared.
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On the massive stellar population of the super star cluster Westerlund 1

On the massive stellar population of the super star cluster Westerlund 1

However, these values were determined solely from anal- ysis of the distribution of the cluster supergiants, thus exclud- ing both the more massive and evolved WR component, and also the currently undetectable Main Sequence (M ≤ 30 M ; Sect. 5.1). Consequently, we refrain from drawing firm conclu- sions on the extent and cluster density profile. We do not expect the positions of the currently observed post-MS population to accurately represent the underlying stellar population, as many young clusters such as R136/NGC 2070 show evidence of strong mass segregation (e.g., Meylan 1993; Schweizer 2005). Indeed, by analogy with other young massive clusters we might expect the density profile of Wd 1 to be described by a EFF (Elson et al. 1987) King-like profile (Schweizer 2005; Larsen 2004); this will be addressed in a future paper in which we analyse VLT (NAOS CONICA & FORS1) imaging data.
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SDSS IV MaNGA: stellar population gradients as a function of galaxy environment

SDSS IV MaNGA: stellar population gradients as a function of galaxy environment

Fig. 7 shows the light-weighted age and metallicity gradients for central (grey) and satellite (red) galaxies, as a function of local environment. Table 4 shows the numerical results of this analysis. First, Fig. 7 shows that stellar population gradients are indepen- dent of environmental density, as no correlation is evident between stellar population gradient and local density. This can be quan- titively described by fitting a line through the stellar population gradient–environment plane in each panel plot. We find that lumi- nosity and mass-weighted stellar population gradients generally do not correlate with local environment neither for central nor satellite galaxies. We further do not detect any evidence for a difference in gradients between satellite and central galaxies (see also Table 4). We conclude that the galaxy environment, whether measured as lo- cal environmental density or through central/satellite classification, does not appear to have any significant effect on age and metallic- ity gradients in galaxies. This result agrees well with a recent IFU study of nearby massive galaxies as part of the MASSIVE survey, where it is found that even at large radius, internal properties mat- ter more than environment in determining star formation history (Greene et al. 2015).
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Surot Madrid, Francisco Ricardo
  

(2018):


	On the age of the Milky Way bulge stellar population.


Dissertation, LMU München: Fakultät für Physik

Surot Madrid, Francisco Ricardo (2018): On the age of the Milky Way bulge stellar population. Dissertation, LMU München: Fakultät für Physik

under-estimated, respectively. Therefore, the discrepancy could be interpreted under the assumption that the chemical evolution of the bulge is He-enhanced. On the other hand, Haywood et al. (2016) suggested the discrepancy being caused by the effect of the age- -metallicity degeneracy that makes it hard to distinguish in the CMD a young MR star from an old MP one. They compared the MS-TO color spread observed in the CMD of Clarkson et al. (2011) with that of synthetic CMDs obtained by using two different age-metallicity relations: i) the one presented by Bensby et al. (2013), based on a total sample of 59 micorlensed dwarfs, and ii) one that extends from [Fe/H] = −1.35 dex at 13.5 Gyr to [Fe/H] = +0.5 dex at 10 Gyr. When taking into account distance, reddening and metallicity effects, Haywood et al. (2016) showed that the MS-TO color spread of a purely old stellar population would be wider than what is observed, and thus advocating for the presence in the bulge of a conspicuous population of young and intermediate-age stars. Very similar results have been presented by Bernard et al. (2018) who calculated the star formation history (SFH) of four bulge fields, including that of Clarkson et al. (2011). Their findings suggest that over 80% of the stars are older than 8 Gyr, but also the presence of star formation as recent as ∼ 1 Gyr.
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Stellar population trends in S0 galaxies

