Quiescent Early-type Galaxies Research Articles

The Origin of Star Formation in Early-type Galaxies Inferred from Spatially Resolved Spectroscopy

We investigate the origin of star formation activity in early-type galaxies with current star formation using spatially resolved spectroscopic data from the Mapping Nearby Galaxies at Apache Point Observatory in the Sloan Digital Sky Survey (SDSS). We first identify star-forming early-type galaxies from the SDSS sample, which are morphologically early-type but show current star formation activity in their optical spectra. We then construct comparison samples with different combinations of star formation activity and morphology, which include star-forming late-type galaxies, quiescent early-type galaxies, and quiescent late-type galaxies. Our analysis of the optical spectra reveals that the star-forming early-type galaxies have two distinctive episodes of star formation, which is similar to late-type galaxies but different from quiescent early-type galaxies with a single star formation episode. Star-forming early-type galaxies have properties in common with star-forming late-type galaxies, which include stellar population, gas and dust content, mass, and environment. However, the physical properties of star-forming early-type galaxies derived from spatially resolved spectroscopy differ from those of star-forming late-type galaxies in the sense that the gas in star-forming early-type galaxies is more concentrated than their stars, and is often kinematically misaligned with stars. The age gradient of star-forming early-type galaxies also differs from those of star-forming late-type galaxies. Our findings suggest that the current star formation in star-forming early-type galaxies has an external origin including galaxy mergers or accretion gas from the cosmic web.

Morphological Transformation and Star Formation Quenching of Massive Galaxies at 0.5 ≤ z ≤ 2.5 in 3D-HST/CANDELS

Abstract To figure out the effect of stellar mass and local environment on morphological transformation and star formation quenching in galaxies, we use the massive (M * ≥ 1010 M ⊙) galaxies at 0.5 ≤ z ≤ 2.5 in five fields of 3D-HST/CANDELS. Based on the UVJ diagnosis and the possibility of possessing a spheroid, our sample of massive galaxies is classified into four populations: quiescent early-type galaxies (qEs), quiescent late-type galaxies (qLs), star-forming early-type galaxies (sEs), and star-forming late-type galaxies (sLs). It is found that the quiescent fraction is significantly elevated at the high ends of mass and local environmental overdensity, which suggests a clear dependence of quenching on both mass and local environment. Over cosmic time, the mass dependence of galaxy quiescence decreases while the local environment dependence increases. The early-type fraction is found to be larger only at the high-mass end, indicating an evident mass dependence of morphological transformation. This mass dependence becomes more significant at lower redshifts. Among the four populations, the fraction of active galactic nuclei (AGNs) in the qLs peaks at 2 < z ≤ 2.5, and rapidly declines with cosmic time. The sEs are found to have higher AGN fractions of 20%–30% at 0.5 ≤ z < 2 . The redshift evolution of AGN fractions in the qLs and sEs suggests that AGN feedback could have played important roles in the formation of the qLs and sEs.

Early-type galaxies with recent star formation have low metallicities – evidence for star formation triggered by external processes?

ABSTRACTOn the basis of the Galaxy Evolution Explore (GALEX) ultraviolet (UV) data, many studies have demonstrated that recent star formation (RSF) is common in early-type galaxies. In particular, near-ultraviolet (NUV) light is used to investigate star formation activity in early-type galaxies, owing to its high sensitivity to the presence of young stars. This study characterized the stellar population properties of luminous early-type galaxies (Mr ≤ −20.65 mag) in the redshift range 0.005 ≤ z ≤ 0.09 on the basis of the Sloan Digital Sky Survey (SDSS) data. Initially, the most frequently used criterion of NUV $-\, r$ ≤ 5.4 was applied to identify early-type galaxies with RSF, and 19 per cent (172/913) of the sample galaxies met this criterion, in agreement with previous studies. A more robust sample of galaxies with RSF along with a stricter criterion (70 galaxies with NUV $-\, r$ ≤ 5.0) were then used for further analysis, and consequently 7.7 per cent (70/913; lower limit of the RSF fraction) of the sample galaxies were classified as early-type galaxies with RSF. These galaxies tended to exhibit higher H β absorption-line strengths and stronger emission lines than quiescent (QST, NUV $-\, r$ > 5.4) early-type galaxies. The most prominent feature of early-type galaxies with RSF identified in this study was that they were more metal-poor than QST galaxies owing to metal-poor stellar populations that were possibly formed from materials accreted from gas-rich satellites during (minor) mergers or interactions. The results strongly indicate that the observed RSF in the sample galaxies at the present epoch is mainly driven by external processes (i.e. mergers or interactions).

