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Herschel-SPIRE observations of the disturbed galaxy NGC4438

We present Herschel-SPIRE observations of the perturbed galaxy NGC4438 in the Virgo cluster. These images reveal the presence of extra-planar dust up to ~4-5 kpc away from the galaxy's disk. The dust closely follows the distribution of the stripped atomic and molecular hydrogen, supporting the idea that gas and dust are perturbed in a similar fashion by the cluster environment. Interestingly, the extra-planar dust lacks a warm temperature component when compared to the material still present in the disk, explaining why it was missed by previous far-infrared investigations. Our study provides evidence for dust stripping in clusters of galaxies and illustrates the potential of Herschel data for our understanding of environmental effects on galaxy evolution.


Introduction
Clusters of galaxies are extremely hostile environments for starforming galaxies. A plethora of observations and numerical simulations have revealed how gravitational and hydrodynamical interactions can affect cluster spirals (e.g., Boselli & Gavazzi 2006): stars and gas can be stripped, galaxy morphologies can be changed and star formation activity can be enhanced and/or quenched. However, still very little is known about the effects of the environment on the dust content of cluster galaxies. Since the dust is mixed with the interstellar medium (ISM), the common expectation is that the environment should affect the dust in a similar fashion as the gas. However this hypothesis has still to be confirmed observationally. The launch of Herschel (Pilbratt et al. 2010) has opened a new era in the study of dust in galaxies. Thanks to its high spatial resolution and sensitivity to all dust components, Herschel should be able to determine whether dust is stripped from infalling cluster spirals and dispersed into the intra-cluster medium (ICM).
One of the most dramatic examples of the effects of the environment on nearby galaxies is represented by the disturbed galaxy NGC4438, in the Virgo cluster. NGC 4438 is a highly Hi-deficient early-type spiral showing stellar tails (Kenney et al. 1995;Boselli et al. 2005) and extra-planar gas (Vollmer et al. 2009). Numerical simulations show that only a combination of ram-pressure stripping and tidal interactions is able to reproduce the disturbed morphology and kinematics of NGC4438 (Vollmer et al. 2005). However, while Combes et al. (1988) and ⋆ Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA. Vollmer et al. (2005) invoked a high-velocity (∼800 km s −1 ) gravitational interaction with the companion early-type galaxy NGC4435, Kenney et al. (2008) have revealed the presence of a series of Hα+ [NII] filaments connecting NGC4438 to the giant elliptical M86 (see Fig. 1). These new observations favour a more complex scenario in which NGC4438 has recently (∼100 Myr ago) interacted with M86 (Kenney et al. 2008). Although the detailed history of NGC4438 is still unclear, its peculiar properties make it an ideal target to test the power of Herschel in unveiling the effects of the environment on the dust properties of cluster galaxies.
In this Letter we present Herschel-SPIRE (Griffin et al. 2010) observations of NGC4438 obtained as part of the Herschel Science Demonstration (SD) Phase. The observations of the elliptical galaxy M86 are presented in a companion paper (Gomez et al. 2010). We assume for NGC4438 a distance of 17 Mpc (Gavazzi et al. 1999), corresponding to a linear scale of 82 pc/ ′′ .

Observations and data reduction
The region around NGC4438 was observed by the Herschel-SPIRE instrument as part of the SD observations for the Herschel Reference Survey (Boselli et al. 2010a). Eight pairs of cross-linked scan-map observations were carried out over an area of ∼12 ′ ×12 ′ with a nominal scan speed of 30 ′′ /sec. The data have been reduced following the procedure described in The NGC4438/M86 region as seen by SPIRE. This mosaic is obtained by combining the 250µm maps centered on NGC4438 and M86 (Gomez et al. 2010). The blue contours show the extended Hα+[NII] filaments discovered by Kenney et al. (2008). Note that the intensity excess south of NGC4438 (near the edge of the frame) is just a residual of the baseline subtraction.
tion on a scan by scan basis. A robust linear fit with outlier rejection was applied to the first and last fifty sample points of the time line for each bolometer, thus avoiding the galaxies and the extended diffuse emission present across the field. The SPIRE astronomical calibration methods and accuracy are outlined in Swinyard et al. (2010). Since at the time of the data reduction the SPIRE pipeline used a preliminary flux calibration, we followed the recommendation of the SPIRE Instrument Control Center and multiplied the flux densities by 1.02, 1.05, and 0.94 at 250, 350, and 500 µm, respectively 1 . The images have flux calibration uncertainties of 15% and the rms are ∼5.5, 5.8, 6.9 mJy/beam at 250, 350 and 500 µm, respectively. We note that in all three bands the noise is dominated by confusion. The astrometric uncertainty is ∼2 ′′ and the full widths at half maximum of the SPIRE beams are 18.1 ′′ , 25.2 ′′ , 36.9 ′′ at 250, 350 and 500 µm, respectively.

