EDP Sciences
Free Access
Issue
A&A
Volume 581, September 2015
Article Number A82
Number of page(s) 8
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361/201526947
Published online 08 September 2015

© ESO, 2015

1. Introduction

Karachentseva et al. (2010) selected nine candidates for isolated dSphs in the Local Supercluster. Here we study three galaxies (Table 1) from this list. KKH 65 = BTS 23 was discovered by Binggeli et al. (1990) and rediscovered in radio observations by Karachentsev et al. (2001a). KK 180 and KK 227 were first found by Karachentseva & Karachentsev (1998).

As distinct from dwarf irregulars, dSphs tend to be close companions of bright giant galaxies. Many theoretical models explain the formation of early-type dwarf galaxies by interaction of their progenitors with massive neighbors and subsequent loss of gas (e.g., Einasto et al. 1974; Del Popolo 2012, and references therein). Isolated early-type dwarf galaxies are extremely rare within 10 Mpc (Karachentsev et al. 2001b, 2015; Makarov et al. 2012). Their formation mechanism is still poorly understood. Their observational properties are of particular interest. Isolated dSphs were not able to interact with massive neighbors during their lifetimes. Consequently, their existence is evidence in favor of different gas-loss factors, for example, interaction with other small galaxies, gaseous filaments in the intergalactic medium (Benítez-Llambay et al. 2013), or cooling and feedback processes in the early Universe during the epoch of reionization (Bovill & Ricotti 2009). N-body cosmological simulations (e.g., Klypin et al. 1999; Ricotti & Gnedin 2005; Klypin et al. 2014) predict ~30 times more massive isolated dwarf galaxies than are actually observed. dSphs with very low surface brightness (LSB) are good candidates for this missing population because they are barely detectable in the optical and radio bands.

In this paper, we report the results of extensive spectroscopic observations carried out with the Russian 6 m telescope. The observed data allowed us to identify neighbors of these three galaxies and estimate group membership distances and physical parameters for them: sizes, masses, and luminosities.

Table 1

Main data for the galaxies.

Table 2

Log of spectroscopic observations.

thumbnail Fig. 1

One-hour long-slit exposure of KK180, the brightest galaxy of the sample. The vertical axis indicates the position along the slit in arcseconds.

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2. Observations and data reduction

Spectroscopic observations were carried out in 2014 and 2015 with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences (SAO RAS). The SCORPIO spectrograph1 was mounted in the primary focus (Afanasiev & Moiseev 2005). Table 2 includes the date, exposure time, seeing, and heliocentric velocity correction for each galaxy. The atmospheric transparency was very good during the nights. Because of the extremely low brightness of the galaxies (see Fig. 1, Sect. 4), the total exposures for each object were long. The long slit was always 6× 1′′ and the field of view was 6× 6. The instrumental setup included the CCD detector EEV 42-40 with a pixel scale of 0.18 pixel-1 and the grism VPHG1200B with a resolution of FWHM ~ 5 Å and a reciprocal dispersion of 0.9 Å/pix in the spectral range of 3700–5500 Å. Every night, we obtained spectra of twilight, spectrophotometric standard stars, and radial velocity standard stars. The position angle of the slit was always nearly parallactic for all the galaxies except KK 227. Spectra reductions revealed the existence of a faint distant object seen through the stellar body of KK 227 with a spectrum resembling that of a quasi-stellar object (QSO; see Appendix C). Therefore, this dSph was reobserved in 2015 with a different position of the slit (Fig. C.1) to improve the signal-to-noise ratio (S/N) in the one-dimensional total spectrum of KK 227.

thumbnail Fig. 2

Variation of the measured velocity and instrumental velocity dispersion in the twilight spectrum as a function of wavelength.

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thumbnail Fig. 3

Integrated spectra of the stellar light of three dSphs (black) (top: KKH 65, middle: KK 180 and bottom: KK 227) in comparison with a composite model (green). The fitting was carried out using the ULySS program, PEGASE-HR SSP model, the ELODIE stellar library, and the LSF of the spectrograph.

