Chemical Abundances in Virgo Spiral Galaxies II: Effects of Cluster

astro-ph/9511019 03 Nov 95 University of Arizona CHEMICAL Based on Received I I UCOLick Astronomy Department University of observations Steward Observatory University of EFFECTS ABUNDANCES Astronomy Department University of University of California and Observatory and the Smithsonian Church St S E Minneap olis obtained OF shieldsastroasutexasedu skillmanzonspaumnedu Rob ert dennislickucscedu robkasarizonaedu Gregory A Shields at CLUSTER Evan Dennis the Multiple Board C Kennicutt Jr D Skillman Institution and IN Zaritsky Santa of Astronomy and accepted VIRGO Arizona Tucson Mirror Texas Austin TX ENVIRONMENT Cruz CA MN Telescope Minnesota SPIRAL Astrophysics AZ a joint GALAXIES facility of the

ABSTRACT

We present new measurements of chemical abundances in H II regions in

spiral galaxies of the Virgo cluster and a comparison of Virgo galaxies and eld

spirals With these new data there now exist nine Virgo spirals with abundance

measurements for at least four H II regions Our sample of Virgo galaxies ranges

from H I decient ob jects near the core of the cluster to galaxies with normal H I

prop erties far from the cluster core We investigate the relationship b etween H I

disk characteristics and chemical abundances to determine whether dynamical

pro cesses that remove gas from the disk such as ram pressure stripping by the

intracluster medium also aect the chemical abundances

We divide the nine Virgo spirals into three groups of three galaxies each

those with strong H I deciencies intermediate cases and those with no H I

deciencies The three most H I decient Virgo spirals have larger mean

abundances to dex in OH than the spirals on the p eriphery of the

cluster This suggests that dynamical pro cesses asso ciated with the cluster

environment are more imp ortant than cluster membership in determining the

current chemical prop erties of spiral galaxies There is also weak evidence of

shallower abundance gradients in the H I decient Virgo spirals

We also compare the abundance prop erties of our Virgo sample to a

large sample of eld spirals studied by Zaritsky Kennicutt Huchra

Those authors found that the mean abundance of the disk gas increases with

increasing maximum circular velocity increasing luminosity and decreasing

earlier Hubble type but with a large disp ersion in the mean abundances

and abundance gradients The disp ersion in the prop erties of eld galaxies

and the small size of the Virgo sample make it dicult to draw denitive

conclusions ab out any systematic dierence b etween the eld and Virgo spirals

Nevertheless the H I decient Virgo galaxies have larger mean abundances than

eld galaxies of comparable luminosity or Hubble type while the spirals at the

p eriphery of the cluster are indistinguisha ble from the eld galaxies

Simple illustrative chemical evolution mo dels with infall of metalp o or gas

are constructed and compared to mo dels in which the infall is terminated The

mo dels are constrained by comparison with observed gas mass fractions current

star formation rates and gas consumption times The mo del results indicate

that the curtailment of infall of metalp o or gas onto cluster core spirals may

explain part of the enhanced abundance However additional work is needed

particularly mo deling of the eects of truncating the outer gaseous disk within

the context of mo dels with radial gas transp ort

The increased abundances in cluster core latetype spirals relative to eld

galaxies may b e imp ortant in the interpretation of observations of these

galaxies Sp ecically we p oint to p ossible eects on the TullyFisher and

Cepheid variable distance determinations and the interpretation of colors in the

ButcherOemler eect

Subject headings galaxies abundances galaxies clustering galaxies

evolution galaxies interstellar medium galaxies intergalactic medium

galaxies structure galaxies distances nebulae H II regions

INTRODUCTION

Spiral galaxies are mo died by the environment within a rich cluster of galaxies see

reviews by Haynes and Whitmore and the introduction to Henry et al

Such eects as tidal stripping ram pressure stripping and ablation by the intracluster

medium must aect galaxy evolution The cluster environment clearly inuences galaxy

morphology Dressler Postman Geller and so a natural question is whether it

also aects the star formation history and chemical evolution of galaxies Do es the cluster

environment predetermine galaxy morphology by solely inuencing initial conditions or

do es it continually inuence the evolution of a member galaxy Comparing the chemical

abundances of eld and cluster members may provide clues to these fundamental questions

