Table 3.2: Journal of observations
Galaxy Date P.A. (deg) Wavelength range (˚A) Exposure time (s) Slit (00) Airmass NGC 278 15.12.2003 40.5, 93.15, 143.5 6034−7088 3×1200 1.03 1.06 - 1.33 NGC 2500 15.12.2003 24, 79.5, 40.7 6034−7088 2,3×1200 1.03 1.08 - 1.14
NGC 1058 16.12.2003 32.5, 125 6034−7088 3×1800 1.03 1.01 - 1.17
UGC 3574 16.12.2003 0, 27, 122 6034−7088 3×1200; 2,3×1800 1.03 1.14 - 1.28
STARLINK3package DIPSO. The continuum level is simultaneously fitted with a first order polynomial. Following the same process as in Chapter 2
From equation 3.1, neglecting the expansion velocity in equation 3.1 (this is, assuming vexp = 0) and applying the relations between the celestial coordinates (equations 3.2 and 3.3), the vertical velocity component can be expressed as:
VZ =
Vobs−Vsys−Vrot
r
Rcos(φ−P A) sini
cos−1i=
=
Vobs−Vsys−Vrot
sini
p1 + tan2(φ−P A)/cos2i
cos−1i (3.5)
Data for the rotation curves, the inclination angle and the PA of the line of nodes for each galaxy are taken from Epinatet al. (2008), see Table 3.1. But these rotation curves were fitted using a different parametric model than these authors, we use the parametric model of Giovanelli & Haynes (2002) (as for NGC 6946 in Chapter 2),
Vpe(r) =V0(1−e−r/rpe)(1 +α r/rpe) (3.6) where V0 regulates the overall amplitude of the rotation curve, rpe yields a scale length for the inner steep rise, and α sets the slope of the slowly changing outer part. After a least square fit of this model to the data, the parameters obtained for each galaxy are summarized in Table 3.3.
Table 3.3: Parameters from the rotation curve fit for each galaxy.
Galaxy V0 rpe α R
(km s−1) (arcsec) (correlation)
NGC 278 219.73±59.1 1.08±0.36 -0.40±0.17 0.907 NGC 1058 74.5±12.7 0.16±0.10 0.05±0.03 0.668 UGC 3574 155.5±18.5 1.29±0.31 0.024±0.017 0.916 NGC 2500 55.9±3 0.40±0.53 0.04±0.007 0.872
** The rotation curve has been fitted by a least squares fit, following the parametric model of Giovanelli & Haynes (2002): Vpe(r) =V0(1−e−r/rpe)(1 +α r/rpe).
Figure 3.6 shows the observed and vertical velocities, Vobs andVZ, for NGC 278. Top panels show the observed radial velocities together with the fitted rotation curve, to be subtracted from the latter, and for each position angle. The resulting vertical components of the velocity, equation 3.5, are represented versus galactocentric distance at each position
angle (with different color code) along the slit in the middle panels; zero marks the galactic center position. The Hαemission line flux is also plotted, with the aim of looking for some relation or correlation between the velocity and flux peaks.
As the rotational velocity is fitted with least squares, Vrot = V(R) +erot, the errors in the fitting can be clearly observed in some cases, middle panels of Fig. 3.6, where the rotational residues are not completely removed. So, what it is actually being represented is
n
Vobs−Vsys−V(R)f(φ, P A) sinio
cos−1i=erotf(φ, P A) tani+VZ (3.7) wheref(φ, P A) = (1+tan2(φ−P A)/cos2i)−12. The left hand side of the above equation, that we assume as VZ, is then an approximation of the actual perpendicular component.
To remove this effect, the vertical velocity component has been detrended,∆VZ, by fitting a linear component to the gentle rise for each rotation curve side, the approaching and the receding. Therefore, the global trend from the rotational residues is removed, remaining only the local fluctuations or oscillations of the vertical velocities.
∆n
Vobs−Vsys−V(R)f(φ, P A) sinio
cos−1i'VZactual (3.8) This analysis is done for the rest of galaxies, Figures 3.7, 3.8, and 3.9 show the variations of the vertical velocity component across the disk, through the spiral arms and for different slit positions. In many cases a remarkably well-defined wavelike structure is clear.
Analyzing the detrended vertical velocities∆VZfor NGC 278, bottom panels of Fig. 3.6, we find the same kinematical behaviour than that found by Alfaroet al. 2001 in NGC 5427.