Results
In Fig. 1 we show a mosaic of the 250 µm SPIRE maps of NGC4438 and M86 (Gomez et al. 2010). In addition to NGC4438, also the companion S0 galaxy NGC4435 is clearly detected in all the three SPIRE bands. This is not surprising since Panuzzo et al. (2007) showed that this galaxy has just experienced a burst of star formation and it is confirmed by the fact that the SPIRE flux density ratios ( f (250)/ f (350) ∼2.9 and f (350)/ f (500) ∼2.6) are consistent with the values observed in star-forming galaxies (Boselli et al. 2010b). More intriguing is the presence of a few bright (i.e., peak f (250) ≥0.05 Jy/beam) extended sources possibly associated with NGC4438. The most remarkable feature is the extended emission to the N-NW of NGC4435 (Plume in Fig. 1). This plume is approximately 8 ′ long and 3 ′ wide and it is detected (at least in part) in all the three SPIRE bands. Although its origin (i.e., Galactic or extragalactic) is still debated, Cortese et al. (2010a) have recently shown that the properties of this plume 1 See also http://herschel.esac.esa.int/SDP wkshops/presentations/ IR/3 Griffin SPIRE SDP2009.pdf. are more consistent with Galactic cirrus than tidal debris at the distance of the Virgo cluster.
Two additional elongated structures (E1 and E2 in Fig. 2) are detected within the optical radius of NGC4438. E1 appears as a tail extending from the bulge of NGC4438. However a careful comparison of the SPIRE data with Spitzer images reveals that it is just the result of blending of point sources visible at 3.6 and 24 µm, possibly background galaxies. This seems also supported by the lack of any counterpart in UV, Hi or Hα+ [NII]. Less clear is the origin of E2, which is composed of three different knots. The southern and central knots coincide with an Hi cloud discovered by Hota et al. (2007) and the whole feature apparently follows one of the Hα+[NII] filaments pointing towards M86 (Kenney et al. 2008), suggesting that it might be associated with NGC4438 (see Fig. 2). If so, this would represent a unique example of stripped intra-cluster dust. However, caution is required before interpreting E2 as a dust stream in Virgo. Firstly, the southern knot is offset (∼15 ′′ north) from the Hα+[NII] stream, thus it is not clear whether the two features are related. Secondly, Spitzer 3.6 and 24 µm point sources are found within ∼6-9 ′′ from each of the three knots, thus we cannot completely exclude the possibility that E2 is just the result of blending of background sources, like E1. So, although E2 is a very intriguing system, only future investigations will reveal whether it is associated with NGC4438. We note that, in this case, E2 would represent the only case of submillimetre (submm) emission associated with the intra-cluster Hα+[NII] streams connecting M86 and NGC4438 (see also Gomez et al. 2010).
Stronger evidence supporting dust stripping is found in the main body of NGC4438. In Fig. 2 we compare the distribution of the cold dust as revealed by Herschel with those of stars, hot/warm dust and warm ionized 2 , cold atomic and molecular hydrogen. Interestingly, despite the presence of a young stellar population, no submm emission is detected in the northern and southern tidal tails. Although the 250-500 µm emission is mainly concentrated within the central ∼4.5 kpc of the galaxy, extended emission is visible on the west side of NGC4438. Particularly remarkable are the two bright regions highlighted in Fig. 2: a tail (A) extending to the south up to ∼9 kpc from the center of NGC4438, and an elongated structure (B) ∼4.5 kpc from the plane of the disk and almost completely detached from the main body of NGC4438. The tail A is detected at 8, 24, 70 µm and in Hi. It extends up the southern stellar tail of NGC4438 and is composed of at least two disjoint bright knots. Although it follows very closely the southernmost Hα+[NII] filament in NGC4438, a careful comparison between the 8, 24 µm and Hα+ [NII] images reveals that two features do not spatially coincide. The fact that the Hi in correspondence of tail A has a recessional velocity (−40< V < +20 km s −1 , Hota et al. 2007) significantly lower than the ionized gas (V ∼ −85 km s −1 , Chemin et al. 2005) suggests that the two components might just be projected along the same line-of-sight.
Although the presence of submm emission up to the stellar tidal tail is consistent with a dust stripping scenario, it remains uncertain whether tail A is just part of what is left of the disk of NGC4438 or a stripped dust tail.
In comparison, the formation history of knot B appears a little bit clearer. This feature coincides with the extra-planar dustlane visible in optical images and it clearly follows the distribution of the extra-planar atomic (Hota et al. 2007) and molecular (Vollmer et al. 2005) hydrogen detected in this region. While the extra-planar CO(1-0) emission is mainly segregated around knot B, the Hi extends further south following closely the diffuse 250 µm emission. Although low surface brightness Hα+[NII] emission is observed in correspondence of the diffuse 250 µm to the west of NGC4438, only a single Hii region (slightly offset from the submm emission peak) is observed in knot B. Previous investigations have shown that the disturbed morphology and kine-matics of the gas in this region is consistent with a stripping scenario (Kenney et al. 1995;Combes et al. 1988;Vollmer et al. 2005). So, it is likely that we are directly witnessing dust in the process of being removed from the disk of NGC4438. Contrary to tail A, knot B is not entirely detected by Spitzer: only the single Hii region and low surface brightness emission are visible at 8 and 24 µm. The integrated f (350)/ f (24) flux density ratio of knot B (∼47) is a factor ∼4 higher than the value observed in the main body of NGC4438, confirming that this feature is missing a warm dust component and it is not associated with active star formation. This is additionally supported by the reddening of the f (250)/ f (350) ratio (from ∼2.7 to ∼2.1) when moving from the center of NGC4438 to knot B.

