Research

In a Nutshell:

Active galactic nuclei (AGN) diagnostics: X-ray, Infrared, Radio observations
Infrared & radio-driven star formation in galaxies
Wide-field VLBI techniques & surveys
Empirical modeling of black hole and galaxy growth
Dual AGN
Multi-wavelength spectral energy distributions of galaxies and AGN
Compton-Thick AGN
Survey science: (VLA-)COSMOS, XXL, EMU, MeerKAT, VLBA, multi-wavelength follow-up

Below is a detailed list of my main current and past research topics:

Unveiling the composite nature of the faint radio sky with the VLBA

I am leading a large VLBA programme (120 hr) within the COSMOS field, targeting >500 star-forming galaxies at 0.5<z<5 (red area in the Figure on the left). By scanning through each galaxy with ultra-deep (rms=3.7uJy/beam at 1.4 GHz) observations and extremely high angular resolution (0.01”), the main goal is to provide radio morphological information required to separate star formation and AGN emission within individual high-z galaxies. This study will be instrumental for taking a genuine census of star formation and AGN activity in the distant Universe.

VLBA observations are on-going, so...stay tuned!

Watch my recorded talk from the virtual conference The Past, Present, and Future of the VLA: Celebrating 40 Years

A new "infrared-radio correlation" of star-forming galaxies

Local star forming galaxies follow a strikingly tight (σ~0.25 dex) correlation between total infrared (rest-frame 8-1000 μm, L_IR) and (rest-frame 1.4 GHz, L_1.4) radio luminosity arising from star formation, usually referred to as "infrared-radio correlation" (IRRC) and parametrised via "q_IR", that is log(L_IR/L_1.4).

For the first time, in Delvecchio et al. (2021) we have explored how q_IR varies as a function of both stellar mass (M*) and redshift, starting from a M*-selected galaxy sample. Intriguingly, the q_IR evolves primarily with M*, with more massive galaxies displaying a systematically lower q_IR (i.e. brighter radio emission at fixed L_IR) than less massive counterparts. After correcting for radio-excess AGN contamination, this M* dependence remains significant, and supports a non-linearity of the IRRC across galaxies of different M*.

This study highlights that future ultra-deep radio surveys (i.e. with the SKA) will require M*-dependent recipes in order to convert radio detections into accurate L_IR, thus star formation rates.

The evolving (X-ray) AGN duty cycle since z~3

In Delvecchio et al. (2020) I have built a new modeling of the X-ray luminosity function (XLF) of active galactic nuclei (AGNs) out to z∼3, dissecting the contributions of main-sequence (MS, green) and starburst (SB, blue) galaxies. By matching the observed XLF, we find that the cosmic BHAR density (Figure on the left) is predominantly made by massive (10^10<M*<10^11 Msun) MS galaxies, while SB-driven BH accretion, possibly associated with galaxy mergers, accounts for about 20-30% of the global BHAR density, and it becomes dominant only in the brightest AGN, with:

log(Lx [erg/s]) > 44.36 + 1.28 × (1+z)

As confirmed by previous studies, the observed XLF can only be reproduced through an intrinsic flattening of the λ_edd distribution and with a positive shift of the break λ* (see Figure on the left) with increasing redshift, consistent with an anti-hierarchical behavior of the X-ray AGN population.

As shown on the left (circles), the probability of finding highly accreting (λ_edd>10%) AGNs significantly increases with redshift, from 0.4% (3.0%) at z=0.5, out to 6.5% (15.3%) at z=3 for MS (SB) galaxies, implying a longer (X-ray) AGN duty cycle towards the early universe.

SMBH-host scaling relations: mass dependence and evolution

Building up on the empirical modeling of the XLF, we were able to constrain that the BHAR/SFR ratio evolves positively with galaxy M*, in the form:

BHAR/SFR ~ M*^(0.73 [+0.22, -0.29])

This relation is valid for galaxies on the MS and does not significantly evolve since z~3.

Based on the above BHAR/SFR relation, in Delvecchio et al. (2019), we have built an empirically-motivated model that tracks SMBH and galaxy mass growth, assuming a random seed distribution of galaxies and BHs at z~10 that grows over time. In this formalism, we assumed that galaxies increase their M* along the MS relation.

Our simple recipe naturally describes the BH–galaxy buildup in two stages. At first, the SMBH lags behind the host that evolves along the MS. Later, as the galaxy grows in M*, we observe a superlinear BH growth, as M_BH ~ M*^(1.7). The Figure on the left shows the extrapolation of our empirical trend down to z~0 (coloured circles), which displays a good agreement with both model predictions and local intrinsic M_BH –M* scaling relations (e.g. Shankar et al. 2016).

We speculate that the observed nonlinear BH–galaxy buildup is reflected in a twofold behavior of BHAR/SFR with dark matter halo mass (M_DM), displaying a clear turnover at M_DM∼2×10^12 Msun.