A systematic displacement between the velocities and the emission line peaks, where the approaching (negative values) peaks of ∆VZ occur in the convex border of the arms, and the receding maxima (positive values) are located behind the Hαemission maxima, in the concave side. This kinematical behavior is similar to the response of a gas flow into a spiral density wave in a thick and magnetized gaseous disk, described by the Martos & Cox (1998) and Martoset al. (1999) galactic bore models.
A remarkable feature is found in the slit position PA=143.5◦, where a strong vertical velocity peak at∼0.8kpc from the galactic center is related with a very faint Hαemission
peak. As it weren’t associated with a spiral arm. At this slit position however it is observed that the Hαemission is weaker in one side (positive distances from the center) than in the other. The other two slits have both sides with similar Hαintensities.
The kinematical relation described above, between the vertical velocities and the Hαemission peaks, can be also found in NGC 1058 (Fig. 3.7). In this case the vertical displacements of the velocity seem to be smaller than in NGC 278, in the majority of order 10 km s−1, with some peaks no bigger than 15 km s−1. The vertical displacement appears more continuously, with a more sinusoidal wave like aspect than for NGC 278.
It seems than this kinematical relation is more associated with the encounter with a spiral arm, than with its intensity. Such as it can be observed from these two last galaxies, the magnitude order of the vertical displacements,VZ, is not related with the Hαintensity emission.
The other two galaxies of the sample do not show so clearly this kinematical behaviour.
The Hαemission line was fitted pixel by pixel across the slits for NGC 278 and NGC 1058, however in NGC 2500 and UGC 3574 there are some regions with no Hαemission or where it was too faint to be fitted, see Figures 3.8 and 3.9. This is more noticeable in UGC 3574, loosing therefore a global view of the vertical velocity deviations, that only can be discerned, with respect the Hαemission peaks. So, VZ has not been detrended for this galaxy.
For NGC 2500 this discontinuity in the Hαemission line fit is not so extreme. There are only small gaps in the Hαfit, for the slit positions of 40.7◦ and 79.5◦, but theVZ structure can be clearly distinguished.
However, we find in the slit with PA=24◦, where the Hαemission line is fitted continuously pixel by pixel along the slit, that the relation between VZ, or ∆VZ, with the Hαemission peaks does not seem to hold as for NGC 278 and NGC 1058. The clearest features differing are the most negative peak (approaching peak) of ∆VZ, at a distance of ∼ −1.8 kpc, that occurs in the concave side of the arms, instead of appearing in the convex border. And its preceding positive peak of∆VZ (receding peak) is located in front of a Hαemission maxima, not behind.
In the other two slits, with PA=40.7◦and 79.5◦, the central regions do not show neither a clear structure of∆VZ, between distances of -1.8 to 1.5 and -2.5 to 2.5 kpc respectively.
In this regions the Hαemission is much more fainter than in the extremes, the error bars are therefore bigger, and we also find gaps in the Hαemission line fit.
In the ends of these two slits the ∆VZ structure is clearer and its relation with the Hαemission peaks. But we find that for the slit with PA=40.7◦such relation is more similar with that of PA=24◦, differing with the galactic bore model of NGC 278 and NGC 1058.
Only in the slit with PA= 79.5◦, between distances of∼2.8 to 4.2 kpc, this kinematical relation seems to agree with the latter.
This part of the work is still in an analysis stage, and more study about the kinematical properties of these galaxies is required before trying to give any firm conclusion or result.
3.5 Diagnostic Diagrams
A brief introduction about the Diagnostic Diagrams (DD) can be found in Section 2.5 of previous Chapter 2. To complete the kinematic analysis in this chapter, DD are included to determine the ionizing mechanisms in the observed features. The ionization and excitation process can be determined by diagnostic of ratios of [NII]/Hαversus [OIII]/Hβ, depending on its location in such diagrams (Baldwin, Philips & Terlevich 1981).
As Hβ and [OIII] data are available, the blue spectra, [NII]/Hαversus [OIII]/Hβ can be plotted, as one of the best options among all the possible emission line ratios (Veilleux &
Osterbrock 1987), and therefore one of the most commonly and widely used DD.
In any case, both this DD and that used for NGC 6946, [SII]/Hαversus [NII]/Hα, are applied for determining the excitation mechanism. They are found in Figure 3.10 for each galaxy. The color code is the same as in previous Figures 3.6 to 3.9, for clarity and coherence.