Moreover a systematic abundance dierential b etween cluster and eld galaxies would have

implications for the stellar p opulations and could impact a number of currently p opular

distance indicators

The Virgo cluster due to its proximity provides the logical starting p oint for the study

of environmental eects on abundances in cluster spirals The Virgo cluster is irregular

and in some resp ects this compromises the study of environmental eects However the

extensive catalog of the p ositions morphologies and radial velocities of galaxies by

Binggeli Sandage Tammann allows a denition of the clusters structure In

addition the structure of the hot gas comp onent which has b een revealed through sensitive

mapping of the Xray emission made p ossible by ROSAT Boringer et al is very

similar to the galaxy distribution

Substantial evidence p oints to dierences b etween the spiral galaxies in the core of the

Virgo cluster and eld spiral galaxies Davies Lewis rst p ointed to a deciency in

H I gas in Virgo spirals when compared to eld spirals Giovanelli Haynes showed

that galaxies in the outer parts of the Virgo cluster and eight other clusters were less

decient in H I than the galaxies in the core of the cluster From H I synthesis observations

of Virgo cluster spirals Warmels concluded the H I deciencies could b e attributed

to a depletion of the H I at large galacto centric radii This was supp orted by Cayatte et al

CKBG

Because the star formation history of a galaxy is related to its gas content it is logical

to ask if the chemical abundances in the cluster spirals are dierent from those of eld

galaxies Shields Skillman Kennicutt SSK presented sp ectrophotometry of H II

regions in ve Virgo spirals The Virgo spirals app eared to b e considerably more metal

rich dex than the typical galaxy in their eld sample SSK suggested that such a

dierence could result from dierences in infall or radial inow rates Henry et al

HPLC and Henry Pagel Chincarini HPC conducted intensive studies of two

Virgo spirals and found the evidence of a Virgoeld dierential less convincing

Hence the issue of chemical abundance anomalies in the Virgo cluster is unresolved

The existing data sets have b een to o small to address the most fundamental physical

question whether the chemical prop erties of the H I decient galaxies in the core of the

Virgo cluster are dierent from those of undisturb ed galaxies in Virgo or elsewhere We

have undertaken a larger investigation which is aimed at addressing these problems in four

ways by enlarging the Virgo sample choosing galaxies within Virgo that span the

range of observed H I prop erties using published data on the H I distributions within

galaxies to examine the relationship b etween the H I distribution and chemical abundance

and by comparing the prop erties of the Virgo galaxies to a much larger sample of eld

spirals than was previously available

This study includes measurements of more H II regions in the strongly H I decient

galaxies NGC and NGC for which SSK had only one H II region each and in

several spirals with mo derate or no H I deciency Our work increases the number of Virgo

spirals with abundance measurements of four or more H II regions to nine For a comparison

eld sample we use the data for eld spirals recently published by Zaritsky Kennicutt

Huchra ZKH

In this pap er we describ e the observations and discuss the implications for the questions

raised ab ove In Section we discuss the sample selection new observations data reduction

pro cedures and OH abundance determinations In Section we discuss the abundance

prop erties of the Virgo spirals test for any dep endence of abundance prop erties on cluster

lo cation or H I deciency and use the ZKH results to determine whether there are any

systematic dierences b etween the eld and cluster galaxies In Section we characterize

and quantify the abundance dierential seen for the H I decient spiral galaxies In Section

we assess these results in the context of a simple mo del for environmental eects on

chemical evolution and in Section we outline future observational pro jects and discuss

