arXiv subjects

Sara L. Ellison

Publications and source records attributed to Sara L. Ellison.

At least 163 records · Page 9Linked to original sources

arXiv2013

A catalog of bulge, disk, and total stellar mass estimates for the Sloan Digital Sky Survey

We present a catalog of bulge, disk, and total stellar mass estimates for ~660,000 galaxies in the Legacy area of the Sloan Digital Sky Survey Data Release 7. These masses are based on a homogeneous catalog of g- and r-band photometry described by Simard et al. (2011), which we extend here with bulge+disk and Sersic profile photometric decompositions in the SDSS u, i, and z bands. We discuss the methodology used to derive stellar masses from these data via fitting to broadband spectral energy distributions (SEDs), and show that the typical statistical uncertainty on total, bulge, and disk stellar mass is ~0.15 dex. Despite relatively small formal uncertainties, we argue that SED modeling assumptions, including the choice of synthesis model, extinction law, initial mass function, and details of stellar evolution likely contribute an additional 60% systematic uncertainty in any mass estimate based on broadband SED fitting. We discuss several approaches for identifying genuine bulge+disk systems based on both their statistical likelihood and an analysis of their one-dimensional surface-brightness profiles, and include these metrics in the catalogs. Estimates of the total, bulge and disk stellar masses for both normal and dust-free models and their uncertainties are made publicly available here.

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arXiv2013

The Dynamics of Galaxy Pairs in a Cosmological Setting

We use the Millennium Simulation, and an abundance-matching framework, to investigate the dynamical behaviour of galaxy pairs embedded in a cosmological context. Our main galaxy-pair sample, selected to have separations under 250 kpc/h, consists of over 1.3 million pairs at redshift z = 0, with stellar masses greater than 10^9 Msun, probing mass ratios down to 1:1000. We use dark matter halo membership and energy to classify our galaxy pairs. In terms of halo membership, central-satellite pairs tend to be in isolation (in relation to external more massive galaxies), are energetically- bound to each other, and are also weakly-bound to a neighbouring massive galaxy. Satellite-satellite pairs, instead, inhabit regions in close proximity to a more massive galaxy, are energetically-unbound, and are often bound to that neighbour. We find that 60% of our paired galaxies are bound to both their companion and to a third external object. Moreover, only 9% of our pairs resemble the kind of systems described by idealised binary merger simulations in complete isolation. In sum, we demonstrate the importance of properly connecting galaxy pairs to the rest of the Universe.

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arXiv2013

Quasars Probing Quasars VI. Excess HI Absorption Within One Proper Mpc of z~2 Quasars

With close pairs of quasars at different redshifts, a background quasar sightline can be used to study a foreground quasar's environment in absorption. We use a sample of 650 projected quasar pairs to study the HI Lya absorption transverse to luminous, z~2 quasars at proper separations of 30kpc < R < 1Mpc. In contrast to measurements along the line-of-sight, regions transverse to quasars exhibit enhanced HI Lya absorption and a larger variance than the ambient intergalactic medium, with increasing absorption and variance toward smaller scales. Analysis of composite spectra reveals excess absorption characterized by a Lya equivalent width profile W = 2.3A (R/100kpc)^-0.46. We also observe a high (~60%) covering factor of strong, optically thick HI absorbers (HI column log NHI > 17.3) at separations R<200kpc, which decreases to ~20% at R~1Mpc, but still represents a significant excess over the cosmic average. This excess of optically thick absorption can be described by a quasar-absorber cross-correlation function xi_QA(r) = (r/r_0)^gamma with a large correlation length r_0 = 12.5+2.7-1.4 Mpc/h (comoving) and gamma = 1.68+0.14-0.30. The HI absorption measured around quasars exceeds that of any previously studied population, consistent with quasars being hosted by massive dark matter halos Mhalo~10^12.5 Msun at z~2.5. The environments of these massive halos are highly biased towards producing optically thick gas, and may even dominate the cosmic abundance of Lyman limit systems and hence the intergalactic opacity to ionizing photons at z~2.5. The anisotropic absorption around quasars implies the transverse direction is much less likely to be illuminated by ionizing radiation than the line-of-sight, which we interpret in terms of the same obscuration effects frequently invoked in unified models of active galactic nuclei.