Stellar population trends in S0 galaxies

Differences in handling uncertain physics by model makers leads to large differences in the interpretation of galaxy colors from varying SSP models. Although it is difficult to estimate uncertainties in the derived stellar population parameters (like age and metallicity), comparing the BC03 models that we use here with other available models suggests how much our results are dependent on our choice of stellar population mod- els. The treatment of advanced stages in evolution, such as the Thermal-Pulsating Asymptotic Giant Branch phase (TP-AGB), has received much attention in the past decade. TP-AGB stars are extremely bright and dominate the NIR light of a galaxy following a burst of star formation, but are difficult to model theoretically because of the combined effects of thermal pulses, changes from heavy element dredge-up, and mass loss (BC03). The stellar population synthesis models of Maraston (2005) use a different prescription for the TP-AGB phase than BC03 and the effect on the model colors has been demonstrated in the literature (Tonini et al. 2009). A revised ver- sion of the Bruzual and Charlot stellar population synthesis code has been developed (Charlot & Bruzual 2009, private communication) which includes a new prescription for TP-AGB evolution of low and intermediate mass stars following Marigo & Girardi (2007) and uses tracks from models with updated input physics from Bertelli et al. (2008). Eminian et al. (2008) has demonstrated a significant change with the new Bruzual and Charlot models in NIR model colors for intermediate populations.
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P MaNGA : full spectral fitting and stellar population maps from prototype observations

P MaNGA : full spectral fitting and stellar population maps from prototype observations

In this section we compare the results of the BPT classifica- tion with stellar ages and metallicities derived in this paper from full spectral fitting. We make use of the emission lines maps cre- ated by Belfiore et al. (2014) and plot in Fig. 17 the position of all the regions in the 14 galaxies considered by Belfiore et al. (2014) in the BPT diagram. Belfiore et al. (2014) discuss a comparison with D4000 and Hδ absorption. Here we extend this analysis by directly comparing with stellar population ages and metallicities. We make use of the demarcation lines of Kewley et al. (2001) and Kauffmann et al. (2003). We refer the reader to Belfiore et al. (2014) for a care- ful discussion of the assumptions and caveats implicit in the use of these demarcation lines. We expect galactic regions (Voronoi bins) which lie below the demarcation Kauffmann 2003 line to harbour on-going star-formation in H II regions. This is confirmed by the study of the stellar population ages, as evident in the left-hand-side of Figure 17, where the regions lying below the Kauffmann 2003 line present younger stellar populations, consistently with a signif- icant population of O and B stars, capable of ionising classical H II
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The spatially resolved stellar population and ionized gas properties in the merger LIRG NGC 2623

The spatially resolved stellar population and ionized gas properties in the merger LIRG NGC 2623

the early phases of mergers (Teyssier et al. 2010; Hopkins et al. 2013; Renaud et al. 2015). In this sense, integral field spec- troscopy (IFS) is a very promising technique, because it can provide relevant information to characterize the extent of star formation, and how/when it is produced. Extended star forma- tion has also been observationally reported in many early-stage mergers, mainly in the form of widespread star clusters, most of which are located at the intersections between progenitors and/or tidal structures (Wang et al. 2004; Elmegreen et al. 2006; Smith et al. 2016), and through stellar population analysis re- lying on IFS (Cortijo-Ferrero et al. 2017; hereafter CF17). In fact, the results of the two early-stage merger LIRGs reported in CF17, IC 1623 W and NGC 6090, are compared throughout the paper with the merger LIRG NGC 2623. The advantage of NGC 2623 is that it is a more advanced system, in the merger stage, where a triggering of the star formation is expected to oc- cur, but at the same time, it also keeps a fossil record in the stellar populations of previous star formation bursts (i.e., when it was at the early-stage merger phase). Therefore, NGC 2623 represents an interesting nearby LIRG to study the role that major mergers play in galaxy evolution using spatially resolved spectroscopy.
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Resolved Stellar Population Properties in Galaxies During the Peak Epoch of Star Formation

Resolved Stellar Population Properties in Galaxies During the Peak Epoch of Star Formation

The evolution of stellar populations within each galaxy is an integral part of galaxy evo- lution as a whole. The distribution of stars within each galaxy governs the morphology we observe. In elliptical galaxies, there is very little star formation and a low gas fraction. El- liptical galaxies typically hold a population of small red dwarf stars. These stars burn their fuel much more slowly than massive stars on the main sequence, causing many red dwarfs to be about the age of the Universe at z = 0 (Laughlin, Bodenheimer, & Adams 1997). Unlike ellipticals, disk galaxies typically hold populations of both young and old stars. Older stars were formed from past bursts of star formation, and typically reside in the nuclear bulge and thick disk. The young population of stars are associated with ongoing star formation from the molecular gas supply within the disk.
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Stellar population gradients in isolated, local group dwarf galaxies