A possible interpretation is that, while supernovae-driven feedback suppresses SMBH growth in smaller halos, above that M_DM threshold cold gas inflows might be fueling both SMBH accretion and star formation in a similar fashion.

Radio-excess AGN

Owing to the unprecedented sensitivity of the VLA-COSMOS 3-GHz Large Project, I identied a substantial population (~1300 objects, 20% of the radio sample) of jet-mode AGN candidates, being totally elusive in X-ray and mid-IR emission, but showing a strong excess (>4x) in radio light relative to their star formation. Studying such population of "radio-excess AGN" at different redshifts might help us shed light on the impact of jet-mode AGN feedback onto the evolution of galaxies.

Building on my radio-excess criterion, in Delvecchio et al. (2018) I studied how their average BHAR (measured from deep Chandra data) changes as a function of redshift and radio AGN power. This study revealed that the average BHAR of radio AGN at z>1.5 was 10x higher than in the local Universe, at fixed L_1.4, implying that radio AGN at z>1.5 were consistent with radiatively-efficient SMBHs (i.e. Eddington ratio λ_edd > 1%).

Interestingly, also the fraction of "blue" galaxies increases with redshift (at fixed L_1.4) in a strikingly similar fashion (Figure on the left) irrespective of whether a radio AGN is present or not. These results suggested that: (1) Radio AGN show enhanced SMBH accretion and star formation at z>1.5; (2) Radio AGN activity does not show clear evidence for "quenching" in high-z galaxies.

The VLA COSMOS 3-GHz Large Project

Radio emission can trace both on-going star formation and relativistic jets (from pc to Mpc scales) originated from SMBH accretion. The unprecedented sensitivity (rms~2.3uJy/beam at 3GHz) reached by the VLA-COSMOS 3-GHz Large Project (PI: V. Smolcic) has allowed us to study faint radio AGN activity in the distant Universe.

Relative fraction of radio-excess AGN vs star-forming galaxies in the VLA-COSMOS 3GHz Large Project data, as a function of 1.4 GHz flux (scaled from 3GHz). Sources were separated based on the dominant source of radio emission. Adapted from Novak et al. (2018).

I was deeply involved in the processing, analysis and release of all the core and value added data. These data were all made publicly available to the scientic community through a press release in an A&A special issue and broadcasted worldwide through the COSMOS webpage. In Delvecchio et al. (2017), I employed panchromatic diagnostics, including X-ray, mid-IR and radio selection criteria, to separate radio AGN from star-forming galaxies and understand the origin of their radio emission.

Mapping (X-ray) AGN activity across the SFR-M* plane

During my PhD, I spent 6 months at MPE (Garching), where I explored the correlation between SMBH accretion rate (BHAR) and star formation rate (SFR). While X-ray data offer a clean and unambiguous view of on-going SMBH accretion, short X-ray AGN variability (over a few years) might affect the (single-epoch) flux measurements, thus biasing the derived BHAR towards the brightest phases of the SMBH lifecycle. I tackled this issue from the opposite approach. In Delvecchio et al. (2015), I selected a large (>8,600) sample of star-forming galaxies with Herschel, and then I measured their average BHAR via stacking of deep Chandra imaging, thus smoothing possible BHAR fluctuations over galaxy timescales (>100 Myr).

This approach enabled me to unveil the (time-averaged) relationship between BHAR and SFR (at fixed stellar mass M* and redshift). The main result of this work was that a positive correlation between BHAR and SFR exists at least since z~2 . My findings supported a scenario of AGN-galaxy co-evolution in which AGN accretion and star formation broadly trace each other, possibly induced by a common gas fuelling mechanism.

(IR) AGN accretion history since z~3

I gained extensive experience in fitting and decomposing the broad-band spectral energy distributions (SEDs) of AGN and galaxies (see Figure above). In Delvecchio et al. (2014) I employed this approach for a sample of 4,500 Herschel-selected galaxies in the COSMOS and GOODS-South fields. This wealth of ancillary data allowed me to isolate the AGN emission relative to the global SED of each source. I constructed the Herschel-based AGN bolometric luminosity function, and inferred the dust-unbiased growth of active SMBH growth since z~3, for the first time from an infrared (IR) perspective (Figure on the left). My IR-based derivation showed a very good agreement with previous independent estimates based on X-ray data (see Figure on the left; e.g. Hopkins et al. 2007; Merloni & Heinz 2008; Ueda et al. 2014), displaying a common peak at z~2 (10 Gyr ago) and decreasing towards the local Universe. These findings demonstrated that an IR-based selection, combined with broad-band SED decomposition, can be a powerful dust-unbiased tool for exploring the hidden SMBH growth over cosmic time .

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