Are All Post-starbursts Mergers? HST Reveals Hidden Disturbances in the Majority of PSBs

Abstract How do galaxies transform from blue, star-forming spirals to red, quiescent early-type galaxies? To answer this question, we analyzed a set of 26 gas-rich, shocked post-starburst galaxies with Hubble Space Telescope (HST) imaging in B, I, and H bands and Sloan Digital Sky Survey (SDSS) i-band imaging of similar depth but lower resolution. We found that post-starbursts in our sample have intermediate morphologies between disk- and bulge-dominated (Sérsic n = 1.7 − 0.0 + 0.3 ) and have red bulges, likely due to dust obscuration in the cores. A majority of galaxies in our sample are more morphologically disturbed than regular galaxies (88%, corresponding to >3σ significance) when observed with HST, with asymmetry and Sérsic residual flux fraction being the most successful measures of disturbance. Most disturbances are undetected at the lower resolution of SDSS imaging. Although ∼27% galaxies are clear merger remnants, we found that disturbances in another ∼30% of the sample are internal, caused by small-scale perturbations or dust substructures rather than tidal features, and require high-resolution imaging to detect. We found 2.8σ evidence that asymmetry features fade on timescales ∼200 Myr, and may vanish entirely after ∼750 Myr, so we do not rule out a possible merger origin of all post-starbursts given that asymmetric features may have already faded. This work highlights the importance of small-scale disturbances, detected only in high-resolution imaging, in understanding structural evolution of transitioning galaxies.

Signatures of quenching in dwarf galaxies in local galaxy clusters

Context. The transformation of late-type galaxies has been suggested as the origin of early-type dwarf galaxies (typically M⋆ ≤ 109 M⊙) in galaxy clusters. Based on deep images, Venhola and colleagues analysed correlations between colour and surface brightness for galaxies in the Fornax cluster binned by luminosity or stellar mass. In the bins with M⋆ < 108 M⊙, the authors identified a correlation of redness with fainter surface brightness and interpreted it as a consequence of the quenching of star formation by ram pressure stripping in the dwarf galaxies. Aims. This study carries out a similar analysis for the Virgo cluster. The analysis for both clusters is then used to compare the Virgo and Fornax clusters, for which the ram pressure is expected to have different strengths. The purpose of this is to scrutinise the ram pressure interpretation from the other study and search for differences between the clusters that reflect the different ram pressure efficiencies, which would either support or weaken this interpretation. Ultimately, this could help weigh the importance of ram pressure stripping relative to other transformative processes in the shaping of the dominant early-type dwarf galaxy population. Methods. We extend the analysis of colour versus surface brightness binned by stellar mass to higher masses and a wider range of optical colours. The results, in particular at low stellar mass, are compared to predictions of stellar evolution models. Benefitting from larger sample sizes, we also analyse late- and early-type galaxies separately. This analysis is carried out for the Virgo and Fornax clusters, and the colour versus surface brightness relation, as well as other properties of the two clusters’ galaxy populations, are compared. Results. While the colour–surface brightness diagrams are remarkably similar for the two clusters, only the low-mass late-type galaxies are found to have slopes consistent with a fading and reddening following the quenching of star formation. For the early-type galaxies, there are no (or only weak) correlations between colour and surface brightness in all mass bins. Early- and late-type galaxies in both clusters have comparable sizes below a stellar mass of M⋆ ≲ 108 M⊙. The colour and size scaling relations are very similar for the Virgo and Fornax clusters. However, Virgo features a lower fraction of early-type or red galaxies despite its higher mass. Conclusions. The similarity of early-type dwarfs and low-mass late types in size at the masses M⋆ ≲ 108 M⊙ as well as the overall consistency of the colour–surface brightness correlation with fading stellar populations support a scenario of transformation via the quenching of star formation, for example by gas removal. However, the lack of this imprint of an ageing stellar population on the early-type dwarfs themselves calls for some additional explanation. Finally, the Virgo cluster is an atypical cluster with a comparably low fraction of quiescent early-type galaxies at all galaxy masses despite its large cluster mass.