Stellar population gradients in isolated, local group dwarf galaxies

Figure 1 shows the color-magnitude diagrams (CMDs) of Cetus, Tucana, LGS-3, and Phoenix of four regions defined as a function of the scale length of each galaxy. The CMDs of each galaxy, except those of Phoenix, contain the same number of stars. It is interesting to note that there is no a clear change with radius of the blue-plume of stars of Cetus and Tucana. This stars have been identify as a blue straggler star population by [8]. However, in the case of LGS-3 and Phoenix, there is a clear change in the main-sequence of young stars being less populated toward the outer regions. Finally, all the CMDs of the galaxies looks very similar beyond two scale lengths.
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The Red MSX Source Survey: The Massive Young Stellar Population of Our Galaxy

The Red MSX Source Survey: The Massive Young Stellar Population of Our Galaxy

We can also compare our catalog with other published lists of young massive stars. A complete catalog for UCHII regions exists from the recently completed CORNISH blind radio survey (Purcell et al. 2013). A high-reliability compact H ii region catalog (Urquhart et al. 2013b) with clear ATLASGAL counterparts (Schuller et al. 2009) has recently been produced. This catalog identified 207 compact and UCHII regions that lie within the union of the boundaries of CORNISH and ATLASGAL. We recover 160 of these in our final list of good RMS sources. An additional 18 have other MSX counterparts within 10 arcsec, with 10 with a 21 μm flux below our limit, seven with brighter 8 μm emission than 14 μm emission (suggestive of strong polycyclic aromatic hydrocarbon, PAH, emission commonly seen around H ii regions, but excluded from our color selection), and one only detected by MSX at 21 μm (which, as noted above, we excluded from consideration for the RMS catalog). Notably, many of these 18 sources also have stronger emission at 12 μm than at either 8 μm or 14 μm, indicative of strong 12.8 μm [Ne ii] emission from their nebulae. Sources such as these are often extended in MSX, and would have been classified by us as diffuse H ii regions. Inspection of the Spitzer GLIMPSE and MIPSGAL images for the sources that have no MSX counterparts reveal an equal combination of extended sources that would have also been classified by us as diffuse H ii regions and faint sources in complex backgrounds. Overall, we conclude that the use of our color constraints misses about 10% of the total H ii region population within our size limits. We used the modeling of Davies et al. (2011) to estimate the fraction missing due to the size constraint itself. Fewer than 5% of UCHII regions are missing anywhere in the Galaxy, consistent with our comparison with the CORNISH catalog. The larger compact H ii regions are less well represented, with more than 50% missing, although presumably some fraction of these in reality make up our diffuse H ii region category. Therefore, our overall size constraint is strictly only useful in terms of detecting UCHII regions.
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Galaxy modelling through stellar population synthesis

Galaxy modelling through stellar population synthesis

From this method we find that the data are not well fitted by a unique bulge population simulated by a Ferrer’s ellipsoid. But allowing to fit the sum of two ellipsoids gives a very good fit to the overall star counts and to the CMDs. Figure 1 shows the comparison of 2MASS data with star counts simulated by several models: first a model assuming a single bulge/bar population, second a model with two distinct populations. We show [29] that the model with two populations (a bulge and a bar) reproduces best 2MASS data. With these 2 populations simulated CMDs are well in agreement with the data, as shown for example in figure 2 in Baade’s window. Moreover, this model reproduces also the bimodality which is seen in metallicity distribution in fields along the minor axis from [51].
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Binary population and spectral synthesis version 2 1 : construction, observational verification and new results

Binary population and spectral synthesis version 2 1 : construction, observational verification and new results

