Zoom-in on the dust-obscured phase of galaxy formation with gravitational lenses

Over the last 20 years gravitational lensing has become an essential instrument to investigate the structures within the Universe and the Universe itself. It directly traces the gravity of matter, whether baryonic or not, and so it’s essential for a systematic study of dark matter and its distribution on both small and large scales. Moreover, the magnification generated by a foreground lensing system, like a massive elliptical galaxy, on a background source allows us to study high-redshift galaxy structures down to scales difficult to probe with the largest telescope at present, and to detect intrinsically faint objects. In this PhD thesis I describe the advantages that gravitational lensing offers in the study of high-redshift (z > 1.5) dusty star forming galaxies (DSFGs), progenitors of the early-type (ETGs) observed in the local Universe. DSFGs are the major contributors to the cosmic star formation activity in the Universe and, as such, they represent the key to understand the build-up of galaxies. Dust absorption of UV/optical radiation from newborn stars is re-emitted in the far-infrared/sub-mm bands, making DSFGs particularly bright at those wavelengths. In order to extract information from the galaxy-galaxy strong lensing events involving DSFGs, I have written a Python code performing lens modelling and source reconstruction, based on the Regularized Semilinear Inversion method by Warren & Dye (2003), as outlined in Enia et al. (2018). This method reconstructs the intrinsic (i.e. un-lensed) surface brightness of the background galaxy without any analytic pre-assumption on its distribution, while searching in the parameters space for the mass distribution of the lens. Since DSFGs are the main focus of this project, and since they are very bright at FIR/sub-mm wavelengths, I have extended the formalism to the uv plane, in order to deal with interferometric data. In fact, interferometry is the best observational technique to achieve high resolution imaging in the sub-mm/mm bands, thanks to facilities like the Atacama Large Millimeter/sub-millimeter Array (ALMA). Furthermore, since the lens galaxy is usually a massive elliptical, the main emission in the sub-mm/mm is due almost exclusively from the background galaxy. DSFGs show extremely steep number counts, so that any DSFGs with a very high flux density (e.g. above ∼ 100mJy at 500μm) is expected to be lensed. The selection of gravitationally lensed galaxies based on a simple cut in flux density has proven to be extremely efficient in the search for these sources in wide area extragalactic surveys such as the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). This survey found 80 candidate lensed galaxies, 20 of which have already been confirmed to be lensing systems by a number of follow-up observations mainly with the Sub-Millimeter Array (SMA), the Hubble Space Telescope and the Keck telescope. In Enia et al. (2018) I have applied my code to the SMA observations of 12 strongly lensed galaxies from the H-ATLAS in order to derive their morphologies, sizes and magni cations. The derived lens model parameters are in general consistent with previous findings (i.e. Bussmann et al., 2013), however the estimated magnification factors, ranging from 3 to 10, are lower. These discrepancies are observed in particular where the reconstructed source hints at the presence of multiple knots of emission. An appropriate estimate of the magnification factor is essential to properly retrieve the physical properties of the sources, i.e. CO line luminosities, star formation rates, or SFR surface densities. In Massardi, Enia et al., 2018 multiwavelength observations of two strongly lensed sources are presented. H-ATLAS J090740.0-004200, also known as SDP.9, and H-ATLAS J091043.1-000322, also known as SDP.11, both come from the H-ATLAS sample. The observations were carried out with Chandra, HST and ALMA, covering a large portion of the electromagnetic spectrum. These multiwavelength observations probed the presence of highly obscured nuclear activity in the galaxy, with X-ray emissions generated in the nuclear area, allowing an insight on the co-evolution between the central SMBH and the galaxy, as predicted by various evolutionary theories for galaxy formation and evolution. I applied the code to SDP.9, reconstructing the background source in the different bands, obtaining a clear cospatiality in the source plane between the sub-mm emission, tracing the star formation, and the X-ray signal, tracing the nuclear activity, within a circle of ∼ 400 pc diameter. This analysis will be further exploited in the future thanks to the large number of follow-up campaigns in different wavelength ranges currently ongoing. In Rodighiero, Enia et al., ApJL submitted, a study of the statistical properties of a sample of dusty sources with very efficient star formation rates (SFR) is performed, in order to understand the role of enhanced SFR in triggering the Black-Hole Accretion Rate. These sources are Herschel-selected in the COSMOS field, with SFRs elevated 4× above the star-forming ”main sequence”, classifying them as starbursts (SB). Here, by means of a multicomponent spectral energy distribution fitting analysis, the contribution of stars, AGN torus, and star formation to the total emission at different wavelengths is separated, spanning the range from the UV to the far-IR. The sample is divided into active SBs (dominated by an AGN emission, SBs-AGN) and purely star-forming SBs (SBs-SFR). From visual inspection of the HST-UV morphology, the two classes have statistically different morphologies: SBs-SFR are generally irregular systems, while a large majority (∼ 65%) of SBs-AGN are instead dominated by regular compact and symmetric morphologies. Searching in the ALMA public archive, I found continuum counterparts with a secure detection above 3σ for 24 galaxies (10 SBs-AGN and 14 SBs-SFR). Then, dust and total molecular gas masses are computed, finding that SBs turn to be gas rich systems (fgas =45%−85%),with similar gas fractions in the two classes, and therefore no direct evidence of AGN feedback depleting the parent hosts. This results are discussed in the context of the co-evolution scenario. The SB population is consistent with a mixture of: low-mass primordial galaxies, rapidly accreting their M∗ together with their MBH (mainly the more compact SBs-AGN), and a class of highly star-forming merging systems (dominating the SBs-SFR). Anyway, feedback effects have not reduced yet the fgas of the objects. Alternatively, feedback processes (in form of galactic outflows from the SMBH) are not efficient enough to significantly deplete the gas masses of the host galaxies