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The standard spectral data reduction was performed using the European Southern Observatory Munich Image Data Analysis System (MIDAS; Banse et al. 1983) and the Image Reduction and Analysis Facility (IRAF) software system (Tody 1993). The accuracy of the wavelength calibration was ~0.16 Å.

3. Spectroscopic results

To derive radial velocities of our objects, we used the ULySS package (Koleva et al. 2008, 2009) with the PEGASE-HR model grids (Le Borgne et al. 2004), the Salpeter IMF (Salpeter 1955), and the ELODIE (Prugniel & Soubiran 2001) stellar library. To extract kinematic information from the spectra, we first determined the line spread function (LSF) of our spectrograph using the prescriptions of Koleva et al. (2008). The LSF was approximated by comparing a model spectrum of the Sun with the twilight spectra taken during the same night. The result of the comparison is shown in Fig. 2. It demonstrates the change with wavelength in the measured radial velocity and instrumental velocity dispersion. These dependences were taken into account when we computed radial velocities using ULySS. Figure 3 shows the results of the spectral fitting for the galaxies. One-dimensional total spectra of the galaxies have a quite low S/N per pixel in the middle of the spectral range: S/N ~ 20 for KK 180, ~10 for KKH 65, and ~8 for KK 227. Our spectra allowed us to derive radial velocities, metallicities, and approximate velocity dispersions of KKH 65, KK 180, and KK 227 (Table 1).

4. Surface photometry results

We performed surface photometry of the galaxies using Sloan Digital Sky Survey (SDSS) images in the g, r, and i bands and fitted their surface brightness profiles using the Sersic function (Sersic 1968). The results are shown in Figs. A.2A.4 and are summarized in Tables 1 (rows 46), 3, and A.1. Table 3 contains the following columns: (1) galaxy name; (2) apparent V magnitude (mag) and mean color (VI); (3) colors (BV) and (RI), (mag); (4) Sersic index n of the major-axis profile fit in the V band; (57) the corresponding V-band central SB, μ0s in mag arcsec-2), effective radius (arcsec), effective SB in mag arcsec-2 and apparent V magnitude integrated within the effective radius. The photometric data in Table 3 were corrected for Galactic extinction. The procedure of the data analysis is described in Appendix A.

Table 3

Surface photometry and Sersic function fitting results for the galaxies.

We estimated the Ks magnitudes of KKH 65, KK 180, and KK 227 using the transformations between the SDSS and 2MASS photometric systems (Bilir et al. 2011): Ks0 = g−1.907·(gr)−1.654·(ri)−0.684. The Ks luminosities were converted into stellar masses of the galaxies using MKs = 3.29 (Blanton & Roweis 2007) and (M/L)Ks = 0.9 in solar units (Maraston 2005).

5. Group membership distances of KKH 65, KK 180, and KK 227 and association with other galaxies

Identifying possible neighbors of the three dSphs was an important step of the work. Understanding the real isolation status of the galaxies and their physical properties requires information on their redshift-independent distances. To perform this task, we first searched for possible neighbors within a projected distance of ~500 kpc and with differences in radial velocities slower than ~500 km s-1. Next, we used the group-finding method by Makarov & Karachentsev (2011, hereafter MK11). The MK11 algorithm uses radial velocities of objects, projected distances between them, and their masses. To apply this method, we used the data obtained in our study for the three dSphs and the corresponding data for the surrounding galaxies extracted from the literature using the Aladin sky atlas (Bonnarel et al. 2000) and the HyperLEDA database (Makarov et al. 2014). We used the value of the Hubble constant: H0 = 73 km s-1 Mpc-1. Appendix B contains the detailed results of group member searches for our galaxies. Tables B.1 and B.2 list the data for the projected neighbors of KKH 65 and KK180.

6. Discussion and conclusions

The estimated metallicities and approximate velocity dispersions (Table 1) of KKH 65, KK 180, and KK 227 are similar of dSphs in the LV (e.g., Gritschneder & Lin 2013). In the following, we examine how typical the other derived parameters are.

