Multi-long-slit Spectroscopy for Kinematic Studies II.Initial Results for Edge-on Galaxies NGC891 and NGC4244

Jiehae Choi, Sophia Cisneros, Cat Wu, Maria T. Patterson, Rene Walterbos

New Mexico State University, Las Cruces, NM 88003

Fraternali, F., Binney, J.J., 2006, MNRAS 366, 449.Heald, G.H., etal., 2006, ApJ 647, 1018.Hoopes, C.G., Walterbos, R.A.M., Rand, R.J., 1999, ApJ 522, 669.Kamphuis, P. etal., 2007, A&A 498, 951.Oosterloo, T., Fraternali, F., Sancisi, R., 2007, AJ 134, 1019.

References

This research was supported by an award from Research

Corporation.

We acknowledge early discussions about this mode of spectroscopy with Robert Braun

and the late Michael Ledlow.

Acknowledgements Grey-scale image of a combined 100 min exposure of NGC4244 with

the slits perpendicular to the major axis. We show an area of 3.8' by 3.4'. Slit 8 was centered on the stellar nucleus. In this medium-depth spectrum, we can already trace DIG to a higher vertical extent (> 1 kpc above the disk in some slits) than was done in the deep imaging study by Hoopes et. al. (1999), which set the most sensitive limits thus far. We will obtain deeper exposures to measure the velocities of this faint gas.

Fig. 2

Sky subtracted 3 hours exposed spectrum for the Northern half of NGC891. We show the central 14 slits, each of which is 3.3' long. Sky subtraction was optimized for the faint sky line, no longer visible, that was just to the right of the Hα line. This led to over-subtraction of a neighboring sky line farther from Hα. Stars are smoothed somewhat due to small position changes between the 7 exposures. Slit 1 is the rightmost slit; slitnumbers increase to the left.

Fig. 4

Images in Hα of the two edge-on galaxies NGC891 and NGC4244, and a few other edge-ons, on the same linear scale. These images demonstrate the difference in the brightness and spatial extent of the ionized gas halos in these two systems. NGC4244 is a late-type disk galaxy, while NGC891 is a prominent Sb with a high star formation rate per unit area. The bar shown in the lower left of the image is 1 kpc in length. (Figure taken from Hoopes et. al., 1999)

Fig. 1

APO

Velocity measurements for slits oriented perpendicular to the major axis of NGC 891. Left panel: Velocities for all 16 slits. The velocities near the mid-plane often show a pronounced "peak," possibly due to an HII region closer to our line of sight than the tangential point in the disk or to extinction near the mid-plane. Gas above the disk generally rotates slower, as observed before, but this only becomes readily evident above 2500 pc. Our final data will have 3 times higher S/N than these spectra, and they will extend further above the disk than the 1997 Rand spectrum. Bottom panel: Our velocities (with offsets) compared to those measured by Rand (1997) from a 6-hour, single-long-slit KPNO 4-m spectrum in the same area (located between two of our slits).

Fig. 3

Radial Velocities of the ionized gas measured in the disk-halo interface of NGC4244. Slit 8 was centered on the nucleus. The distance between slits is 15” or 225 pc.

The trend towards lagging rotation for the halo gas is expected to be most prominent for slits farthest away from the center. We see the signature in slits 1, 13, 15, and 16 where the gas at higher distance above the plane is closer to the systemic radial velocity of NGC4244.

Fig. 5

We present initial results of multi-long-slit spectroscopy of two edge-on galaxies. We demonstrate that we can measure velocities of ionized gas in the halo out to 4 kpc in the case of NGC 891. From the velocity fields, it appears that the halo gas lags disk rotation, but this effect is only clearly present 2 kpc or more above the disk. This is in agreement with other measurements (Heald et.al., 2006; Kamphuis et.al., 2007) and with the velocities of the HI halo gas (Oosterloo et.al., 2007).

For NGC 4244, which has a very faint DIG halo, we detect ionized gas to higher distances above the plane than seen before. We have selected a sample of 12 edge-on galaxies for further study, which will be done in collaboration with R. Rand and B. Benjamin. Through longer exposure times, a higher resolution grating, on-chip binning, and new filters, our future data will be 2.5 to 4 times more sensitive than shown here. This provides a new avenue for studying the origin and dynamics of gas in thick disks and halos, and it may help in unravelling the origin of ionization of the gas.

Summary

Deep optical imaging over the past 15 years has shown that several spiral galaxies possess thick ionized gas disks and, in some cases, halos. The extent and brightness of this gas is generally correlated with the level of star formation in the disk, suggesting a "galactic fountain" origin for the gas. However, preliminary kinematic studies do not always agree with a fountain model (e.g. Heald et al., 2006, Fraternali & Binney 2006) and some of the gas may stem from continuing infall. The ionization would still be provided by star formation in the disk, but the gas itself might have a different origin. One way to address this is to determine in more detail the velocities of the gas for a larger sample of galaxies.

We present initial results of observations of the edge-on galaxies NGC891 and NGC4244 using a multi-long-slit setup on the DIS spectrograph on the ARC 3.5m telescope. In combination with a 25Å Hα filter, 16 slits of 2" by 4.5' spaced 15" apart were observed simultaneously over a 4' by 4.5' field of view. In the case of NGC891, we obtained a medium-deep spectrum of the Northern half of the halo. We tested the accuracy of our velocity measurements by comparing them with previous observations, and we expanded on the spatial coverage of velocity measurements of ionized gas in the halo. For NGC4244, deep optical imaging has not shown a very extended diffuse ionized gas halo. For this galaxy, our principal goal is to use our spectroscopic data to improve upon the sensitivity limits set in the imaging studies and to see if there is ionized gas above the mid-plane.

Introduction