the implications of elevated abundances for other studies of cluster spirals

OBSERVATIONS

Galaxy Selection

The primary ob jective of this program is to measure H II region abundances for a large

number of Virgo cluster spirals that span a larger range of H I deciency Following SSK

we chose to restrict our sample to latetype spirals those classied as type Sb c or later in

the Revised ShapleyAmes Catalog RSA Sandage Tammann This was done to

limit the variation in abundances due to variations in galaxy type Oey Kennicutt

OK ZKH and to take advantage of the brighter H II regions in latetype galaxies

We compiled a list of spirals in this type range that were mapp ed in H I by Warmels

or Cayatte et al We then selected a subset of galaxies exhibiting the full

range of H I prop erties from galaxies clearly decient in H I NGC NGC NGC

to intermediate ob jects NGC NGC NGC and nally to those

lo cated in the outskirts of the cluster and exhibiting apparently normal H I distributions

NGC NGC NGC By limiting our interest to Sb c and Sc galaxies our

sample excludes the most H I decient spirals closest to the Virgo core These ob jects

are earlytype spirals SaSb which generally contain much fainter H II regions than the

latetype spirals Kennicutt Edgar Ho dge OK Their H II regions are to o faint for

this program future observations of these ob jects will provide an imp ortant additional test

for environmentally induced changes in the chemical evolution of spirals Figure shows a

map of the p ositions of the Virgo cluster spirals relative to the main structural features

identied and discussed by Binggeli Tammann Sandage see their Figure

Table summarizes the prop erties of the Virgo sample Listed are the pro jected

separation from M radial velocity RSA and RC types de Vaucouleurs et al

luminosity rotation velocity as determined from the H I rotation curve optical diameter as

dened in the RC and H I to optical diameter ratio where the H I diameter is dened as

the diameter at which the H I surface density falls b elow M p c following Warmels

Warmels has argued that the latter ratio is a go o d indicator of the degree to which

H I has b een removed from the disk Following ZKH we have adopted a distance of

Mp c for the Virgo cluster galaxies Tully which is in go o d agreement with the

recently measured Cepheid distance of Mp c for M Freedman et al Figure

shows the variation of D D for a sample of eld spirals compiled by Warmels

H O

plotted as a function of RC Ttype the error bars indicate the disp ersion among eld

spirals of the same type The corresp onding values for the Virgo spirals in our sample are

plotted as p oints and lab eled This plot demonstrates the large range in D D for a given

H O

Ttype in the eld sample the general tendency for smaller H I diameters among Virgo

spirals as a whole and the variation in D D within the Virgo sample

H O

The radial H I surface proles of the Virgo spirals are shown in Figure The p oints

represent the azimuthally averaged H I surface densities from Warmels and CKBG

and for NGC from van der Hulst et al while the solid lines represent the

mean distributions for eld spirals of the same morphological type from CKBG CKBG

demonstrate that the radial H I surface density distribution of a eld spiral galaxy is

determined by its morphological type if it is normalized to the optical size of the galaxy

We have normalized the observed proles of the Virgo spirals in the same manner using

the D diameters from the RC Figure conrms the variation in H I deciency within

the Virgo sample esp ecially at the extremes eg compare NGC and NGC

with NGC We have ordered the panels in Figure in increasing D D relative

H O

to the mean for the appropriate Ttype with the result that the galaxies group into the

deciency groups discussed ab ove This sub division into three groups is in general

agreement with an indep endent classication by CKBG

Sp ectrophotometry

Sp ectrophotometry for H II regions in the Virgo spirals was obtained in April

and May using the Blue Channel Sp ectrograph on the Multiple Mirror Telescope

MMT The H II regions were identied from H images obtained by RCK using a fo cal

reducer CCD imager on the Steward Observatory m telescop e The images also provided

precise co ordinates and nuclear osets for the sp ectrophotometric observations Preference

in selection was given to bright H II regions that sample the greatest p ossible radial extent