Moreover, thanks to the availability of Hβ data, the flux ratios are corrected from reddening following the relation
IHα
IHβ = FHα
FHβ ×100.4(AHα−AHβ) = FHα
FHβ ×100.4AV(kHα−kHβ) (3.9)
Figure 3.6: NGC 278, for the three slits positions. Top panels: The observed velocities for NGC 278, derived from the Hαemission lines (see Section 3.3). The red line corresponds with the projected rotation curve, and the red dot marks the systemic velocity. The Hαintensity is drawn in the right-hand ordinate, with a dotted line. Thex-axisrepresents the distance to the reference pixel, in kpc units;Middle panels:
The perpendicular velocityVZ, at each pixel position across the slit, calculated as Equation 3.5;Top panels:
The detrended perpendicular velocity, after fitting a linear component to the global trend. A dashed line marking the zero value emphasizes that the global trend has been practically removed, remaining only the local oscillations ofVZ.
Figure 3.7: NGC 1058, PA = 32.5◦. Top panels: The observed velocities for NGC 1058, derived from the Hαemission lines (see Section 3.3). The red line corresponds with the projected rotation curve, and the red dot marks the systemic velocity. This PA slit position is very close to the minor axis and the rotation curve projection appears to be rather constant, equal to the Vsys, since the rotational velocity is negligible. The Hαintensity is drawn in the right-hand ordinate, with a dotted line. Thex-axisrepresents the distance to the reference pixel, in kpc units;Middle panels: The perpendicular velocityVZ, at each pixel position across the slit, calculated as Equation 3.5; Bottom panels: The detrended perpendicular velocity after fitting a linear component to the global trend, residuals still from the fits of the rotational velocity. The removal of such global trend can be checked by just comparing with the central plot.
where Iλ is the un-reddened emission at a certain wavelength from the source, and Fλ is the observed flux, affected by dust absorption. Aλ = kλAV is the extinction factor at a particular waveband λ, and kλ and AV are the extinction curve and the absorption in the V band respectively. A detailed explanation of the reddening and its correction is described in Chapter 5. The Balmer decrement is a way for reddening correction, deduced from the comparison of the observed Hαto Hβ flux ratio with the theoretical value of2.86 (Osterbrock 1989), based on case B recombination. Applying the Balmer decrement we have that,
AV = log(2.86)−log(FHβ/FHα)
0.4(kHα−kHβ) (3.10)
Figure 3.8: Top panels: The observed velocities for UGC 3574, derived from the Hαemission lines (see Section 3.3). The red line corresponds with the projected rotation curve, and the red dot marks the systemic velocity. For this galaxy only some regions of the slit were fitted, due to its fainter emission as it can be clearly observed in Figure 3.4. The Hαintensity is drawn in the right-hand ordinate, with a dotted line.
Thex-axisrepresents the distance to the reference pixel, in kpc units;Bottom panels: The perpendicular velocityVZ, calculated as Equation 3.5, is only represented where it was possible to fit the Hαemission line.
So, the unreddened flux ratios can be obtained as
log Iλ1
Iλ2
=log Fλ1
Fλ2
+ 0.4AV(kλ1−kλ2) =
=logFλ1 Fλ2
+C1logFHα FHβ
−C2 (3.11)
whereC1= kkλ2−kλ1
Hα−kHβ, andC2=log(2.86)C1.
From Figure 3.10 we can deduce that photoionization, from high energetic photons, is the main ionization mechanism. Although there is a small portion of the gas that appears to be ionized by low-velocity shocks. In NGC 1058, for the slit with PA= 125◦, this portion is more important.
Two clear different patterns appear in the [NII]/Hαversus [OIII]/Hβ diagnostic
Figure 3.9: Top panels: The observed velocities for NGC 2500, derived from the Hαemission lines (see Section 3.3). The red line corresponds with the projected rotation curve, and the red dot marks the systemic velocity. The Hαintensity is drawn in the right-hand ordinate, with a dotted line. Thex-axisrepresents the distance to the reference pixel, in kpc units;Bottom panels: The detrended perpendicular velocity∆VZ, at each pixel position across the slit, calculated as Equation 3.5
diagrams. NGC 278 and NGC 1058 have all their pixels, for the three slit positions, concentrated at the same location in the DD. Whereas for NGC 2500 and UGC 3574 each slit position occupies a different location in the DD, nearly covering the theoretical curve separating the different ionization mechanisms, as metallicity gradient.
It should be interesting to analyze how the kinematical behaviour found in last section, between the vertical velocitiesVZ location respect the Hαemission peaks (coinciding with a galactic bore model or not), is related with these two patterns in the DD. Since those galaxies where the kinematical behaviour clearly coincided with a galactic bore model, NGC 278 and NGC 1058, occupy the same location in the DD clearly differentiate from the other two galaxies, NGC 2500 and UGC 3574, where such kinematical relation can not be established.