Chandra and ALMA observations of the nuclear activity in two strongly lensed star-forming galaxies

Abstract

Aims. According to coevolutionary scenarios, nuclear activity and star formation play relevant roles in the early stages of galaxy formation. We aim at identifying them in high redshift galaxies by exploiting high-resolution and sensitivity X-ray and millimetre-wavelength data to confirm the presence or absence of star formation and nuclear activity and their relative role in shaping their SEDs and contributing to their energy budget. Methods. In the current paper we present the data, model and analysis in the X-ray and mm bands for two strongly lensed galaxies, SDP.9 (HATLAS J090740.0-004200) and SDP.11 (HATLAS J091043.1-000322) that we selected in the Herschel-ATLAS catalogues as having an excess emission in the mid-IR regime at redshift > ∼ 1.5, suggesting the presence of a nuclear activity in the early stages of galaxy formation. We observed both of them with Chandra ACIS-S in the X-ray regime and analyzed the high resolution mm data available in the ALMA Science Archive for SDP9, and, by combining the information available in mm, optical and X-ray bands we reconstructed the source morphology.Results. Both the targets were detected in the X-ray, strongly indicating the presence of highly obscured nuclear activity. ALMA observations for SDP9 for continuum and CO(6-5) spectral line with high resolution (0.02arcsec corresponding to ∼ 65 pc at galaxy distance) allowed us to estimate the lensed galaxy redshift to a better accuracy than pre-ALMA estimates (1.5753±0.0003) and to model the emission of the optical, millimetric, and X-ray band emission for this galaxy. We demonstrated that the X-ray emission is generated in the nuclear environment and it strongly support the presence of nuclear activity in this object. On the basis of the X-ray data, we attempted an estimate of the BH properties in these galaxies. Conclusions. By taking advantage of the lensing magnification we identified weak nuclear activity associated with high-z galaxies with large star formation rates, useful to extend the investigation of the relationship between star formation and nuclear activity to two intrinsically less luminous, high-z star forming galaxies than was possible so far. Given our results only for two objects, they solely cannot constrain the evolutionary models, but provide us with interesting hints and set an observational path towards addressing the role of star formation and nuclear activity in forming galaxies.

The Dramatic Size and Kinematic Evolution of Massive Early-type Galaxies

“…Our expectations are consistent with the measured sizes of z ≈ 2 star-forming galaxies (see Barro et al 2016aBarro et al , 2017Hodge et al 2016;Genzel et al 2017;Tadaki et al 2017b;Massardi et al 2018;van Dokkum et al 2015;Talia et al 2018). Specifically, sizes inferred from near-IR/optical data (light blue symbols) are seen to be located in between R Q and R rot , while sizes inferred from mid/far-IR data (dark blue symbols) lie around and within R rot .…”

“…High-resolution, multi-band observations (e.g., Negrello et al 2014;Massardi et al 2018) of strongly lensed dusty star-forming galaxies have also highlighted a clear spatial segregation between the UV and far-IR emissions, with the latter being substantially more concentrated. Note that the extremely large dispersion in the data points for star-forming galaxies is in part spuriously due to this difference between near-IR and far-IR sizes; however, even when considering data with homogenous selection, the dispersion remains substantial, in agreement with our expectation regarding the scatter on R Q and R rot .…”

“…X-ray followup observations of dusty star-forming galaxies have revealed the growth of the central supermassive BH, before it attains a high enough mass and power to manifest as a quasar, to quench star formation and to evacuate gas and dust from the host (e.g., Alexander & Hickox 2012 and references therein; Mullaney et al 2012;Page et al 2012;Johnson et al 2013;Delvecchio et al 2015;Rodighiero et al 2015;Stanley et al 2015Stanley et al , 2017; intriguing correlations between the nuclear power and the host stellar mass and SFR have been established. Recently, even earlier stages in the growth of the BH have been revealed by targeted X-ray observations in a gravitationally lensed, far-IR selected galaxy at z ∼ 2 (Massardi et al 2018).…”

Resolving Star Formation on Subkiloparsec Scales in the High-redshift Galaxy SDP.11 Using Gravitational Lensing*

“…Additionally, the differences in the calculated star formation rates could indicate the presence of an AGN, which boosts the IR luminosity and artificially inflates the star formation rate derived from that quantity, while leaving the SFR calculated from the radio free-free emission, which is disentangled from the nonthermal AGN contribution, unaffected. Recently, X-Ray observations conducted with the Chandra X-Ray Observatory (Massardi et al 2018), and dense-gas tracers observed with ALMA (Oteo et al 2017), have both been detected to be co-spatial with the peak of the dust continuum, suggesting the presence of an AGN within SDP.11, making this scenario plausible.…”

CO, H2O, H2O+ line and dust emission in a z = 3.63 strongly lensed starburst merger at sub-kiloparsec scales

“…However, most of these studies are limited in spatial resolution and only investigate their globally averaged properties. Spatially resolved observations with angular resolution approaching the characteristic scales of star-forming regions are still rare (e.g., Swinbank et al 2015;Cañameras et al 2017;Dye et al 2018;Sharda et al 2018;Massardi et al 2018). In order to understand the detailed physical properties of high-redshift SMGs, especially their complex intrinsic structures, it is crucial to acquire high angular resolution images.…”