within each galaxy

The observing setup at the MMT was identical to that describ ed in SSK The

sp ectrograph was used with the channel element intensied Reticon photon

counting detector When used with a gpm grating A longpass blo cking lter and

00 00

ap erture it provided a wavelength coverage of A at approximately A

FWHM resolution Ob ject and sky ap ertures were b eamswitched every sec and total

integration times ranged from to sec dep ending on the brightness of the ob ject

and sky conditions Ob jects were observed at high zenith angles to avoid problems with

dierential refraction Observing conditions ranged from clear to thin clouds and b ecause

of the variable conditions and small ap erture sizes only relative line uxes are rep orted here

The sp ectra were calibrated using observations of standard stars from Filipp enko

Greenstein and Massey et al as well as the usual internal wavelength and

ateld lamp calibrations Fluxes and equivalent widths for the principal sp ectral lines

were then measured using the SPLOT package in IRAF We conrmed that the SPLOT

measurements agreed with the automated measuring techniques of ZKH Line strength

uncertainties were determined from the rms deviations in the adjacent continuum In

some cases the sky ap erture fell within the galaxy so the the ob ject sp ectrum constructed

by subtracting o the sky sp ectrum is contaminated This can articially increase the

emission line equivalent widths and the uncertainties in the line strengths and this eect

has not b een accounted for in our error estimates The Balmer lines were corrected for

underlying stellar absorption with an assumed constant equivalent width of A McCall

Rybski Shields MRS and then the reddening was determined from the relative

Balmer line strengths assuming the values for an electron temp erature of K and an

electron density of cm Bro cklehurst All line strengths were then corrected for

reddening using the Galactic reddening law compiled by Seaton as parameterized

by Howarth The resultant line strengths and asso ciated uncertainties are listed in

IRAF is distributed by the National Optical Astronomy Observatories which is op erated

by the Asso ciation of Universities for Research in Astronomy Inc AURA under co op erative

agreement with the National Science Foundation

Table

Abundance Determinations

The new sp ectra of H II regions rep orted here when combined with previously

measured sp ectra from SSK MRS HPLC and HPC provide data for a total of H II

regions in the Virgo galaxies listed in Table Because accurate measurements of the

electron temp erature are not available for the regions we have used the oxygen excitation

index R O II O III H I H to estimate the

nebular abundances Edmunds Pagel

To ensure consistency among the abundances measured for the various eld galaxies

and the ZKH eld galaxy sample we have applied the same empirical R abundance

calibration as used by ZKH for all of the Virgo cluster H II regions The ZKH calibration

is an average of three calibrations by Edmunds Pagel MRS and Dopita

Evans Since we are mainly interested in dierential comparisons of the abundance

prop erties of dierent ob jects the precise choice of the R calibration is not critical and

as argued by ZKH the use of three separate calibrations provides an explicit although

incomplete estimate of the error in the derived oxygen abundances Typically these

calibrations are based on directly measured electron temp eratures in the low abundance

regime and photoionizatio n mo del ts for high abundances low excitation Presumably

the uncertainties are greatest at the high abundance end see discussions in OK Henry

and Shields Kennicutt which applies to most of the Virgo cluster H II regions

In any case the derived abundance is monotonic in R over the excitation range of interest

here and it should b e meaningful at the very least as a relative abundance sequencing

parameter

Table lists the abundances derived in this way for the entire Virgo H II region sample

