Galaxy parameters

Characterizing the gas, stellar and structural properties of galaxies, as well as their dark matter halos, is one of the objectives of my research. With my colleagues, we characterize galaxies following different methodologies, including machine learning procedures.

Determining stellar masses, stellar population and structural parameters is crucial for a series of topics, as constraining the dark matter profile and dark matter fraction (see more in the tab Dark matter and IMF), study the size-mass relation and select particular classes of galaxies (see e.g. the tab on Ultra-compact massive galaxies and Relics), or determine and study how colour and stellar population gradients change with mass (see tab on Colour gradients).

1. We have determined photometric redshifts for KiDS-DR2 in Cavuoti et al. (2015), exploring a cooperative method, joining machine learning with SED fitting, in Cavuoti et al. (2017). We have also developped a method to calculate the probability density function (PDF) of photometric redshifts (Cavuoti et al. 2017) and applied it to KiDS-DR3 data (Amaro et al. 2019). In Li et al. (2022) we have developped a method which applies convolutional neural networks and deep learning neural networks to galaxy images and catalogs to determine their photometric redshifts.
2. We have determined structural parameters for KiDS-DR3 using classical Sérsic modelling in Roy et al. (2018) and began to develop a machine learning approach in Li et al. (2022)
3. In Bisigello et al. (2020) you will find how to select quiscent galaxies and star-forming galaxies with Euclid and ground-based data. Deep learning neural networks and CNNs can be used to determine physical parameters (stellar mass and star formation rate) using Euclid and Rubin data (Bisigello et al. 2022).
4. Machine learning can also be used to determine the dark matter properties of galaxies, starting from their stellar, structural and kinematical properties (von Marttens et al. 2022).

Ultra-compact massive galaxies

Massive Early-Type Galaxies (ETGs) are believed to form through a two-phase formation scenario (Oser et al. 2010). An intense and fast dissipative series of processes forms their central mass bulk at z > 2 generating, after the star formation quenches, a compact massive quiescent galaxy with size a factor of ∼4 smaller than local massive galaxies. These are the so-called red nuggets. Then, a second phase, dominated by mergers and gas inflows, is responsible for their dramatic structural evolution and size growth (e.g. van Dokkum et al. 2010). Nevertheless, a small fraction of red nuggets survives intact until the present day Universe, without experiencing any merger or interaction, massive (M_★ > 8 × 10¹⁰ M_☉) and compact (R_e < 1.5 kpc): the so-called Relic Galaxies (Trujillo et al. 2009). These old Ultra Compact Massive Galaxies (UCMGs) are supposedly made of "in situ only" pristine stellar populations. As such they provide a unique opportunity to track the formation of this specific galaxy stellar component, which is mixed with the accreted one in normal massive ETGs. Relic galaxies are the perfect systems to study in great detail the processes that shaped the mass assembly of massive galaxies in the high-z universe. In the local universe one true relic has been found and studied in great details (NGC 1277, Trujillo et al. 2009, 2014; Ferré-Mateu et al. 2017). Even if UCMGs remain extremely rare at very low redshifts, their number is increasing at larger redshifts (0.2 < z < 0.7), thus it is crucial to increase their number statistics in this redshift range.

We have started a long-term program to perform the census of UCMGs in the KiDS survey, and different follow-ups: spectroscopic validation and higher-resolution photometry and spectroscopy.

Finding UCMGs

With KiDS we are concentrating on the search of the UCMGs (Tortora et al. 2016, 2018). This analysis requires large samples of UCMGs, discovered thanks to the large areas observed, and spectroscopic follow-ups to determine the spectroscopic redshifts, provide their validation, and study the systematic sources in the searching strategy. KiDS offers the opportunity to observe large areas of the sky (1350 sq. deg. at the end of the survey) with a good seeing (FWHM ∼ 0.65, on average, for r-band). Among the others, the KiDS image quality makes the data very suitable for measuring structural parameters of galaxies (we have studied the size evolution of ETGs and LTGs in Roy et al. 2018), including compact ones. We have performed the first census of UCMGs in KiDS in Tortora et al. (2016). Then, we have upgraded analysis and results in a second paper of the series (Tortora et al. 2018), selecting UCMGs within 333 sq. deg. of the third data release of KiDS, discussing the first results from our spectroscopic campaign and plotting the abundances in terms of redshift. We find ∼3 UCMG candidates per square degrees, which corresponds to ∼1 per cent of the whole galaxy population at M_★ > 8 × 10¹⁰ M_☉. The number density of UCMGs is reduced of about ten times among redshift z = 0.5 and z = 0, which is a large variation if compared with normal-sized galaxies, suggesting a size-dependent evolution. In the attached figure (extracted from Tortora et al. 2018) I present the number density as a function of redshift, and the results are compared with other works in the literature. Read Tortora et al. (2018) for more details. In Tortora et al. (2020) we have investigated the role of environment, finding that UCMGs (and likely relics overall) are not special because of the environment effect on their nurture, but rather they are just a product of the stochasticity of the merging processes regardless of the global environment in which they live.