Illuminating the Dark Side of Cosmic Star Formation II. A second date with RS-NIRdark galaxies in COSMOS

About 12 billion years ago, the Universe was first experiencing light again after the dark ages, and galaxies filled the environment with stars, metals and dust. How efficient was this process? How fast did these primordial galaxies form stars and dust? We can answer these questions by tracing the Star Formation Rate Density (SFRD) back to its widely unknown high redshift tail, traditionally observed in the Near-InfraRed (NIR), Optical and UV bands. Thus, the objects with a high amount of dust were missing. We aim to fill this knowledge gap by studying Radio Selected NIR-dark (\textit{RS-NIRdark}) sources, i.e. sources not having a counterpart at UV-to-NIR wavelengths. We widen the sample by Talia et al. (2021) from 197 to 272 objects in the COSMic evolution Survey (COSMOS) field, including also photometrically contaminated sources, previously excluded. Another important step forward consists in the visual inspection of each source in the bands from u* to MIPS-24$\mu$m. According to their "environment" in the different bands, we are able to highlight different cases of study and calibrate an appropriate photometric procedure for the objects affected by confusion issues. We estimate that the contribution of RS-NIRdark to the Cosmic SFRD at 3$<$z$<$5 is $\sim$10--25$\%$ of that based on UV-selected galaxies

ALMA reveals the molecular gas properties of five star-forming galaxies across the main sequence at 3

International audienceWe present the detection of CO(5-4) with S/N> 7 - 13 and a lower CO transition with S/N > 3 (CO(4-3) for 4 galaxies, and CO(3-2) for one) with ALMA in band 3 and 4 in five main sequence star-forming galaxies with stellar masses 3-6x10^10 M/M_sun at 3 < z < 3.5. We find a good correlation between the total far-infrared luminosity LFIR and the luminosity of the CO(5-4) transition L'CO(5-4), where L'CO(5-4) increases with SFR, indicating that CO(5-4) is a good tracer of the obscured SFR in these galaxies. The two galaxies that lie closer to the star-forming main sequence have CO SLED slopes that are comparable to other star-forming populations, such as local SMGs and BzK star-forming galaxies; the three objects with higher specific star formation rates (sSFR) have far steeper CO SLEDs, which possibly indicates a more concentrated episode of star formation. By exploiting the CO SLED slopes to extrapolate the luminosity of the CO(1-0) transition, and using a classical conversion factor for main sequence galaxies of alpha_CO = 3.8 M_sun(K km s^-1 pc^-2)^-1, we find that these galaxies are very gas rich, with molecular gas fractions between 60 and 80%, and quite long depletion times, between 0.2 and 1 Gyr. Finally, we obtain dynamical masses that are comparable with the sum of stellar and gas mass (at least for four out of five galaxies), allowing us to put a first constraint on the alpha_CO parameter for main sequence galaxies at an unprecedented redshift

Characterisation of Herschel-selected strong lens candidates through HST and sub-mm/mm observations