Cool circumgalactic gas of passive galaxies from cosmological inflow

The circumgalactic medium (CGM) of galaxies consists of a multi-phase gas with components at very different temperatures, from 104 K to 107 K. One of the greatest puzzles about this medium is the presence of a large amount of low-temperature (T ∼ 104 K) gas around quiescent early-type galaxies (ETGs). Using semi-analytical parametric models, we describe the cool CGM around massive, low-redshift ETGs as the cosmological accretion of gas into their dark matter halos, resulting in an inflow of clouds from the external parts of the halos to the central galaxies. We compare our predictions with the observations of the COS-LRG collaboration. We find that inflow models can successfully reproduce the observed kinematics, the number of absorbers and the column densities of the cool gas. Our MCMC fit returns masses of the cool clouds of about 105 M⊙ and shows that they must evaporate during their journey due to hydrodynamic interactions with the hot gas. We conclude that the cool gas present in the halos of ETGs likely cannot reach the central regions and feed the galaxy star formation, thus explaining why these passive objects are no longer forming stars.

Sub-Eddington Supermassive Black Hole Activity in Fornax Early-type Galaxies

Abstract We characterize the incidence and intensity of low-level supermassive black hole activity within the Fornax cluster, through X-ray observations of the nuclei of 29 quiescent early-type galaxies. Using the Chandra X-ray Telescope, we target 17 galaxies from the Hubble Space Telescope Fornax Cluster Survey, down to a uniform (3σ) limiting X-ray luminosity threshold of 5 · 1038 erg s−1, which we combine with deeper, archival observations for an additional 12 galaxies. A nuclear X-ray point source is detected in 11 out of 29 targets. After accounting for the low-mass X-ray binary contamination to the nuclear X-ray signal, the X-ray active fraction is measured at 26.6% ± 9.6%. The results from this analysis are compared to similar investigations targeting quiescent early types in the Virgo cluster, as well as the field. After correcting for the different mass distributions, the measured Fornax active fraction is less than the field fraction, at more than 3σ, confirming that the funneling of gas to the nuclear regions of cluster members is inhibited compared to those galaxies in the field. At the same time, we find no statistically significant difference between Fornax and Virgo galaxies, with only marginal evidence for a lower active fraction in Fornax (1σ); if real, owing to Fornax's higher galaxy number density, this could indicate that galaxy–galaxy interactions are more effective at gas removal than galaxy-gas effects.

The unexpectedly large dust and gas content of quiescent galaxies at z > 1.4

Early type galaxies (ETG) contain most of the stars present in the local Universe and, above a stellar mass of ~5e10 Msun, vastly outnumber spiral galaxies like the Milky Way. These massive spheroidal galaxies have, in the present day, very little gas or dust, and their stellar populations have been evolving passively for over 10 billion years. The physical mechanisms that led to the termination of star formation in these galaxies and depletion of their interstellar medium remain largely conjectural. In particular, there are currently no direct measurements of the amount of residual gas that might be still present in newly quiescent spheroids at high redshift. Here we show that quiescent ETGs at z~1.8, close to their epoch of quenching, contained 2-3 orders of magnitude more dust at fixed stellar mass than local ETGs. This implies the presence of substantial amounts of gas (5-10%), which was however consumed less efficiently than in more active galaxies, probably due to their spheroidal morphology, and consistently with our simulations. This lower star formation efficiency, and an extended hot gas halo possibly maintained by persistent feedback from an active galactic nucleus (AGN), combine to keep ETGs mostly passive throughout cosmic time.