The OH abundances listed in Table show two asso ciated errors The rst error is

the result of propagating the formal errors of the emission line uxes into the empirical

calibration of R by ZKH The second set of errors in parentheses reect the uncertainty

of the R calibration adopted by ZKH In most cases the latter are signicantly larger

than the former Following ZKH we use these larger errors throughout our analysis unless

noted otherwise The derived abundances are plotted as a function of galacto centric radius

normalized to the eective radius following McCall in Figure The ordering of the

galaxies is identical to that used in Figure

Following ZKH we have tted a linear relation in log OH versus radius to the

abundances of the individual H II regions to determine the mean value of OH at a

ducial radius Like ZKH we use a fraction of the isophotal radius R as the ducial

radius ZKH found this preferable b ecause it is less sensitive than other choices to various

selection eects see discussion in ZKH The results from these ts are given in Table

and plotted in Figure Hereafter we will refer to the mean value as that taken at

R The errors in the mean abundances were determined in the standard manner for a

weighted linear leastsquares t where the weighting was inversely prop ortional to the

formal errors in R the rst set of errors in Table and normalized to give Chisquared

equal to one This prevents the uncertainties in the R calibration from propagating into

the uncertainty in the the mean abundance ie we are interested in the uncertainty in the

abundance dierences b etween galaxies and not the absolute uncertainty in the abundance

scale For calculating the gradients the second set of errors in Table the larger ones

were used since the uncertainty in the absolute calibration is relevant to the uncertainty in

the magnitude of the gradient

Note that the outermost H II region in NGC lies well ab ove the trend established

by the other H II regions As a result the calculated mean abundance is slightly higher and

the abundance gradient is slightly shallower than would result from a determination based

on the inner H II regions At this time we have no reason to drop this discrepant p oint

from the t or to choose a more complex tting function From a comparison of the results

in SSK HPLC and HPC it is quite p ossible for an error in the abundance of a single H II

region of dex and an error of this size would bring the outermost H II region in NGC

into line with the trend established in the inner galaxy However if the inection in

the abundance distribution in NGC is real then this galaxy deserves a more detailed

abundance study

THE EFFECT OF H I DEPLETION ON THE ABUNDANCES IN VIRGO

SPIRALS

General Trends in the Virgo Spirals

We b egin our analysis with a qualitative discussion of the abundances in the Virgo

galaxies based on a comparison of Figures and We fo cus on the question of whether

the H I decient Virgo galaxies have dierent abundance prop erties than the Virgo galaxies

with normal H I disks Figures and are generally consistent as to which galaxies are

decient in H I although the D D parameter is a limited indicator NGC NGC

H O

and NGC are clearly decient the latter two even at small radii NGC

NGC and NGC are outlying galaxies with normal H I disks The remaining

three galaxies are intermediate cases Comparing the H I data with the abundance patterns

in Figure we note that the p eripheral galaxies have strong radial gradients in OH and

reach rather low values log OH at the largest radii In contrast the three

most H I decient galaxies show high abundances with little evidence for radial gradients

but note that the radial range of H II regions is very limited in the H I decient spirals

This is further illustrated in Figure which shows the characteristic OH values

and gradients from Table for the Virgo galaxies as a function of D D In the top

H O

panel of Figure a trend of decreasing OH with increasing D D is evident for the

H O

Virgo galaxies The top half of Figure shows that our grouping into three categories is

somewhat arbitrary as all nine Virgo spirals display a continuum of decreasing mean OH

with decreasing H I deciency

The b ottom panel of Figure illustrates the relationship b etween oxygen abundance

gradient and H I deciency Here we tentatively see weak evidence for a trend of stronger

gradient with decreasing H I deciency The most notable exception is NGC but as

noted ab ove a single linear t to the radial abundance trend in NGC is a p o or t

The evidence for a trend in abundance gradient trend is weak in part b ecause we have

conservatively included the uncertainty in the R calibration in the calculation of the

uncertainty in the gradient However the main problem with this comparison is that only

a small number of H II regions sampling a very restricted range of radius were measured in

the H I decient Virgo spirals Note that this comparison is further complicated b ecause

the range of radii prob ed in each galaxy is dierent and b ecause the radial normalization

the isophotal radius may also b e aected by the cluster environment

The number of Virgo galaxies available is obviously less than one would like and the

number of H II regions sampled p er galaxy is less than satisfactory for all but p erhaps two

or three of the galaxies It is p ossible that the trends shown by the Virgo galaxies in Figure

are simply statistical ukes On the other hand one cannot make these trends go away

simply by discarding any one galaxy The three H I decient galaxies have signicantly

higher characteristic abundances and p ossibly smaller gradients than the three galaxies

with normal H I prop erties

Comparison of the Virgo and Field Spirals

Do Virgo galaxies have dierent abundance prop erties than comparable eld galaxies