KKH 65. The nearest bright galaxy to KKH 65 in projection to the sky is the peculiar S0-galaxy NGC 3414. The mean radial velocity of the NGC 3414 group with respect to the LG is VLG = 1298 ± 117 km s-1 (MK11). This value agrees well with the velocity of KKH 65 VLG = 1301 ± 50 km s-1 found by us. Thus, KKH 65 is a probable member of the group. We here adopted the distance to the NGC 3414 group measured by Tonry et al. (2001) using SB fluctuations: D = 25.2 Mpc. The corresponding projected separation between KKH 65 and NGC 3414 is 188 kpc, the value typical for dSphs (Karachentsev et al. 2005). At this distance, KKH 65 is larger and more luminous (MV = −15.08 mag, Re,V = 2.4 kpc) than dSph neighbors of our Galaxy (e.g., McConnachie 2012). The luminous mass of KKH 65 calculated using Kso = 15.3 mag (Sect. 4) is 9 × 107M.

KK 180. The nearest bright galaxy to KKH 65 in projection to the sky is the SBc galaxy UGC 8036. Its radial velocity with respect to the LG, VLG = 844 km s-1, is very close to the velocity of KK 180 VLG = 609 ± 40 km s-1 that we found. A small galaxy group around UGC 8036 resides in the outskirts of the Virgo cluster (e.g., Tully et al. 2008). We here adopted the distance to KK 180 equal to the distance to the Virgo cluster: D = 16.4 Mpc (Ferrarese et al. 2000). Based on this, KK 180 is located at a projected distance of 1.39 Mpc from M87, that is, within the virial radius of the Virgo cluster Rv = 1.8 Mpc (Hoffman et al. 1980). Its luminosity, effective radius, and mass are MV = −14.98 mag, Kso = 14.14 mag, Re,V = 1.6 kpc, MassKs = 1.1 × 108M. KK 180 is as luminous as KKH 65, but by 1.5 times more compact.

KK 227. This galaxy most likely belongs to the NGC 5371 group of 55 galaxies (MK11) according to its radial velocity and position in the sky (Table 1). The distance to the Sbc galaxy NGC 5371 was estimated using the Tully-Fisher relation by Tully & Trentham (2008): D = 29 Mpc. The radial velocity of NGC 5371 with respect to the center of the LG is VLG = 2640 km s-1 (van Driel et al. 2001). A mean velocity of the group is VLG = 2615 km s-1, and its dispersion is σV = 195 km s-1 (MK11). If we accept the group membership distance D ~ 29 Mpc for KK 227, then the projected separation between KK 227 and NGC 5371 is ~74 kpc, MV = −15.22 mag, Kso = 15.0 mag, Re,V = 2.75 kpc, and MassKs = 1.6 × 108M.

Colors, metallicities, and surface brightnesses of KH 65, KK 180, and KK 227 are similar to those of dSphs in the Local Group, but the sizes are larger. Objects with similar properties have recently been discovered in the Coma cluster (van Dokkum et al. 2015). Ultra-diffuse galaxies (UDGs) are large objects of extremely low density containing old stellar populations. They are characterized by the following parameters (van Dokkum et al. 2015): effective radii Reff = 1.5−4.6 kpc, absolute magnitudes −16.0 ≤ Mg ≤ −12.5, central surface brightnesses μg,0 = 24−26 mag arcsec-2, masses 1 × 107−3 × 108M, and colors gi ⟩ = 0.8 ± 0.1. The photometric data, masses, and metallicities of KKH 65 and KK 227 are in an excellent agreement with these values. Therefore, these two dSphs may be classified as UDGs. KK 180 is slightly brighter in the center than typical UDGs.

Our spectroscopic study and grouping analysis allow us to measure radial velocities of KKH 65, KK 180, and KK 227 and derive their group membership distances. We conclude that these three galaxies are non-isolated.

