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ÈDensity map of the Virgo Cluster at a simulated distance of 200 Mpc. Darker regions are indicative of higher galaxy density. Contours appropriate to the various model clusters at 200 Mpc have been placed over the map, as described in ° 6.1. The locations of individual galaxies are indicated with a plus sign. Slight o †sets between galaxy positions and contour/density peaks are related to binning e †ects and are inconsequential to the Ðnal comparative density analysis.
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Using V-band imagery, we present results from a comparative density analysis of the local environments surrounding nine elliptical galaxies, with the objective of validating that these objects reside in extremely isolated environments. We have developed a technique that compares galaxy number density enhancements within well-studied clusters and lo...
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Citations
... Different definitions have been used in the literature for selecting IfEs (Aars, Marcum, & Fanelli 2001;Reda et al. 2004;Smith et al. 2004;Lacerna et al. 2016). The difference mainly occurs both in the selection of galaxies and in the isolation criteria. ...
... Even though the criteria used for detecting IfEs are diverse, the main idea remains the same, central galaxies with less crowded surroundings or with faint companion galaxies. Aars et al. (2001) identified nine IfE from the catalogue of Karachentseva (1973) and compared their environments with those of loose groups. They showed that half of their IfEs have similarities with loose groups in terms of environmental densities. ...
We present a catalogue of isolated field elliptical (IfE) galaxies drawn from the W1 field of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). 228 IfEs were identified from a flux-limited $(r<21.8)$ galaxy catalogue which corresponds to a density of 3 IfE/sq.deg. For comparison we consider a sample of elliptical galaxies living in dense environments, based on identification of the brightest cluster galaxies (BGCs) in the same survey. Using the same dataset for the comparison sample ensures a uniform selection, including in the redshift range as IfEs (i.e. $0.1 ). A comparison of elliptical galaxies in different environments reveals that IfEs and BCGs have similar behaviours in their colours, star formation activities, and scaling relations of mass–size and size–luminosity. IfEs and BCGs have similar slopes in the scaling relations with respect to cluster ellipticals within the $-24 \leq M_{r} \leq -22$ magnitude and $10.2< \textrm{log}(M_{*}/ \textrm M_\odot)\leq12.0$ mass ranges. Three IfEs identified in this study can be associated with fossil groups found in the same survey area which gives clues for future studies.
... Different definitions have been used in the literature for selecting IfEs (Aars et al., 2001;Reda et al., 2004;Smith et al., 2004;Lacerna et al., 2016). The difference mainly occurs both in the selection of galaxies and in the isolation criteria. ...
... Even though the criteria used for detecting IfEs are diverse, the main idea remains the same, central galaxies with less crowded surroundings or with faint companion galaxies. Aars et al. (2001) identified nine IfE from the catalogue of Karachentseva (1973) and compared their environments with those of loose groups. They showed that half of their IfEs have similarities with loose groups in terms of environmental densities. ...
We present a catalogue of isolated field elliptical (IfE) galaxies drawn from the W1 field of the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS). 228 IfEs were identified from a flux-limited (r<21.8) galaxy catalogue which corresponds to a density of 3 IfE/sq.deg. For comparison we consider a sample of elliptical galaxies living in dense environments, based on identification of the brightest cluster galaxies (BGCs) in the same survey. Using the same dataset for the comparison sample ensures a uniform selection, including in the redshift range as IfEs (i.e. 0.1 < z < 0.9). A comparison of elliptical galaxies in different environments reveals that IfEs and BCGs have similar behaviours in their colours, star formation activities, and scaling relations of mass-size and size-luminosity. IfEs and BCGs have similar slopes in the scaling relations with respect to cluster ellipticals within the $-24 \leq M_{r} \leq -22$ magnitude and $10.2< \textrm{log}( \textrm M_{*}/ \textrm M_\odot)\leq12.0$ mass ranges. Three IfEs identified in this study can be associated with fossil groups found in the same survey area which gives clues for future studies.
... The observed mean optical color of IEGs is bluer than that of group and cluster ETGs, likely due to higher star formation rates (Aars et al. 2001;Marcum et al. 2004). The observed differences in star formation rates of ETGs can primarily be attributed to their gas supply (Kauffmann 1996). ...
