Studying the effects of interstellar dust in nearby galaxies

Στη παρούσα διατριβή διερευνώ τις ιδιότητες της κοσμικής σκόνης και την αλληλεπίδρασής της με το αστρικό πεδίο ακτινοβολίας (που προέρχεται από διαφορετικούς αστρικούς πληθυσμούς) για 814 γαλαξίες στο κοντινό Σύμπαν (γαλαξίες που βρίσκονται σε απόσταση μικρότερη των 40 Mpc) οι οποίοι έχουν παρατηρηθεί από το διαστημικό τηλεσκόπιο Herschel του Ευρωπαϊκού Οργανισμού Διαστήματος. Για το σκοπό αυτό έγινε χρήση του ευρέως διαδεδομένου κώδικα CIGALE, ο οποίος είναι κατάλληλα διαμορφωμένος ώστε να συμπεριλαμβάνει το προηγμένο μοντέλο για την εκπομπή της σκόνης, THEMIS. Χρησιμοποιώντας τα δεδομένα της βάσης δεδομένων του DustPedia, μου επέτρεψε να προσδιορίσω τις φυσικές ιδιότητες των γαλαξιών, όπως τη μάζα των αστρικών πληθυσμών και της σκόνης, τον ρυθμό σχηματισμού νέων αστέρων, τη συνολική φωτεινότητα κάθε αστρικού πληθυσμού, καθώς και την ενέργεια που απορροφάται εξαιτίας της σκόνης, τόσο για τους γηραιούς (> 200 Myr) όσο και για τους νεαρούς (< 200 Myr) αστρικούς πληθυσμούς. Επιπλέον, ένας από τους κύριους στόχους της διατριβής ήταν η δημιουργία τρισδιάστατων μοντέλων διάδοσης ακτινοβολίας για γαλαξίες με υψηλή διακριτική ικανότητα. Με τη συγκεκριμένη μέθοδο επιχείρησα να λύσω το πρόβλημα διάδοσης της ακτινοβολίας. Από δισδιάστατες εικόνες παρατηρήσεων στα διάφορα μήκη κύματος, παρήγαγα τις τρισδιάστατες κατανομές των αστεριών και της σκόνης. Για να μοντελοποιήσω τις σύνθετες γεωμετρίες, χρησιμοποιήσα το SKIRT, έναν σύγχρονο κώδικα Monte Carlo, τρισδιάστατης διάδοσης ακτινοβολίας που σχεδιάστηκε για να προσομοιώνει την απορρόφηση, τη σκέδαση, και τη θερμική εκπομπή από τη σκόνη για αυθαίρετες τρισδιάστατες κατανομές. Ανέλυσα τη συμβολή των διαφόρων αστρικών πληθυσμών στις διαδικασίες θέρμανσης της σκόνης σε τέσσερις κοντινούς, ραβδωτούς γαλαξίες: NGC1365, M83, M95, και M100; καθώς κι έναν γαλαξία οποίος διαθέτει έναν ενεργό γαλαξιακό πυρήνα: NGC1068. Σκοπός της ανάλυσης αυτής ήταν να ποσοτικοποιήσω το ποσοστό που σχετίζεται άμεσα με τον ρυθμό παραγωγής νέων αστέρων, καθώς και την έκταση της συνεισφοράς του ενεργού γαλαξιακού πυρήνα στη θέρμανση της διάχυτης σκόνης στις τρεις διαστάσεις.The present thesis is investigating the properties of cosmic dust and its interaction with the stellar radiation field (originating from different stellar populations) on global scales, for 814 galaxies in the local Universe, all observed by the Herschel Space Observatory. I took advantage of the widely used fitting code CIGALE, properly adapted to include the state-of-the-art dust model THEMIS. Using the DustPedia photometry I have determined the physical properties of the galaxies, such as the dust and stellar mass, the star-formation rate, the bolometric luminosity, the unattenuated and the absorbed by dust stellar light, for both the old (> 200 Myr) and young (< 200 Myr) stellar populations. In addition, one of the main goals of this thesis, was to construct detailed 3D dust radiative transfer models for spatially resolved, nearby galaxies. From 2D images I have derived the 3D distributions of stars and dust. To model the complex geometries, I have used SKIRT, a state-of-the-art 3D Monte Carlo radiative transfer code designed to self-consistently simulate the absorption, scattering, and thermal re-emission by the dust for arbitrary 3D distributions. By analysing the contribution of the different stellar populations to the radiative dust heating processes in four nearby face-on barred galaxies: NGC1365, M83, M95, and M100; and a galaxy hosting an AGN: NGC1068, has enabled me to quantify the fraction directly related to star-formation, as well as the extent of the contribution of the AGN to the diffuse dust heating in three dimensions