We have carried out HST snapshot observations at 1.1 $\mu$m of 281 candidate strongly lensed galaxies identified in the wide-area extragalactic surveys conducted with the Herschel space observatory. Our candidates comprise systems with flux densities at $500\,\mu$m$S_{500}\geq 80$ mJy. We model and subtract the surface brightness distribution for 130 systems, where we identify a candidate for the foreground lens candidate. After combining visual inspection, archival high-resolution observations, and lens subtraction, we divide the systems into different classes according to their lensing likelihood. We confirm 65 systems to be lensed. Of these, 30 are new discoveries. We successfully perform lens modelling and source reconstruction on 23 systems, where the foreground lenses are isolated galaxies and the background sources are detected in the HST images. All the systems are successfully modelled as a singular isothermal ellipsoid. The Einstein radii of the lenses and the magnifications of the background sources are consistent with previous studies. However, the background source circularised radii (between 0.34 kpc and 1.30 kpc) are $\sim$3 times smaller than the ones measured in the sub-mm/mm for a similarly selected and partially overlapping sample. We compare our lenses with those in the SLACS survey, confirming that our lens-independent selection is more effective at picking up fainter and diffuse galaxies and group lenses. This sample represents the first step towards characterising the near-IR properties and stellar masses of the gravitationally lensed dusty star-forming galaxies.Comment: 57 pages, 18 figures, 11 tables. Accepted for publication in MNRA

Unveiling the distant Universe: Characterizing z≥9z\ge9 Galaxies in the first epoch of COSMOS-Web

We report the identification of 15 galaxy candidates at $z\ge9$ using the initial COSMOS-Web JWST observations over 77 arcmin$^2$ through four NIRCam filters (F115W, F150W, F277W, F444W) with an overlap with MIRI (F770W) of 8.7 arcmin$^2$. We fit the sample using several publicly-available SED fitting and photometric redshift codes and determine their redshifts between $z=9.3$ and $z=10.9$ ($\langle z\rangle=10.0$), UV-magnitudes between M$_{\rm UV}$ = $-$21.2 and $-$19.5 (with $\langle$M$_{\rm UV}\rangle=-20.2$) and rest-frame UV slopes ($\langle \beta\rangle=-2.4$). These galaxies are, on average, more luminous than most $z\ge9$ candidates discovered by JWST so far in the literature, while exhibiting similar blue colors in their rest-frame UV. The rest-frame UV slopes derived from SED-fitting are blue ($\beta\sim$[$-$2.0, $-$2.7]) without reaching extremely blue values as reported in other recent studies at these redshifts. The blue color is consistent with models that suggest the underlying stellar population is not yet fully enriched in metals like similarly luminous galaxies in the lower redshift Universe. The derived stellar masses with $\langle \log_{\rm 10} ($M$_\star/$M$_\odot)\rangle\approx8-9$ are not in tension with the standard $\Lambda$CDM model and our measurement of the volume density of such UV luminous galaxies aligns well with previously measured values presented in the literature at $z\sim9-10$. Our sample of galaxies, although compact, are significantly resolved.Comment: Submitted to Ap