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arXiv2013

Galaxy pairs in the Sloan Digital Sky Survey - VIII: The observational properties of post-merger galaxies

In order to investigate the effects of galaxy mergers throughout the interaction sequence, we present a study of 10,800 galaxies in close pairs and a smaller sample of 97 post-mergers identified in the Sloan Digital Sky Survey. We find that the average central star formation rate (SFR) enhancement (x 3.5) and the fraction of starbursts (20 per cent) peak in the post-merger sample. The post-mergers also show a stronger deficit in gas phase metallicity than the closest pairs, being more metal-poor than their control by -0.09 dex. Combined with the observed trends in SFR and the timescales predicted in merger simulations, we estimate that the post-mergers in our sample have undergone coalescence within the last few hundred Myr. In contrast with the incidence of star-forming galaxies, the frequency of active galactic nuclei (AGN) peaks in the post-mergers, outnumbering AGN in the control sample by a factor of 3.75. Moreover, amongst the galaxies that host an AGN, the black hole accretion rates in the closest pairs and post-mergers are higher by a factor of ~3 than AGN in the control sample. These results are consistent with a picture in which star formation is initiated early on in the encounter, with AGN activity peaking post-coalescence.

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arXiv2013

A Search for Boron in Damped Lyα Systems

We present the first systematic study of boron beyond the Local Group. This analysis is performed on a sample of 30 damped Lyα systems (DLAs) with strong metal-lines, which are expected to trace the interstellar medium of high z galaxies. We report on two boron detections at > 3σ significance; one new detection and one confirmation. The ratios of B/O and, for the first time, B/S are compared with previous stellar and interstellar measurements in the Milky Way and Small Magellanic Cloud. The novel comparison with sulphur, which tracks oxygen's abundance, alleviates the uncertainty associated with stellar oxygen measurements. For both detections, the inferred B/S ratio is in excess of the prediction of primary boron production from spallation processes. Possible sources of contamination are discussed, as well as physical effects that could impact the observed ratios. However taken at face value, the implication of these measurements suggest potentially higher cosmic ray fluxes in DLAs. The prospects for future boron detections in other high redshift DLAs to confirm our results is also discussed.

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arXiv2013

The Chandra Multi-Wavelength Project: Optical Spectroscopy and the Broadband Spectral Energy Distributions of X-ray Selected AGN

From optical spectroscopy of X-ray sources observed as part of ChaMP, we present redshifts and classifications for a total of 1569 Chandra sources from our targeted spectroscopic follow up using the FLWO, SAAO, WIYN, CTIO, KPNO, Magellan, MMT and Gemini telescopes, and from archival SDSS spectroscopy. We classify the optical counterparts as 50% BLAGN, 16% NELG, 14% ALG, and 20% stars. We detect QSOs out to z~5.5 and galaxies out to z~3. We have compiled extensive photometry from X-ray to radio bands. Together with our spectroscopic information, this enables us to derive detailed SEDs for our extragalactic sources. We fit a variety of templates to determine bolometric luminosities, and to constrain AGN and starburst components where both are present. While ~58% of X-ray Seyferts require a starburst event to fit observed photometry only 26% of the X-ray QSO population appear to have some kind of star formation contribution. This is significantly lower than for the Seyferts, especially if we take into account torus contamination at z>1 where the majority of our X-ray QSOs lie. In addition, we observe a rapid drop of the percentage of starburst contribution as X-ray luminosity increases. This is consistent with the quenching of star formation by powerful QSOs, as predicted by the merger model, or with a time lag between the peak of star formation and QSO activity. We have tested the hypothesis that there should be a strong connection between X-ray obscuration and star-formation but we do not find any association between X-ray column density and star formation rate both in the general population or the star-forming X-ray Seyferts. Our large compilation also allows us to report here the identification of 81 XBONG, 78 z>3 X-ray sources and 8 Type-2 QSO candidates. Also we have identified the highest redshift (z=5.4135) X-ray selected QSO with optical spectroscopy.