A simple spectroscopic technique to identify rejuvenating galaxies

Rejuvenating galaxies are unusual galaxies that fully quench and then subsequently experience a "rejuvenation" event to become star-forming once more. Rejuvenation rates vary substantially in models of galaxy formation: 10%-70% of massive galaxies are expected to experience rejuvenation by z = 0. Measuring the rate of rejuvenation is therefore important for calibrating the strength of star formation feedback mechanisms. However, these observations are challenging because rejuvenating systems blend in with normal star-forming galaxies in broadband photometry. In this paper, we use the galaxy spectral energy distribution (SED)-fitting code Prospector to search for observational markers that distinguish normal star-forming galaxies from rejuvenating galaxies. We find that rejuvenating galaxies have smaller Balmer absorption line equivalent widths (EWs) than star-forming galaxies. This is analogous to the well-known "K + A" or post-starburst galaxies, which have strong Balmer absorption due to A-stars dominating the light: in this case, rejuvenating systems have a lack of A-stars, instead resembling "O - A" systems. We find star-forming galaxies that have H$\beta$, H$\gamma$, and/or H$\delta$ absorption EWs $\lesssim 3${\AA} corresponds to a highly pure selection of rejuvenating systems. Interestingly, while this technique is highly effective at identifying mild rejuvenation, "strongly" rejuvenating systems remain nearly indistinguishable from star-forming galaxies due to the well-known stellar outshining effect. We conclude that measuring Balmer absorption line EWs in star-forming galaxy populations is an efficient method to identify rejuvenating populations, and discuss several techniques to either remove or resolve the nebular emission which typically lies on top of these absorption lines.Comment: Published in ApJ, 16 pages, 9 figure

Less is less: photometry alone cannot predict the observed spectral indices of z∼1z\sim1 galaxies from the LEGA-C spectroscopic survey

We test whether we can predict optical spectra from deep-field photometry of distant galaxies. Our goal is to perform a comparison in data space, highlighting the differences between predicted and observed spectra. The Large Early Galaxy Astrophysics Census (LEGA-C) provides high-quality optical spectra of thousands of galaxies at redshift $0.6<z<1$. Broad-band photometry of the same galaxies, drawn from the recent COSMOS2020 catalog, is used to predict the optical spectra with the spectral energy distribution (SED) fitting code Prospector and the MILES stellar library. The observed and predicted spectra are compared in terms of two age and metallicity-sensitive absorption features (H$\delta_\mathrm{A}$ and Fe4383). The global bimodality of star-forming and quiescent galaxies in photometric space is recovered with the model spectra. But the presence of a systematic offset in the Fe4383 line strength and the weak correlation between the observed and modeled line strength imply that accurate age or metallicity determinations cannot be inferred from photometry alone. For now we caution that photometry-based estimates of stellar population properties are determined mostly by the modeling approach and not the physical properties of galaxies, even when using the highest-quality photometric datasets and state-of-the-art fitting techniques. When exploring a new physical parameter space (i.e. redshift or galaxy mass) high-quality spectroscopy is always needed to inform the analysis of photometry.Comment: 13 pages, 8 figures, accepted 26 October 202

Probing the spectral shape of dust emission with the DustPedia galaxy sample

The objective of this paper is to understand the variance of the far-infrared (FIR) spectral energy distribution (SED) of the DustPedia galaxies, and its link with the stellar and dust properties. An interesting aspect of the dust emission is the inferred FIR colours which could inform us about the dust content of galaxies, and how it varies with the physical conditions within galaxies. However, the inherent complexity of dust grains as well as the variety of physical properties depending on dust, hinder our ability to utilise their maximum potential. We use principal component analysis (PCA) to explore new hidden correlations with many relevant physical properties such as the dust luminosity, dust temperature, dust mass, bolometric luminosity, star-formation rate (SFR), stellar mass, specific SFR, dust-to-stellar mass ratio, the fraction of absorbed stellar luminosity by dust (f_abs), and metallicity. We find that 95% of the variance in our sample can be described by two principal components (PCs). The first component controls the wavelength of the peak of the SED, while the second characterises the width. The physical quantities that correlate better with the coefficients of the first two PCs, and thus control the shape of the FIR SED are: the dust temperature, the dust luminosity, the SFR, and f_abs. Finally, we find a weak tendency for low-metallicity galaxies to have warmer and broader SEDs, while on the other hand high-metallicity galaxies have FIR SEDs that are colder and narrower

Rest-Frame Near-Infrared Radial Light Profiles up to z=3 from JWST/NIRCam: Wavelength Dependence of the S\'ersic Index

