Signatures of Interstellar-Intracluster Medium Interactions: Spiral Galaxy Rotation Curves in Abell 2029
ABSTRACT We investigate the rich cluster Abell 2029 (z~0.08) using optical imaging and long-slit spectral observations of 52 disk galaxies distributed throughout the cluster field. No strong emission-line galaxies are present within ~400 kpc of the cluster center, a region largely dominated by the similarly-shaped X-ray and low surface brightness optical envelopes centered on the giant cD galaxy. However, two-thirds of the galaxies observed outside the cluster core exhibit line emission. H-alpha rotation curves of 14 cluster members are used in conjunction with a deep I band image to study the environmental dependence of the Tully-Fisher relation. The Tully-Fisher zero-point of Abell 2029 matches that of clusters at lower redshifts, although we do observe a relatively larger scatter about the Tully-Fisher relation. We do not observe any systematic variation in the data with projected distance to the cluster center: we see no environmental dependence of Tully-Fisher residuals, R-I color, H-alpha equivalent width, and the shape and extent of the rotation curves. Comment: 22 pages, 6 figures, 3 tables; to appear in the August 2000 Astronomical Journal
arXiv:astro-ph/0004148v1 11 Apr 2000
Signatures of Interstellar-Intracluster Medium Interactions:
Spiral Galaxy Rotation Curves in Abell 2029
DANIEL A. DALE
IPAC, California Institute of Technology 100-22, Pasadena, CA 91125
JUAN M. USON
National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903
We investigate the rich cluster Abell 2029 (z ∼ 0.08) using optical imaging
and long-slit spectral observations of 52 disk galaxies distributed throughout
the cluster field. No strong emission-line galaxies are present within ∼ 400 kpc
of the cluster center, a region largely dominated by the similarly-shaped X-ray
and low surface brightness optical envelopes centered on the giant cD galaxy.
However, two-thirds of the galaxies observed outside the cluster core exhibit line
emission. Hα rotation curves of 14 cluster members are used in conjunction with
a deep I band image to study the environmental dependence of the Tully-Fisher
relation. The Tully-Fisher zero-point of Abell 2029 matches that of clusters
at lower redshifts, although we do observe a relatively larger scatter about
the Tully-Fisher relation. We do not observe any systematic variation in the
data with projected distance to the cluster center: we see no environmental
dependence of Tully-Fisher residuals, R − I color, Hα equivalent width, and the
shape and extent of the rotation curves.
Subject headings: galaxies: clusters: individual (Abell 2029) — galaxies:
evolution — galaxies: intergalactic medium
The evolution of galaxies in clusters is affected by ram-pressure stripping, tidal
interactions, mergers, accretion, and cooling flows. These processes are expected to be
particularly effective in the richest clusters where they are likely to erase any memory
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of their initial conditions (Dressler 1984). A rich cluster typically has a conspicuous
intracluster medium and a regular, elliptical-dominated core (Sarazin 1986). The spiral
galaxies of a rich cluster are predominantly distributed in the periphery of the cluster, and
the closer a spiral disk is to the cluster center, the less likely it is to contain neutral hydrogen
gas (Giovanelli & Haynes 1985). Moreover, the frequency and strength of optical emission
lines are lower in cluster galaxies, as first suggested by Osterbrock (1960) and later verified
with large samples of field and cluster galaxies (Gisler 1978; Dressler et al. 1985; Balogh
et al. 1999). This trend has been shown to correlate with cluster-centric distance, and is
not solely due to morphological segregation (Balogh et al. 1997). This lack of interstellar
gas within cluster galaxies may be due to evaporation into the hotter intracluster gas, or it
may be attributed to stripping originating from either tidal galaxy-galaxy interactions or
ram pressure ablation on intracluster gas. Ram pressure ablation, which involves the loss
of interstellar gas due to rapid motion through intracluster gas, was first pointed out by
Gunn and Gott (1972) as the likely cause of mass loss of spiral galaxies in clusters, and
optical and 21 cm observations give direct evidence of this process (Haynes 1990; Kenney
& Koopman 1999). In fact, spiral galaxies that pass through the centers of rich clusters are
likely to loose up to 90% of their interstellar H I (Roberts & Haynes 1994). For example,
H I observations of the Virgo cluster and of Abell 2670 (located at a redshift of z ∼0.08
and considerably richer than Virgo) show them to be quite different. Indeed, the “stripping
radius” (the distance from the cluster center inside which spiral galaxies are H I deficient)
is two to three times larger in Abell 2670 than in the Virgo cluster (van Gorkom 1996).