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arXiv2013

Galaxy pairs in the Sloan Digital Sky Survey - VI. The orbital extent of enhanced star formation in interacting galaxies

We use pair and environmental classifications of ~ 211,000 star-forming galaxies from the Sloan Digital Sky Survey, along with a suite of merger simulations, to investigate the enhancement of star formation as a function of separation in galaxy pairs. Using a new technique for distinguishing between the influence of nearby neighbours and larger scale environment, we find a clear enhancement in star formation out to projected separations of ~ 150 kpc, beyond which there is no net enhancement. We find the strongest enhancements at the smallest separations (especially < 20 kpc), consistent with earlier work. Similar trends are seen in the simulations, which indicate that the strongest enhancements are produced in highly disturbed systems approaching final coalescence, whereas the more modest enhancements seen at wider separations are the result of starburst activity triggered at first pericentre passage, which persists as the galaxies move to larger separations. The absence of any net enhancement beyond 150 kpc provides reassurance that the detected enhancements are due to galaxy-galaxy interactions, rather than larger scale environmental effects or potential pair selection biases. A rough census indicates that 66 per cent of the enhanced star formation in our pair sample occurs at separations > 30 kpc. We conclude that significant interaction-induced star formation is not restricted to merger remnants or galaxies with close companions; instead, a larger population of wider separation pairs exhibit enhanced star formation due to recent close encounters.

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arXiv2013

The Connection between Star Formation and Metallicity Evolution in Barred Spiral Galaxies

We have performed a series of chemodynamical simulations of barred disc galaxies. Our goal is to determine the physical processes responsible for the increase in the central gas-phase metallicity and of the central SFR observed in SDSS. We define a 2kpc diameter central aperture to approximate the integrated spectroscopic fibre measurements from the SDSS. The chemical evolution observed within this central region depends critically upon the relative size of the bar and the aperture, which evolves strongly with time. At t~0.5Gyr, a strong bar forms, whose length is considerably longer than the 2kpc aperture. The stars and gas lose angular momentum and follow elongated orbits that cause an intense mixing of the gas between the central region and its surroundings. During the next 1.5Gyr, the orbits of the gas contract significantly until the entire gas bar is contained in the 2kpc aperture, resulting in a net flux of gas into the central region. During this period, the metallicity in the central region increases steadily, and this enrichment is dominated by metal-rich gas that is flowing into the central region. The main result of this work is therefore that the observed enrichment in the centres of barred galaxies is not dominated by in-situ enrichment by stars formed in the centre. Rather, star formation occurs along the full length of the bar, much of which occurs initially outside the 2kpc aperture. About 50% of the metals that end up in the central region originate from this extended bar-long star formation, but flow into the central region due to loss of angular momentum. We conclude that there is no direct connection between central SFR and central metallicity. The central metallicity does not originate exclusively from central stars. Instead, the global SFR (especially along the bar) and the large-scale flow of enriched gas play a major role.

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arXiv2013

Galaxy pairs in the Sloan digital sky survey - VII: The merger -- luminous infra-red galaxy connection

We use a sample of 9397 low z galaxies with a close companion to investigate the connection between mergers and luminous infra-red (IR) galaxies (LIRGs). The pairs are selected from the SDSS and have projected separations rp < 80 kpc, relative velocities dv < 300 km/s and stellar mass ratios within a factor 1:10. The IR luminosities (LIR) of galaxies in the pair and (mass, z and environment-matched) control samples are determined from the SDSS -- IRAS matched catalog of Hwang et al. (2010). We find a trend for increasing LIRG fraction towards smaller pair separations, peaking at a factor of ~ 5--10 above the median control fraction at the smallest separations (rp < 20 kpc), but remaining elevated by a factor ~ 2--3 even out to 80 kpc (the widest separations in our sample). We also find that LIRGs are most likely to be found in high mass galaxies which have an approximately equal mass companion. We confirm the results of previous studies that both the active galactic nucleus (AGN) fraction and merger fraction increase strongly as a function of IR luminosity. About 7% of LIRGs are associated with major mergers, as defined within the criteria and mass completion of our sample. Finally, we quantify a SFR offset (Delta SFR) as the enhancement (or decrement) relative to star-forming galaxies of the same mass and redshift. We demonstrate that there is a clear connection between the Delta SFR and the classification of a galaxy as a LIRG that is mass dependent. Most of the LIRGs in our merger sample are relatively high mass galaxies (log M* > 10.5), likely because the SFR enhancement required to produce LIRG luminosities is more modest than at low masses. The Delta SFR offers a redshift-independent metric for the identification of the galaxies with the most enhanced star forming rates that does not rely on fixed LIR boundaries.