We examine the wavelength dependence of radial light profiles based on S\'ersic index $n$ measurements of 1067 galaxies with M$_*\geq$ 10$^{9.5}$M$_\odot$ and in the redshift range $0.5 < z < 3$. The sample and rest-frame optical light profiles are drawn from CANDELS$+$3D-HST; rest-frame near-infrared light profiles are inferred from CEERS JWST/NIRCam imaging. $n$ shows only weak dependence on wavelength, regardless of redshift, galaxy mass and type: on average, star-forming galaxies have $n = 1-1.5$ and quiescent galaxies have $n = 3-4$ in the rest-frame optical and near-infrared. The strong correlation at all wavelengths between $n$ and star-formation activity implies a physical connection between the radial stellar mass profile and star-formation activity. The main caveat is that the current sample is too small to discern trends for the most massive galaxies (M$_* > 10^{11}M_\odot$).Comment: 15 pages, 7 figures, Submitted to ApJ. Comments welcom

A census of star formation histories of massive galaxies at 0.6 < z < 1 from spectro-photometric modeling using Bagpipes and Prospector

We present individual star-formation histories of $\sim3000$ massive galaxies (log($\mathrm{M_*/M_{\odot}}$) > 10.5) from the Large Early Galaxy Astrophysics Census (LEGA-C) spectroscopic survey at a lookback time of $\sim$7 billion years and quantify the population trends leveraging 20hr-deep integrated spectra of these $\sim$ 1800 star-forming and $\sim$ 1200 quiescent galaxies at 0.6 < $z$ < 1.0. Essentially all galaxies at this epoch contain stars of age < 3 Gyr, in contrast with older massive galaxies today, facilitating better recovery of previous generations of star formation at cosmic noon and earlier. We conduct spectro-photometric analysis using parametric and non-parametric Bayesian SPS modeling tools - Bagpipes and Prospector to constrain the median star-formation histories of this mass-complete sample and characterize population trends. A consistent picture arises for the late-time stellar mass growth when quantified as $t_{50}$ and $t_{90}$, corresponding to the age of the universe when galaxies formed 50\% and 90\% of their total stellar mass, although the two sets of models disagree at the earliest formation times (e.g. $t_{10}$). Our results reveal trends in both stellar mass and stellar velocity dispersion as in the local universe - low-mass galaxies with shallower potential wells grow their stellar masses later in cosmic history compared to high-mass galaxies. Unlike local quiescent galaxies, the median duration of late-time star-formation ($\tau_{SF,late}$ = $t_{90}$ - $t_{50}$) does not consistently depend on the stellar mass. This census sets a benchmark for future deep spectro-photometric studies of the more distant universe.Comment: 25 pages, 12 figures, 4 tables, submitted to Ap

High-resolution, 3D radiative transfer modelling : V. A detailed model of the M 51 interacting pair

Context. Investigating the dust heating mechanisms in galaxies provides a deeper understanding of how the internal energy balance drives their evolution. Over the last decade radiative transfer simulations based on the Monte Carlo method have emphasised the role of the various stellar populations heating the diffuse dust. Beyond the expected heating through ongoing star formation, older stellar populations (>= 8 Gyr) and even active galactic nuclei can both contribute energy to the infrared emission of diffuse dust.Aims. In this particular study we examine how the radiation of an external heating source, such as the less massive galaxy NGC 5195 in the M 51 interacting system, could affect the heating of the diffuse dust of its parent galaxy NGC 5194, and vice versa. Our goal is to quantify the exchange of energy between the two galaxies by mapping the 3D distribution of their radiation field.Methods. We used SKIRT, a state-of-the-art 3D Monte Carlo radiative transfer code, to construct the 3D model of the radiation field of M 51, following the methodology defined in the DustPedia framework. In the interest of modelling, the assumed centre-to-centre distance separation between the two galaxies is similar to 10 kpc.Results. Our model is able to reproduce the global spectral energy distribution of the system, and it matches the resolved optical and infrared images fairly well. In total, 40.7% of the intrinsic stellar radiation of the combined system is absorbed by dust. Furthermore, we quantify the contribution of the various dust heating sources in the system, and find that the young stellar population of NGC 5194 is the predominant dust-heating agent, with a global heating fraction of 71.2%. Another 23% is provided by the older stellar population of the same galaxy, while the remaining 5.8% has its origin in NGC 5195. Locally, we find that the regions of NGC 5194 closer to NGC 5195 are significantly affected by the radiation field of the latter, with the absorbed energy fraction rising up to 38%. The contribution of NGC 5195 remains under the percentage level in the outskirts of the disc of NGC 5194. This is the first time that the heating of the diffuse dust by a companion galaxy is quantified in a nearby interacting system

High-resolution, 3D radiative transfer modelling : IV. AGN-powered dust heating in NGC 1068