Discussion and conclusions
The SPIRE data alone are not sufficient to determine whether the extra-planar dust is in the process of being removed from NGC4438 or, for example, falling back onto the disk. However, by combining multiwavelength observations with detailed numerical simulations, Vollmer et al. (2005Vollmer et al. ( , 2009) have shown that the distribution and kinematics of the different components of the ISM in NGC4438 can only be reproduced via a combination of tidal interaction and ram-pressure stripping. Although the details of the gravitational interaction are still unclear (i.e., M86 and/or NGC4435, Vollmer 2009), it appears that strong on-going ram-pressure (in addition to tidal forces) is necessary to reproduce the properties of the extra-planar gas component in knot B. In this case, whatever the exact mechanisms affecting NGC4438, it is likely that at least part of the dust in the west side of the galaxy is in the process of being removed from the disk thus providing evidence of dust stripping by environmental effects. If completely removed, the stripped dust will be dispersed in the ICM contributing to its metal enrichment.
Since in galaxies the dust is associated with the gaseous component of the ISM, it is generally expected that when the gas is stripped part of the dust can be removed as well. What is still unclear is how much dust follows the fate of the stripped hydrogen. Unfortunately, the SPIRE fluxes alone are not sufficient to accurately quantify the amount and temperature of the dust in knot B. Nevertheless, we can at least try to estimate the dust mass by using the fluxes at 250 and 350 µm, where knot B is clearly resolved, and assuming M dust = ( f ν D 2 )/[κ ν B(ν, T )], where f ν is the flux density (∼0.98 and ∼0.46 Jy at 250 and 350 µm, respectively), D is the distance and κ ν is the absorption cross section per mass of dust. We adopt κ ν =4 and 1.9 cm 2 g −1 at 250 and 350 µm respectively (Draine 2003). The total mass of dust in knot B is in the range ∼2×10 6 -2×10 7 M ⊙ for a dust temperature between 10 and 20 K. In order to determine the total gas mass in the extraplanar cloud, we combined the estimates of M(H 2 )∼4.7×10 8 M ⊙ (including helium) and M(Hi) ∼1.5×10 8 M ⊙ obtained by Vollmer et al. (2005) and Hota et al. (2007), respectively. The total gas-to-dust ratio of knot B is in the range ∼30-300, not significantly different from the values observed in nearby galaxies (e.g., Draine et al. 2007;Galliano et al. 2008) and across the disk of star-forming spirals (e.g., Muñoz-Mateos et al. 2009;Bendo et al. 2010b;Pohlen et al. 2010). This seems to support a scenario in which a cloud of gas does not lose a significant amount of its heavy elements when removed from the disk. It is in fact quite remarkable how well the cold dust follows the spatial distribution of the cold hydrogen across the whole galaxy, even in such a highly perturbed system like NGC4438.
Interestingly, the high degree of spatial correlation between cold dust and cold hydrogen in NGC4438 is different from what is observed by Herschel in M86 (Gomez et al. 2010). In M86 the dust is mainly associated with the Hα+[NII] emitting gas and only mildly correlated with the cold atomic component. This is intriguing and might suggest different dust properties (e.g., stripping mechanism, heating source, etc.) in the two galaxies.
Finally, we note that it is unclear whether the extra-planar dust is missing a hot component just because the interstellar radiation field outside the plane is not strong enough to keep it hot, or if the cloud was already dominated by a cold dust component while in the disk. Although the f (250)/ f (350) flux ratio is lower than the value observed in the central part of NGC4438, it is not significantly different from what is observed in the outer parts of the disk of M81 (Bendo et al. 2010a), M99 andM100 (Pohlen et al. 2010).
In summary, in this paper we provide evidence for dust stripping by environmental effects in NGC4438. The high spatial resolution and sensitivity to cold dust of the SPIRE camera allowed us to discover an extra-planar cold dust component completely missed by previous far-infrared surveys. The strong spatial correlation between cold dust, atomic and molecular hydrogen suggests that when the gas is removed also the dust is pulled out the galactic disk. These results provide interesting insights into the evolution of an extreme case among perturbed cluster galaxies. Once combined with the discovery of truncated dust disks in Hi-deficient Virgo cluster spirals (Cortese et al. 2010b), our analysis clearly highlights the great potential of Herschel for our understanding of the effects of the cluster environment on the dust properties of galaxies.