A massive, quiescent galaxy at a redshift of 3.717.

Finding massive galaxies that stopped forming stars in the early Universe presents an observational challenge because their rest-frame ultraviolet emission is negligible and they can only be reliably identified by extremely deep near-infrared surveys. These surveys have revealed the presence of massive, quiescent early-type galaxies appearing as early as redshift z ≈ 2, an epoch three billion years after the Big Bang. Their age and formation processes have now been explained by an improved generation of galaxy-formation models, in which they form rapidly at z ≈ 3-4, consistent with the typical masses and ages derived from their observations. Deeper surveys have reported evidence for populations of massive, quiescent galaxies at even higher redshifts and earlier times, using coarsely sampled photometry. However, these early, massive, quiescent galaxies are not predicted by the latest generation of theoretical models. Here we report the spectroscopic confirmation of one such galaxy at redshift z = 3.717, with a stellar mass of 1.7 × 1011 solar masses. We derive its age to be nearly half the age of the Universe at this redshift and the absorption line spectrum shows no current star formation. These observations demonstrate that the galaxy must have formed the majority of its stars quickly, within the first billion years of cosmic history in a short, extreme starburst. This ancestral starburst appears similar to those being found by submillimetre-wavelength surveys. The early formation of such massive systems implies that our picture of early galaxy assembly requires substantial revision.

SHOCKED POSTSTARBUST GALAXY SURVEY. I. CANDIDATE POST-STARBUST GALAXIES WITH EMISSION LINE RATIOS CONSISTENT WITH SHOCKS

ABSTRACT There are many mechanisms by which galaxies can transform from blue, star-forming spirals, to red, quiescent early-type galaxies, but our current census of them does not form a complete picture. Recent observations of nearby case studies have identified a population of galaxies that quench “quietly.” Traditional poststarburst searches seem to catch galaxies only after they have quenched and transformed, and thus miss any objects with additional ionization mechanisms exciting the remaining gas. The Shocked POststarburst Galaxy Survey (SPOGS) aims to identify transforming galaxies, in which the nebular lines are excited via shocks instead of through star formation processes. Utilizing the Oh-Sarzi-Schawinski-Yi (OSSY) measurements on the Sloan Digital Sky Survey Data Release 7 catalog, we applied Balmer absorption and shock boundary criteria to identify 1067 SPOG candidates (SPOGs*) within z = 0.2. SPOGs* represent 0.2% of the OSSY sample galaxies that exceed the continuum signal-to-noise cut (and 0.7% of the emission line galaxy sample). SPOGs* colors suggest that they are in an earlier phase of transition than OSSY galaxies that meet an “E+A” selection. SPOGs* have a 13% 1.4 GHz detection rate from the Faint Images of the Radio Sky at Twenty Centimeters Survey, higher than most other subsamples, and comparable only to low-ionization nuclear emission line region hosts, suggestive of the presence of active galactic nuclei (AGNs). SPOGs* also have stronger Na i D absorption than predicted from the stellar population, suggestive of cool gas being driven out in galactic winds. It appears that SPOGs* represent an earlier phase in galaxy transformation than traditionally selected poststarburst galaxies, and that a large proportion of SPOGs* also have properties consistent with disruption of their interstellar media, a key component to galaxy transformation. It is likely that many of the known pathways to transformation undergo a SPOG phase. Studying this sample of SPOGs* further, including their morphologies, AGN properties, and environments, has the potential for us to build a more complete picture of the initial conditions that can lead to a galaxy evolving.

