H I and H 2 properties and star formation efficiency of NGC 4654 in the cluster environment

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1 Publ. Astron. Soc. Jpn (014) 66 (1), 11 (1 10) doi: /pasj/pst011 Advance Access Publication Date: 014 February H I and H properties and star formation efficiency of NGC 4654 in the cluster environment Eun Jung CHUNG and Sungeun KIM Department of Astronomy and Space Science, Sejong University, Seoul , Korea * ejchung@sejong.edu (EJC);sek@sejong.ac.kr (SK) (Corresponding authors) Received 013 April 9; Accepted 013 September 6 Abstract NGC 4654, an Scd galaxy in the Virgo cluster, is asymmetric in its H I distribution, with a sharp cutoff in the north-west and a long tail in the south-east, while its CO is extended to the north-west where the H I is compressed and hence its surface density is increased. This galaxy is reported to have experienced tidal interaction about 500 Myr ago, and to have been undergoing ram pressure continuously until now. To investigate the environmental effects on the interstellar medium, we make a point-to-point comparison between the H I, H, and star formation rate surface densities. The mean H I surface density at the north-west is about 5 M pc, which is higher than the threshold where H I surface density begins to saturate. The ratio of molecular to atomic hydrogen (R mol ) in the north-west region is lower than that of the other regions. Star formation efficiency with respect to the molecular gas (SFE H ) at the north-west appears to be higher than the other regions with the same total gas surface density. We discuss the high SFE H at the north-west region and propose the possibility that the intracluster medium (ICM) pushes the low-metal gas from the outer radius into the north-west region. In a low-metallicity environment, it has been reported that high H I can exist without saturation and stars can be formed from the cold atomic hydrogen phase rather than from the molecular phase. Suggestion of inflow of the outer metal-poor gas into the north-west explains well the high H I and SFE H of the north-west region. We suggest another possibility, of H I conversion into H due to the increase of gas surface density and midplane pressure due to the ICM pressure at the north-west. Key words: galaxies: evolution galaxies: individual (NGC 4654) galaxies: interactions galaxies: ISM ISM: evolution 1 Introduction The properties of galaxies in rich cluster environments differ from those of field galaxies; for example, morphology, star formation activity, and gas contents can all differ. The fraction of spiral galaxies dramatically decreases in the cores of rich clusters (Dressler 1980; Whitmore et al. 1993). Also, cluster galaxies are found to be optically redder (Kennicutt 1983;Hogg et al.004) and poorer in atomic gas (Chamaraux et al. 1980; Giovanelli & Haynes 1983). In fact, to date, a substantial amount of observational evidence for H I gas stripping in the cluster environment has been found (e.g., Bravo-Alfaro et al. 000; Chung et al. C The Author 014. Published by Oxford University Press on behalf of the Astronomical Society of Japan. All rights reserved. For Permissions, please journals.permissions@oup.com

2 11- Publications of the Astronomical Society of Japan, (014), Vol. 66, No. 1 Fig. 1. Left: H I contours overlaid on the grayscale of 1 CO(J = 1 0) of NGC The optical center is shown with a white cross. H I and CO data are taken from VIVA (Chung et al. 009a) and 1 CO(J = 1 0) from the OTF mapping survey of the Virgo cluster spirals (Chung et al. 009b), respectively. Right: CO intensity maps of 14 Virgo spirals (Chung et al. 009b). NGC 4654 is shown within the box in the north-east of the cluster center (M 87). Individual galaxies are 0 times enlarged. The radii of the dashed circles are 3, 5, and 10 degrees. North is up and east is to the left. Table 1. General properties of NGC Right Ascension (J000) 1 h 43 m 56 ṣ 6 Declination (J000) Heliocentric radial velocity, V hel 1046 km s 1 Morphological Type (RC3) SAB(rs)cd Optical diameter, D Optical B-band magnitude, BT mag K-band luminosity, L K L Inclination, i 51 H I mass M H I deficiency, def H I 0.1 H mass M Chung et al. (009a). Chung et al. (009b). A distance to source of 17 Mpc is applied for the H I and H masses, and a CO-to-H conversion factor of mol cm (K km s 1 ) 1 is used