As part of an ongoing study of isolated early-type galaxies (IEG), we present neutral hydrogen (H i) observations of six IEGs obtained with the Green Bank Telescope. Two of the six IEGs presented in this paper have detected H i emission (KIG 870 and SDSS J102145.89+383249.8). KIG 870 has an H i emission profile that is strongly asymmetric about the optical systemic velocity with a redshifted double-horned profile and a blueshifted single-peaked component. KIG 870 is likely an advanced merger system. SDSS J102145.89+383249.8 has a Gaussian-like profile, indicating that the H i is not strongly rotating, is in a face-on disk, or is in a thick-disk similar to a dwarf galaxy. Our parent sample of H i observations is composed of 12 IEGs, 7 of which have now been detected in H i. The dwarf and luminous IEGs in our parent sample have median H i-mass-to-blue-luminosity ratios that are each three times larger than that of their non-cluster ETG counterparts, indicating that IEGs in our sample are significantly more gas rich than non-cluster ETGs.
... IEGs tend to have significantly bluer colors than typical ETGs (Aars et al. 2001;Marcum et al. 2004;Niemi et al. 2010). The blue colors suggest recent global star formation, indicating that the IEGs have gas reservoirs fueling the activity. ...
We present the results of single-dish atomic hydrogen (H i) observations of six highly isolated early-type galaxies. These objects are a representative subset of galaxies previously studied at optical wavelengths and selected to be separated by at least 2.5 Mpc from companions brighter than M V = −16.5 mag. Each galaxy was observed with a single pointing using the NRAO Green Bank Telescope L-band receiver. Five of these systems were strongly detected in H i. These five galaxies exhibit H i profiles with a range of properties: single Gaussian-like peaks, separate double peaks, and double horn-like profiles. The four bluest galaxies (B−V < 0.54) all contain significant gas with H i masses ranging from 1.1 × 10⁸ to 1.4 × 10⁹.
... Specifically, bright IEGs are rare within extreme low-density environments. The lack of bright IEGs separated by at least 2.5 Mpc from neighboring galaxies is supported by the study of Aars et al. (2001), for which only nine IEGs were identified, using a much larger search volume. The brightest isolated early-type systems are typically found to be isolated out to 1–1.5 Mpc (Kuntschner et al. 2002; Reda et al. 2004). ...
We describe the properties of a sample of extremely isolated early-type galaxies (IEGs) selected from the spectroscopic Sloan Digital Sky Survey. Sample galaxies are isolated from nearest neighbors more luminous than M{sub V} = -16.5 by a minimum distance corresponding to 2.5 Mpc and 350 km s{sup -1} in redshift space. The candidate IEGs exhibit a number of unusual features as compared to bulge-dominated galaxies in cluster and group environments, including fainter luminosities, blue colors suggesting possible recent star formation, and smaller physical sizes. The paper is the first in a series analyzing this isolated galaxy sample.
We report on the neutral hydrogen gas content (21-cm emission) of eight extremely isolated early-type galaxies (IEGs) using the Green Bank Telescope. Emission is detected in seven of the eight objects. This paper is the third in a series that collectively present new HI observations for 20 IEGs. Among the 14 HI detections in our observations, eight exhibit a Gaussian-like HI line profile shape, four are double-peaked, one is triple-peaked, and another has a plateaued rectangular shape. Five additional IEGs observed in previous surveys were added to our analysis, bringing the total number of IEGs with HI observations to 25. Of these objects, emission is detected in 19 (76%). The 25 IEGs in our combined study have gas masses that are systematically larger than their luminosity-matched comparison galaxy counterparts. The isolated early-type galaxies presented here follow a trend of increasing gas-richness with bluer B-V colors. This correlation is also observed in a comparison sample drawn from the literature composed of loose group and field early-type galaxies. Two IEGs, KIG164 and KIG870, exhibit properties highly anomalous for spheroidal systems: luminous (M_B = -20.5, -20.1) and blue (B-V = 0.47, 0.48) respectively, with substantial neutral gas, M_HI = 4.1 & 5.5x10^9 M_(sun). Other IEG systems may represent early-type galaxies continuing to assemble via quiescent HI accretion from the cosmic web or relaxed merged systems.
Isolated galaxies in low-density regions are significant in the sense that
they are least affected by the hierarchical pattern of galaxy growth, and
interactions with perturbers, at least for the last few Gyr. To form a
comprehensive picture of the star formation history of isolated galaxies, we
constructed a catalog of isolated galaxies and their comparison sample in
relatively denser environments. The galaxies are drawn from the SDSS DR7 in the
redshift range of $0.025<z<0.044$. We performed a visual inspection and
classified their morphology following the Hubble classification scheme. For the
spectroscopic study, we make use of the OSSY catalog. We confirm most of the
earlier understanding on isolated galaxies. The most remarkable additional
results are as follows. Isolated galaxies are dominantly late type with the
morphology distribution (E: S0: S: Irr) = (9.9: 11.3: 77.6: 1.2)\%. The
frequency of elliptical galaxies among isolated galaxies is only a third of
that of the comparison sample. Most of the photometric and spectroscopic
properties are surprisingly similar between isolated and comparison samples.