UV-IR color profiles of the outer regions of 2K nearby S4G galaxies

AbstractWe present our new, spatially-resolved, photometry in FUV and NUV from images obtained by GALEX, and IRAC1 (3.6 μm) photometry obtained by the Spitzer Space Telescope. We analyzed the surface brightness profiles μFUV, μNUV, μ[3.6], as well as the radial evolution of the (FUV-NUV), (FUV - [3.6]), and (NUV - [3.6]) colors in the Spitzer Survey of Stellar Structures in Galaxies (S4G) galaxies (d < 40 Mpc) sample. We defined the GALEX Blue Sequence (GBS) and GALEX Red Sequence (GBR) from the (FUV - NUV) versus (NUV - [3.6]) color-color diagram, populated by late-type star forming galaxies and quiescent early-type galaxies respectively. While most disk becomes radially bluer for GBS galaxies, and stay constant for GRS galaxies, a large fraction ( > 50%) of intermediary GALEX Green Valley (GGV) galaxies’ outer disks are becoming redder. An outside-in quenching mechanism such as environmentally-related mechanisms such as starvation or ram-pressure-stripping could explain our results.

GLACE survey: OSIRIS/GTC tuneable filter Hαimaging of the rich galaxy cluster ZwCl 0024.0+1652 atz = 0.395

The cores of clusters at 0 $\lesssim$ z $\lesssim$ 1 are dominated by quiescent early-type galaxies, whereas the field is dominated by star-forming late-type ones. Galaxy properties, notably the star formation (SF) ability, are altered as they fall into overdense regions. The critical issues to understand this evolution are how the truncation of SF is connected to the morphological transformation and the responsible physical mechanism. The GaLAxy Cluster Evolution Survey (GLACE) is conducting a study on the variation of galaxy properties (SF, AGN, morphology) as a function of environment in a representative sample of clusters. A deep survey of emission line galaxies (ELG) is being performed, mapping a set of optical lines ([OII], [OIII], H$\beta$ and H$\alpha$/[NII]) in several clusters at z $\sim$ 0.40, 0.63 and 0.86. Using the Tunable Filters (TF) of OSIRIS/GTC, GLACE applies the technique of TF tomography: for each line, a set of images at different wavelengths are taken through the TF, to cover a rest frame velocity range of several thousands km/s. The first GLACE results target the H$\alpha$/[NII] lines in the cluster ZwCl 0024.0+1652 at z = 0.395 covering $\sim$ 2 $\times$ r$_{vir}$. We discuss the techniques devised to process the TF tomography observations to generate the catalogue of H$\alpha$ emitters of 174 unique cluster sources down to a SFR below 1 M$_{\odot}$/yr. The AGN population is discriminated using different diagnostics and found to be $\sim$ 37% of the ELG population. The median SFR is 1.4 M$_{\odot}$/yr. We have studied the spatial distribution of ELG, confirming the existence of two components in the redshift space. Finally, we have exploited the outstanding spectral resolution of the TF to estimate the cluster mass from ELG dynamics, finding M$_{200}$ = 4.1 $\times$ 10$^{14}$ M$_{\odot} h^{-1}$, in agreement with previous weak-lensing estimates.

Testing the modern merger hypothesis via the assembly of massive blue elliptical galaxies in the local Universe

The modern merger hypothesis offers a method of forming a new elliptical galaxy through merging two equal-mass, gas-rich disk galaxies fuelling a nuclear starburst followed by efficient quenching and dynamical stabilization. A key prediction of this scenario is a central concentration of young stars during the brief phase of morphological transformation from highly-disturbed remnant to new elliptical galaxy. To test this aspect of the merger hypothesis, we use integral field spectroscopy to track the stellar Balmer absorption and 4000\AA\ break strength indices as a function of galactic radius for 12 massive (${\rm M_{*}}\ge10^{10}{\rm M_{\odot}}$), nearby (${\rm z}\le0.03$), visually-selected plausible new ellipticals with blue-cloud optical colours and varying degrees of morphological peculiarities. We find that these index values and their radial dependence correlate with specific morphological features such that the most disturbed galaxies have the smallest 4000\AA\ break strengths and the largest Balmer absorption values. Overall, two-thirds of our sample are inconsistent with the predictions of the modern merger hypothesis. Of these eight, half exhibit signatures consistent with recent minor merger interactions. The other half have star formation histories similar to local, quiescent early-type galaxies. Of the remaining four galaxies, three have the strong morphological disturbances and star-forming optical colours consistent with being remnants of recent, gas-rich major mergers, but exhibit a weak, central burst consistent with forming $\sim5\%$ of their stars. The final galaxy possesses spectroscopic signatures of a strong, centrally-concentrated starburst and quiescent core optical colours indicative of recent quenching (i.e., a post-starburst signature) as prescribed by the modern merger hypothesis.