for the H mass. 009a). This gas stripping may eventually affect the star formation rate. However, the results from previous studies on molecular gas contents of cluster galaxies are not conclusive. Most studies do not find any significant differences in molecular gas contents between the field and the cluster populations (Stark et al. 1986; Kenney & Young 1986, 1989; Casoli et al. 1991; Boselli et al. 00), while some groups find cluster galaxies to be molecular gas deficient (Rengarajan & Iyengar 199; Fumagalli et al. 009; Corbelli et al. 01; Pappalardo et al. 01). Molecular gas is usually located well within the optical disk where the galactic potential is high. Also, the surface Fig.. Top: Hα image of NGC 4654 from Koopmann, Kenny, and Young (001), showing remarkable Hα enhancement in the north-west region. Apart from the large Hα knots of the north-west region, most star forming activities are found along its spiral arms like the normal field spiral galaxies. Bottom: Star formation rate map from 1.4 GHz radio continuum data (Condon et al. 1998) with H I (black) and CO (white) contours. The scale of the star formation rate (SFR) is shown at the top in units of M yr 1 kpc. density of molecular gas in the optical disk is higher than the atomic gas surface density (e.g., Helfer et al. 003; Kuno et al. 007). Hence, molecular gas is not expected to be easily stripped away. However, Fumagalli et al. (009) have shown that about 40% of the H I-deficient nearby spiral galaxies are also depleted in molecular hydrogen. By examining radial distributions of H I and CO, they found that galaxies stripped of atomic gas within the stellar disk are deficient in molecular gas. Indeed, recent high-resolution CO observations showed that molecular gas can also be affected by the environment (e.g., Vollmer et al. 005, 008, 01).

3 Publications of the Astronomical Society of Japan, (014), Vol. 66, No Fig. 3. H I (blue) and H (red) contours overlaid on a MASS K s -band image. H I data are taken from VIVA, and 1 CO(J = 1 0) from CARMA STING. The spatial resolutions for H I and CO are 15 and 3,respectively. The regions where both CO and H I data are available and hence investigated in this work are marked: the central, the north-west, and the other regions are indicated with open circles, solid squares, and crosses, respectively. The H I contours shown in the map correspond to 1, 5, 10, 0, and 30 M pc. The H contours are 10, 30, 50, and 100 M pc. It shows asymmetric H I and CO distribution, but in opposite directions. H I is extended to the south-east and compressed and increased in surface density at the north-west. On the contrary, CO extends to the north-west. This different morphology of H I and CO is presented in figure 4 with the same symbols. Chung et al. (009b) have carried out a 1 CO(J = 1 0) On-the-Fly (OTF) mapping survey of 8 Virgo cluster spirals with the Five College Radio Astronomy Observatory (FCRAO) 14-m antenna. In most cases, CO is found to be well confined toward the galactic center, with no evidence of being affected by the environments (see figure 1, Chung et al. 009b). However, several galaxies show noticeably asymmetric CO morphology, as presented in the left-hand side of figure 1 with the example of NGC NGC 4654 is located at 3. 3 north-east of M 87 the center of the Virgo cluster. The H I gas is compressed in the north-west, but very extended on the opposite side. The CO is extended to the north-west, coinciding with the compressed H I. The Hα morphology is also asymmetric, showing an enhancement at the north-west, indicating active recent star formation in this region (top panel of figure ; see also Koopmann et al. 001). In the bottom panel of figure, the star formation map derived from 1.4 GHz radio continuum emission (see subsection 3., Condon et al. 1998) also shows slightly extended star formation activity to the north-west, consistent with Hα image. Vollmer (003) proposed that this galaxy has been affected by both tidal interaction and the intracluster medium (ICM). Based on his model, this galaxy likely interacted with NGC 4639 about 500 Myr ago, and has also undergone ICM pressure until now. This one-sided long H I tail has been suggested as evidence of the ram pressure acting on the galaxy as it falls into the cluster, passing through the ICM (Chung et al. 007). The general properties of NGC 