The star formation rate and the mass of interstellar medium (ISM) have a high predictive power for the future evolution of a galaxy. Nevertheless, deriving such properties is not straightforward. Dust emission, an important diagnostic of star formation and ISM mass throughout the Universe, can be powered by sources unrelated to ongoing star formation. In the framework of the DustPedia project we set out to disentangle the radiation of the ongoing star formation from that of the older stellar populations. This is done through detailed 3D radiative transfer simulations of face-on spiral galaxies. We take special care in modelling the morphological features present for each source of radiation. In this particular study, we focus on NGC 1068, which in addition contains an active galactic nucleus (AGN). The effect of diffuse dust heating by an AGN (beyond the torus) has so far only been investigated for quasars. This additional dust heating source further contaminates the broadband fluxes that are used by classic galaxy modelling tools to derive physical properties. We aim to fit a realistic model to the observations of NGC 1068 and quantify the contribution of the several dust-heating sources. Our model is able to reproduce the global spectral energy distribution of the galaxy. It matches the resolved optical and infrared images fairly well, but deviates in the UV and the submillimetre (submm). This is partly due to beam smearing effects, but also because the input dust distribution is not sufficiently peaked in the centre. We find that AGN contamination of the broadband fluxes has a strong dependency on wavelength. It peaks in the mid-infrared, drops in the far-infrared, and then rises again at submm wavelengths. We quantify the contribution of the dust-heating sources in each 3D dust cell and find a median value of 83% for the star formation component. The AGN contribution is measurable at the percentage level in the disc, but quickly increases in the inner few hundred parsecs, peaking above 90%. This is the first time the phenomenon of an AGN heating the diffuse dust beyond its torus is quantified in a nearby star-forming galaxy. NGC 1068 only contains a weak AGN, meaning this effect could be stronger in galaxies with a more luminous AGN. This could significantly impact the derived star formation rates and ISM masses for such systems

The resolved scaling relations in DustPedia: Zooming in on the local Universe

We perform a homogeneous analysis of an unprecedented set of spatially resolved scaling relations (SRs) between ISM components and other properties in the range of scales 0.3-3.4 kpc. We also study some ratios: dust-to-stellar, dust-to-gas, and dust-to-metal. We use a sample of 18 large, spiral, face-on DustPedia galaxies. All the SRs are moderate/strong correlations except the dust-HI SR that does not exist or is weak for most galaxies. The SRs do not have a universal form but each galaxy is characterized by distinct correlations, affected by local processes and galaxy peculiarities. The SRs hold starting from 0.3 kpc, and if a breaking down scale exists it is < 0.3 kpc. By evaluating all galaxies at 3.4 kpc, differences due to peculiarities of individual galaxies are cancelled out and the corresponding SRs are consistent with those of whole galaxies. By comparing subgalactic and global scales, the most striking result emerges from the SRs involving ISM components: the dust-total gas SR is a good correlation at all scales, while the dust-H2 and dust-HI SRs are good correlations at subkpc/kpc and total scales, respectively. For the other explored SRs, there is a good agreement between small and global scales and this may support the picture where the main physical processes regulating the properties and evolution of galaxies occur locally. Our results are consistent with the hypothesis of self-regulation of the SF process. The analysis of subgalactic ratios shows that they are consistent with those derived for whole galaxies, from low to high z, supporting the idea that also these ratios could be set by local processes. Our results highlight the heterogeneity of galaxy properties and the importance of resolved studies on local galaxies in the context of galaxy evolution. They also provide observational constraints to theoretical models and updated references for high-z studies.Comment: 42 pages, 11 figures and 5 tables in the main text, 2 figures and 1 table in Appendix. Accepted for publication in A&

Reproducing the Universe: a comparison between the EAGLE simulations and the nearby DustPedia galaxy sample

We compare the spectral energy distributions (SEDs) and inferred physical properties for simulated and observed galaxies at low redshift. We exploit UV-submillimetre mock fluxes of ~7000 z=0 galaxies from the EAGLE suite of cosmological simulations, derived using the radiative transfer code SKIRT. We compare these to ~800 observed galaxies in the UV-submillimetre range, from the DustPedia sample of nearby galaxies. To derive global properties, we apply the SED fitting code CIGALE consistently to both data sets, using the same set of ~80 million models. The results of this comparison reveal overall agreement between the simulations and observations, both in the SEDs and in the derived physical properties, with a number of discrepancies. The optical and far-infrared regimes, and the scaling relations based upon the global emission, diffuse dust and stellar mass, show high levels of agreement. However, the mid-infrared fluxes of the EAGLE galaxies are overestimated while the far-UV domain is not attenuated enough, compared to the observations. We attribute these discrepancies to a combination of galaxy population differences between the samples, and limitations in the subgrid treatment of star-forming regions in the EAGLE-SKIRT post-processing recipe. Our findings show the importance of detailed radiative transfer calculations and consistent comparison, and provide suggestions for improved numerical models.Comment: 17 pages, 14 figures, accepted for publication in MNRA