It is worth noting that seven of nine candidates for isolated dSphs from the list of Karachentseva et al. (2010) have been investigated up to now, including our three sample objects. Karachentsev et al. (2014) found out that KK 258 is a very isolated transitional-type LSB dwarf galaxy. Karachentsev measured radial velocity of KKH9 and confirmed it as a probable isolated dSph (priv. comm.). KKR8 was not resolved into stars on the HST images. It is much more distant than has been thought before (L. N. Makarova, priv. comm.). KKR9 is a Galactic cirrus. Therefore, only two of the seven studied objects are isolated galaxies. In this work we did not expand the list of known isolated dSphs. Objects of this class are extremely rare. More observational efforts are needed to establish the exact frequency of their occurrence in the Local Supercluster.


Acknowledgments

This work was performed with the support of the Russian Science Foundation grant No. 14-12-00965. We thank Dodonov S.N. for the technical support of our observations. We acknowledge the usage of the HyperLeda database (http://leda.univ-lyon1.fr). Funding for the SDSS and SDSS-II was provided by the Alfred P. Sloan Foundation, the Participating institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web Site is http://www.sdss.org

References

Online material

Appendix A: SDSS surface photometry of KKH 65, KK 180, and KK 227

The data reduction and analysis were conducted using the MIDAS package. First, all images were cleaned of all foreground and background objects. Then we used the SURFPHOT program from the MIDAS package to perform all the steps of the photometric procedure: the sky background subtraction, the subsequent ellipse fitting and integration of the light in the obtained ellipses. The algorithm of this software is based on the formulas of Bender (1987) and Bender & Moellenhoff (1987). The FIT/FLAT_SKY task was used to approximate the sky background by a surface created with a two-dimensional

polynomial and the least-squares method. All pixels of the residual image with values that differed by more than two sigma from the mean value of the sky were not used in the calculation of the background level with the FIT/BACKGROUND program. The typical accuracy of the sky brightness estimation was better than 1%. This corresponds to the level SB ~ 27–28 mag arcsec-2 in the B band. The ellipse fitting was carried out using the sky-subtracted images and the task FIT/ELL3. The INTEGRATE/ELLIPSE program was used to integrate the light in the successive ellipses. To transform SDSS magnitudes into the Johnson-Cousins system, we used the empirical color transformations by Jordi et al. (2006).

thumbnail Fig. A.1

SDSS images of the three dSphs KKH 65, KK 180, and KK 227 (from left to right).

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Table A.1

Surface photometry using SDSS images in the g, r, i bands and Sersic function fitting results for KKH 65, KK 180, and KK 227.

thumbnail Fig. A.2

Sersic function fits to the equivalent profiles of KKH 65 in the B, V, R, I bands.

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thumbnail Fig. A.3

Same as Fig. A.2, but for KK 180.

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thumbnail Fig. A.4

Same as Fig. A.3, but for KK 227.

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Appendix B: Members of the groups containing KKH 65 and KK180

Appendix B.1: NGC 3414 group

The algorithm developed by MK11 identified eight members of the NGC 3414 group. Using the same group-finding algorithm and new observational data (radial velocities from SDSS DR9 and spectroscopic and photometric data for KKH 65), we found 19 galaxies with differences in radial velocities smaller than 600 km s-1 and within the projected radius from NGC 3414 kpc (Table B.1). Eleven of them are members of the group with ii< 1, and eight objects are candidate field galaxies with ii> 1. Some of log (ii) values are close to zero. This means that the actual group size may change if accurate redshift-independent distances will be available. After selecting the group members, we compared the properties of KKH 65 and the surrounding galaxies. There are three candidate early-type group member galaxies fainter than KKH 65, and six candidate field early-type objects comparable to KKH 65 (Table B.1). SDSS images and our photometric study indicate that KKH 65 is the most diffuse among all these objects.

Table B.1 lists the data for the projected neighbors of KKH 65 with the differences in radial velocities between NGC 3414 and each individual galaxy that are smaller than 600 kms-1: (1) galaxy name; (2) equatorial coordinates; (3) morphological type in numerical code according to the RC3 catalog (de Vaucouleurs et al. 1991); (4) integrated magnitude in the Ks band, corrected for Galactic extinction; (5) radial velocity with respect to the centroid of the LG; (6) projected separation in kpc from NGC 3414 assuming that the galaxies are located at their Hubble distances; (7) logarithmic isolation index calculated with respect to NGC 3414 calculated according to MK11.