4654 are listed in table 1. In this paper, we probe the influence of the cluster environment on both the atomic and the molecular gas of NGC In particular, we investigate the effect on the midplane pressure of the environment, including changes in the atomic and molecular gas properties within the disk. We measured the midplane pressure and made a point-to-point comparison. In section, we describe the data. In section 3, we examine the H I and H surface density with respect to the stellar surface density and star formation activity, and estimate the midplane pressure in different positions from the H I and CO surface densities. In section 4, we discuss how the ICM pressure might have affected the midplane pressure and also the ISM properties. In section 5, we summarize our findings. Data We employed the HI data from the VIVA survey (VLA Imaging of Virgo spirals in Atomic gas: Chung et al. 009a). The H I distribution of NGC 4654 is well resolved down to 15 (1. kpc at the Virgo distance). The H I surface density, H I, is calculated by H I (M pc ) = m p N H I cos(i), (1) where m p is the assumed mean particle mass of 1.36 m H in M, accounting for the presence of helium and metals, and i is the inclination of the galactic disk. The H I column density, N H I, is measured by F H I N H I (pc ) = , () B maj B min where F HI is the H I flux in Jy beam 1 km s 1,andB maj and B min are the major and minor axes of the beam in arcseconds, respectively (e.g., Walter et al. 008). For the molecular gas, we utilize 1 CO(J = 1 0) data from CARMA STING 1 (CARMA Survey Toward Infraredbright Nearby Galaxies), which provides data of a much higher resolution ( for NGC 4654) than the FCRAO OTF data of Chung et al. (009b, 45 ). CARMA STING is an extragalactic CO survey with the CARMA 1

4 11-4 Publications of the Astronomical Society of Japan, (014), Vol. 66, No. 1 Fig. 4. H I and H surface densities ( H I vs. H ). The data for the center and north-west regions are shown with circles and squares, respectively. The other regions are indicated with crosses. interferometer (A. D. Bolatto et al. in preparation). Its full sensitivity field of view is, a hexagonal mosaicking pattern is carried out with 19 points and spacing of 6,and the angular resolution varies between 3 and 5 among galaxies (e.g., Rahman et al. 01). For comparison, we convolved the CO data to the H I resolution. The H surface density, H, is calculated by H (M pc ) = F CO B maj B min cos(i), (3) where F CO is the CO flux in Jy beam 1 km s 1. We adopted the CO-to-H conversion factor of X CO = mol cm (K km s 1 ) 1 and a mean particle mass of 1.36 m H per hydrogen nucleus. We measured the total gas surface density as the sum of the H I and the H surface densities (i.e., H I+H = H I + H ). We defined R mol as the ratio of the molecular to atomic hydrogen gas surface density (R mol = H / H I ). Both quantities were derived only for the regions where both H I and CO data are available. Figure 3 shows the H I and CO contours on the K s -band image. The central, north-west, and remaining regions probed in this work are indicated with open circles, solid squares, and crosses, respectively. The north-west region, in which the gas might be directly affected by the environment (likely both the tidal interaction and the ICM pressure) is our main interest. It is compared with the central and the other galactic regions. Fig. 5. The ratio of molecular to atomic hydrogen surface density ( H / H I, R mol ) as a function of stellar surface density ( ) and of total gas surface density ( H I+H ) in the top and bottom panels, respectively. The symbols are the same as in figure 4. somewhere in the central region at H I 1 M pc. The north-west is clearly distinct from the rest, showing H I 5 M pc.sincehi surface density tends to saturate at 10M pc (Wong & Blitz 00; Bigiel et al. 008; Leroy et al. 008; Bigiel et al. 011), the higher H I at the north-west is indeed unusual. Leroy et al. (008) show that R mol is linearly dependent on the stellar surface density. We present R mol as a function of stellar surface density in the top panel of figure 5. We used the K s image in the Two Micron All Sky Survey (MASS) Large Galaxy Atlas (Jarrett et al. 003) and calculated the stellar surface density following Blitz and Rosolowsky (006): log M pc = 0.4μ K s log (cos i), (4) 3 Results 3.1 H I and H surface densities Figure 4 shows the relation between the H I surface density ( H I )andtheh surface density ( H ). H peaks where μ Ks is the K s band surface brightness in magnitudes per square arcsecond, and i is the inclination. A fixed massto-light ratio (M K /L K = 0.5 M /L ) was adopted from a large variety of star formation histories (Bell & de Jong 001). The major uncertainty of is the mass-to-light