Such dramatic environmental effects could affect a variety of observations. Whitmore,
Forbes, & Rubin (1988) showed that spiral galaxies within clusters exhibit falling rotation
curves, as opposed to the asymptotically flat or rising rotation curves usually seen in
galaxies located in the periphery of clusters as well as in the field; Adami et al. (1999) show
similar results for late-type spiral galaxies. Furthermore, they find that rotation curves
of cluster galaxies may be of lower amplitude than those of field galaxies. They offer the
explanation that the falling (and lower amplitude) rotation curves are due to mass loss—the
inner galaxies have had their dark matter halos stripped—or that the cluster environment
simply inhibits halo formation. They also find a monotonic increase in the mass to light
ratio with distance to the cluster center which they ascribe to the changing shape of the
rotation curves with cluster position. This view has been contested, however, by Amram et
al. (1993) and Vogt (1995) who find little evidence for any gradients in the outer portions of
rotation curves. Clusters, the peaks of the density hierarchy, have undergone strong merger
activity, both in terms of large-scale subclumps (Girardi et al. 1997) and at the galaxy
level, as in the formation of cDs. In short, there is a large body of work that suggests that
the spiral galaxy population in dense clusters is fundamentally different to spiral systems
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found in regions of lower density.
Abell 2029 is one of the densest and richest clusters in the Abell catalog of rich clusters
of galaxies and thus provides an important laboratory in which to study the effects of the
intracluster medium. It is located at a distance of ∼240h−1Mpc (we write the Hubble
constant in the form 100h km s−1Mpc−1), and extensive redshift studies have determined
its velocity dispersion to be ∼1500 km s−1(Dressler 1981; Bower, Ellis, & Efstathiou 1988).
In addition, it has been (re)classified as an Abell richness class 4.4 cluster (Dressler 1978).
The cluster is a textbook example of a compact, relaxed, cD galaxy-dominated cluster with
a high intracluster X-ray luminosity (1.1 ×1045h−2ergs s−1; David et al. 1993). The cD
galaxy is one of the largest galaxies known, with low surface brightness emission detected
out to a radius of 0.6h−1Mpc (Uson, Boughn, & Kuhn 1991).
We have obtained a large set of rotation curves of galaxies located in the field of
Abell 2029 in order to study the environmental effects due to the cluster by comparing
our data to the I band Tully-Fisher template relation for clusters obtained by Giovanelli
et al. (1997) and Dale et al. (1999; hereafter G97 and D99 respectively). This relation
was derived from the application of the Tully-Fisher relation to more than 1000 galaxies
located in 76 clusters, of which 75 are Abell richness class 2 or lower. Our observations are
described in Section 2 and the results are presented in Section 3. The implications of this
work are discussed in Section 4.
2. The Data
2.1. Optical Spectroscopy
We obtained long-slit spectroscopy to derive optical rotation curves of galaxies in
Abell 2029. The observations were carried out at the Mt. Palomar 5 m telescope during the
nights of 1998 April 27–29. We used the red camera of the Double Spectrograph (Oke and
Gunn 1982) to observe the Hα (6563˚ A), [N II] (6548, 6584˚ A), and [S II] (6717, 6731˚ A)
emission lines. The spatial scale of CCD21 (10242) was 0.′′468 pixel−1. The combination of
the 1200 lines mm−1grating and a 2′′wide slit yielded a dispersion of 0.65˚ A pixel−1and a
spectral resolution of 1.7˚ A (equivalent to 75 km s−1at 6800˚ A). The grating angle allowed
us to observe Hα in galaxies with recessional velocities between 7,600 and 38,100 km s−1.
We were fortunate to enjoy extremely mild atmospheric conditions at Mt. Palomar.
All three nights were photometric and dark. The seeing was remarkably sharper and more
stable than typically encountered at the site; we estimate the median seeing to have been
1′′, but at times the seeing dropped to 0.′′6. Such excellent spatial resolution is important
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to obtain high sensitivity rotation curves at the redshifts of the target galaxies. Besides
yielding higher signal-to-noise per pixel, a sharper seeing also allows a more accurate
placement of the slit. This is important because slit offsets and incorrect estimations of
the position angles of galaxy disks can lead to serious errors in the inferred velocity widths
(Bershady 1998, Giovanelli et al. 2000; hereafter G00). We did not obtain absolute flux
calibrations as they were not necessary for the purpose of this paper.