Appendix B.2: UGC 8036 group

Table B.2 lists galaxies located within the projected vicinity from KK 180. The results of the MK11 algorithm (Table B.2) favor an extremely weak gravitational influence between the UGC 8036 group members. At the same time, the gravitational attraction from the Virgo cluster is strong. (1) Galaxy name; (2) equatorial coordinates; (3) morphological type in numerical code according to de Vaucouleurs et al. (1991); (4) integrated magnitude in the Ks band; (5) radial velocity with respect to the centroid of the LG; (8) projected separation in kpc from UGC 8036; (9) isolation index with respect to UGC 8036; (10) projected separation in Mpc from M 87; (11) isolation index calculated with respect to the center of the Virgo cluster with the total mass of 8 × 1014M (Karachentsev et al. 2014). The projected separations are calculated assuming that the galaxies are at their Hubble distances.

Table B.1

S0 galaxy NGC 3414 and galaxies with differences in radial velocities smaller than 600 km s-1 and within the projected radius from NGC 3414 kpc.

Table B.2

Most massive galaxy in the Virgo cluster M 87 and six galaxies around KK 180 with a radial velocity difference smaller than km s-1 and within the projected radius from KK180 kpc.

Appendix C: QSO projected on KK 227

The long-slit observations revealed a distant extended object seen through the stellar body of KK 227 with a QSO-like spectrum. It was detected in both slit positions. We estimated its redshift z = 0.535 using only one clearly seen line MgII 2798 Å. The orientation of the slit in the case of KK 227 and the location of the quasar are shown in Fig. C.1.

thumbnail Fig. C.1

Left: settings of the slit on the preliminary short-exposure image of KK 227 in the B band. The approximate location of a quasar is circled. Right: one-dimensional total spectrum of the quasar after observations in the two positions of the slit.

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All Tables

Table 1

Main data for the galaxies.

Table 2

Log of spectroscopic observations.

Table 3

Surface photometry and Sersic function fitting results for the galaxies.

Table A.1

Surface photometry using SDSS images in the g, r, i bands and Sersic function fitting results for KKH 65, KK 180, and KK 227.

Table B.1

S0 galaxy NGC 3414 and galaxies with differences in radial velocities smaller than 600 km s-1 and within the projected radius from NGC 3414 kpc.

Table B.2

Most massive galaxy in the Virgo cluster M 87 and six galaxies around KK 180 with a radial velocity difference smaller than km s-1 and within the projected radius from KK180 kpc.

All Figures

thumbnail Fig. 1

One-hour long-slit exposure of KK180, the brightest galaxy of the sample. The vertical axis indicates the position along the slit in arcseconds.

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In the text
thumbnail Fig. 2

Variation of the measured velocity and instrumental velocity dispersion in the twilight spectrum as a function of wavelength.

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In the text
thumbnail Fig. 3

Integrated spectra of the stellar light of three dSphs (black) (top: KKH 65, middle: KK 180 and bottom: KK 227) in comparison with a composite model (green). The fitting was carried out using the ULySS program, PEGASE-HR SSP model, the ELODIE stellar library, and the LSF of the spectrograph.

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In the text
thumbnail Fig. A.1

SDSS images of the three dSphs KKH 65, KK 180, and KK 227 (from left to right).

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In the text
thumbnail Fig. A.2

Sersic function fits to the equivalent profiles of KKH 65 in the B, V, R, I bands.

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In the text
thumbnail Fig. A.3

Same as Fig. A.2, but for KK 180.

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In the text
thumbnail Fig. A.4

Same as Fig. A.3, but for KK 227.

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In the text
thumbnail Fig. C.1

Left: settings of the slit on the preliminary short-exposure image of KK 227 in the B band. The approximate location of a quasar is circled. Right: one-dimensional total spectrum of the quasar after observations in the two positions of the slit.

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In the text

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