5 Publications of the Astronomical Society of Japan, (014), Vol. 66, No ratio, and it varies 0.1 to 0. dex from galaxy to galaxy (Bell & de Jong 001). The molecular gas ratio is well correlated to the stellar surface density in the top panel of figure 5. The north-west does not show any discrepancy from the other regions. This can be interpreted as that the gas asymmetries are affected by the gravitational interaction as well as ICM pressure. However, NGC 4654 shows overall higher R mol than that of Leroy et al. (008) within the same stellar surface density by a factor of 3 to 5. Since we use the same conversion factor, X CO, as Leroy et al. (008), the difference might be caused by the different H I content of NGC 4654 in the cluster environment. The ratio of molecular to atomic hydrogen gas surface density shows a linear relation with the total gas surface density (Schruba et al. 011; Vollmer et al. 01). In the bottom panel of figure 5, R mol is shown as a function of HI + H. In this relationship, the northwest region is well separated from the other regions, in the sense that R mol is lower by a factor of 5 to 10 for a given H I+H. This can be interpreted as evidence for a higher proportion of atomic hydrogen gas or a lower proportion of molecular hydrogen gas than other regions, or a combination of the two effects. Meanwhile, the presence of two sequences in the relation between H I+H and the molecular gas fraction is presented for M 33, and the difference in metallicity in the inner and outer regions is suggested as a cause of the two sequences (Tosaki et al. 011). It has been reported that lowmetal galaxies show higher H I than the threshold where the atomic gas saturates (typically 10 M pc ; see, e.g., Vidal-Madjar et al. 000; Walter et al. 007; Ektaetal. 008). Krumholz et al. (009) also show that extremely high H I can exist with the condition of low metallicity in their simulations ( H I 30 M pc for Z 0.1Z and H I 100 M pc for Z 0.01Z ). The higher H I at the north-west of NGC 4654 can also be related with low metallicity. Metallicity is closely related to the star formation history and is expected to vary along the galactic radius. It might be higher at the galactic center and decrease with increasing radius. Hence, if the ram pressure pushes the outer low-metal gas into the inner region of the north-west, then atomic gas of higher surface density at the north-west can exist without saturation. 3. Star formation activity Since stars are formed from the ISM, the variation of ISM properties caused by ICM pressure may also affect star formation activity in the disk. We used the 1.4 GHz radio continuum data of the NRAO VLA Sky Survey (NVSS: Condon et al. 1998) to estimate the star formation rate surface density. The synthesized beam size is 45 and we regrid the data into 15 to keep the number of sample points. The star formation rate is calculated following Hopkins et al. (003): SFR 1.4GHz (M yr 1 ) = fl 1.4GHz (W Hz 1 ). (5) The calibration factor f is1forl 1.4GHz > L c and f = [ (L 1.4GHz /L c ) 0.3] 1, (6) for L 1.4GHz L c, where L c = WHz 1. The total luminosity at 1.4 GHz (L 1.4GHz ) of NGC 4654 is less than L c, and the calibration factor of equation (6) is adopted. The rms brightness fluctuation is 0.45 mjy beam 1 (Condon et al. 1998), and the rms uncertainty is less than 1%. Figure 6 presents the star formation rate surface density (SFR 1.4 GHz ) as a function of integrated CO intensity (I CO ) and star formation efficiency with respect to the molecular gas derived with constant X CO (SFE H [yr 1 ] = SFR 1.4GHz / H ) as a function of the total gas surface density ( H I+H ). The spatially resolved SFR 1.4 GHz of NGC 4654 is in the range M kpc yr 1. This result is well consistent with Vollmer et al. (01), in which SFE is investigated in terms of cluster environmental effect on gas surface density. Vollmer et al. (01) used GALEX farultraviolet data and Spitzer IRAC and MIPS data to derive