Follow-ups

We have started a multi-site and multi-facility spectroscopic campaign in the North and South hemisphere, to cover the whole KiDS area during the entire solar year. The multi-site approach allows us to cover the two KiDS patches (KiDS-North from La Palma and KiDS-South from Chile), while the multi-facility allows us to optimize the exposure time according to the target brightness (ranging from r ∼ 18.5 to 20.5). We have planned to observe our UCMG candidates at 3-4m and 8-10m class telescopes (for brighter and fainter targets, respectively). In Tortora et al. (2018) we have presented the first 28 UCMG candidates observed with NTT and TNG, and we present the data for further 33 galaxies, observed with TNG and INT, in Scognamiglio et al. (2020).

Thanks to the ESO Large Program INSPIRE, we will also obtain higher-resolution spectra for a subsample of UCMGs using X-Shooter, which will allow us to measure the stellar population parameters and the Initial Mass Function. We aim at proving that the age of these systems is comparable with the age of the Universe, confirming their relic nature of relic galaxies. We will also derive estimates of the metal content and velocity dispersion, and constraints on the Initial Mass Function, to test the two-phase scenario, and to understand the origin of such rare and peculiar objects, tracing the evolutionary history of the elliptical galaxies.

In the pilot paper (Spiniello et al. 2020), we present the discovery of the first two relic galaxies at 0.15 < z < 0.5. In Spiniello et al. (2021) we present the data for 19 relic candidates, confirming 10 of these as relic galaxies.

Visit the home-page of the INSPIRE project.

Our list of papers:

1. The INvestigate Stellar Population In RElics (INSPIRE) Project - Scientific Goals and Survey Design (Spiniello, Tortora et al. 2021, Msngr, 184, 26)
2. INSPIRE: INvestigating Stellar Population In RElics. II. First data release (DR1) (Spiniello, Tortora et al. 2021, A&A, 654, 136)
3. INSPIRE: INvestigating Stellar Population In RElics - I. Survey presentation and pilot program (Spiniello, Tortora et al. 2020, A&A, 646, 28)
4. Nature versus nurture: relic nature and environment of the most massive passive galaxies at z < 0.5 (Tortora et al. 2020, A&A, 638L, 11)
5. Building the Largest Spectroscopic Sample of Ultracompact Massive Galaxies with the Kilo Degree Survey (Scognamiglio et al. 2020, ApJ, 893, 4)
6. The first sample of spectroscopically confirmed ultra-compact massive galaxies in the Kilo Degree Survey (Tortora et al. 2018, MNRAS, 481, 4728)
7. Evolution of galaxy size-stellar mass relation from the Kilo-Degree Survey (Roy et al. 2018, MNRAS, 480, 1057)
8. Towards a census of supercompact massive galaxies in the Kilo Degree Survey (Tortora et al. 2016, MNRAS, 457, 2845)

Dark matter and Initial Mass Function

In the concordance cosmological model, structures form hierarchically (in a bottom-up fashion), starting from the amplification via gravitational instability of primordial small density fluctuations in the dark matter (DM). Within these potential wells, the gas is falling down, condensates and starts producing stars. These structures further evolve hierarchically, galaxy merging induces the size evolution that is observed in the most massive galaxies. It is therefore natural to aim at quantifying the amount of DM in galaxies. This task can be addressed when stellar population information are joined with dynamics and/or gravitational lensing.

I have provided a strong contribution to this field of research during the past years, deriving constraints on the central DM content, the total mass density slope and the Initial Mass Function (IMF).

We have analyzed DM fraction within the effective radius as a function of mass, effective radius, average galaxy density and galaxy age, studying the impact of IMF and total mass profile in both local (Tortora et al. 2009, 2012, 2013, 2014a, 2016; Napolitano, Romanowsky & Tortora 2010) and higher-z (Cardone et al. 2009, Cardone & Tortora 2010, Cardone et al. 2011, Tortora et al. 2010, 2014b, 2018) early-type galaxies (ETGs).