Early-type galaxy star formation histories in different environments

We use very high signal-to-noise ratio stacked spectra of ∼29 000 nearby quiescent early-type galaxies (ETGs) from the Sloan Digital Sky Survey to investigate variations in their star formation histories (SFHs) with environment at fixed position along and perpendicular to the Fundamental Plane. We define three classifications of local group environment based on the ‘identities’ of galaxies within their dark matter haloes: central ‘Brightest Group Galaxies’ (BGGs); Satellites; and Isolateds (those ‘most massive’ in a dark matter halo with no Satellites). We find that the SFHs of quiescent ETGs are almost entirely determined by their structural parameters σ and ΔIe. Any variation with local group environment at fixed structure is only slight: Satellites have the oldest stellar populations, 0.02 dex older than BGGs and 0.04 dex older than Isolateds; differences in Fe enrichment with local group environment are consistent with zero (<2σ); there are no differences in Mg enhancement between BGGs, Isolateds, and Satellites. Our observation that, to zeroth-order, the SFHs of quiescent ETGs are fully captured by their structures places important qualitative constraints on the degree to which late-time evolutionary processes (those which occur after a galaxy's initial formation and main star-forming lifetime) can alter their SFHs/structures.

Larger sizes of massive quiescent early-type galaxies in clusters than in the field at 0.8 &lt; z &lt; 1.5

[abridged] The mass-size relation of early-type galaxies (ETGs) has been largely studied in the last years to probe the mass assembly of the most massive objects in the Universe. In this paper, we focus on the mass-size relation of quiescent massive ETGs (Mstar/Msol > 3*10^10) living in massive clusters (M200 ~ 10^14 Mstar) at 0.8< z <1.5, as compared to those living in the field at the same epoch. Our sample contains ~ 400 ETGs in clusters and the same number in the field. Therefore, our sample is approximately an order of magnitude larger than previous studies in the same redshift range for galaxy clusters. We find that ETGs living in clusters are between ~30-50% larger than galaxies with the same stellar mass residing in the field. We parametrize the size using the mass-normalized size, gamma=Re/Mstar^0.57. The gamma distributions in both environments peak at the same position but the distributions in clusters are more skewed towards larger sizes. Since this size difference is not observed in the local Universe, the size evolution at fixed stellar mass from z~1.5 of cluster galaxies is less steep ((1+z)-0.53pm0.04) than the evolution of field galaxies ((1+z)-0.92pm0.04). The size difference seems to be essentially driven by the galaxies residing in the clusters cores (R<0.5*R200). If part of the size evolution is due to mergers, the difference we see between cluster and field galaxies could be due to higher merger rates in clusters at higher redshift, probably during the formation phase of the clusters when velocity dispersions are lower. We cannot exclude however that the difference is driven by newly quenched galaxies which are quenched more efficiently in clusters. The implications of these results for the hierarchical growth of ETGs will be discussed in a companion paper.