Zoom-in on the dust-obscured phase of galaxy formation with gravitational lenses

Over the last 20 years gravitational lensing has become an essential instrument to investigate the structures within the Universe and the Universe itself. It directly traces the gravity of matter, whether baryonic or not, and so it’s essential for a systematic study of dark matter and its distribution on both small and large scales. Moreover, the magnification generated by a foreground lensing system, like a massive elliptical galaxy, on a background source allows us to study high-redshift galaxy structures down to scales difficult to probe with the largest telescope at present, and to detect intrinsically faint objects. In this PhD thesis I describe the advantages that gravitational lensing offers in the study of high-redshift (z > 1.5) dusty star forming galaxies (DSFGs), progenitors of the early-type (ETGs) observed in the local Universe. DSFGs are the major contributors to the cosmic star formation activity in the Universe and, as such, they represent the key to understand the build-up of galaxies. Dust absorption of UV/optical radiation from newborn stars is re-emitted in the far-infrared/sub-mm bands, making DSFGs particularly bright at those wavelengths. In order to extract information from the galaxy-galaxy strong lensing events involving DSFGs, I have written a Python code performing lens modelling and source reconstruction, based on the Regularized Semilinear Inversion method by Warren & Dye (2003), as outlined in Enia et al. (2018). This method reconstructs the intrinsic (i.e. un-lensed) surface brightness of the background galaxy without any analytic pre-assumption on its distribution, while searching in the parameters space for the mass distribution of the lens. Since DSFGs are the main focus of this project, and since they are very bright at FIR/sub-mm wavelengths, I have extended the formalism to the uv plane, in order to deal with interferometric data. In fact, interferometry is the best observational technique to achieve high resolution imaging in the sub-mm/mm bands, thanks to facilities like the Atacama Large Millimeter/sub-millimeter Array (ALMA). Furthermore, since the lens galaxy is usually a massive elliptical, the main emission in the sub-mm/mm is due almost exclusively from the background galaxy. DSFGs show extremely steep number counts, so that any DSFGs with a very high flux density (e.g. above ∼ 100mJy at 500μm) is expected to be lensed. The selection of gravitationally lensed galaxies based on a simple cut in flux density has proven to be extremely efficient in the search for these sources in wide area extragalactic surveys such as the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS). This survey found 80 candidate lensed galaxies, 20 of which have already been confirmed to be lensing systems by a number of follow-up observations mainly with the Sub-Millimeter Array (SMA), the Hubble Space Telescope and the Keck telescope. In Enia et al. (2018) I have applied my code to the SMA observations of 12 strongly lensed galaxies from the H-ATLAS in order to derive their morphologies, sizes and magni cations. The derived lens model parameters are in general consistent with previous findings (i.e. Bussmann et al., 2013), however the estimated magnification factors, ranging from 3 to 10, are lower. These discrepancies are observed in particular where the reconstructed source hints at the presence of multiple knots of emission. An appropriate estimate of the magnification factor is essential to properly retrieve the physical properties of the sources, i.e. CO line luminosities, star formation rates, or SFR surface densities. In Massardi, Enia et al., 2018 multiwavelength observations of two strongly lensed sources are presented. H-ATLAS J090740.0-004200, also known as SDP.9, and H-ATLAS J091043.1-000322, also known as SDP.11, both come from the H-ATLAS sample. The observations were carried out with Chandra, HST and ALMA, covering a large portion of the electromagnetic spectrum. These multiwavelength observations probed the presence of highly obscured nuclear activity in the galaxy, with X-ray emissions generated in the nuclear area, allowing an insight on the co-evolution between the central SMBH and the galaxy, as predicted by various evolutionary theories for galaxy formation and evolution. I applied the code to SDP.9, reconstructing the background source in the different bands, obtaining a clear cospatiality in the source plane between the sub-mm emission, tracing the star formation, and the X-ray signal, tracing the nuclear activity, within a circle of ∼ 400 pc diameter. This analysis will be further exploited in the future thanks to the large number of follow-up campaigns in different wavelength ranges currently ongoing. In Rodighiero, Enia et al., ApJL submitted, a study of the statistical properties of a sample of dusty sources with very efficient star formation rates (SFR) is performed, in order to understand the role of enhanced SFR in triggering the Black-Hole Accretion Rate. These sources are Herschel-selected in the COSMOS field, with SFRs elevated 4× above the star-forming ”main sequence”, classifying them as starbursts (SB). Here, by means of a multicomponent spectral energy distribution fitting analysis, the contribution of stars, AGN torus, and star formation to the total emission at different wavelengths is separated, spanning the range from the UV to the far-IR. The sample is divided into active SBs (dominated by an AGN emission, SBs-AGN) and purely star-forming SBs (SBs-SFR). From visual inspection of the HST-UV morphology, the two classes have statistically different morphologies: SBs-SFR are generally irregular systems, while a large majority (∼ 65%) of SBs-AGN are instead dominated by regular compact and symmetric morphologies. Searching in the ALMA public archive, I found continuum counterparts with a secure detection above 3σ for 24 galaxies (10 SBs-AGN and 14 SBs-SFR). Then, dust and total molecular gas masses are computed, finding that SBs turn to be gas rich systems (fgas =45%−85%),with similar gas fractions in the two classes, and therefore no direct evidence of AGN feedback depleting the parent hosts. This results are discussed in the context of the co-evolution scenario. The SB population is consistent with a mixture of: low-mass primordial galaxies, rapidly accreting their M∗ together with their MBH (mainly the more compact SBs-AGN), and a class of highly star-forming merging systems (dominating the SBs-SFR). Anyway, feedback effects have not reduced yet the fgas of the objects. Alternatively, feedback processes (in form of galactic outflows from the SMBH) are not efficient enough to significantly deplete the gas masses of the host galaxies