We have faced the problem of the IMF (Tortora et al. 2013, 2014a), constraining the stellar mass normalization using the Jeans equations. We show that the IMF is not universal in ETGs, and changes in terms of velocity dispersion and mass. And we also demonstrate that IMF is not universal if the gravity framework is changed (MOND, Tortora et al. 2014; Emergent gravity, Tortora et al. 2018). In Tortora et al. (2016) we have provided the first complete analysis of DM content and IMF for dwarf ellipticals.

In Tortora et al. (2014a) we have found that the slope of the total mass distribution in the central regions of ETGs is not universal and is varying with stellar mass, which may be related to a varying role of dissipation and galaxy mergers with galaxy mass (as pointed out by simulations, Remus et al. 2013). We already found some indication of such "non-homology" in the total mass density profile in Tortora et al. (2009), by comparing observations with N-body simulation predictions. Combining the results for massive ETGs and dwarf ellipticals, we show that DM fraction follows a U-shape trend with stellar mass, with a minimum at M_★ ≈ 3 × 10¹⁰ M_☉. The same mass scale is emerging from the analysis of colour gradients (Tortora et al. 2010) and total mass density slopes (Tortora et al. 2019). In Tortora et al. (2019) we also show that the total mass density slope of late-type galaxies is steepening as a function of mass. The difference of the slope in dwarf ellipticals and late-type galaxies can be understood by stellar feedback from a more prolonged star formation period in the latter systems, causing a transformation of the initial steep density cusp to a more shallow profile. These results are summarized in the figure on the right (from Tortora et al. 2019). By studying the evolution with redshift of the central DM content of very massive ETGs, in Tortora et al. (2014b, 2018), we show that ETGs are larger and more DM dominated at lower redshift (if the IMF is left fixed) and find that minor mergers can explain these results.

List of papers:

1. SEAGLE - III: Towards resolving the mismatch in the dark-matter fraction in early-type galaxies between simulations and observations (Mukherjee, S. et al. 2022, MNRAS, 509, 1245)
2. Central dark matter in early-type galaxies (Tortora, C. & Napolitano, N.R. 2021, Frontiers in Astronomy and Space Sciences, 8, 197)
3. The dark matter halo masses of elliptical galaxies as a function of observationally robust quantities (Sonnenfeld, A. et al. 2021, arXiv:211011966S)
4. The weak lensing radial acceleration relation: Constraining modified gravity and cold dark matter theories with KiDS-1000 (Brouwer M.M. et al., 2021, A&A, 650, 113)
5. The dichotomy of dark matter fraction and total mass density slope of galaxies over five dex in mass (Tortora et al. 2019, MNRAS, 489, 5483)
6. Testing Verlinde's emergent gravity in early-type galaxies (Tortora et al. 2018, MNRAS, 473, 2324)
7. The last 6 Gyr of dark matter assembly in massive galaxies from the Kilo Degree Survey (Tortora et al. 2018, MNRAS, 473, 969)
8. Dark matter and IMF normalization in Virgo dwarf early-type galaxies (Tortora et al. 2016, MNRAS, 455, 308)
9. Evolution of central dark matter of early-type galaxies up to z ~ 0.8 (Tortora et al. 2014, MNRAS, 445, 162)
10. Systematic variations of central mass density slopes in early-type galaxies (Tortora et al. 2014, MNRAS, 445, 115)
11. MOND and IMF variations in early-type galaxies from ATLAS^3D (Tortora et al. 2014, MNRAS, 438L, 46)
12. An Inventory of the Stellar Initial Mass Function in Early-type Galaxies (Tortora et al. 2013, ApJ, 765, 8)
13. SPIDER - VI. The central dark matter content of luminous early-type galaxies: Benchmark correlations with mass, structural parameters and environment (Tortora et al. 2012, MNRAS, 425, 577)
14. Secondary infall model and dark matter scaling relations in intermediate-redshift early-type galaxies (Cardone et al. 2011, MNRAS, 416, 1822)
15. Dark matter scaling relations in intermediate z haloes (Cardone & Tortora 2010, MNRAS, 409, 1570)
16. Central Dark Matter Trends in Early-type Galaxies from Strong Lensing, Dynamics, and Stellar Populations (Tortora et al. 2010, ApJ, 721, L1)
17. The central dark matter content of early-type galaxies: scaling relations and connections with star formation histories (Napolitano et al. 2009, MNRAS, 405, 2351)
18. The global mass-to-light ratio of SLACS lenses (Cardone et al. 2009, A&A, 504, 769)
19. Central mass-to-light ratios and dark matter fractions in early-type galaxies (Tortora et al. 2009, MNRAS, 396, 1132)

Color, stellar population and M/L gradients

The different processes which drive the galaxy evolution might rule the star formation at the global galaxy scale, or act at subgalactic scales (e.g. the nuclear regions versus outskirts) such that they are expected to introduce a gradient in the main stellar properties with the radius, that shall leave observational signatures in galaxy colours.