The environments of luminous radio galaxies and type-2 quasars

We present the results of a comparison between the environments of 1) a complete sample of 46 southern 2Jy radio galaxies at intermediate redshifts (0.05 < z < 0.7), 2) a complete sample of 20 radio-quiet type-2 quasars (0.3 < z < 0.41), and 3) a control sample of 107 quiescent early-type galaxies at 0.2 < z < 0.7 in the Extended Groth Strip (EGS). The environments have been quantified using angular clustering amplitudes (Bgq) derived from deep optical imaging data. Based on these comparisons, we discuss the role of the environment in the triggering of powerful radio-loud and radio-quiet quasars. When we compare the Bgq distributions of the type-2 quasars and quiescent early-type galaxies, we find no significant difference between them. This is consistent with the radio-quiet quasar phase being a short-lived but ubiquitous stage in the formation of all massive early-type galaxies. On the other hand, PRGs are in denser environments than the quiescent population, and this difference between distributions of Bgq is significant at the 3 sigma level. This result supports a physical origin of radio loudness, with high density gas environments favouring the transformation of AGN power into radio luminosity, or alternatively, affecting the properties of the supermassive black holes themselves. Finally, focussing on the radio-loud sources only, we find that the clustering of weak-line radio galaxies (WLRGs) is higher than the strong-line radio galaxies (SLRGs), constituting a 3 sigma result. 82% of the 2Jy WLRGs are in clusters, according to our definition (Bgq > 400) versus only 31% of the SLRGs.

Black hole wind speeds and the M − σ relation

We derive an M_bh-sigma relation between supermassive black hole mass and stellar velocity dispersion in galaxy bulges, that results from self-regulated, energy-conserving feedback. The relation is of the form M_bh v_w \propto sigma^5, where v_w is the velocity of the wind driven by the black hole. We take a sample of quiescent early-type galaxies and bulges with measured black hole masses and velocity dispersions and use our model to infer the wind speeds they would have had during an active phase. This approach, in effect, translates the scatter in the observed M_bh-sigma relation into a distribution of v_w. There are some remarkable similarities between the distribution of black hole wind speeds that we obtain and the distributions of outflow speeds observed in local AGN, including a comparable median of v_w = 0.035 c.

An X-Ray Detected Group of Quiescent Early-Type Galaxies at z = 1.6 in the Chandra Deep Field South

Abstract We report the discovery of an X-ray group of galaxies located at a high redshift of $ z$$ =$ 1.61 in the Chandra Deep Field South. Based on 4 Ms Chandra data, the group is first identified as an extended X-ray source. We have used a wealth of deep multi-wavelength data to identify the optical counterpart—our red sequence finder detects a significant over-density of galaxies at $ z$$ \sim$ 1.6. The brightest group galaxy is spectroscopically confirmed at $ z$$ =$ 1.61, based on published spectroscopic redshifts. Using this as a central redshift of the group, we measure an X-ray luminosity of $ L_{0.1{\rm-}2.4\rm keV}$$ =$ (1.8 $ \pm$ 0.6) $ \times$ 10$ ^{43}$ erg s$ ^{-1}$ , which then translates into a group mass of (3.2 $ \pm$ 0.8) $ \times$ 10$ ^{13}$$ M_{\odot}$ . This is the lowest-mass group ever confirmed at $ z$$ &amp;gt;$ 1.5. Deep optical–nearIR images from CANDELS reveal that the group exhibits a surprisingly prominent red sequence, and most of the galaxies are consistent with a formation redshift of $ z_{\rm f}$$ =$ 3. A detailed analysis of the spectral energy distributions of the group member candidates confirms that most of them are indeed passive galaxies. Furthermore, their structural parameters measured from near-IR CANDELS images show that they are morphologically early-type. The newly identified group at $ z$$ =$ 1.61 is dominated by quiescent early-type galaxies, and the group appears to be similar to those in the local Universe. One possible difference is the high fraction of AGN—38$ ^{+23}_{\hskip.4mm-20}$ % of the bright group member candidates are AGN, which might indicate a role for AGN in the quenching of star formation. However, a statistical sample of high-$ z$ groups is needed to draw a general picture of groups at this redshift. Such a sample will hopefully be available in near-future surveys.