dust extinction corrected SFR. In the results of Vollmer et al. (01), NGC 4654 shows SFR between and 5 10 M kpc yr 1, which covers a much lower SFR than our result. Since we only use the data points where both H I and CO are available, we cannot cover the low SFR which is shown in a large radius. The SFR map of NGC 4654 from FUV+TIR data (Vollmer et al. 01) shows peak SFR at the center and the north-west regions. The SFR map from the 1.4 GHz radio continuum in this paper also shows high SFR at the center and slightly extended SFR to the north-west. We attribute this to the poor spatial resolution of radio continuum data. The relationship of SFR 1.4 GHz to I CO shows a large scatter, but a slightly increasing trend of SFR 1.4 GHz with increasing I CO exists. The north-west tends to have a high star formation rate, as seen by comparison with other points having similar I CO values. In addition, the CO surface brightness at the north-west is lower than the surface brightness at the center by a factor of 5 on average, but the star formation rate is comparable between them. The range of SFE H is between 0.5 and 0 Gyr 1.This is higher than the previously reported SFE H of field and cluster galaxies. SFE H is reported to be yr 1 in the case of normal nearby spiral galaxies (Bigiel et al.

6 11-6 Publications of the Astronomical Society of Japan, (014), Vol. 66, No. 1 Fig. 6. Left panel: Relation between star formation rate and CO surface brightness. Right panel: Star formation efficiency with respect to the molecular gas as a function of the total gas surface density. The average SFR 1.4 GHz and SFE H in bins of I CO and H I+H, respectively, for disk regions are denoted with errorbar. 008; Leroy et al. 008) and to be between and yr 1 in the case of the Virgo spirals (Vollmer et al. 01). Vollmer et al. (01) reported that any significant increase of SFE H at the windward side is not observed in the gas-stripped galaxies. However, the star formation timescale (inverse of star formation efficiency) map of NGC 4654 in Vollmer et al. (01) shows that the center and the north-west regions have similar ranges of star formation efficiency with respect to the total gas. In our result, the north-west shows an overall higher SFE H than that of the other regions having the same H I+H. Our question is why the star formation efficiency is higher even though the molecular gas ratio is lower at the north-west. If the low-metallicity gas of the outer radii has been pushed into the north-western region, higher X CO should be used, unless the molecular gas at the northwest were underestimated, and the star formation efficiency with respect to H might be affected, too. In the following section, we test the CO-to-H conversion factor. 3.3 CO-to-H conversion factor The CO-to-H conversion factor (X CO ) varies by up to a factor of 10, and higher values of X CO are generally observed at low metallicities and in strong UV radiation fields (Polk et al. 1988; Strong et al. 1988; Allen & Lequeux 1993; Rubio et al. 1993; Digel et al. 1996; Neininger et al. 1996). The gradient of X CO along the galactocentric distance also exists in the sense that X CO increases with increasing radius (Sodroski et al. 1995; Arimoto et al.1996; Blanc et al. 013). Boissier et al. (003) have compared the molecular gas mass with constant X CO (H [X CO = const]) and with the metallicity-dependent X CO (H [X CO = f (Z)]) for several Fig. 7. Relation between the CO surface brightness and stellar surface brightness. nearby galaxies, and found the increasing radial gradient for the ratio of H [X CO = const] and H [X CO = f (Z)] by a factor of to 3 at the center. NGC 4654 also shows the difference by a factor of 3 between the inner and outer regions (Boissier et al. 003). If the north-west has a low metallicity, then the dependency of X CO on metallicity could have affected H, i.e., H could have been underestimated at the north-west by the use of constant X CO. To investigate whether X CO affects H at the north-west, we examine the relationship of CO surface brightness to stellar surface density and star formation rate. CO surface brightness is usually well correlated with the stellar surface brightness. In figure 3, the CO appears to be well confined to the stellar disk. Figure 7 also shows that I CO is linearly correlated with in NGC The slope of the disk is equal to that of the center, and the northwest also shows no distinction in this relationship from