Ricostruzione di sorgenti gravitazionalmente lensate selezionate nel sub-millimetrico

Nel presente lavoro di tesi viene descritto il metodo della Regularized Semilinear Inversion, un metodo numerico per ricostruire il profilioriginale di una sorgente che ha subito strong lensing, e contestualmente ricavare i parametri dela lente. Il metodo è successivamente applicato a SDP.81, una galassia fortemente oscurata da polveri scoperta nel 2010 all'interno survey H-ATLAS, nei dati forniti da Hubble Space Telescope e dall'interferometro ALMA. Per questi ultimi il metodo viene esteso direttamente al piano UV delle delle visibilità implementando qualità della ricostruzion

Illuminating the Dark Side of Cosmic Star Formation. II. A Second Date with RS-NIRdark Galaxies in COSMOS

International audienceAbout 12 billion years ago, the Universe was first experiencing light again after the dark ages, and galaxies filled the environment with stars, metals, and dust. How efficient was this process? How fast did these primordial galaxies form stars and dust? We can answer these questions by tracing the star formation rate density (SFRD) back to its widely unknown high-redshift tail, traditionally observed in the near-infrared (NIR), optical, and UV bands. Thus, objects with a large amount of dust were missing. We aim to fill this knowledge gap by studying radio-selected NIR-dark (RS-NIRdark) sources, i.e., sources not having a counterpart at UV-to-NIR wavelengths. We widen the sample of Talia et al. from 197 to 272 objects in the Cosmic Evolution Survey (COSMOS) field, including also photometrically contaminated sources, which were previously excluded. Another important step forward consists in the visual inspection of each source in the bands from u* to MIPS 24 μm. According to their "environment" in the different bands, we are able to highlight different cases of study and calibrate an appropriate photometric procedure for the objects affected by confusion issues. We estimate that the contribution of RS-NIRdark sources to the cosmic SFRD at 3 < z < 5 is ~10%-25% of that based on UV-selected galaxies

Mental health impacts of the COVID-19 pandemic on college students

The COVID-19 pandemic is having profound effects on college students, and those with mental health conditions are more vulnerable to the impact of this stress. Objective: To study the impact of the COVID-19 pandemic on college students’ mental health. Participants: Participants (n=489) were mostly female, undergraduate, and aged 18–25. Methods: Participants completed an online survey assessing symptoms of mental health problems including hopelessness, loneliness, sadness, anxiety, sadness, and anger. Results: Approximately 81.6 % self-reported at least one negative mental health symptom. Students reported increased feelings of hopelessness (+7.8%), loneliness (+6.7%), sadness (+8.8%), depression (+2.6%), anxiety (+5.2%), and anger (+14.6%) during the pandemic than before. LGBTQ students and Black students had significantly more mental health symptoms during the pandemic than straight and White students. Conclusions: Results of this study highlight the negative impact of the pandemic and resultant changes on college students’ mental health

A Bayesian chemical evolution model of the DustPedia Galaxy M74

We introduce a new, multi-zone chemical evolution model of the DustPedia galaxy M74, calibrated by means of MCMC methods. We take into account the observed stellar and gas density profiles and use Bayesian analysis to constrain two fundamental parameters characterising the gas accretion and star formation timescale, i.e. the infall timescale tau and the SF efficiency nu, respectively, as a function of galactocentric radius R. Our analysis supports an infall timescale increasing with R and a star formation efficiency decreasing with R, thus supporting an 'Inside-Out' formation for M74. For both tau and nu, we find a weaker radial dependence than in the Milky Way. We also investigate the dust content of M74, comparing the observed dust density profile with the results of our chemical evolution models. Various prescriptions have been considered for two key parameters, i.e. the typical dust accretion timescale and the mass of gas cleared out of the dust by a supernova remnant, regulating the dust growth and destruction rate, respectively. Two models with a different current balance between destruction and accretion, i.e. with equilibrium and dominion of accretion over destruction, can equally reproduce the observed dust profile of M74. This outlines the degeneracy between these parameters in shaping the interstellar dust content in galaxies. Our methods will be extended to more DustPedia galaxies to shed more light on the relative roles of dust production and destruction.Comment: MNRAS, accepted for publication, 19 pages, 14 figure