We have argued ab ove that the H I decient galaxies in the cluster core show higher

abundances than the outlying galaxies Studies of abundance patterns in eld galaxies

demonstrate that the mean abundances of disks are systematically correlated with galaxy

type luminosity and circular velocity eg Pagel Edmunds MRS Garnett

Shields Skillman Kennicutt Ho dge VilaCostas Edmunds VCE

OK ZKH and it is imp ortant to check that the patterns in Figure are not due to

underlying variations in those prop erties For example many of the Virgo spirals are very

luminous galaxies and so they might b e metalrich ob jects solely on that basis indep endent

of cluster environment

To address this issue we have used the ZKH survey to compare the abundances of the

Virgo galaxies to those of the eld galaxies as functions of absolute magnitude maximum

disk circular velocity V and morphological Ttype In Figure we have added the Virgo

spirals to the eld spirals from Figure of ZKH We exclude strongly barred spirals RC

B type from the comparison b ecause bars have b een shown to aect the gradient slop e

Pagel et al VCE ZKH Martin Roy Friedli Benz Kennicutt

Note that although none of the Virgo sample spirals are strongly barred four are classied

as transitional X types NGC NGC NGC and NGC In Figure

a we plot the mean OH and gradient as a function of M The H I decient Virgo

galaxies have higher oxygen abundances than the eld galaxies as exp ected from the earlier

discussion This is seen again in Figure b where the mean OH value is plotted against V

and in Figure c where the galaxies are plotted as a function of Ttype The outstanding

impression rendered by Figure is that the three H I decient Virgo spirals are al l at the top

of the abundance distributions of galaxies with similar properties

This is supp orted by a simple statistical test Each of the three H I decient Virgo

galaxies has the largest characteristic abundance for galaxies of its corresp onding Ttype

cf Figure c The N probability of this happ ening for each galaxy if the abundances

are distributed randomly within galaxies of the same Ttype are and

for NGC NGC and NGC resp ectively Because each of these presumably

represents an indep endent event the probability that all three would have the largest

abundances within galaxies of their Ttype by chance is We discuss other p ossible

factors in greater detail b elow but we conclude here that there is strong evidence that the

H I decient galaxies have elevated characteristic abundances

In making this comparison b etween Virgo spirals and eld spirals we need to b e careful

that we have not introduced a bias in the comp osition of the sample For example our

simple statistical test could b e biased by other factors that aect abundance ie we know

that luminosity and abundance are correlated This is related not only to the question of

the reality of any Virgo abundance dierential but also to the broader issue of how Hubble

types are inuenced by cluster membership In Figure we plot M and V versus Ttype

B C

There is no evidence that the H I decient group suers a bias in comparison to the eld

sample Among the H I decient Virgo spirals NGC is near the top of the distribution

in M and V but NGC and NGC are near to the middle of the distributions

B C

in V and nearer the b ottom in M Thus their higher mean abundances do not app ear

C B

to b e due to their intrinsic prop erties of luminosity or mass However the subsample of

intermediate galaxies may b e skewed All three intermediate galaxies are near the top of

the distributions in M and V for their Ttypes

B C

Finally we checked our result from the preceding section showing a dep endence of

OH on D D in the Virgo spirals ie we needed to check for such a trend in eld

H O

spirals Constructing a comparison for the eld galaxies is not straightforward Only ab out

twothirds of the galaxies in the ZKH sample have published H I synthesis observations and

unlike the uniform H I observations of the Virgo cluster sample the H I observations of the

eld galaxies were obtained with a large range in linear resolution Despite such diculties

a comparison was constructed and we found no evidence of a trend in OH with D D in