In Tortora et al. (2010) we have investigated colour and stellar population gradients for a sample of local SDSS galaxies. The colour (and metallicity) gradients of late-type galaxies (LTGs) decrease systematically with mass while the trend for early-type galaxies (ETGs) inverts near a mass of M_★ ∼ 3 × 10¹⁰ M_☉: systematically decreasing and increasing at lower and higher masses, respectively. The overall observational picture is interpreted in the context of differential feedback efficiency of supernovae at low masses and galaxy mergers, AGN feedback at large masses (see a cartoon in the picture on the right, where colour-mass plane is plotted, and main physical processes in place are shown). In Tortora et al. (2011), from the fitted synthetic spectra we have also derived the stellar mass-to-light (M/L) gradients and we have discussed the trends as a function of colour gradients and mass. We find that M/L gradients are tightly correlated with colour gradients, and generally follow patterns of variation with stellar mass and galaxy type found for colour and metallicty gradients. M/L gradients in LTGs would have a larger effect on dark matter inferences, while ETGs have, generally, very shallow gradients.

In Tortora & Napolitano (2012) we have investigated the role of the galaxy mergers in massive galaxies. For central galaxies in groups, we find that both optical colour and M/L gradients are shallower in central galaxies residing in denser environments. On the other hand, satellites do not show any differences in terms of the environment. In central galaxies, we show that the observed trends can be explained with the occurrence of dry mergings (more numerous in denser environments), which produce shallower colour gradients because of more uniform metallicity distributions due to the mixing of stellar populations.

The steepening of metallicity gradients with mass in low mass galaxies is also confirmed by the analysis of a sample of group and cluster satellite galaxies from a N-body-hydrodynamical simulation in Tortora et al (2011). Environment processes are important, we find that dwarf galaxies in clusters have steeper negative gradients with respect to the dwarfs in groups. Finally, Tortora et al (2013) have analyzed the evolution of colour gradients predicted by the hydrodynamical models of ETGs in Pipino et al. (2008), reproducing fairly well the chemical abundance pattern, the metallicity and colour gradients of ETGs at z < 1.

1. Colour gradients of high-redshift early-type galaxies from hydrodynamical monolithic models (Tortora et al 2013, MNRAS, 435, 786)
2. Stellar population gradients from cosmological simulations: dependence on mass and environment in local galaxies (Tortora et al 2011, MNRAS, 411, 627)
3. Population Gradients in the SDSS Galaxy Catalog. The role of merging (Tortora & Napolitano 2012, MNRAS, 421, 2478)
4. Stellar mass-to-light ratio gradients in galaxies: correlations with mass (Tortora et al. 2011, MNRAS, 418, 1557)
5. Colour and stellar population gradients in galaxies: correlation with mass (Tortora et al. 2010, MNRAS, 407, 144)

Alternatives to dark matter

Dark matter is one of the biggest puzzles in the modern astrophysics and cosmology, since it is thought to dominate the mass density of galaxies and clusters of galaxies in the Universe, but is elusive, interacting very weakly with visible matter and has not yet detected by any experiment. Thus, alternative ways to solve the missing mass problem have been suggested.

We have analyzed early-type galaxies (ETGs) within the MOdified Newtonian Dynamics (MOND) and the new revolutionary proposition by Verlinde (2016), i.e. Emergent Gravity. In Tortora et al. (2014) and Tortora et al. (2018), we show, for the first time, and with unique and systematic analyses, that within a modified gravity scenario, IMF is not universal, as in the standard gravity. I have also provided a contribution in the study of the weak-field limit of f(R) theories. In this regime, though an additional Yukawa term in the gravitational potential modifies dynamics with respect to the standard Newtonian limit of General Relativity, the motion of massless particles (e.g. photons) results unaffected thanks to suitable cancellations in the post-Newtonian limit (Lubini et al. 2011). We have also successfully fitted the observed velocity dispersion profiles of three elliptical galaxies with a Yukawa-like potential (Napolitano et al. 2012).