7 Publications of the Astronomical Society of Japan, (014), Vol. 66, No other regions. This means that the lower R mol of the northwest in figure 5 is not due to an unusually low CO in the north-west. Next, we ask whether H is underestimated when it is derived from I CO, i.e., due to the use of a constant X CO. The star formation rate in galaxies has been reported to be proportional to the molecular gas mass (e.g., Bigiel et al. 008). We can examine whether the molecular gas surface density of the north-west region is underestimated using the relation betweensfr and CO surface brightness. As shown in the previous section, the north-west region shows a higher trend of star formation rate and efficiency, and more molecular gas is expected. Hence, it is likely that H in the north-west is underestimated by the CO measurement and the use of a constant X CO in our study. We simply applied lower and higher X CO by a factor of to the center and the north-west, respectively, than that to the disk region from figure 1 of Boissier et al. (003). The distinct features of the north-west in the relation between H I+H and R mol (figure 5) are not changed. This is because the application of large X CO increases not only R mol but also H I+H. However, the discrepancy of the north-west region from other regions in the relation between SFE H and H I+H as shown in the right-hand panel of figure 6 disappears within the uncertainty. Therefore, the molecular gas at the north-west can be underestimated at least by a factor of due to the use of constant X CO. 3.4 Molecular to atomic hydrogen ratio and midplane pressure The molecular to atomic hydrogen ratio has been known to be linearly correlated with the midplane pressure: ( ) P α R mol =, (7) P 0 where P 0 is the midplane pressure where the H I and H surface densities become identical (R mol = 1: Blitz & Rosolowsky 004, 006; Leroy et al. 008). In this subsection, we examine the relation between R mol and midplane pressure for NGC Elmegreen (1989) gives the midplane pressure for a gas and stellar disk: P h π G gas ( gas + σ ) g, (8) σ,z where gas and are the gas and stellar surface density, respectively, σ g is the velocity dispersion of the gas, and σ,z is the vertical stellar velocity dispersion. We adopt a σ g of 8kms 1, which is the observed value in several galaxies (e.g., Dickey et al. 1990; Malhotra 1995). σ,z is derived Fig. 8. Hydrostatic midplane pressure (P h ) vs. the molecular gas ratio (R mol ). The least-squares fit of the data excluding those of the northwest region is shown as a solid line. The result of Leroy et al. (008) is shown as a dotted line. The open squares are for the north-west region with R mol and P h calculated by adopting H I = 10M pc instead of the observed H I (see section 4 for more details). following Leroy et al. (008), who assumed an unvarying exponential stellar scale height with radius, a linear relation of stellar scale length and vertical scale height, an isothermal disk in the z-direction, and a fixed ratio of radial and vertical stellar velocity dispersion. It can be simply written as: σ,z = πgh, (9) where h is the vertical stellar scale height; 60 pc is adopted, which is estimated from the relation between the radial scale length (4.5 kpc for NGC 4654) and the vertical scale height of the stellar disk (Kregel et al. 00). The uncertainty of P h is expected to be accurate within 0% at the highest estimate with the assumed constant uncertainties of 0.3 dex for gas, 0. dex for, and 10% for,z. Figure 8 shows R mol as a function of P h /k in different regions of NGC Except for the north-west region, R mol appears to be roughly linearly correlated with P h /k in a log log relation: ( ) Ph /k 0.8 R mol =. (10) The index, α, is well matched with Leroy et al. (008). NGC 4654 shows an overall higher P 0 /k compared to the sample of Leroy et al. (008). The higher P 0 /k might be due to the different H I and H properties of NGC 4654, which is being affected by the cluster environment. Indeed, it is reported that interacting galaxies show different relations of molecular gas ratio and midplane pressure from those of isolated galaxies (Blitz & Rosolowsky 006), and NGC 4654 agrees well with the interacting sample galaxies of Blitz and Rosolowsky (006).