The Cygnus OB2 association (Wright et al., 2015) and Upper Scorpius (Pecaut et al., 2012) are best fit in our models at stellar population ages log(age/years)=6.8 and 7.0 respectively, while NGC 6067 (Alonso-Santiago et al., 2017) is fit at log(age/years)=8.0. This age for Cygnus OB2 is at the higher end of the range suggested by previous studies, as we would expect with a binary population. For Up- per Sco, the strongest constraint is given by the exis- tence of Antares, and this fixes our models to an ap- proximate age of 10 Myr, very similar to the 11 Myr found by Pecaut et al. (2012). Our age for NGC 6067 agrees with previous estimates. The HR diagrams for single star models are presented on the left of Figure 23 and binaries on the right. Interacting binaries broaden the range of properties (and hence colours) expected for cluster members. If we used single star models for these comparisons we would have to use a much younger pop- ulation to fit the bulk of stars, but we would not be able to match the luminosity of the WR stars. The binary population spreads out the most luminous stars signifi- cantly in temperature at late ages. In older clusters blue stragglers typically form a very clear group above the main sequence turn-off; in younger clusters it is diffi- cult to separate these out and identify the true, non- interacting, main-sequence stars. Colour-magnitude di- agrams for these clusters extending from the optical to the near-infrared are shown in Figure 24. The stellar main sequences predicted from the HR diagrams are coincident with the location of stars in their colour- magnitude diagrams albeit subject to some uncertain- ties in extinction effects (which may be present in the blue bands, particularly for Upper Sco), providing a ver- ification test for the atmosphere models which dominate in young stellar populations.
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The Dark Energy Survey: more than dark energy – an overview

The Dark Energy Survey: more than dark energy – an overview

Galaxies at z ∼ 3 are usually singled out as ‘dropouts’ (a.k.a. Lyman-break galaxies, LBGs) through successive passbands; this makes use of the strong Lyman-break signature in their SEDs (e.g. Steidel, Pettini & Hamilton 1995). The multiband data from DES will be sufficiently deep that they can be used to select galaxies from z ∼ 4 to 6 in three redshift bins as g, r, i-dropouts. If young, massive galaxies have already assembled at high redshift, then DES will be able to detect them. This observation is exciting as the presence of such objects is expected from the fossil record of local massive galaxies (Thomas et al. 2011), but could not be easily accommodated in theories of hierarchical galaxy formation based on cold dark matter cosmogonies (e.g. Baugh et al. 2005). Davis et al. (2013) investigate the potential for DES to detect such objects in a study based on stellar population modelling and the DES simulated data. They show that very young massive galaxies would be found, if they exist and are not heavily dust obscured. The combination of sky coverage and photometric depth of DES is the key. Narrow, deep surveys, usually designed for the detection of high- z galaxies (Stanway, Bunker & McMahon 2003; Bouwens et al. 2010), do not cover enough volume to find rare massive galaxies. DES will play
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Raiter, Anna
  

(2010):


	Emission nebulae at high redshift.


Dissertation, LMU München: Fakultät für Physik

Raiter, Anna (2010): Emission nebulae at high redshift. Dissertation, LMU München: Fakultät für Physik

Apart from the Lyα line itself, multi-wavelength data are also used to interpret the nature of the observed sources. Large number statistics allowed the studies of stellar mass assembly and the discovery of the “downsizing” effect (Cowie et al. 1996), morphology and star formation history and their dependence on the environment, AGN activity, dust properties, luminosity functions etc. from only 1 Gyr after the Big Bang (z ∼ 6) until now. The galaxy observations are usually interpreted by comparing them to the stellar pop- ulation synthesis models, where one of the most popular are those of Bruzual & Charlot (2003). The models are constructed based on observed stellar spectra. The luminosity, colours and spectrum of the stellar population as a function of age is obtained by sum- ming up the individual stellar templates after weighting them by the initial mass function (IMF). The most commonly used is the Salpeter IMF (Salpeter 1955, see the comparison of popular IMFs in Fig. 1.1) which is a pure power law with slope x = 1.35. Additionally, a certain star formation history is used. These kinds of models are treated as templates to be compared to the observed galaxy spectral energy distribution (SED). The observed SED is composed of a number of photometric magnitudes in different filters over a wide wavelength range and can be treated as a very low resolution spectrum. The SED fitting technique gives as output a number of best-fitting parameters characterising the galaxy. With the best-fitting template SED one obtains the age, metallicity, star formation history (SFH), star formation rate (SFR), extinction E(B-V) and also the photometric redshift.
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Realistic ionizing fluxes for young stellar populations from 0.05 to 2 ZÔ