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arXiv2013

A search for HI 21cm absorption towards a radio-selected quasar sample II: a new low spin temperature DLA at high redshift

We report results from a deep search for redshifted HI 21cm absorption from eight damped Lyman-$α$ absorbers (DLAs) detected in our earlier optical survey for DLAs towards radio-loud quasars. HI 21cm absorption was detected from the $z = 2.192$ DLA towards TXS2039+187, only the sixth case of such a detection at $z > 2$, while upper limits on the HI 21cm optical depth were obtained in six other DLAs at $z > 2$. Our detection of HI 21cm absorption in the eighth system, at $z = 2.289$ towards TXS0311+430, has been reported earlier. We also present high spatial resolution images of the background quasars at frequencies close to the redshifted HI 21cm line frequency, allowing us to estimate the covering factor of each DLA, and then determine its spin temperature $T_s$. For three non-detections of HI 21cm absorption, we obtain strong lower limits on the spin temperature, $T_s \gtrsim 790$ K, similar to the bulk of the high-$z$ DLA population; three other DLAs yield weak lower limits, $T_s > 140-400$ K. However, for the two DLAs with detections of HI 21cm absorption, the derived spin temperatures are both low $T_s = (160 \pm 35) \times (f/0.35)$ K for the $z = 2.192$ DLA towards TXS2039+187 and $T_s = (72 \pm 18) \times (f/0.52)$ K for the $z = 2.289$ DLA towards TXS0311+430. These are the first two DLAs at $z > 1$ with $T_s$ values comparable to those obtained in local spiral galaxies. Based on the observed correlations between spin temperature and metallicity and velocity spread and metallicity in DLAs, we suggest that the hosts of the two absorbers are likely to be massive, high-metallicity galaxies.

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arXiv2012

Towards a physical picture of star-formation quenching: the photometric properties of recently-quenched galaxies in the Sloan Digital Sky Survey

We select a sample of young passive galaxies from the Sloan Digital Sky Survey Data Release 7 in order to study the processes that quench star formation in the local universe. Quenched galaxies are identified based on the contribution of A-type stars to their observed (central) spectra and relative lack of ongoing star formation; we find that such systems account for roughly 2.5 per cent of all galaxies with log M_sun >= 9.5, and have a space density of ~2.2x10^-4 Mpc^-3. We show that quenched galaxies span a range of morphologies, but that visual classifications suggest they are predominantly early-type systems. Their visual early-type classification is supported by quantitative structural measurements Sersic indices that show a notable lack of disk-dominated galaxies, suggesting that any morphological transformation associated with galaxies' transition from star-forming to passive--e.g. the formation of a stellar bulge--occurs contemporaneously with the decline of their star-formation activity. We show that there is no clear excess of optical AGN in quenched galaxies, suggesting that: i) AGN feedback is not associated with the majority of quenched systems or ii) that the observability of quenched galaxies is such that the quenching phase in general outlives any associated nuclear activity. Comparison with classical post-starburst galaxies shows that both populations show similar signatures of bulge growth, and we suggest that the defining characteristic of post-starburst galaxies is the efficiency of their bulge growth rather than a particular formation mechanism.

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arXiv2012

Galaxy Pairs in the Sloan Digital Sky Survey - V. Tracing changes in star formation rate and metallicity out to separations of 80 kpc

We present a sample of 1899 galaxies with a close companion taken from the SDSS DR7. The galaxy pairs are selected to have velocity differences < 300 km/s, projected separations (rp) < 80 kpc/h, mass ratios between 0.1 and 10, and robust measurements of star formation rates and gas-phase metallicities. We match the galaxies in total stellar mass, redshift, and local density to a set of 10 control galaxies per pair galaxy. For each pair galaxy we can therefore calculate the statistical change in star formation rate (SFR) and metallicity associated with the interaction process. Relative to the control sample, we find that galaxies in pairs show typical SFR enhancements that are, on average, 60% higher than the control sample at rp < 30 kpc/h. In addition, the pairs demonstrate more modest SFR enhancements of ~30% out to at least 80 kpc/h (the widest separations in our sample). Galaxies in both major and minor mergers show significant SFR enhancements at all rp, although the strongest starbursts (with SFR enhancements of a factor of ~10) appear to be found only in the major mergers. For the first time, we are also able to trace the metallicity changes in galaxy pairs as a function of projected separation. The metallicity is generally diluted in galaxy pairs by ~0.02 dex, with an average metallicity decrement of -0.03 dex at the smallest separations. The SFR and metallicity trends with projected separation are interpreted through a comparison with theoretical models. These simulations indicate that the peak in SFR enhancements at small separations is due to systems near the end of the merger process. The extended plateau in SFR enhancements out to at least 80 kpc/h is dominated by galaxies that have made a pericentric passage and are now experiencing triggered star formation on their trajectory towards apogalacticon, or on a subsequent close approach. (Abridged)