8 11-8 Publications of the Astronomical Society of Japan, (014), Vol. 66, No. 1 For the north-west region, we find the following relation: ( ) Ph /k 1.38 R mol =. (11) The north-west region has even higher P 0 /k and α than the other regions. Every four points of the north-west region are located at the lower boundary of the R mol and P h relation, and three points are out of the range of the other regions in figure 8, which we attribute to unusually high H I. 4 Discussion NGC 4654 is likely to be affected by both tidal interaction and the cluster medium. Vollmer (003) has probed the stellar and gas distributions, and successfully reconstructed its morphology by combining past interactions with NGC 4639 ( 500 Myr ago) and ongoing weak ram pressure due to the ICM. Although it seems to be located in the low ICM density environment, the ICM pressure in its current location could be sufficient to strip the gas in the outer disk, depending mainly on its orbital velocity (Yoon 013). As suggested by the model of Vollmer (003), the tidal interaction with NGC 4639 could help to pull material from NGC 4654, which would then be more easily stripped by ram pressure. Unlike tidal interactions, ram pressure affects only the gas disk, yielding a very asymmetric morphology depending on the orbital parameters. On the side where the ICM wind is acting, the gas will be compressed and increased in density, while on the opposite side, the stripped gas can form a long gas tail, as seen in NGC Intriguingly, the molecular gas distribution is also very asymmetric in this case, being extended to the north-west along the H I compression. The high star formation rate and efficiency in the northwest of NGC 4654 (shown in the previous section) matches the results provided by Vollmer et al. (01). The tidal perturbation makes the gas lose its angular momentum and fall into the inner disk, and central starburst can occur (Mihos & Hernquist 1996; Springel 000; Di Matteo et al. 007; Hopkins et al. 013). Besides, the enhancement of star formation due to ram pressure has been shown in a number of simulations (e.g., Fujita & Nagashima 1999; Vollmer et al. 001; Tonnesen & Bryan 01). In particular, edgeon ram pressure (low inclination between galactic disk and orbit plane) can increase the central gas surface density within a few 10 7 yr, and enhance the local star formation rate up to a factor of by N-body simulations (Vollmer et al. 001). NGC 4654 has been experiencing both tidal interaction with its companion and ICM pressure, and the star formation activity at the north-west is likely enhanced by the cluster environment. The question arises, why does the north-west region show such a high star formation efficiency even with a low molecular gas fraction? The first possibility is that the compression of H I and the increase in H I due to the ICM pressure directly forms stars from H I. The second possibility is that the atomic hydrogen is being transformed into molecular hydrogen, promoting star formation in the north-west region. The former is very interesting because star formation in the outer HI disk has been reported for isolated galaxies whose outer disk gas was gravitationally very stable (e.g., Barnes et al. 011, 01), but not yet for any disturbed or interacting galaxies. However, Krumholz (01) showed that star formation can occur in a cold atomic phase rather than a molecular phase at extremely low metallicities. Hence, as mentioned in subsection 3.1, if the low-metallicity gas is flowing into the north-west region by ICM pressure and stars are formed from H I, the star formation efficiency with respect to the molecular gas appears to be higher than the other disk regions. For the latter case, the question is the timescale of transition from H I to H, and finally to stars or the amount of transformed H from H I. The theory of the transition of H I to H has been studied in photodissociation regions, where the photodissociation of interstellar atoms and molecules due to stellar radiation is dominant (Hollenbach & Tielens 1999 and references therein). Krumholz, McKee, and Tumlinson (009) have suggested that an H I surface density higher than 10 M pc can shield the interstellar material from strong radiation. In this case, the H I density is sufficient to form molecular hydrogen which is well protected from stellar radiation. Lee et al. (01) have supported these results in their observational study on the Perseus molecular cloud. The H I surface density in the north-west region of NGC 4654 ( 5M pc atoms cm ) is well above the threshold suggested by Krumholz, McKee, and Tumlinson (009) or Lee et al. (01). However, if the timescale is too long or the transformed mass is too small, the change might hardly be detected. The typical timescale of H formation in equilibrium is Myr (e.g., Goldsmith et al. 007; Liszt007), and it is expected to be longer for NGC 4654, which continues to experience ram pressure. The amount of atomic gas possibly transformed into molecular form could be estimated by assuming H I = 10M pc for the north-west region and using the relationship between R mol and P h /k. The high H I of the north-west is clearly due to the ICM pressure. H I tends to saturate at a value of 10M pc in molecular gas-rich