H O

the eld spirals This suggests that the trend is due to the eects of the cluster environment

and not a characteristic of galaxies in general

Are the H I Decient Virgo Spirals Chemically Dierent

From a comparison of ve Virgo spirals and nine eld spirals SSK found the Virgo

spirals to b e considerably more metal rich dex than comparable eld spirals

HPLC added observations of NGC and HPC added observations of NGC These

observations were found to b e in general agreement with those of SSK From a comparison

of the three Virgo spirals with more than observed H II regions with ve eld spirals

HPC concluded that the Virgo spirals might b e more metal rich than the eld spirals by

ab out dex SSK HPLC and HPC all suggested that a uniform eld sample and

an understanding of eects due to galaxy mass and structure were needed b efore rm

conclusions ab out environmental inuences could b e drawn

We have addressed these concerns by increasing the number of observed Virgo galaxies

to nine increasing the number of observed H II regions in these galaxies and making the

comparison to the large uniform sample of ZKH By concentrating on the comparison

b etween the H I decient Virgo spirals and the eld and p eripheral cluster galaxies we have

claried the evidence of an abundance dierential In retrosp ect the need to separate the

Virgo galaxies into H I decient and H I normal classes is an obvious one The intended

exp eriment is to test for the eects of cluster environment on the ISM abundances of cluster

spirals so it makes sense to search for the eects in those galaxies which show evidence of

altering by the cluster environment

We conclude that H I decient spirals in the cluster core have larger abundances

than comparable eld spirals or outlying Virgo spirals It is noteworthy that the outlying

Virgo spirals are indistinguis habl e from eld galaxies of comparable type and luminosity

Quantifying the size of the abundance dierential is problematic due to the small sample

size the disp ersion in the eld comparison sample the change in b oth mean abundance and

gradient and the variation in dierential with the degree of H I deciency We will return

to this in the next section

QUANTIFYING THE ABUNDANCE DIFFERENTIAL

To mo del the eect of cluster environment on spiral abundance patterns it is necessary

to quantify the size of the abundance dierential that we claim to see For this we

concentrate on the three most H I decient galaxies in the Virgo sample Taking the

characteristic abundances group ed by morphological type and averaging the logarithmic

values we nd an average characteristic abundance of for the T galaxies

from the ZKH sample This is dex or lower than the characteristic value for

NGC The T galaxies yield an average of which is dex or

lower than NGC The T galaxies have an average of which

is dex or lower than NGC although with only comparison galaxies a

statistical comparison for NGC do es not seem justiable

The ab ove analysis fo cuses on characteristic abundances but it is useful to compare

entire chemical abundance distributions to ensure that the choice of a ducial radius is

not biasing our result For example by restricting our comparison to the characteristic

abundances it is p ossible that we could underestimate the chemical comp osition dierence

in the H I decient Virgo spirals since the observed shallower gradients means that the

eect is largest at the last measured p oint To investigate this p ossibili ty we have

constructed Figure where the radial abundance patterns of the three most H I decient

Virgo spirals are compared to the abundance patterns of the nonbarred eld spiral galaxies

of comparable morphological type from the ZKH compilation The most striking case is

NGC shown in the top panel of Figure NGC has measured H II regions out

to R and for which the data are consistent with no gradient Comparing the OH

at the last measured p oint for NGC with the median value for the comparison eld

galaxies the abundance dierential is again roughly dex The comparison for NGC

is less dramatic A similar comparison of the OH at the last measured p oint to the

median for the comparison sample yields a dierential of roughly dex Note however

that NGC has a lower luminosity than all but one of the comparison eld galaxies

and should therefore b e exp ected to have lower abundances than the comparison galaxies

NGC has a small comparison sample but shows a dierential similar to that seen in

NGC Thus using characteristic abundances and outermost measured p oints give