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arXiv2012

HI content, metallicities and spin temperatures of damped and sub-damped Lyman alpha systems in the redshift desert (0.6 < z < 1.7)

The HI 21cm absorption optical depth and the N(HI) derived from Lya absorption can be combined to yield the spin temperature (Ts) of DLAs. Although Ts measurements exist for samples of DLAs with z <0.6 and z >1.7, the intermediate redshift regime currently contains only 2 HI 21cm detections, leading to a `redshift desert' that spans 4 Gyrs of cosmic time. To connect the low and high z regimes, we present observations of the Lya line of six 0.6<z<1.7 HI 21cm absorbers. The dataset is complemented by both VLBA observations (to derive the absorber covering factor, f), and optical echelle spectra from which metal abundances are determined. Our dataset therefore not only offers the largest statistical study of HI 21cm absorbers to date, and bridges the redshift desert, but is also the first to use a fully f-corrected dataset to look for metallicity-based trends. In agreement with trends found in Galactic sightlines, we find that the lowest N(HI) absorbers tend to be dominated by warm gas. In the DLA regime, spin temperatures show a wider range of values than Galactic data, as may be expected in a heterogenous galactic population. However, we find that low metallicity DLAs are dominated by small cold gas fractions and only absorbers with relatively high metallicities exhibit significant fractions of cold gas. Using a compilation of HI 21cm absorbers which are selected to have f-corrected spin temperatures, we confirm an anti-correlation between metallicity and Ts at 3.4 sigma significance. Finally, one of the DLAs in our sample is a newly-discovered HI 21cm absorber (at z=0.602 towards J1431+3952), which we find to have the lowest f-corrected spin temperature yet reported in the literature: Ts=90+-23 K. The observed distribution of Ts and metallicities in DLAs and the implications for understanding the characteristics of the interstellar medium in high redshift galaxies are discussed.

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arXiv2012

The dependence of galaxy group star formation rates and metallicities on large scale environment

We construct a sample of 75,863 star forming galaxies with robust metallicity and star formation rate measurements from the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), from which we select a clean sample of compact group (CG) galaxies. The CGs are defined to be close configurations of at least 4 galaxies that are otherwise apparently isolated. Our selection results in a sample of 112 spectroscopically identified compact group galaxies, which can be further divided into groups that are either embedded within a larger structure, such as a cluster or large group, or truly isolated systems. The compact groups then serve as a probe into the influence of large scale environment on a galaxy's evolution, while keeping the local density fixed at high values. We find that the star formation rates (SFRs) of star forming galaxies in compact groups are significantly different between isolated and embedded systems. Galaxies in isolated systems show significantly enhanced SFR, relative to a control sample matched in mass and redshift, a trend not seen in the embedded systems. Galaxies in isolated systems exhibit a median SFR enhancement at fixed stellar mass of +0.07 \pm 0.03 dex. These dependences on large scale environment are small in magnitude relative to the apparent influence of local scale effects found in previous studies, but the significance of the difference in SFRs between our two samples constrains the effect of large scale environment to be non-zero. We find no significant change in the gas-phase interstellar metallicity for either the isolated or embedded compact group sample relative to their controls. However, simulated samples that include artificial offsets indicate that we are only sensitive to metallicity changes of log O/H >0.13 dex (at 99% confidence), which is considerably larger than the typical metallicity differences seen in previous environmental studies.

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arXiv2011

Galaxy pairs in the Sloan Digital Sky Survey - IV: Interactions trigger AGN

Galaxy-galaxy interactions are predicted to cause gas inflows leading to enhanced nuclear star formation. In this paper we test the further prediction that the gas inflows lead to enhanced accretion onto the central supermassive black hole, triggering activity in the nucleus. Based on a sample of 11,060 SDSS galaxies with a close companion (rp < 80 kpc, Delta V < 200 km/s), we classify AGN based either on emission line ratios or on spectral classification as a quasar. The AGN fraction in the close pairs sample is compared to a control sample of 110,600 mass- and redshift-matched control galaxies with no nearby companion. We find a clear increase in the AGN fraction in close pairs of galaxies with projected separations < 40 kpc by up to a factor of 2.5 relative to the control sample (although the enhancement depends on the chosen S/N cut of the sample). The increase in AGN fraction is strongest in equal mass galaxy pairings, and weakest in the lower mass component of an unequal mass pairing. The increased AGN fraction at small separations is accompanied by an enhancement in the number of `composite' galaxies whose spectra are the result of photoionization by both AGN and stars. Our results indicate that AGN activity occurs (at least in some cases) well before final coalescence and concurrently with ongoing star formation. Finally, we find a marked increase at small projected separations of the fraction of pairs in which BOTH galaxies harbour AGN. We demonstrate that the fraction of double AGN exceeds the expected random fraction, indicating that some pairs undergo correlated nuclear activity. Taken together with complimentary studies, we favour an interpretation where interactions trigger AGN, but are not the only cause of nuclear activity.