We used deep R and I band images to select candidate galaxies as well as to estimate
their position angles. We discuss these data in the next section. We observed all probable
disk-like systems on the reference images that might be members of the cluster, did not
appear to be face-on, and were free of contamination from foreground stars. The limited
resolution of the reference images precluded unambiguous identification of appropriate
Tully-Fisher candidates. Our observing strategy began with a five minute test-exposure on
each spectroscopic target. That way we were able to estimate “on the fly” the exposure
time required in order to sample adequately the outer disk regions. Furthermore, the test
exposure determined whether the galaxy was even useful to our work; a galaxy may lie in
the foreground or background of the cluster or it may contain little or no Hα emission. If
the observation was deemed useful, a second exposure typically ranged between 15 and
45 minutes. We detected line emission in half of the 52 observed galaxies. We list the
galaxies observed in Table 1, sorting the entries by Right Ascension. The table contains:
Col. 1: Identification names corresponding to a coding number in our database, referred to
as the Arecibo General Catalog.
Cols. 2 and 3: Right Ascension and Declination in the 1950.0 epoch. Coordinates have
been obtained from the Digitized Sky Survey catalog and are accurate to < 2′′.
Col. 4: The galaxy radial velocity as measured in the heliocentric reference frame. The
redshift measurements of the galaxies without emission lines were obtained from the NED1
database. They have been previously derived by others using absorption-line spectra.
Col. 5: An indication of the usefulness of the optical emission lines in order to apply the
Tully-Fisher relation: 0=no lines present; 1=strong emission lines throughout much of the
disk; 2=weak or nuclear emission only.
Rotation curves are extracted as discussed in Dale et al. (1997 and 1998; hereafter D97
and D98). We use the Hα emission line to map the rotation curve except in the case of
the galaxy AGC 251909 where the emission of the [N II] line (6584˚ A) extends to a larger
1The NASA/IPAC Extragalactic Database is operated by the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics and Space Administration.
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distance than that of the Hα emission. We center the rotation curve kinematically by
assigning the velocity nearest to the average of the 10% and 90% velocities to be at radius
zero, where an N% velocity is greater than N% of the velocity data points in the rotation
curve. The average of the 10% and 90% velocities is taken to be the galaxy’s recessional
velocity. We define the observed rotational velocity width to be Wobs≡ V90%− V10%. We
filled-in small portions of the Hα rotation curve of two galaxies (AGC 251913 and AGC
251912) using data from the [N II] rotation curve in order to provide information on the
shape of the inner parts and to ensure consistent estimates of Wobs.
The rotation curves in our sample vary in physical extent, and more importantly, they
do not all reach the optical radius, Ropt, the distance along the major axis to the isophote
containing 83% of the I band flux. This radius is reported by Persic & Salucci (1991) and
G00 to be the most useful radius at which to measure the velocity width of rotation curves.
We have extrapolated the rotation curves, and hence made adjustments to Wobs, when they
did not reach Ropt. The resulting correction, ∆sh, depends on the shape of the rotation
curve and only exceeded 4% for AGC 251831 where the correction was large (∼ 44%).
To recover the actual velocity widths, a few more corrections are necessary. The first is
the factor 1/sini to convert the width observed when a disk is inclined to the line of sight
at an angle i to what would be observed if the disk were edge–on, and the second is the
factor 1/(1+z) to correct the cosmological broadening of W. A final correction, fslit< 1.05,
accounts for the finite width of the slit of the spectrograph (G00). The corrected optical
rotational velocity width is
(1 + z)sinifslit. (1)
A discussion of the errors in the velocity widths can be found in D97.
Figure 1 is a display of the rotation curves observed in the field of Abell 2029. Entries
in the figure are sorted by Right Ascension. The name of the galaxy is given along with the
CMB radial velocity. Two dashed lines are drawn: the vertical line is at Ropt; the horizontal
line indicates the adopted half-velocity width, W/2, which in some cases arises from an
extrapolation to the rotation curve (see Table 1). Overlayed are the fits used to infer
W(Ropt). Details of the fitting procedure can be found in G00. The error bars include both
the uncertainty in the wavelength calibration and the routine used to fit the rotation curve.
Notice that the data are highly correlated due to seeing and guiding jitter. This is properly
taken into account by the fitting routines (see D97 and references therein for details).