Realistic ionizing fluxes for young stellar populations from 0.05 to 2 ZÔ

observations at various wavelengths. In the satellite ultravio- let, the stellar wind spectral features of a population of mas- sive stars can be synthesized to provide information on the star formation rates, the slope of the initial mass function (IMF), and ages (e.g. Robert, Leitherer & Heckman 1993; Leitherer, Robert & Heckman 1995; de Mello, Leitherer & Heckman 2000). In the optical region, the properties of in- tegrated stellar populations are usually derived indirectly from their total radiative energy outputs using nebular diag- nostic line ratios (e.g. Garc´ıa-Vargas, Bressan & D´ıaz 1995; Stasi´ nska & Leitherer 1996; Stasi´ nska, Schaerer & Leitherer 2001). This method uses the theoretical ionizing fluxes from a population of massive stars as a function of age as input into a photoionization code. The ability of evolutionary syn- thesis models to predict correctly the properties of a young stellar population from nebular emission line ratios there- fore depends heavily on the accuracy of the evolutionary and atmospheric models developed for single massive stars. Early attempts to model stellar populations using evo- lutionary synthesis coupled with photoionization codes re- lied mainly on Kurucz (1992) plane-parallel LTE model at- mospheres (e.g. Garc´ıa-Vargas et al. 1995). Gabler et al. (1989), however, showed that the presence of a stellar wind has a significant effect on the emergent ionizing flux in the neutral and ionized helium continua of O stars. Non-LTE effects depopulate the ground state of He ii , leading to a de- crease in the bound-free opacity above 54 eV, and hence a larger flux in the He ii continuum by up to 3–6 orders of magnitude. The emergent spectrum is flattened in the re- gion of the He i continuum and has a higher flux due to the presence of the stellar wind.
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MUSE observations of the counter-rotating nuclear ring in NGC 7742

MUSE observations of the counter-rotating nuclear ring in NGC 7742

One can separate the rotation of the old component from that of the newly-formed stars in the ring by fitting two LOSVDs in the pPXF code, as done in Coccato et al. (2011) and more re- cently in Morelli et al. (2017). For this we used our 12 high-S / N ring apertures and the result of our stellar-population analysis (see Sect. 3.5) in order to create a “bulge” and a “ring” template by combining all single-age templates older and younger than 1 Gyr, respectively, according to their best fitting mass weights (see Fig. 9). Due to the low inclination of NGC 7742, and the consequent small velocity separation between the old and young stellar component, we found it necessary to impose to the young stars in the ring the same velocity as observed for the ionised (Hα) gas in order to converge to sensible results.
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FRB 121102 Is Coincident with a Star-forming Region in Its Host Galaxy

FRB 121102 Is Coincident with a Star-forming Region in Its Host Galaxy

The resulting half-light radius corresponds to an HWHM ( Gaussian HWHM, 1.1774s ) of 0. 20 1  ( ). At a redshift of z = 0.193, an angle of 1 corresponds to a projected distance of 3.31 kpc, hence the half-light radius is 0.68 3 ( ) kpc. Under the assumption that the diffuse emission due to the underlying stellar population can be represented by a Gaussian ( which, given the irregular nature of dwarf galaxies may not be valid ) , the knot is located 0. 57  ( 1.9 kpc ) from the nominal centroid of the diffuse emission, which itself has a half-light diameter ( Gaussian FWHM ) between 1. 5  and 2. 0  ( 5 to 7 kpc ) . The higher spatial resolution and greater depth of the HST observations improve upon the  4 kpc diameter we estimated in Tendulkar et al. ( 2017 ) . The centroid of the knot, as measured in the drizzled F110W image, is located at
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