9 Publications of the Astronomical Society of Japan, (014), Vol. 66, No Table. before H and enhanced H mass of the north-west region. before H obs H M excess H Position at NW upper-left point upper-right point lower-left point lower-right point Expected H before the ram pressure with the assumed H I in M pc. Observed H in M pc. Excess of H mass in 10 6M. galaxies like NGC 4654 (Wong & Blitz 00), and hence it is valid to adopt H before = I 10M pc in the north-west region in the past before the galaxy was affected. We calculate R mol and P h with H before = I 10M pc for the northwest points, and the results are denoted by open squares in figure 8. The amount of R mol (= H obs / H before ) at the northwest (the open squares in figure 8) appears to be similar I to or higher than other regions of NGC 4654, and close to the mean of the isolated sample galaxies of Leroy et al. (008), which can be interpreted as the currently observed H being slightly enhanced. The excess of H, H (= H obs H before ), can be calculated using the relation between R mol and P h /k of the other regions [equation (11)], where H before is: before H ( ) = H before Ph /k 0.8. (1) I Since P h is a function of H itself, we used the iteration method and found an approximate value of H before.the assumed before H I of 10 M pc and the calculated before H are supposed to be ideal values for the north-west before the atomic gas surface density increased as a consequence of the environmental effects. The estimated H before values of the north-west region are tabulated in table. When we consider the total area of the north-west region, the expected H mass is M, which we can adopt as M H in this region before any environmental effects acted on NGC The observed M H is 10 8 M. Hence, we infer that M of H could have recently formed in the north-west. 5 Summary In this study, we probed the spatially resolved atomic and molecular gas properties of NGC This galaxy, located at the north-east of Virgo ( 1. R vir away from M 87, where R vir is the virial radius of the cluster), is highly asymmetric in both H I and CO, but in the opposite sense. Its H I is more extended than the stellar disk to the southeast, with a sharp cutoff in the north-west. Its CO is more extended to the north-west, in the same direction as the H I compression. This galaxy is likely to have tidally interacted with its neighbor about 500 Myr ago and to have been experiencing weak ram pressure to the present day. We investigated the H I and H properties with star formation activity and midplane pressure under the ICM pressure. The atomic and molecular hydrogen gas properties of the north-west region are distinct from those of the other regions. First, the atomic hydrogen gas surface density is quite high ( 5M pc ) at the north-west. Secondly, R mol is linearly correlated with the stellar surface density and also with the total gas surface density. However, the north-west has lower R mol than other points having similar H I+H. Lastly, the star formation rate of the north-west appears to be high compared to the other points having similar I CO, and the star formation efficiency with respect to H is also high at the north-west in the relation between SFE H and H I+H. We examined the underestimation of H in the northwest due to the use of constant X CO. The relation between I CO and indicates that CO is not deficient at the northwest. This implies that H in the north-west can be underestimated from CO due to the use of constant X CO. However, the use of radial-dependent X CO (Boissier et al. 003) does not change the distinctly lower R mol at the north-west. In the relation between R mol and P h,ther mol of the north-west region is lower than that of the other regions for given P h, and this is not because of the lower H but because of the higher H I of the north-west region. We suggest that the inflow of low-metal gas into the north-west by the ICM pressure can cause the unusually high H I gas surface density without saturation. The metalpoor ISM can also explain the lower R mol but the slightly higher star formation rate at the north-west due to star formation from atomic phase hydrogen rather than molecular phase. The other possible influence of the ICM pressure on ISM, that H I is transformed into H in this region, is suggested. We calculate the enhanced M H at the north-west from the relation between R mol and P h, and star formation might be promoted by H formation from H I. Acknowledgments We thank A. Chung for useful comments on an early draft of this paper. We thank Alberto Bolatto for his kind help in using the CARMA STING data. We thank the anonymous referees and the editor for thorough reading of the manuscript and very helpful suggestions to improve the paper. This research was supported in part by a Mid-career Researcher Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science, and Technology,

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