However, early-type isolated galaxies are less massive by 50\% and younger (by
H$\beta$) by 20\% than their counterparts in the comparison sample. This can be
explained as a result of different merger and star formation histories for
differing environments in the hierarchical merger paradigm. We provide an
on-line catalog for the list and properties of our sample galaxies.
We perform an exhaustive comparison among central galaxies from Sloan Digital Sky Survey catalogs in different local environments at 0.01 ≤ z ≤ 0.08. The central galaxies are separated into two categories: group centrals (host halos containing satellites) and field centrals (host halos without satellites). From the latter, we select two subsamples: isolated centrals and bright field centrals, both with the same magnitude limit. The stellar mass (M
s) distributions of the field and group central galaxies are different, which explains why in general the field central galaxies are mainly located in the blue cloud/star-forming regions, whereas the group central galaxies are strongly biased to the red sequence/passive regions. The isolated centrals occupy the same regions as the bright field centrals since both populations have similar M
s distributions. At parity of M
s, the color and specific star formation rate (sSFR) distributions of the samples are similar, especially between field and group centrals. Furthermore, we find that the stellar-to-halo mass (M
s-M
h) relation of isolated galaxies does not depend on the color, sSFR, and morphological type. For systems without satellites, the M
s-M
h relation steepens at high halo masses compared to group centrals, which is a consequence of assuming a one-to-one relation between group total stellar mass and halo mass. Under the same assumption, the scatter around the M
s-M
h relation of centrals with satellites increases with halo mass. Our results suggest that the mass growth of central galaxies is mostly driven by the halo mass, with environment and mergers playing a secondary role.
We have conducted a BVR imaging survey of nine early-type galaxies previously verified to exist in extremely isolated environments. Our goals are to establish a baseline of morphological and photometric properties for spheroidal systems evolving in extremely low-density environments and to compare these properties with signatures predicted for merged galaxy groups. We find that these isolated systems are underluminous by at least a magnitude compared with objects identified as merged group remnants in other studies. Image processing techniques sensitive to shell features produced no detections, a result in strong contrast to the high frequency of such structures found in other isolated elliptical galaxies. Two objects, KIG 164 and KIG 870, appear to be merger remnants, as indicated by their disturbed morphology, apparent tidal features, and blue colors. KIG 164 exhibits an asymmetric nuclear morphology and a low surface brightness "bridge" between it and a possible dwarf satellite. KIG 870 shows both fan-shaped emission at large radii and a possible double nucleus. Two other galaxies, KIG 412 and KIG 792, are also blue, but without any morphological peculiarities, suggesting that these systems are advanced mergers, older than KIG 164 and KIG 870. Two systems appear to be isolated lenticular galaxies with no evidence of a merger history. Based on their red colors, good fit to a R1/4-law light distribution, and the lack of morphological peculiarities, two other galaxies, KIG 557 and KIG 824, are found to be excellent candidates for passively evolving primordial elliptical galaxies formed early in cosmic time. Optical data were obtained with the 2.1 m Otto Struve telescope at McDonald Observatory, which is operated by the University of Texas at Austin.
Arp 104 is a pair of luminous interacting galaxies consisting of NGC 5216, an elliptical, and NGC 5218, a disturbed disk galaxy and joined by a stellar bridge. We obtained optical imaging to support photometric and color studies of the system. NGC 5216 lies on the red sequence, while the unusual distribution of stellar population properties in combination with intense central star formation in a dusty region result in NGC 5218 being a nearby example of an intermediate color (green valley) system. The stellar bridge has remarkably uniform optical surface brightness, with colors consistent with its stars coming from the outskirts of NGC 5218, but is relatively gas-poor while the northern tidal tail is rich in HI. While both galaxies contain shells, the shell structures in NGC 5218 are pronounced, and some appear to be associated with counter-rotating gas. This combination of features suggests that Arp 104 could be the product of distinct multiple interactions in a small galaxy group, possibly resulting from a hierarchical merging process, and likely leading to the birth of a relatively massive and isolated early-type galaxy. Comment: 20 pages, 6 figures and 1 table, accepted for publication in The Astrophysical Journal