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arXiv2011

A Catalog of Bulge+Disk Decompositions and Updated Photometry for 1.12 Million Galaxies in the Sloan Digital Sky Survey

We perform two-dimensional, Point-Spread-Function-convolved, bulge+disk decompositions in the $g$ and $r$ bandpasses on a sample of 1,123,718 galaxies from the Legacy area of the Sloan Digital Sky Survey Data Release Seven. Four different decomposition procedures are investigated which make improvements to sky background determinations and object deblending over the standard SDSS procedures that lead to more robust structural parameters and integrated galaxy magnitudes and colors, especially in crowded environments. We use a set of science-based quality assurance metrics namely the disk luminosity-size relation, the galaxy color-magnitude diagram and the galaxy central (fiber) colors to show the robustness of our structural parameters. The best procedure utilizes simultaneous, two-bandpass decompositions. Bulge and disk photometric errors remain below 0.1 mag down to bulge and disk magnitudes of $g \simeq 19$ and $r \simeq 18.5$. We also use and compare three different galaxy fitting models: a pure Sersic model, a $n_b=4$ bulge + disk model and a Sersic (free $n_b$) bulge + disk model. The most appropriate model for a given galaxy is determined by the $F$-test probability. All three catalogs of measured structural parameters, rest-frame magnitudes and colors are publicly released here. These catalogs should provide an extensive comparison set for a wide range of observational and theoretical studies of galaxies.

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arXiv2011

The impact of gas inflows on star formation rates and metallicities in barred galaxies

The star formation rates (SFRs) and metallicities of a sample of 294 galaxies with visually classified, strong, large-scale bars are compared to a control sample of unbarred disk galaxies selected from the Sloan Digital Sky Survey Data Release 4. The fibre (inner few kpc) metallicities of barred galaxies are uniformly higher (at a given mass) than the unbarred sample by ~0.06 dex. However, the fibre SFRs of the visually classified barred galaxies are higher by about 60% only in the galaxies with total stellar mass log M > 10. The metal enhancement at log M<10 without an accompanying increase in the SFR may be due to a short-lived phase of early bar-triggered star formation in the past, compared to on-going star formation rate enhancements in higher mass barred galaxies. There is no correlation between bar length or bar axial ratio with the enhancement of the star formation rate. In order to assess the relative importance of star formation triggered by bars and galaxy-galaxy interactions, SFRs are also determined for a sample of close galaxy pairs. Both mechanisms appear to be similarly effective at triggering central star formation for galaxies with log M>10. However, due to the much lower fraction of pairs than bars, bars account for ~3.5 times more triggered central star formation than interactions.

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arXiv2011

Compact groups in theory and practice -- IV. The connection to large-scale structure

We investigate the properties of photometrically-selected compact groups (CGs) in the Sloan Digital Sky Survey. In this paper, the fourth in a series, we focus on understanding the characteristics of our observed CG sample with particular attention paid to quantifying and removing contamination from projected foreground or background galaxies. Based on a simple comparison of pairwise redshift likelihoods, we find that approximately half of compact groups in the parent sample contain one or more projected (interloping) members; our final clean sample contains 4566 galaxies in 1086 compact groups. We show that half of the remaining CGs are associated with rich groups (or clusters), i.e. they are embedded sub-structure. The other half have spatial distributions and number-density profiles consistent with the interpretation that they are either independently distributed structures within the field (i.e. they are isolated) or associated with relatively poor structures. Comparisons of late-type and red-sequence fractions in radial annuli show that galaxies around apparently isolated compact groups resemble the field population by 300 to 500 kpc from the group centre. In contrast, the galaxy population surrounding embedded compact groups appears to remain distinct from the field out beyond 1 to 2 Mpc, consistent with results for rich groups. We take this as additional evidence that the observed distinction between compact groups, i.e. isolated vs. embedded, is a separation between different host environments.

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