The evolution of the mass–size relation for early-type galaxies from z ∼ 1 to the present: dependence on environment, mass range and detailed morphology

We study the dependence of the galaxy size evolution on morphology, stellar mass and large-scale environment for a sample of 298 group and 384 field quiescent early-type galaxies from the COSMOS survey, selected from z ∼ 1 to the present, and with masses log(M/M⊙) > 10.5. From a detailed morphological analysis we infer that ∼80 per cent of passive galaxies with mass log(M/M⊙) > 10.5 have an early-type morphology and that this fraction does not evolve over the last 6 Gyr. However, the relative abundance of lenticular and elliptical galaxies depends on stellar mass. Elliptical galaxies dominate only at the very high mass end – log(M/M⊙) > 11 – while S0 galaxies dominate at lower stellar masses – 10.5 < log(M/M⊙) < 11. The galaxy size growth depends on galaxy mass range and early-type galaxy morphology, e.g. elliptical galaxies evolve differently than lenticular galaxies. At the low-mass end – 10.5 < log(M/M⊙) < 11 – ellipticals do not show strong size growth from z ∼ 1 to the present (10 to 30 per cent depending on the morphological classification). On the other end, massive ellipticals – log(M/M⊙) > 11.2 – approximately doubled their size. Interestingly, lenticular galaxies display different behaviour: they appear more compact on average and they do show a size growth of ∼60 per cent since z = 1 independent of stellar mass range. We compare our results with state-of-the art semi-analytic models. While major and minor mergers can account for most of the galaxy size growth, we find that with present data and the theoretical uncertainties in the modelling we cannot state clear evidence favouring either merger or mass-loss via quasar and/or stellar winds as the primary mechanism driving the evolution. The galaxy mass–size relation and size growth do not depend on environment in the halo mass range explored in this work (field to group mass log(Mh/M⊙) < 14), i.e. group and field galaxies follow the same trends. At low redshift, where we examine both Sloan Digital Sky Survey and COSMOS groups, this result is at variance with predictions from some current hierarchical models that show a clear dependence of size growth on halo mass for massive ellipticals (log(M*/M⊙) > 11.2). In future work, we will analyse in detail if this result is specific of the observations and model used in this work. Brightest Cluster Galaxies (BCG) and satellite galaxies lie on the same mass–size relation, at variance with predictions from hierarchical models, which predict that BCGs should have larger sizes than satellites because they experience more mergers in groups over the halo mass range probed.

The importance of galaxy interactions in triggering type II quasar activity

We present deep Gemini Multi-Object Spectrograph-South optical broad-band images for a complete sample of 20 Sloan Digital Sky Survey selected type II quasars taken from Zakamska et al., with redshifts in the range 0.3 < z < 0.41 and [O iii]λ5007 emission-line luminosities . The images were taken with the aim of investigating the interaction status of the quasar host galaxies, in order to determine the significance of galaxy interactions in triggering nuclear activity. We find that 15 of our sample of 20 (75 per cent) show evidence for interaction in the form of tails, shells, fans, irregular features, amorphous haloes and double nuclei. The median surface brightness of the features is and the range is . We find a similar rate of interaction signatures in the type II quasars as in a comparison sample of quiescent early-type galaxies at similar redshift (67 per cent) taken from Ramos Almeida et al. (RA11). However the surface brightness of the detected features is up to 2 mag brighter for the type II quasars than for the quiescent early types, which have surface brightnesses in the range . Despite the relatively small sample size, this may indicate that the mergers witnessed in the comparison sample galaxies could have different progenitors, or we may be viewing the interactions at different stages. We also compare our results with those of Ramos Almeida et al. (RA12) who made a similar analysis using a complete sample of radio-loud active galactic nuclei (AGN). They find a higher rate of interaction signatures in the radio-loud AGN (95 per cent) than the type II quasars, but a very similar range of surface brightnesses for the morphological features , possibly indicating a similarity in the types of triggering interactions. The wide range of features detected in the type II quasar sample suggests that AGN activity can be triggered before, during or after the coalescence of the black holes, with six of the 20 objects (30 per cent) having double nuclei. Overall, the results presented here are consistent with the idea that galaxy interaction plays an important role in the triggering of quasar activity. We also use time-scale arguments to show that it is unlikely that most radio-quiet quasars cycle through a radio-loud phase as part of a single quasar triggering event.