1. Testing Verlinde's emergent gravity in early-type galaxies (Tortora et al. 2018, MNRAS, 473, 2324)
2. MOND and IMF variations in early-type galaxies from ATLAS^3D (Tortora et al. 2014, MNRAS, 438L, 46)
3. Testing Yukawa-like Potentials from f(R)-gravity in Elliptical Galaxies (Napolitano et al. 2012, ApJ, 748, 87)
4. Probing the dark matter issue in f(R)-gravity via gravitational lensing (Lubini et al. 2011, EPJC, 71, 1834)
5. The modified Newtonian dynamics Fundamental Plane (Cardone et al. 2011, MNRAS, 412, 2617)

Baryon cycling

I have been recently involved in the PRIN-SKA project ESKAPE-HI, a national funded project which aims at exploring stellar and gas properties in galaxy populations from low to high redshift, in terms of mass and environment, and pave the way for SKA HI surveys. I am working on both predictions for HI detection in SKA and scaling relations at low- and high-redshift.

Metallicity and gas content are intimately related in the baryonic exchange cycle of galaxies. To quantify this relation and obtain information about physical processes shaping it, in Ginolfi et al. (2019) we have collected "MAGMA" (Metallicity And Gas for Mass Assembly), a sample of ∼400 local galaxies, which covers an unprecedented range in parameter space: it spans more that 5 orders of magnitudes in stellar mass, star formation rate and gas mass, and almost a factor 2 in metallicity. We find that the relations between M_★, SFR, Metallicity (Z) and M_gas (including HI and H2 gas) require only two dimensions to describe the hypersurface. In particular, to accommodate the curvature in the M_★-Z, we have applied a piecewise 3D PCA that successfully predicts observed metallicities to an accuracy of ∼0.07 dex. The break mass used in the piecewise PCA, i.e. M_break ∼ 2 × 10¹⁰ M_☉, is the value which minimizes the scatter. This value is similar to the characteristic mass which emerges from other observational probes (colour gradients, Tortora et al. 2010; dark matter fraction and mass density slope, Tortora et al. 2016, 2019). We also present a new relation to express M_gas as a linear combination of M_★ and SFR, to an accuracy of ∼0.2 dex.

In the second paper of the series (Hunt et al. 2020) we have investigated gas content and star formation efficiency, disentangling the contribution of atomic and molecular gas. We have focussed on the analysis of preventive feedback, i.e. the (lack of) availability of cold baryons from the host halo, and inefficiency of the star-formation process, the conversion of the available gas into stars. We confirm that atomic gas plays a key role in baryonic cycling, and is a fundamental ingredient for current and future star formation, especially in dwarf galaxies.

In Tortora et al. (2021), the third paper of the series, we have investigated ejective feedback, the production of energy and momentum, and the expulsion of material. In particular, we adopt a standard galactic chemical evolution model, with which we can quantify stellar-driven outflows. The resulting model reproduces very well the local mass-metallicity relation, and the observed trends of metallicity with gas fraction. Although the difference in mass loading between accreted and expelled gas is extremely difficult to constrain, we find indications that, on average, the amount of gas acquired through accretion is roughly the same as the gas lost through bulk stellar outflows, a condition roughly corresponding to a \gas equilibrium" scenario. In agreement with previous work, the wind metal-loading factor shows a steep increase toward lower mass and circular velocity, indicating that low-mass galaxies are more efficient at expelling metals, thus shaping the mass-metallicity relation

1. Scaling relations and baryonic cycling in local star-forming galaxies. III. Outflows, effective yields and metal loading factors (Tortora et al. 2021, arXiv:2110.06946)
2. Scaling relations and baryonic cycling in local star-forming galaxies. II. Gas content and star-formation efficiency (Hunt et al. 2020, A&A, 643, 180)
3. Scaling relations and baryonic cycling in local star-forming galaxies. I. The sample (Ginolfi et al. 2020, A&A, 638, 4)

A look at the magellanic clouds

In my research activities there are not only galaxies, but also star clusters and tidal structures in the local Universe.

1. We have recently characterized the structural parameters of 170 star clusters in the Small Magellanic Cloud (SMC, Gatto et al. 2021).
2. We have discovered YMCA-1, a low mass, low luminosity star cluster which is probably associated with the Large Magellanic Cloud (LMC), it is found to reside in a transition region of the plane luminosity vs. half-light radius, in between the ultrafaint dwarf galaxies and the classical old clusters (Gatto et al. 2021, Gatto et al. 2022).
3. We have demonstrated that KMHK 1762 is the third or fourth confirmed age gap star cluster in the LMC (Gatto et al. 2022).
4. Around the LMC, we have discovered the Northeast structure (NES), a new diffuse stellar substructure extending for more than 5 degree from the northeast border of the LMC (Gatto et al. 2022).